What Scientists Expect to Learn From Cassini’s Upcoming Plunge Into Saturn

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Saturn as imaged from above by Cassini last year. Over the next five months, the spacecraft will orbit closer and closer to the planet and will finally plunge into its atmosphere. (NASA)

Seldom has the planned end of a NASA mission brought so much expectation and scientific high drama.

The Cassini mission to Saturn has already been a huge success, sending back iconic images and breakthrough science of the planet and its system.  Included in the haul have been the discovery of plumes of water vapor spurting from the moon Encedalus and the detection of liquid methane seas on Titan.  But as members of the Cassini science team tell it, the end of the 13-year mission at Saturn may well be its most scientifically productive time.

Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory (JPL) put it this way: “Cassini will make some of its most extraordinary observations at the end of its long life.”

This news was first announced last week, but I thought it would be useful to go back to the story to learn more about what “extraordinary” science might be coming our way, with the help of Spilker and NASA headquarters Cassini program scientist Curt Niebur.

And the very up close encounters with Saturn’s rings and its upper atmosphere — where Cassini is expected to ultimately lose contact with Earth — certainly do offer a trove of scientific riches about the basic composition and workings of the planet, as well as the long-debated age and origin of the rings.  What’s more, everything we learn about Saturn will have implications for, and offer insights into, the vast menagerie of  gas giant exoplanets out there.

“The science potential here is just huge,” Niebur told me.  “I could easily conceive of a billion dollar mission for the science we’ll get from the grand finale alone.”

The Cassini spacecraft will make 22 increasingly tight orbits of Saturn before it disappears into the planet’s atmosphere in mid-September, as shown in this artist rendering.  (NASA/JPL-Caltech)

The 20-year, $3.26 billion Cassini mission, a collaboration of NASA, the European Space Agency and the Italian Space Agency,  is coming to an end because the spacecraft will soon run out of fuel.  The agency could have just waited for that moment and let the spacecraft drift off into space, but decided instead on the taking the big plunge.

This was considered a better choice not only because of those expected scientific returns, but also because letting the dead spacecraft drift meant that theoretically it could be pulled towards Titan or Enceladus — moons that researchers now believe just might support life.

Because the spacecraft wasn’t sterilized before launch, scientists didn’t want to take the chance that it might carry some earthly bacteria that could possibly contaminate the moons with our life.

So instead Cassini will be sent on 22 closer and closer passes around Saturn, into the region between the innermost ring and the atmosphere where no spacecraft has ever gone.  On April 26, Cassini will make the first of those dives through a 1,500-mile-wide  gap between Saturn and its rings as part of the mission’s grand finale.

As it makes those terminal orbits, the spacecraft will have to be maneuvered with precision so it doesn’t actually fly into one of the rings.  They consist of water ice, small meteorites and dust, and are sufficiently dense to fatally damage Cassini.

“Based on our best models, we expect the gap to be clear of particles large enough to damage the spacecraft. But we’re also being cautious by using our large antenna as a shield on the first pass, as we determine whether it’s safe to expose the science instruments to that environment on future passes,” said Earl Maize, Cassini project manager at the NASA Jet Propulsion Lab. “Certainly there are some unknowns, but that’s one of the reasons we’re doing this kind of daring exploration at the end of the mission.”

Then in mid-September, following a distant encounter with Titan and its gravity, the spacecraft’s path will be bent so that it dives into the planet itself.  The final descent will occur in mid September, when Cassini enters the atmosphere where it will soon begin to spin and tumble, lose radio contact with Earth, and then ultimately explode due to pressures created by the enormous planet.

All the while it will be taking pioneering measurements, and sending back images predicted to be spectacular.

The age and origin of the rings of Saturn remains a subject of a great debate that may soon come to an end. Ring particle sizes range from tiny, dust-sized icy grains to a few particles as large as mountains. Two tiny moons orbit in gaps (Encke and Keeler gaps) in the rings and keep the gaps open. (NASA)

While the Cassini team has to keep clear of the rings, the spacecraft is expected to get close enough to most likely answer one of the most long-debated questions about Saturn:  how old are those grand features, unique in our solar system?

One school of thought says they date from the earliest formation of the planet, some 4.6 billion years ago.  In other words, they’ve been there as long as the planet has been there.

But another school says they are a potentially much newer addition.  They could potentially be the result of the break-up of a moon (of which Saturn has 53-plus) or a comet, or perhaps of several moons at different times.  In this scenario, Saturn may have been ring-less for eons.

As Niebur explained it, the key to dating the rings is a close view of, essentially, how dirty they are.  Because small meteorites and dust are a ubiquitous feature of space, the rings would have significantly more mass if they have been there 4.6 billion years.  But if they are determined to be relatively clean, then the age is likely younger, and perhaps much younger.

“Space is a very dirty place, with dust and micro-meteorites hitting everything.  Over significant time scales this stuff coats things.  So if the rings the rings are old, we should find very dirty ice.  If there is little covering of the ice, then the rings must be young.  We may well be coming to the end of a great debate.”

A corollary of the question of the age of Saturn’s rings is, naturally, how stable they are.

Curt Neibur, lead program scientist at NASA headquarters for the Cassini mission. (NASA)

If they turn out to be as old as the planet, then they are certainly very stable.  But if they are not old, then it is entirely plausible that they could be a passing phenomenon and will some day disappear — to perhaps re-appear after another moon is shattered or comet arrives.

Another way of looking at the rings is that they may well have been formed at different times.

As Cassini Project Scientist Linda Spilker explained in an email, Cassini’s measurements of the mass of the rings will be key.  “More massive rings could be as old as Saturn itself while less massive rings must be young.  Perhaps a moon or comet got too close and was torn apart by Saturn’s gravity.”

The voyage between the rings will also potentially provide some new insights into the workings of the disks present at the formation of all solar systems.

“The rings can teach us about the physics of disks, which are huge rings floating majestically and with synchronicity  around the new sun,” Niebur said.  “That said, the rings of Saturn have a very active regime, with particles and meteorites and micrometeorites smacking into each other.  It’s an amazing environment and has direct relevance to the nebular model of planetary formation.”

This recently released Cassini image show’s moon Daphnis, which is embedded within a ring.  The moon
kicks up waves as it orbits within what is called the Keeler gap. This mosaic combines several previous images to show more waves in the gap edges. (NASA/JPL-Caltech)

Another open question that scientists hope will be answered during the plunge is how long, precisely, is a day on Saturn.

The saturnine day is often given as between 10.5 and 11 hours, but that lack of precision is unique in our solar system.

The usual way to determine a planet’s rotation is to look for a distinctive point and watch to see how long it takes to reappear.   But Saturn has thousands of miles of thick clouds between the rings and the core, and so no distinctive points have been found.

The planet’s inner rocky core and outer core of metallic hydrogen create magnetic fields that potentially could be traced to measure a full rotation. But competing magnetic fields in the complex Saturn ring and moon system make that also difficult.

