Water Worlds, Aquaplanets and Habitability

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This artist rendering may show a water world — without any land — or an aquaplanet with lots of more shallow water around a rocky planet. (NASA)

 

The more exoplanet scientists learn about the billions and billions of celestial bodies out there, the more the question of unusual planets — those with characteristics quite different from those in our solar system — has come into play.

Hot Jupiters, super-Earths, planets orbiting much smaller red dwarf stars — they are all grist for the exoplanet mill, for scientists trying to understand the planetary world that has exploded with possibilities and puzzles over the past two decades.

Another important category of planets unlike those we know are the loosely called “water worlds” (with very deep oceans) and their “aquaplanet” cousins (with a covering of water and continents) but orbiting stars very much unlike our sun.

Two recent papers address the central question of habitability in terms of these kind of planets — one with oceans and ice hundreds of miles deep, and one particular and compelling planet (Proxima Centauri b, the exoplanet closest to us) hypothesized to have water on its surface as it orbits a red dwarf star.

The question the papers address is whether these watery worlds might be habitable.  The conclusions are based on modelling rather than observations, and they are both compelling and surprising.

In both cases — a planet with liquid H20 and ice many miles down, and another that probably faces its red dwarf sun all or most of the time — the answers from modelers is that yes, the planets could be habitable.   That is very different from saying they are or even might be inhabited.  Rather,  the conclusions are based on computer models that take into account myriad conditions and come out with simulations about what kind of planets they might be.

This finding of potential watery-world habitability is no small matter because predictions of how planets form point to an abundance of water and ice in the planetesimals that grow into planets.

As described by Eric Ford, co-author of one of the papers and a professor of astrophysics at Pennsylvania State University, “Many scientists anticipate that planets with oceans much deeper than Earths could be a common outcome of planet formation. Indeed, one of the puzzling properties of Earth is that it has oceans that are just skin deep” compared to the radius of the planet.

“While some planets very close to their star might loose all their water, it would take a delicate balancing act to remove many ocean’s worth of water and to leave a planet with oceans as shallow as those on Earth.”

An interesting place to start.

 

Artist’s conception of a planet covered with a global ocean. A new study finds that these wate rworlds could maintain stable climates and perhaps sustain life under certain conditions. (ESO/M. Kornmesser)

 

It should first be said that many scientists are dubious that extreme water worlds can support life or can support detectable life.  My colleague Elizabeth Tasker wrote a column — Can You Overwater a Planet? — focused on this view last year.

The first of the two new exoplanets/ocean papers involves planets with very deep oceans. Written by Edwin Kite of the University of Chicago and Ford of Penn State, the paper in the Astrophysical Journal concludes that even a planet with such super deep oceans could — under certain conditions — provide habitable conditions.

This finding is at odds with previous simulations, and Kite says that is part of its significance. The scientific community has largely assumed that planets covered in a deep ocean would not support the cycling of minerals and gases that keeps the climate stable on Earth, and thus wouldn’t be friendly to life.

But the Kite and Ford study found that ocean planets (with 10 to 1000 times as much water as Earth) could remain habitable much longer than previously assumed. The authors performed more than a thousand simulations to reach that conclusion.

Eric Ford is a professor astrophysics at Penn State and a specialist in planet formation. (Penn State)

“This really pushes back against the idea you need an Earth clone—that is, a planet with some land and a shallow ocean,” said Edwin Kite, assistant professor of geophysical sciences at the University of Chicago and lead author of the study.

Edwin Kite is an assistant professor of planetary sciences at the University of Chicago. (Univ. of Chicago)

Because life needs an extended period to evolve — and because the light and heat on planets can change as their stars age — scientists usually look for planets that have both some water and some way to keep their climates stable over time. The method for achieving this steady state that we know is, of course, how it works on Earth. Over eons, our planet has cooled itself by drawing down atmospheric greenhouse gases into minerals and warms itself up by releasing them via volcanoes.

But this model doesn’t work on a water world, with deep water covering the rock and suppressing volcanoes.

Kite and Ford wanted to know if there was another way to achieve a balance. They set up a simulation with thousands of randomly generated planets, and tracked the evolution of their climates over billions of years.

“The surprise was that many of them stay stable for more than a billion years, just by luck of the draw,” Kite said. “Our best guess is that it’s on the order of 10 percent of them.”

These planets sit in the right location around their stars. They happened to have the right amount of carbon present, and they don’t have too many minerals and elements from the crust dissolved in the oceans that would pull carbon out of the atmosphere. They have enough water from the start, and they cycle carbon between the atmosphere and ocean only, which in the right concentrations is sufficient to keep things stable.