“The truth is that we don’t know how long a day is on Saturn,” Niebur said.  “But after the finale, we will finally know.”

The answer will hopefully come by measuring the expected “wobble” of the magnetic field inside the rings. Since Cassini will pass beyond the magnetic interference of those rings, the probe should get the most precise magnetic readings ever taken.

Project scientist Spilker is optimistic.  “With the magnetic field we’ll be able to get, for the first time, the length of day for the interior of Saturn. If there’s just a slight tilt to the magnetic field, then it will wobble around and give us the length of a day.”

Artist rendering of Cassini over Saturn’s north pole, with it huge hexagon-shaped storm. (NASA/JPL-Caltech)

Perhaps the most consequential findings to come out of the Cassini finale are expected to involve the planet’s internal structure and composition.

The atmosphere is known to contain hydrogen, helium, ammonia and methane, but Niebur said that other important trace elements are expected to be present.  The probe will use its mass spectrometer to “taste” the chemistry of the gases on the outermost edge of Saturn’s atmosphere and return the most detailed information ever about Saturn’s high-altitude clouds, as well as about the ring material.

Instruments will also measure Saturn’s powerful winds (which blow up to 1,000 miles an hour), and determine how deep they go in the atmosphere.  Like much about Saturn, that basic fact falls in the “unknown” category.

For both Spilker and Niebur, the biggest prize is probably determining the size and mass of Saturn’s rocky core, made up largely of iron and nickel.  That core is estimated to be 9 to 22 times the mass of the Earth, and to have a diameter of perhaps 18,000 miles. 

Cassini project scientist Linda Spilker of JPL was on the Voyager team in the 1970s. She has a long-standing research interest in Saturn’s rings. (Bill Youngblood, Caltech)

But these are broad estimates, and neither the size nor mass is really known.  Those thousands of miles of thick clouds atop the atmosphere and the planet’s chaotic magnetic fields have made the necessary readings impossible.

The Cassini instruments, however, are expected to make those measurements during its final months.  As Cassini makes its close-in passes and then enters the atmosphere for the final plunge, it will send back the data needed to make detailed maps of Saturn’s inner magnetic and gravitational fields.  These are what scientists need to understand the core and other structures that lay beneath the planet’s atmosphere.

This work will compliment the parallel efforts underway at Jupiter, where the Juno mission is collecting data on that planet’s core as well.  If scientists can measure the sizes and masses of both cores, they will be able to use that new information to answer many other questions about our solar system and beyond.

“A better understanding Saturn’s interior, coupled with what Juno mission learns about the interior of Jupiter, will lead to (new insights into) how the planets in our solar system formed, and how our solar system itself formed,”  Spilker said in an email.

“This is then related to how exoplanets form around other stars.  Studying our own giant planets will help us understand giant planets around other stars.”

In other words, Saturn and Jupiter are planetary types expected to be found across the galaxies.  And it’s our good fortune to be able to touch and learn from them, and to use that information to analyze distant planets that we can only indirectly detect or just barely see.

 

An animated video about Cassini’s final chapter is available here.

 

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Some Spectacular Images (And Science) From The Year Past

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A rose made of galaxies

This is a golden era for space and planetary science, a time when discoveries, new understandings, and newly-found mysteries are flooding in.  There are so many reasons to find the drama intriguing:  a desire to understand the physical forces at play, to learn how those forces led to the formation of Earth and ultimately us, to explore whether parallel scenarios unfolded on planets far away, and to see how our burgeoning knowledge might set the stage for exploration.

But always there is also the beauty; the gaudy, the stimulating, the overpowering spectacle of it all.

Here is a small sample of what came in during 2016:

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The Small Magellanic Cloud, a dwarf galaxy that is a satellite of our Milky Way galaxy, can be seen only in the southern hemisphere.  Here, the Hubble Space Telescope captured two nebulas in the cloud. Intense radiation from the brilliant central stars is heating hydrogen in each of the nebulas, causing them to glow red.

Together, the nebulas are called NGC 248 and are 60 light-years long and 20 light-years wide. It is among a number of glowing hydrogen nebulas in the dwarf satellite galaxy, which is found approximately 200,000 light-years away.

The image is part of a study called Small Magellanic Cloud Investigation of Dust and Gas Evolution (SMIDGE). Astronomers are using Hubble to probe the Milky Way satellite to understand how dust is different in galaxies that have a far lower supply of heavy elements needed to create that dust.  {NASA.ESA, STSci/K. Sandstrom (University of California, San Diego), and the SMIDGE team}

This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope.

Probably the biggest exoplanet news of the year, and one of the major science stories, involved the discovery of an exoplanet orbiting Proxima Centauri, the star closest to our own.

This picture combines a view of the southern skies over the European Space Observatory’s 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left).

The planet Proxima Centauri b is thought to lie within the habitable zone of its star.  Learning more about the planet, the parent star and the two other stars in the Centauri system has become a focus of the exoplanet community.

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We all know about auroras that light up our far northern skies, but there’s no reason why they wouldn’t exist on other planets shielded by a magnetic field — such as Jupiter.  Astronomers using the Hubble Space Telescope have found them on the poles of our solar system’s  largest planet, and produced far ultraviolet light images taken as the Juno spacecraft approached the planet.

Auroras are formed when charged particles in the space surrounding the planet are accelerated to high energies along the planet’s magnetic field. When the particles hit the atmosphere near the magnetic poles, they cause it to glow like gases in a fluorescent light fixture. Jupiter’s magnetosphere is 20,000 times stronger than that of Earth.

The full-color disk of Jupiter in this image was separately photographed at a different time by Hubble’s Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project that annually captures global maps of the outer planets.

Inside the Crab Nebula

Peering deep into the core of the Crab Nebula, this close-up image reveals the heart of one of the most historic and intensively studied remnants of a supernova, an exploding star. The inner region sends out clock-like pulses of radiation and tsunamis of charged particles embedded in magnetic fields.

The neutron star at the very center of the Crab Nebula has about the same mass as the sun but compressed into an incredibly dense sphere that is only a few miles across. Spinning 30 times a second, the neutron star shoots out detectable beams of energy that make it look like it’s pulsating.

The NASA Hubble Space Telescope image is centered on the region around the neutron star (the rightmost of the two bright stars near the center of this image) and the expanding debris surrounding it. Intricate details of glowing gas are shown in red and the blue glow is radiation given off by electrons spiraling at nearly the speed of light in the powerful magnetic field around the crushed stellar core.

Observations of the Crab supernova were recorded by Chinese astronomers in 1054 A.D. The nebula, bright enough to be visible in amateur telescopes, is located 6,500 light-years away in the constellation Taurus.  (NASA, ESA)

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The Gemini Planet Imager provides some of the earliest high-resolution, high-contrast direct imaging of exoplanets.  Using a coronagraph inside the telescope to block out the light of the star, the GPI can then allow researchers to see the region surrounding that star — in other words, where exoplanets might be.