None of this means that such a planet exists — our ability to detect oceans worlds is in its infancy.  The issue is rather that Kite and Ford conclude that a deep ocean planet could potentially be habitable if other conditions were met.

 

Artist rendering of Proxima Centauri b orbiting its red dwarf host star. (ESO/L.Calçada/Nick Risinger)

 

Anthony Del Genio and his team of modelers at NASA’s Goddard Institute for Space Studies in New York used their state-of-the-art climate simulations to look at another aspect of the exoplanet water story, and they chose Proxima Centauri b as their subject.  The roughly Earth-sized planet was discovered in 2016 and is the closest exoplanet to Earth.

Scientists determined early on that it is a rocky (as opposed to gaseous) planet and that it orbits its host star every 11 days.  If that star was as powerful as our sun, there would be no talk of possible habitability on close-in Proxima b.  But the star is a red dwarf and puts out only a fraction of the radiation coming from a host star like our sun.

Still, the case for habitability on Proxima b was initially considered to be weak, in part because the planet is tidally locked by its closeness to the host star.  In other words, it would most likely not spin to create days and nights as it orbits, but rather would have a sun-facing side and a space-facing side — making the temperature differences great.

Our ability to characterize a small planet like Proxima b remains very limited, and so it is unknown whether the planet has water or whether it has an atmosphere.  So those two essential components of the habitability question are missing.

But Del Genio’s team decided to model the dynamics of Proxima b with a presumed ocean, though not one that is many miles deep.  In Earth science parlance, what Del Genio referred to as an “aquaplanet.” And using their sophisticated models, they would simulate “dynamic” oceans with currents like our own, rather than the stationary oceans modeled earlier on exoplanets.

And rather to their surprise, they reported in the journal Astrobiology that their model of ocean behavior showed that the planet would not have only small areas of potential habitability — the earlier proposed habitable “eyeball” scenario — but rather much of the planet could be habitable.  That could include some of the normally space-facing side.

 

One type of possible water world is an “eyeball” planet, where the star-facing side is able to maintain a liquid-water ocean, while the rest of the surface is ice. (Image via eburacum45/DeviantArt)

 

“Our group said let’s hook up an atmosphere to a dynamic ocean rather than a static one,” Del Genio said.  “That way you get ocean currents like those on our coasts, and they move water of different temperatures around.

“If you have a real and dynamic ocean in your model, then we found that the eyeball goes away.  Usually the currents go west to east and they carry warmer water even to the night side.”

Anthony Del Genio, leader of NASA’s GISS team that  uses cutting edge Earth climate models to better understand conditions on exoplanets.

So using this more sophisticated model, not insignificant areas of Proxima b, or any other planet like it orbiting a red dwarf star, could be habitable, they concluded.  But again, that is assuming some pretty big “ifs” — the presence of an ocean and an atmosphere.

And then the team added variables such as a thick nitrogen and carbon atmosphere or a thin one, fresh water or salty water, a planet that is firmly locked and never rotating, or one that rotates a modest amount — giving the dark side some light.  Del Genio said that with all these added factors, a substantial portion of the surface of Proxima b, or a planet like it, would have liquid water and potentially habitable conditions.

This focus on watery worlds — including those that would be extreme compared with Earth today — makes sense in the context of the history of Earth.

While there is no direct evidence of this, many scientists think that the very early Earth was covered for a period of time with water with little or no land.

And then after land appeared, it still took some three billion years for any life form — bacteria, early planets — to colonize the land, and another half billion years for animals to come ashore.  Yet the oceans were long habitable and inhabited, as early a 3.8 billion years ago.

So until astronomy and exoplanet science develop the needed instruments and scientists acquire the observed knowledge of conditions on water worlds, progress will come largely from modelling that tells us what might be possible.

 

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A National Strategy for Finding and Understanding Exoplanets (and Possibly Extraterrestrial Life)

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The National Academies of Science, Engineering and Medicine took an in-depth look at what NASA, the astronomy community and the nation need to grow the burgeoning science of exoplanets — planets outside our solar system that orbit a star. (NAS)

 

An extensive, congressionally-directed study of what NASA needs to effectively learn how exoplanets form and whether some may support life was released today, and it calls for major investments in next-generation space and ground telescopes.  It also calls for the adoption of an increasingly multidisciplinary approach for addressing the innumerable questions that remain unanswered.