This image includes a wide-angle view of the star HD 106906 taken by the Hubble Space Telescope and a close-up view from the Planet Imager, which operates on the Gemini South telescope in Chile’s Atacama Desert.  The image reveals a disturbed system of comets near the star, which may be responsible for the orbit of the the unusually distant giant planet (upper right).

The GPI Exoplanet Survey is operated by a team of astronomers from the University of California at  Berkeley and 23 other institutions, and is targeting 600 young stars to understand how planetary systems evolve over time.

Paul Kalas of UC Berkeley is responsible for the image and led the team that wrote about it. That paper actually came out in the Astrophysical Journal in late 2015 but, hey, that’s almost 2016.

Astronomers have regularly found a galaxy or star that is the furthest from us ever to be detected.  But the record is there to be broken, and in 2016 it was astronomers from the Great Observatories Origins Deep Survey (GOODS) who made the discovery.

Galaxy GN-z11, shown in the inset, was imaged as it was 13.4 billion years in the past, just 400 million years after the big bang.  That means the universe was only three percent of its current age when the light left that galaxy.

The galaxy has many blue stars that are bright and young, but it looks red in this image because its light has been stretched to longer spectral wavelengths by the expansion of the universe.

(NASA, ESA, P. Oesch (Yale University), G. Brammer ( STScI)), P. van Dokkum (Yale University), and G. Illingworth (University of California, Santa Cruz)

Biggest announce of discovred exoplanets by Kepler. (No, those are not real images, but still...)

No, these are not images of actual exoplanets, but they represent the continuing work of one of NASA’s most pioneering and productive missions, the Kepler Space Telescope. In May the Kepler team announced the detection of 1284 more planets or planet candidates as part of its newest catalog, the largest number announced at once in the mission.

To date, Kepler has identified unconfirmed 4,696 planet candidates, 2,331 confirmed planets, and 21 confirmed small planets in a habitable zone. In addition, the follow-on K2 mission has identified 458 candidate planets and 173 confirmed.

The Kepler spacecraft stared fixedly at a small portion of the sky for four years, looking to identify miniscule dimmings in the brightness of stars that would indicate that a planet was passing between the telescope and the star. In this way, Kepler has established a census of exoplanets that has been extrapolated to show the presence of billions and billions of planets around other stars.

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The 21-foot array that will collect photons for the James Webb Space Telescope was finished and put on display in November at the Goddard Space Flight Center. It will be the largest mirror to go into space, and will likely make the JWST into the most powerful and far-seeing observatory ever.

It will observe in the infrared portion of the spectrum because its goals include peering deep into the past of the universe, which is now most visible in the infrared. This means the JWST will have to be cooled to -364 degrees F, just 50 degrees above absolute zero.  To achieve that temperature, it’s insulated from the sun by five membrane layers, each no thicker than a human hair. Placing those membranes was finished in November, marking an end to construction of the telescope “mirror.”

The project has been enormously ambitious, and with that has come long delays and budget overruns that almost resulted in it being scrapped. Just this month, some early vibrating tests – designed to simulate launch conditions – experienced an anomaly that NASA engineers are working on now.  The JWST is scheduled to launch in late 2018.

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This composite image shows suspected plumes of water vapor erupting at the 7 o’clock position of Jupiter’s moon Europa. The plumes, photographed by NASA’s Hubble’s Space Telescope Imaging Spectrograph, were seen in silhouette as the moon passed in front of Jupiter.

While the plumes spitting out of Saturn’s moon Enceladus are much better known now — the Cassini spacecraft flew through them in 2015, after all — the growing scientific consensus that Europa also has some plumes may be of even greater importance.  That moon is much larger, its ice-covered oceans have been determined to hold more water than all the oceans of Earth, and those oceans have clearly been around for a long time.

Hubble’s ultraviolet sensitivity allowed for the detection of the plumes, which rise more than 100 miles above Europa’s icy surface. The image of Europa, superimposed on the Hubble data, is assembled from data from the Galileo and Voyager missions. (NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center.)

compounds being created in the xxx nebula

How the fundamentals needed for life are created in space has been a longstanding mystery.  The cosmos, after all, began with hydrogen and helium, and that was about it.  But life needs carbon atoms connected to hydrogen, oxygen, nitrogen and other elements

Astronomers and astrochemists have been making progress in recent years and now understand the basics of how the heavier elements are formed in space.  New data from the European Space Agency’s Herschel Space Observatory has gone further and has established that ultraviolet light from stars plays a key role in creating these molecules.  Previously, scientists thought that turbulence created by “shock” events was the driving force.

This image is of the Orion nebula, where scientists studied carbon chemistry of a major star-forming region. Herschel probed an area of the electromagnetic spectrum — the far infrared, associated with cold objects — that no other space telescope has reached before so it could take into account the entire Orion Nebula instead of individual stars.

The result was a better understanding of how carbon and hydrogen reach the states necessary to bond and form the basic carbon chemistry of the cosmos (and of life.)

Within the inset image, the emission from ionized carbon atoms (C+), overlaid in yellow, was isolated and mapped out from spectrographic data.

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Following a successful close flyby of Enceladus, the NASA-ESA Cassini spacecraft captured this image of the moon with Saturn’s rings beyond.

The image was taken in visible light with the Cassini spacecraft wide-angle camera when it was about 106,000 miles away from Enceladus. That flyby turned into a fly-through as well, when Cassini entered the plumes of water vapor and dust that shoot out of the bottom of the moon.

Scientists already know that an array of organic and other chemicals are in the plumes, but the field is awaiting word about the presence (or absence) of molecular hydrogen, which is formed when water comes into contact with rocks in hydrothermal vents.  Many think that Enceledus is habitable and should be tested for signs of life because biosignatures could potentially exist in the relatively easy-to-access geysers.

moon-halo-5-13-2016-yuri-beletsky-chile-pano

While Yuri Beletsky is a staff astronomer at the Las Campanas Observatory in Chile, he is also a noted astrophotographer who specializes in capturing the beauty of nighttime scenes — usually connecting the celestial with the terrestrial.

In this 2016 photo, the moon is surrounded by a halo caused by the presence of millions of ice crystals in the upper atmosphere.  Great conditions for an astrophotographer, but pretty much useless for an astronomer.

The star within the halo is Regulus, brightest object in the constellation Leo the Lion. On the left outside the halo is Procyon from Canis Minor and on the right is the planet Jupiter.

As is so often the case in this line of endeavor, it’s quite a sight to see.

 

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Waiting on Enceladus

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NASA's Cassini spacecraft completed its deepest-ever dive through the icy plume of Enceladus on Oct. 28, 2015. Credits: NASA/JPL-Caltech
NASA’s Cassini spacecraft completed its deepest-ever dive through the icy plume of Enceladus on Oct. 28, 2015. (NASA/JPL-Caltech)

Of all the possible life-beyond-Earth questions hanging fire, few are quite so intriguing as those surrounding the now famous plumes of the moon Enceladus:  what telltale molecules are in the constantly escaping jets of water vapor, and what dynamics inside the moon are pushing them out?