While the recommendations were many, the top line calls were for a sophisticated new space-based telescope for the 2030s that could directly image exoplanets, for approval and funding of the long-delayed and debated WFIRST space telescope, and for the National Science Foundation and to help fund two of the very large ground-based telescopes now under development.

The study of exoplanets has seen remarkable discoveries in the past two decades.  But the in-depth study from the private, non-profit National Academies of Sciences, Engineering and Medicine concludes that there is much more that we don’t understand than that we do, that our understandings are “substantially incomplete.”

So the two overarching goals for future exoplanet science are described as these:

 

  • To understand the formation and evolution of planetary systems as products of star formation and characterize the diversity of their architectures, composition, and environments.
  • To learn enough about exoplanets to identify potentially habitable environments and search for scientific evidence of life on worlds orbiting other stars.

 

Given the challenge, significance and complexity of these science goals, it’s no wonder that young researchers are flocking to the many fields included in exoplanet science.  And reflecting that, it is perhaps no surprise that the NAS survey of key scientific questions, goals, techniques, instruments and opportunities runs over 200 pages. (A webcast of a 1:00 pm NAS talk on the report can be accessed here.)

 


Artist’s concept showing a young sun-like star surrounded by a planet-forming disk of gas and dust.
(NASA/JPL-Caltech/T. Pyle)

These ambitious goals and recommendations will now be forwarded to the arm of the National Academies putting together 2020 Astronomy and Astrophysics Decadal Survey — a community-informed blueprint of priorities that NASA usually follows.

This priority-setting is probably most crucial for the two exoplanet direct imaging missions now being studied as possible Great Observatories for the 2030s — the paradigm-changing space telescopes NASA has launched almost every decade since the 1970s.

HabEx (the Habitable Exoplanet Observatory) and LUVOIR (the Large UV/Optical/IR Surveyor) are two direct-imaging exoplanet projects in conception phase that would indeed significantly change the exoplanet field.

Both would greatly enhance scientists’ ability to detect and characterize exoplanets. But the more ambitious LUVOIR in particular, would not only find many exoplanets in all stages of formation, but could readily read chemical components of the atmospheres and thereby get clear data on whether the planet was habitable or even if it supported life.  The LUVOIR would provide either an 8 meter or a record-breaking 15-meter space telescope, while HabEx would send up a 4 meter mirror.

HabEx and LUVOIR are competing with two other astrophysics projects for that Great Observatory designation, and so NAS support now and prioritizing later is essential if they are to become a reality.

 

An artist notional rendering of an approximately 15-meter telescope in space. This image was created for an earlier large space telescope feasibility project called ATLAST, but it is similar to what is being discussed inside and outside of NASA as a possible great observatory after the James Webb Space Telescope and the Wide-Field Infrared Survey Telescope. (NASA)

These two potential Great Observatories will be costly and would take many years to design and build.  As the study acknowledges and explains, “While the committee recognized that developing a direct imaging capability will require large financial investments and a long time scale to see results, the effort will foster the development of the scientific community and technological capacity to understand myriad worlds.”

So a lot is at stake.  But with budget and space priorities in flux, the fate of even the projects given the highest priority in the Decadal Survey remains unclear.

That’s apparent in the fact that one of the top recommendations of today’s study is the funding of the number one priority put forward in the 2010 Astronomy and Astrophysics Decadal Survey — the Wide Field Infrared Survey Telescope (WFIRST.)

The project — which would boost the search for exoplanets further from their stars than earlier survey mission using microlensing– was cancelled in the administration’s proposed 2019 federal budget.  Congress has continued funding some development of this once top priority, but its future nonetheless remains in doubt.

WFIRST could have the capability of directly imaging exoplanets if it were built with technology to block out the blinding light of the star around which exoplanets would be orbiting — doing so either with internal coronagraph or a companion starshade.  This would be novel technology for a space-based telescope, and the NAS survey recommends it as well.

 

An artist’s rendering of a possible “starshade” that could be launched to work with WFIRST or another space telescope and allow the telescope to take direct pictures of other Earth-like planets. (NASA/JPL-Caltech)

The list of projects the study recommends is long, with these important additions:

That “ground-based astronomy – enabled by two U.S.-led telescopes – will also play a pivotal role in studying planet formation and potentially terrestrial worlds, the report says. The future Giant Magellan telescope (GMT) and proposed Thirty Meter Telescope (TMT) would allow profound advances in imaging and spectroscopy – absorption and emission of light – of entire planetary systems. They also could detect molecular oxygen in temperate terrestrial planets in transit around close and small stars, the report says.”

The committee concluded that the technology road map to enable the full potential of GMT and TMT in the study of exoplanets is in need of investments, and should leverage the existing network of U.S. centers and laboratories. To that end, the report recommends that the National Science Foundation invest in both telescopes and their exoplanet instrumentation to provide all-sky access to the U.S. community.

And for another variety of ground-based observing the study called for the funding of a project to substantially increase the precision of instruments that find and measure exoplanets using the detected “wobble” of the host star.  But stars are active with or without a nearby exoplanet, and so it has been difficult to achieve the precision that astronomers using this “radial velocity” technique need to find and characterize smaller exoplanets.

Several smaller efforts to increase this precision are under way in the U.S., and the European Southern Observatory has a much larger project in development.

Additionally, the report recommends that the administrators of the James Webb Space Telescope give significant amounts of observing time to exoplanet study, especially early in its time aloft (now scheduled to begin in 2021.)  The atmospheric data that JWST can potentially collect could and would be used in conjunction with results coming from other telescopes, and to further study of exoplanet targets that are already promising based on existing theories and findings.

 

Construction has begun on the Giant Magellan Telescope at the Carnegie Institution’s Las Campanas Observatory in Chile. This artist rendering shows what the 24.5 meter (80 foot) segmented mirror and observatory will look like when completed, estimated to be in 2024. (Mason Media Inc.)

 

While the NAS report gives a lot of attention to instruments and ways to use them, it also focuses as never before on astrobiology — the search for life beyond Earth.

Much work has been done on how to determine whether life exists on a distant planet through modeling and theorizing about biosignatures.  The report encourages scientists to expand that work and embraces it as a central aspect of exoplanet science.

The study also argues that interdisciplinary science — bringing together researchers from many disciplines — is the necessary way forward.  It highlights the role of the Nexus for Exoplanet System Science, a NASA initiative which since 2015 has brought together a broad though limited number  of science teams from institutions across the country to learn about each other’s work and collaborate whenever possible.

The initiative itself has not required much funding, instead bringing in teams that had been supported with other grants.   However, that may be changing. One of the study co-chairs, David Charbonneau of Harvard University, said after the release of the study that the “promise of NExSS is tremendous…We really want that idea to grow and have a huge impact.”

The NAS study itself recommends that “building on the NExSS model, NASA should support a cross-divisional exoplanet research coordination network that includes additional membership opportunities via dedicated proposal calls for interdisciplinary research.”

The initiative, I’m proud to say, sponsors this interdisciplinary column in addition to all that interdisciplinary science.

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15,000 Galaxies in One Image

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Astronomers have just assembled one of the most comprehensive portraits yet of the universe’s evolutionary history, based on a broad spectrum of observations by the Hubble Space Telescope and other space and ground-based telescopes.  Each of the approximately 15,000 specks and spirals are galaxies, widely distributed in time and space. (NASA, ESA, P. Oesch of the University of Geneva, and M. Montes of the University of New South Wales)

Here’s an image to fire your imagination: Fifteen thousand galaxies in one picture — sources of light detectable today that were generated as much as 11 billion years ago.

Of those 15,000 galaxies, some 12,000 are inferred to be in the process of forming stars.  That’s hardly surprising because the period around 11 billions years ago has been determined to be the prime star-forming period in the history of the universe.  That means for the oldest galaxies in the image, we’re seeing light that left its galaxy but three billion years after the Big Bang.

This photo mosaic, put together from images taken by the Hubble Space Telescope and other space and ground-based telescopes, does not capture the earliest galaxies detected. That designation belongs to a galaxy found in 2016 that was 420 million years old at the time it sent out the photons just collected. (Photo below.)

Nor is it quite as visually dramatic as the iconic Ultra Deep Field image produced by NASA in 2014. (Photo below as well.)

But this image is one of the most comprehensive yet of the history of the evolution of the universe, presenting galaxy light coming to us over a timeline up to those 11 billion years.  The image was released last week by NASA and supports an earlier paper in The Astrophysical Journal by Pascal Oesch of Geneva University and a large team of others.

And it shows, yet again, the incomprehensible vastness of the forest in which we are a tiny leaf.

Some people apparently find our physical insignificance in the universe to be unsettling.  I find it mind-opening and thrilling — that we now have the capability to not only speculate about our place in this enormity, but to begin to understand it as well.