Seldom, if ever before, have scientists been given such an opportunity to investigate the insides of a potentially habitable celestial body from the outside.

The Cassini mission to Saturn made its closest to the surface (and last) plume fly-through a year ago, taking measurements that the team initially said they would report on within a few weeks.

That was later updated by NASA to include this guidance:  Given the important astrobiology implications of these observations, the scientists caution that it will be several months before they are ready to present their detailed findings.

The reference to “important astrobiology implications” certainly could cover some incremental advance, but it does seem to at least hint of something more.

I recently contacted the Jet Propulsion Lab for an update on the fly-through results and learned that a paper has been submitted to the journal Nature and that it will hopefully be accepted and made public in the not-too-distant future.

All this sounds most interesting but not because of any secret finding of life — as some might infer from that official language.  Cassini does not have the capacity to make such a detection, and there is no indication at this point that identifiable byproducts of life are present in the plumes.

What is intriguing is that the fly-through was only 30 miles above the moon’s surface — the closest pass through a plume ever by Cassini — and so presumably its instruments produced some new and significant findings.

The scientists writing the paper could not, of course, discuss their findings before publication.  But Jonathan Lunine, a Cornell University planetary scientist and physicist on the Cassini mission with a longtime and deep interest in Enceladus, was comfortable discussing what is known about the moon and what Cassini (and future missions) still have to explain.

And thanks to that briefing, it became apparent that whatever new findings are coming, they will not make or break the case for the moon as a habitable place. Rather, they will essentially add to a strong case that has already been made.

“I think the evidence shows that Enceladus is the most promising target (for finding life beyond Earth) in the solar system,” Lunine told me.

enceladus geyser
Icy cyrogeysers erupt at the southern pole of Enceladus. (NASA/JPL-Caltech/Space Science Institute)

Any new findings from the October 28, 2015 fly-through would certainly be useful in terms of understanding the habitability of the moon, but he said that the logical question to ask now takes the story much further.  “Is the moon inhabited? That’s what I want to know now.”

That Lunine is such an enthusiastic supporter of a habitable Enceladus is not surprising:  He is concept principal investigator for the Enceladus Life Finder, probably the most advanced of several proposed missions looking for NASA support.

What is surprising, at least to those who have not followed Enceladus developments with the intensity they seem to deserve, is how much is already known about the moon and its potential for supporting life.

I’ll lay out Lunine’s case, but first a little background:

Enceladus is one of the 53 (or more) moons of Saturn, and is roughly the width of Colorado– about 310 miles in diameter.  It is one of Saturn’s major inner moons, is covered in ice and as a result reflects a lot of light and is one of the brightest objects in the sky.   But it didn’t attract much scientific attention until 2005 when those water vapor and dust plumes were detected shooting out from its south pole by Cassini.

Further study strongly suggests that Enceladus has a global liquid ocean between its rocky core and its icy surface, and the plumes, or geysers, consist of water pushed out through cracks in that surface.  The ocean is small in comparison to that of Jupiter’s watery moon Europa — the first is roughly the volume of Lake Superior while the latter has more water than all the oceans of the Earth put together — but as Lunine put it, “bacterium could do just fine in a Lake Superior-sized ocean.”

The history of Enceladus and its ocean are little understood in comparison with Europa, which Lunine said has probably had a stable ocean under its ice cover for billions of years.  But unlike Europa, Enceladus has that singular advantage of constantly spitting out its insides for us to study and gradually understand.  (Yes, researchers using the Hubble Space Telescope have detected what they concluded could be some water vapor plumes on Europa, too, but that finding is not confirmed.)

The equatorial surface of Enceladus is a beyond frigid  -340 degrees Fahrenheit,  but the temperatures around the southern polar fractures are a still cold but much warmer -100 to -130 degrees Fahrenheit. What’s important is the huge difference in temperatures — in the range of 200 degrees Fahrenheit.

The presumed sources of the heat are friction caused by gravitational forces from Saturn, and scalding heat from the core that enters the water through hydrothermal vents.

enceladus has a large -- 60/40 or 70/30
Scientists estimate that the ratio of rock to water and ice on Enceladus is in the range of 65 percent rock to 35 percent H2O..  NASA

So, what is known about the geysers being pushed out of Encedadus, and about the dynamics causing the phenomenon?

Already published papers report that the water vapor, which can extend out three times the diameter of the moon, is salty, filled with fine dust particles, and contains molecules including carbon dioxide, methane, molecular nitrogen, propane, acetylene, formaldehyde and traces of ammonia.  While none of these compounds are a biosignature per se, many are associated with life.

Recent analysis of some of the dust particles concluded that they were from the floor of the Enceladus ocean, and based on their characteristics appear to have been formed by the interaction of water and rock.  The most logical site for this kind of interaction is at hydrothermal vents, where heat from the core makes its way up into the water.  Some have argued that life on Earth may well have started at potentially similar hydrothermal vents on early Earth.

Jonathan Lunine is the David C. Duncan Professor of xxx at Cornell University, and Director, Center for Radiophysics and Space Research. He's also a member of the Cassini team.
Jonathan Lunine is the David C. Duncan Professor in the Physical Sciences at Cornell University, and Director of the Cornell Center for Astrophysics and Planetary Science. He’s also a longtime member of the Cassini team. (Cornell)

One of the primary goals of that final close fly-through was to collect data that would allow the Cassini scientists to measure how much hydrothermal activity is occurring within Enceladus.

If substantial amounts can be detected, that increases the chances for the existence around of vents of simple forms of life.  Measurements for hydrothermal activity depend on the detection of methane (which has already occurred) and of molecular hydrogen (which scientists were looking for in that final fly-through.)  Measurements for molecular hydrogen can be difficult to make, which might explain some of the time lag.

At the low altitude, the team also expected to be more sensitive to the possible presence of heavier and more massive molecules — including organics — that would not be observed during previous, higher-altitude passes through the plume.

One potentially complicating issue is that measurements of the pH of the water has come back with quite high alkaline levels.  If that is limited to areas around hydrothermal vents then it isn’t a problem for life, Lunine said.  But if it was far more widespread, it could be.

So these are some of the results we are now awaiting.  But to Lunine (and others on and off the Cassini team) the case for habitability and a possible home for life on Enceladus has already been made.

“What we already know is that the ocean has the general characteristics of habitability.  Obviously, it has liquid water and so there’s an energy source keeping it from all freezing.  It appears to have varied thickness but is still global. It’s salty and has organic molecules, as well as those small grains of silica.  The simplest model for why they exist is that water is cycling through quite warm rock at the base of the ocean, dissolving silica and delivering it to the ocean.

“Put this and more together and you have a signal, a big red arrow pointing to this moon saying it may well support life, and needs to be explored more and soon.”

The plumes of Enceladus originate in the long tiger stripe fractures of the south polar region pictured here. Detailed models support conclusions that the plumes arise from near-surface pockets of liquid water at temperatures of 273 kelvins (0 degrees Celsius). (Cassini Imaging Team, SSI, JPL, ESA, NASA)
The plumes of Enceladus originate in the long tiger stripe fractures of the south polar region pictured here. Detailed models support conclusions that the plumes arise from near-surface pockets of liquid water at temperatures of 273 kelvins (0 degrees Celsius). (Cassini Imaging Team, SSI, JPL, ESA, NASA)

But even if the upcoming Enceladus paper adds significantly to the habitable moon story, another mission to study the plumes may be long in coming.  Limited resources are the major reason why but so too is the congressionally-mandated mission to Europa, a target not dissimilar to Enceladus.

Texas congressman John Culberson has pushed long and hard for the Europa mission (or missions), arguing that the Jovian moon offers our best chance of finding extraterrestrial life in the solar system.  That huge and stable ocean is such a tempting target that the miles of ice encasing it are not seen as an deal-breaking obstacle. (“Thick-icers” and “thin-icers” are in constant debate about how deep that ice might go.)

That Europa is promising in terms of astrobiology is a conclusion that many scientists agree with, and NASA seems eager to cooperate. But it is nonetheless quite unusual to have Congress require NASA to mount a specific and costly mission and to set a timetable for doing it — as Congress did for Europa in 2015.

The congressional requirement follows years of waiting for a Europa mission.  The Galileo mission to Jupiter produced convincing information starting in 1998 that the moon had a large ocean under its ice surface, but almost two decades have gone by without an Europa-specific mission.

Lunine, and others, are pressing to make sure that doesn’t happen with Enceladus.  Last year he proposed a NASA Discovery mission to the moon that wasn’t selected, and has ideas for other sorts of NASA efforts.

“It was a sixteen or seventeen year odyssey to get a mission planned for Europa, and we just hope that doesn’t happen with Enceladus,” he told me.  “We could be testing for bio-activity there and really, where else would that make so much sense?”

 

 

 

 

 

 

 

 

 

 

 

 

 

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How Will We Know What Exoplanets Look Like, and When?

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An earlier version of this article was accidently published last week before it was completed.  This is the finished version, with information from this week’s AAS annual conference.

This image of a pair of interacting galaxies called Arp 273 was released to celebrate the 21st anniversary of the launch of the NASA/ESA Hubble Space Telescope. The distorted shape of the larger of the two galaxies shows signs of tidal interactions with the smaller of the two. It is thought that the smaller galaxy has actually passed through the larger one.
This image of a pair of interacting galaxies called Arp 273 was released to celebrate the 21st anniversary of the launch of the NASA/ESA Hubble Space Telescope. The distorted shape of the larger of the two galaxies shows signs of tidal interactions with the smaller of the two. It is thought that the smaller galaxy has actually passed through the larger one.

Let’s face it:  the field of exoplanets has a significant deficit when it comes to producing drop-dead beautiful pictures.

We all know why.  Exoplanets are just too small to directly image, other than as a miniscule fraction of a pixel, or perhaps some day as a full pixel.  That leaves it up to artists, modelers and the travel poster-makers of the Jet Propulsion Lab to help the public to visualize what exoplanets might be like.  Given the dramatic successes of the Hubble Space Telescope in imaging distant galaxies, and of telescopes like those on the Cassini mission to Saturn and the Mars Reconnaissance Orbiter, this is no small competitive disadvantage.  And this explains why the first picture of this column has nothing to do with exoplanets (though billions of them are no doubt hidden in the image somewhere.)

The problem is all too apparent in these two images of Pluto — one taken by the Hubble and the other by New Horizons telescope as the satellite zipped by.

image

Pluto image taken by Hubble Space Telescope (above) and close up taken by New Horizons in 2015. (NASA)
Pluto image taken by Hubble Space Telescope (above) and close up taken by New Horizons in 2015. (NASA)

Pluto is about 4.7 billion miles away.  The nearest star, and as a result the nearest possible planet, is 25 trillion miles  away.  Putting aside for a minute the very difficult problem of blocking out the overwhelming luminosity of a star being cross by the orbiting planet you want to image,  you still have an enormous challenge in terms of resolving an image from that far away.

While current detection methods have been successful in confirming more than 2,000 exoplanets in the past 20 years (with another 2,000-plus candidates awaiting confirmation or rejection),  they have been extremely limited in terms of actually producing images of those planetary fireflies in very distant headlights.  And absent direct images — or more precisely, light from those planets — the amount of information gleaned about the chemical makeup of their atmospheres  as been limited, too.

But despite the enormous difficulties, astronomers and astrophysicist are making some progress in their quest to do what was considered impossible not that long ago, and directly image exoplanets.

In fact, that direct imaging — capturing light coming directly from the sources — is pretty uniformly embraced as the essential key to understanding the compositions and dynamics of exoplanets.  That direct light may not produce a picture of even a very fuzzy exoplanet for a very long time to come, but it will definitely provide spectra that scientists can read to learn what molecules are present in the atmospheres, what might be on the surfaces and as a result if there might be signs of life.

This diagram illustrates how astronomers using NASA's Spitzer Space Telescope can capture the elusive spectra of hot-Jupiter planets. Spectra are an object's light spread apart into its basic components, or wavelengths. By dissecting light in this way, scientists can sort through it and uncover clues about the composition of the object giving off the light. To obtain a spectrum for an object, one first needs to capture its light. Hot-Jupiter planets are so close to their stars that even the most powerful telescopes can't distinguish their light from the light of their much brighter stars. But, there are a few planetary systems that allow astronomers to measure the light from just the planet by using a clever technique. Such "transiting" systems are oriented in such a way that, from our vantage point, the planets' orbits are seen edge-on and cross directly in front of and behind their stars. In this technique, known as the secondary eclipse method, changes in the total infrared light from a star system are measured as its planet transits behind the star, vanishing from our Earthly point of view. The dip in observed light can then be attributed to the planet alone. To capture a spectrum of the planet, Spitzer must observe the system twice. It takes a spectrum of the star together with the planet (first panel), then, as the planet disappears from view, a spectrum of just the star (second panel). By subtracting the star's spectrum from the combined spectrum of the star plus the planet, it is able to get the spectrum for just the planet (third panel). This ground-breaking technique was used by Spitzer to obtain the first-ever spectra of two planets beyond our solar system, HD 209458b and HD 189733b. The results suggest that the hot planets are socked in with dry clouds high up in the planet's stratospheres. In addition, HD 209458b showed hints of silicates, indicating those high clouds might be made of very fine sand-like particles.
This diagram illustrates how astronomers using NASA’s Spitzer Space Telescope can capture the elusive spectra of hot-Jupiter planets. Spectra are an object’s light spread apart into its basic components, or wavelengths. By dissecting light in this way, scientists can sort through it and uncover clues about the composition of the object giving off the light. (NASA/JPL-Caltech)

There has been lots of technical and scientific debate about how to capture that light, as well as debate about how to convince Congress and NASA to fund the search.  What’s more, the exoplanet community has a history of fractious internal debate and competition that has at times undermined its goals and efforts, and that has been another hotly discussed subject.  (The image of a circular firing squad used to be a pretty common one for the community.)