The Ultra-Deep field composite, which contains approximately 10,000 galaxies.  The images were collected over a nine-year period.  {NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)} 

For those unsettled by the first image, here is the 2014 Ultra Deep Field image, which is 1/14 times the area of the newest image.  More of the shapes in this photo look to our eyes like they could be galaxies, but those in the first image are essentially the same.

In both images, astronomers used the ultraviolet capabilities of the Hubble, which is now in its 28th year of operation.

Because Earth’s atmosphere filters out much ultraviolet light, the space-based Hubble has a huge advantage because it can avoid that diminishing of ultraviolet light and provide the most sensitive ultraviolet observations possible.

That capability, combined with infrared and visible-light data from Hubble and other space and ground-based telescopes, allows astronomers to assemble these ultra deep space images and to gain a better understanding of how nearby galaxies grew from small clumps of hot, young stars long ago.

The light from distant star-forming regions in remote galaxies started out as ultraviolet. However, the expansion of the universe has shifted the light into infrared wavelengths.

These images, then,  straddle the gap between the very distant galaxies, which can only be viewed in infrared light, and closer galaxies which can be seen across a broad spectrum of wavelengths.

The farthest away galaxy discovered so far is called GN-z11 and is seen now as it was 13.4 billion years in the past.  That’s  just 400 million years after the Big Bang.

GN-z11 is surprisingly bright infant galaxy located in the direction of the constellation of Ursa Major. Thus NASA video explains much more:

The farthest away galaxy ever detected — GN-z11. {NASA, ESA, P. Oesch (Yale University, Geneva University), G. Brammer (STScI), P. van Dokkum (Yale University), and G. Illingworth (University of California, Santa Cruz)} 

 

Galaxy formation chronology, showing GN-z11 in context. Hubble spectroscopically confirmed the farthest away galaxy to date. {NASA, ESA, P. Oesch and B. Robertson (University of California, Santa Cruz), and A. Feild (STScI)}

In addition representing cutting-edge science — and enabling much more — these looks into the most distant cosmic past offer a taste of what the James Webb Space Telescope, now scheduled to launch in 2021, is designed to explore.  It will have greatly enhanced capabilities to explore in the infrared, which will advance ultra-deep space observing.

But putting aside the cosmic mysteries that ultra deep space and time astronomy can potentially solve, the images available today from Hubble and other telescopes are already more than enough to fire the imagination about what is out there and what might have been out there some millions or billions of years ago.

A consensus of exoplanet scientists holds that each star in the Milky Way galaxy is likely to have at least one planet circling it, and our galaxy alone has billions and billions of stars.  That makes for a lot of planets that just might orbit at the right distance from its host star to support life and potentially have atmospheric, surface and subsurface conditions that would be supportive as well.

A look these deep space images raises the question of how many of them also house stars with orbiting planets, and the answer is probably many of them.  All the exoplanets identified so far are in the Milky Way, except for one set of four so far.

Their discovery was reported earlier this year by Xinyu Dai, an astronomer at the University of Oklahoma, and his co-author, Eduardo Guerras.  They came across what they report are planets while using NASA’s Chandra X-ray Observatory to study the environment around a supermassive black hole in the center of a galaxy located 3.8 billion light-years away from Earth.

In The Astrophysical Journal Letters , the authors report the galaxy is home to a quasar, an extremely bright source of light thought to be created when a very large black hole accelerates material around it. But the researchers said the results of their study indicated the presence of planets in a galaxy that lies between Earth and the quasar.

Furthermore, the scientists said results suggest that in most galaxies there are hundreds of free-floating planets for every star, in addition to those which might orbit a star.

The takeaway for me, as someone who has long reported on astrobiology and exoplanets, is that it is highly improbable that there are no other planets out there where life occurs, or once occurred.

As these images make clear, the number of planets that exist or have existed in the universe is essentially infinite.  That no others harbor life seems near impossible.

 

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Large Reservoir of Liquid Water Found Deep Below the Surface of Mars

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Artist impression of the Mars Express spacecraft probing the southern hemisphere of Mars, superimposed on a radar cross section of the southern polar layered deposits. The leftmost white line is the radar echo from the Martian surface, while the light blue spots are highlighted radar echoes along the bottom of the ice.  Those highlighted areas measure very high reflectivity, interpreted as being caused by the presence of water. (ESA, INAF. Graphic rendering by Davide Coero Borga )

Far beneath the frigid surface of the South Pole of Mars is probably the last place where you might expect the first large body of Martian liquid water would be found.  It’s -170 F on the surface, there are no known geothermal sources that could warm the subterranean ice to make a meltwater lake, and the liquid water is calculated to be more than a mile below the surface.