But a seemingly much more orderly strategy has been developed in recently years and was on display at the just-completed American Astronomical Society annual meeting in Florida.  The most significant breaking news was probably that NASA has gotten additional funds to support a major exoplanet direct imaging mission in the 2020s, the Wide Field Infrared Survey Telescope (WFIRST), and that the agency is moving ahead with a competition between four even bigger exoplanet or astrophysical missions for the 2030s. The director of NASA Astrophysics, Paul Hertz, made the formal announcements during the conference, when he called for the formation of four Science and Technology Definition Teams to assess in great detail the potentials and plausibilities of the four possibilities.

Paul Hertz, Director of the Astrophysics Division of NASA's Science Mission Directorate.
Paul Hertz, Director of the Astrophysics Division of NASA’s Science Mission Directorate.

Putting it into a broader perspective, astronomer Natalie Batalha, science lead for the Kepler Space Telescope, told a conference session on next-generation direct imaging that  “with modern technology, we don’t have the capability to image a solar system analog.”  But, she said, “that’s where we want to go.”

And the road to discovering exoplanets that might actually sustain life may well require a space-based telescope in the range of eight to twelve meters in radius, she and others are convinced.  Considering that a very big challenge faced by the engineers of the James Webb Space Telescope (scheduled to launch in 2018) was how to send a 6.5 meter-wide mirror into space, the challenges (and the costs) for a substantially larger space telescope will be enormous.

We will come back in following post to some of these plans for exoplanet missions in the decades ahead, but first let’s look at a sample of the related work done in recent years and what might become possible before the 2020s.  And since direct imaging is all about “seeing” a planet — rather than inferring its existence through dips in starlight when an exoplanet transits, or the wobble of a sun caused by the presence of  an orbiting ball of rock (or gas)  — showing some of the images produced so far seems appropriate.  They may not be breath-taking aesthetically, but they are remarkable.

There is some debate and controversy over which planets were the first to be directly imaged. But all agree that a major breakthrough came in 2008 with the imaging of the HR8799 system via ground-based observations.

NASA/JPL-Caltech/Palomar Observatory - http://www.nasa.gov/topics/universe/features/exoplanet20100414-a.html This image shows the light from three planets orbiting a star 120 light-years away. The planets' star, called HR8799, is located at the spot marked with an "X." This picture was taken using a small, 1.5-meter (4.9-foot) portion of the Palomar Observatory's Hale Telescope, north of San Diego, Calif. This is the first time a picture of planets beyond our solar system has been captured using a telescope with a modest-sized mirror -- previous images were taken using larger telescopes. The three planets, called HR8799b, c and d, are thought to be gas giants like Jupiter, but more massive. They orbit their host star at roughly 24, 38 and 68 times the distance between our Earth and sun, respectively (Jupiter resides at about 5 times the Earth-sun distance).
This 2010 image shows the light from three planets orbiting HR8799, 120 light-years away.  The three planets, called HR8799b, c and d, are thought to be gas giants like Jupiter, but more massive. (NASA/JPL-Caltech/Palomar Observatory)

First, three Jupiter-plus gas giants were identified using the powerful Keck and Gemini North infrared telescopes on Mauna Kea in Hawaii by a team led by Christian Marois of the National Research Council of Canada’s Herzberg Institute of Astrophysics. That detection was followed several years later the discovery of a fourth planet and then by the release of the surprising image above, produced with the quite small (4.9 foot) Hale telescope at the Palomar Observatory outside of San Diego.

As is the case for all planets directly imaged, the “pictures” were not taken as we would with our own cameras, but rather represent images produced with information that is crunched in a variety of necessary technical ways before their release.  Nonetheless, they are images in a way similar the iconic Hubble images that also need a number of translating steps to come alive.

Because light from the host star has to be blocked out for direct imaging to work, the technique now identifies only planets with very long orbits. In the case of HR8799, the planets orbit respectively at roughly 24, 38 and 68 times the distance between our Earth and sun.  Jupiter orbits at about 5 times the Earth-sun distance.

In the same month as the HR8799 announcement, another milestone was made public with the detection of a planet orbiting the star Formalhaut.  That, too, was done via direct imagining, this time with the Hubble Space Telescope.

The Hubble images were taken with the Space Telescope Imaging Spectrograph in 2010 and 2012. This false-color composite image, taken with the Hubble Space Telescope, reveals the orbital motion of the planet Fomalhaut b. Based on these observations, astronomers calculated that the planet is in a 2,000-year-long, highly elliptical orbit. The planet will appear to cross a vast belt of debris around the star roughly 20 years from now. If the planet's orbit lies in the same plane with the belt, icy and rocky debris in the belt could crash into the planet's atmosphere and produce various phenomena. The black circle at the center of the image blocks out the light from the bright star, allowing reflected light from the belt and planet to be photographed. Credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute)
The Hubble images the the star Formalhaut and planet Formalhaut b were taken with the Space Telescope Imaging Spectrograph in 2010 and 2012. This false-color composite image reveals the orbital motion of the Fomalhaut b. Based on these observations, astronomers calculated that the planet is in a 2,000-year-long, highly elliptical orbit. The black circle at the center of the image blocks out light from the very bright star, allowed reflected light from the belt and planet to be captured.  Credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute)

Signs of the planet were first detected in 2004 and 2006 by a group headed by Paul Kalas at the University of California, Berkeley, and they made the announcement in 2008.  The discovery was confirmed several years later and tantalizing planetary dynamics began to emerge from the images (all in false color.) For instance, the planet appears to be on a path to cross a vast belt of debris around the star roughly 20 years from now.  If the planet’s orbit lies in the same plane with the belt, icy and rocky debris could crash into the planet’s atmosphere and cause interesting damage.

The region around Fomalhaut’s location is black because astronomers used a coronagraph to block out the star’s bright glare so that the dim planet could be seen. This is essential since Fomalhaut b is 1 billion times fainter than its star. The radial streaks are scattered starlight. Like all the planets detected so far using some form of direct imaging,  Fomalhaut b if far from its host star and completes an orbit every 872 years.

Adaptive optics of the Gemini Planet Imager, at the Gemini South Observatory in Chile, has been successful in imaging exoplanets as well.  The GPI grew out of a proposal by the Center for Adaptive Optics, now run by the University of California system, to inspire and see developed innovative optical technology.  Some of the same breakthroughs now used in treating human eyes found their place in exoplanet astronomy.

 

Discovery image of 51 Eri b with the Gemini Planet Imager taken in the near-infrared light on December 18, 2014. The bright central star has been mostly removed by a hardware and software mask to enable the detection of the exoplanet one million times fainter. Credits: J. Rameau (UdeM) and C. Marois (NRC Herzberg).
Discovery image of 51 Eri b with the Gemini Planet Imager taken in the near-infrared light on December 18, 2014. The bright central star has been mostly removed by a hardware and software mask to enable the detection of the exoplanet one million times fainter. Credits: J. Rameau (UdeM) and C. Marois (NRC Herzberg).

The Imager, which began operation in 2014, was specifically created to discern and evaluate dim, newer planets orbiting bright stars using a different kind of direct imaging. It is adept at detecting young planets, for instance, because they still retain heat from their formation, remain luminous and visible. Using the GPI to study the area around the y0ung (20-million-year-old) star 51 Eridiani, the team made their first exoplanet discovery in 2014.

By studying its thermal emissions, the team gained insights into the planet’s atmospheric composition and found that — much like Jupiter’s — it is dominated by methane.  To date, methane signatures have been weak or absent in directly imaged exoplanets.

James Graham, an astronomer at the University of California, Berkeley, is the project leader for a three-year GPI survey of 600 stars to find young gas giant planets, Jupiter-size and above.

“The key motivation for the experiment is that if you can detect heat from the planet, if you can directly image it, then by using basic science you can learn about formation processes for these planets.”  So by imaging the planets using these very sophisticated optical advances, scientists go well beyond detecting exoplanets to potentially unraveling deep mysteries (even if we still won’t know what the planets “look like” from an image-of-the-day perspective.

The GPI has also detected a second exoplanet, shown here at different stages of its orbit:

 

The animation is a series of images taken between November 2013 and April 2015 with the Gemini Planet Imager (GPI) on the Gemini South telescope in Chile, and shows the exoplanet β Pictoris b, which is more than 60 lightyears from Earth. The star is the black area on the left edge of the frame and is hidden by the Gemini Planet Imager’s coronagraph. We are looking at the planet’s orbit almost edge-on, with the planet closer to the Earth than the star. (M. Millar-Blanchaer, University of Toronto; F. Marchis, SETI Institute)
The animation is a series of images taken between November 2013 and April 2015 with the Gemini Planet Imager (GPI) on the Gemini South telescope in Chile, and shows the exoplanet β Pictoris b, which is more than 60 lightyears from Earth. The star is the black area on the left edge of the frame and is hidden by the Gemini Planet Imager’s coronagraph. (M. Millar-Blanchaer, University of Toronto; F. Marchis, SETI Institute)

A next big step in direct imaging of exoplanets will come with the launch of the James Webb Space Telescope in 2018.  While not initially designed to study exoplanets — in fact, exoplanets were just first getting discovered when the telescope was under early development — it does now include a coronagraph which will substantially increase its usefulness in imaging exoplanets.

As explained by Joel Green, a project scientist for the Webb at the Space Telescope Science Institute in Baltimore, the new observatory will be able to capture light — in the form of infrared radiation– that will be coming from more distant and much colder environments than what Hubble can probe.

“It’s sensitive to dimmer things, smaller planets that are more earth-sized.  And because it can see fainter objects, it will be more help in understanding the demographics of exoplanets.  It uses the infrared region of the spectrum, and so it can look better into the cloud levels of the planets than any telescope so far and see deeper.”

James Webb Space Telescope mirror being inspected at Goddard Space Flight Center, as it nears completion.  The powerful, sophisticated and long-awaited telescope is scheduled to launch in 2018.
James Webb Space Telescope mirror being inspected at Goddard Space Flight Center, as it nears completion. The powerful, sophisticated and long-awaited telescope is scheduled to launch in 2018.

These capabilities and more are going to be a boon to exoplanet researchers and will no doubt advance the direct imaging effort and potentially change basic understandings about exoplanets.  But it is not expected produce gorgeous or bizarre exoplanet pictures for the public, as Hubble did for galaxies and nebulae.  Indeed, unlike the Hubble — which sees primarily in visible light —  Webb sees in what Green said is, in effect, night vision.   And so researchers are still working on how they will produce credible images using the information from Webb’s infrared cameras and translating them via a color scheme into pictures for scientists and the public.

Another compelling exoplanet-imaging technology under study by NASA is the starshade, or external occulter, a metal disk in the shape of a sunflower that might some day be used to block out light from host stars in order to get a look at faraway orbiting planets.  MIT’s Sara Seager led a NASA study team that reported back on the starshade last year in a report that concluded it was technologically possible to build and launch, and would be scientifically most useful.  If approved, the starshade — potentially 100 feet across — could be used with the WFIRST telescope in the 2020s.  The two components would fly far separately, as much as 35,000 miles away from each other, and together could produce breakthrough exoplanet direct images.

An artist's depiction of a sunflower-shaped starshade that could help space telescopes find and characterize alien planets. Credit: NASA/JPL/Caltec
An artist’s depiction of a sunflower-shaped starshade that could help space telescopes find and characterize alien planets.  Credit: NASA/JPL/Caltech

Here is a link to an animation of the starshade being deployed: http://planetquest.jpl.nasa.gov/video/15

The answer, then, to the question posed in the title to this post — “How Will We Know What Exoplanets Look Like, and When?”– is complex, evolving and involves a science-based definition of what “looking like” means.  It would be wonderful to have images of exoplanets that show cloud formations, dust and maybe some surface features, but “direct imaging” is really about something different.  It’s about getting light from exoplanets that can tell scientists about the make-up of those exoplanets and their atmospheres, and ultimately that’s a lot more significant than any stunning or eerie picture.

And with that difference between beauty and science in mind, this last image is one of the more striking ones I’ve seen in some time.

Moon glow over Las Campanas Observatory, run by the Carnegie Institution of Science, in Chile. (Yuri Beretsky)
Moon glow over Las Campanas Observatory, operated by the Carnegie Institution of Science, in Chile. (Yuri Beretsky)

It was taken at the Las Campanas Observatory in Chile last year, during a night of stargazing.  Although the observatory is in the Atacama Desert, enough moisture was present in the atmosphere to create this lovely moon-glow.

But working in the observatory that night was Carnegie’s pioneer planet hunter Paul Butler, who uses the radial velocity method to detect exoplanets.  But to do that he needs to capture light from those distant systems.  So the night — despite the beautiful moon-glow — was scientifically useless.

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Enceladus and Water Worlds

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Glittering geysers of water ice erupt from Saturn's enigmatic moon Enceladus as seen during a previous flyby. The plumes are backlit by the sun, which is almost directly behind the moon. The moon's dark side that we see here is illuminated by reflected Saturn-shine. Today, the Cassini spacecraft flew right through the plumes in order to let its instruments 'taste' them. Credit: NASA/JPL/SSI/Ugarkovich
Glittering geysers of water ice erupt from Saturn’s enigmatic moon Enceladus as seen during a previous flyby. The plumes are backlit by the sun, which is almost directly behind the moon. The moon’s dark side that we see here is illuminated by reflected Saturn-shine.  Credit: NASA/JPL/SSI/Ugarkovich

As if the prospect of billions of potentially habitable exoplanets wasn’t enough to get people excited, what about all those watery exo-moons too?

The question arises as the Cassini mission makes its final pass near the now famous geysers at the south pole of the moon Enceladus ,scheduled for Saturday.  The plumes are currently in darkness and so it’s a perfect time to tease out a particularly compelling aspect of the Enceladus story:  how hot is the inside of the mini-moon.  Earlier measurements of the water ice spray took place when the sun was on that southern pole, so this will be the first time Cassini can measure precisely how much of the already detected heat comes from the moon’s interior.

The expectation is that much of the heat does indeed come from inside, warmed substantially by tidal forces and perhaps hydrothermal vents that together serve to keep liquid a subsurface ocean all around the moon.  As a result, the evolving scientific view is that tiny Enceladus, one of 63 moons of Saturn, just may have the ingredients and characteristics that put it into an improbable habitable zone.

“Step by step, we’re learning about an environment that seemed impossible not long ago,” said Cassini Mission Scientist Linda Spilker.  “We know that Enceladus has some rocky core, and that it touches the liquid water.  We also know that some of the compounds identified in the geysers can only be formed when rock is in contact with hot water, and that must be happening at the bottom of the moon’s ocean.  All the pieces are coming together to tell us that the moon has an ocean that might be able to support life.”

NASA's Cassini spacecraft captured this view as it neared icy Enceladus for its closest-ever dive past the moon's active south polar region. The view shows heavily cratered northern latitudes at top, transitioning to fractured, wrinkled terrain in the middle and southern latitudes. The wavy boundary of the moon's active south polar region -- Cassini's destination for this flyby -- is visible at bottom, where it disappears into wintry darkness. This view looks towards the Saturn-facing side of Enceladus. North on Enceladus is up and rotated 23 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 28, 2015. The view was acquired at a distance of approximately 60,000 miles (96,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 45 degrees. Image scale is 1,896 feet (578 meters) per pixel.
The Cassini spacecraft, sponsored by NASA, the European Space Agency and the Italian space Agency,  captured this view on Oct. 28 as it neared Enceladus. The wavy boundary of the moon’s active south polar region — Cassini’s destination for this flyby — is visible at bottom. The image was taken in visible light with the Cassini spacecraft narrow-angle camera from approximately 60,000 miles away. (Cassini Imaging Team, SSI, JPL, ESA, NASA)

That a moon might have habitable conditions is not a new idea:  science fiction great Arthur C. Clarke (as well as many scientists and now members of Congress) have pressed for a mission to Jupiter’s moon Europa because its internal ocean has been identified as similarly promising.

But what is so compelling about Enceladus is that its potential habitability pretty much came out of nowhere.  While Europa is the sixth largest moon in the solar system, Enceladus is but 370 miles in diameter.  It is covered in ice, but in 2004 its four parallel “tiger stripe” fractures were discovered, leading to the conclusion that some kind of volcanic action was taken place beneath them.  Spilker was a scientist with the Voyager mission that passed by Saturn in 1980-81 and said that Enceladus was then in relative darkness and made little impression on the team.  “We definitely missed the tiger stripes,” she said.

The plumes emerge from the south pole region, not far from the tiger stripe fractures, and appear to come from  near-surface pockets of liquid water. (The oceans of Europa are not nearly as accessible, lying 6 to 20 miles below the icy surface.)  Making Enceladus even more interesting, the 70 geysers spit out organic chemicals known as the building blocks of life along with its water ice.

The chemical composition of the plumes of Enceladus's includes hydrocarbons such as ammonia, methane and formaldehyde in trace amounts similar to the makeup of many comets. (NASA)
The chemical composition of the plumes of Enceladus’s includes hydrocarbons such as ammonia, methane and formaldehyde in trace amounts similar to the makeup of many comets.  The presence of the organic compounds suggests that very interesting chemistry is taking place where the moon’s oceans touch its core. (NASA, ESA)

So Enceladus (and perhaps Europa, too) provide a kind of emerging “proof of concept” that ice-covered water worlds can and do exist elsewhere.  (Jupiter’s giant moons Ganymede and Callisto also have massive ocean, but far below their surfaces and sandwiched between layers of ice.)  In fact, subterranean oceans may be common because, in recent years, scientists have come to understand that water — especially in its vapor and ice stages — is ubiquitous in the solar system, the galaxy and the universe.  Comets, which are generally half water ice and half rock, are one of numerous delivery systems.

And we already have, of course, one good example of what would generally be considered a waterworld without the ice covering — Earth.  But that’s not all, even without leaving our solar system.   We look at planets such as Mars and Venus and now see desiccated landscapes.  Yet it is broadly accepted that Mars once was quite wet on the surface, based on findings from the Curiosity mission and years of satellites imaging, and some have speculated that Venus might once have been wet as well.

So are there many waterworlds or aquaworlds with surface liquid water out there?

“It would be fair to say there is a consensus view that exoplanets and moons with lots of water are all over the place,” said Joel Green, an exoplanet scientist with the Space Telescope Science Institute. “The known presence of so much H2O makes that non-controversial.”

Kepler-62e has been described as being a possible waterworld, with large oceans. UPR Arecibo
Kepler-62e has been described as being a possible waterworld, with large oceans. UPR Arecibo

 

What is indeed controversial is whether any waterworlds, or even potential waterworlds, have been detected. There actually has been wide coverage of waterworld discoveries far, far away,  and even some declared confirmations.  But so far at least, those confirmations have not lasted.  As early as 2004, the Hubble Space Telescope identified the signature of water vapor in the atmosphere of an exoplanet, and similar detections have followed.  But that information and more that scientists have collected and modeled about H2O on exoplanets has never been sufficient to make a confirmation stick.

“It’s a very challenging detection to make, and many don’t think we haven’t gotten there yet,” Green said.  Especially challenging is the detection of liquid water, since it does not show up using optical, infrared, ultraviolet or any other kind of light, and so can’t be identified with a spectroscope. It’s presence can only be inferred based on other conditions.  Water vapor and water ice are, however, detectable via spectroscope.

But waterworld theories abound.  For instance, scientists know that Neptune and Uranus in the outer part of our solar system consist of vast amounts of water ice, and they also know that planets tend to change orbits and sometimes migrate closer to their suns.  Marc Kurchner of NASA’s Goddard Space Flight Center has proposed that if similar planets were to migrate inward in different solar systems, the result could be a very wet planet, with oceans hundreds of miles deep.

There is a general tendency to associate the presence of water with the presence of life, and the ubiquity of water in the galaxies with the likelihood of finding life.   But while life is found everywhere that water exists on Earth, that does not at all mean that the discovery of water elsewhere means life will be present, too.

All it means is that one of many prerequisites for life (as we know it) will have been met. But as Enceladus shows, when water is present, all kinds of interesting things start of happen.

A view of Enceladus’ southern hemisphere in enhanced color (IR-green-UV). The “tiger stripe” fractures, the source of plumes venting gas and dust into space, are prominently visible in the center. {NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)
A view of Enceladus’ southern hemisphere in enhanced color (IR-green-UV). The “tiger stripe” fractures, the source of plumes venting gas and dust into space, are prominently visible in the center. {NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)
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