Yet signs of that liquid water are what a team of Italian scientists detected — a finding that they say strongly suggests that there are other underground lakes and streams below the surface of Mars.  In a Science journal article released today, the scientists described the subterranean lake they found as being about 20 kilometers in diameter.

The detection adds significantly to the long-studied and long-debated question of how much surface water was once on Mars, a subject that has major implications for the question of whether life ever existed on the planet.

Finding the subterranean lake points to not only a wetter early Mars, said co-author Enrico Flamini of the Italian space agency, but also to a Mars that had a water cycle that collected and delivered the liquid water.  That would mean the presence of clouds, rain, evaporation, rivers, lakes and water to seep through surface cracks and pool underground.

Scientists have found many fossil waterways on Mars, minerals that can only be formed in the presence of water, and what might be the site of an ancient ocean.

But in terms of liquid water now on the planet, the record is thin.  Drops of water collected on the leg of NASA’s Phoenix Lander after it touched down in 2008, and what some have described as briny water appears to be flowing down some steep slopes in summertime.  Called recurrent slope lineae or RSLs, they appear at numerous locations when the temperatures rise and disappear when they drop.

This lake is different, however, and its detection is a major step forward in understanding the history of Mars.

Color photo mosaic of a portion of Planum Australe on Mars.  The subsurface reflective echo power is color coded and deep blue corresponds to the strongest reflections, which are interpreted as being caused by the presence of water. (USGS Astrogeology Science Center, Arizona State University, INAF)

The discovery was made analyzing echoes captured by the the radar instruments on the European Space Agency’s Mars Express, a satellite orbiting the planet since 2002.  The data for this discovery was collected from observation made between 2012 and 2015.

 

A schematic of how scientists used radar to find what they interpret to be liquid water beneath the surface of Mars. (ESA)

Antarctic researchers have long used radar on aircraft to search for lakes beneath the thick glaciers and ice layers,  and have found several hundred.  The largest is Lake Vostok, which is the sixth largest lake on Earth in terms of volume of water.  And it is two miles below the coldest spot on Earth.

So looking for a liquid lake below the southern pole of Mars wasn’t so peculiar after all.  In fact, lead author Roberto Orosei of the Institute of Radioastronomy of Bologna, Italy said that it was the ability to detect subsurface water beneath the ice of Antarctica and Greenland that helped inspire the team to look at Mars.

There are a number of ways to keep water liquid in the deep subsurface even when it is surrounded by ice.  As described by the Italian team and an accompanying Science Perspective article by Anja Diez of the Norwegian Polar Institute, the enormous pressure of the ice lowers the freezing point of water substantially.

Added to that pressure on Mars is the known presence of many salts, that the authors propose mix with the water to form a brine that lowers the freezing point further.

So the conditions are present for additional lakes and streams on Mars.  And according to Flamini, solar system exploration manager for the Italian space agency, the team is confident there are more and some of them larger than the one detected.  Finding them, however, is a difficult process and may be beyond the capabilities of the radar equipment now orbiting Mars.

 

Subsurface lakes and rivers in Antarctica. Now at least one similar lake has been found under the southern polar region of Mars. (NASA/JPL)

The view that subsurface water is present on Mars is hardly new.  Stephen Clifford, for many years a staff scientist at the Lunar and Planetary Institute, even wrote in 1987 that there could be liquid water at the base of the Martian poles due to the kind of high pressure environments he had studied in Greenland and Antarctica.

So you can imagine how gratifying it might be to learn, as he put it “of some evidence that shows that early theoretical work has some actual connection to reality.”

He considers the new findings to be “persuasive, but not definitive” — needing confirmation with other instruments.

Clifford’s wait has been long, indeed.  Many observations by teams using myriad instruments over the years did not produce the results of the Italian team.

Their discovery of liquid water is based on receiving particularly strong radar echoes from the base of the southern polar ice — echoes consistent with the higher radar reflectivity of water (as opposed to ice or rock.)

After analyzing the data in some novels ways and going through the many possible explanations other than the presence of a lake, Orosei said that none fit the results they had.  The explanation, then, was clear:  “We have to conclude there is liquid water on Mars.”

The depth of the lake — the distance from top to bottom — was impossible to measure, though the team concluded it was at least one meter and perhaps in the tens of meters.

Might the lake be a habitable?  Orosei said that because of the high salt levels “this is not a very pleasant environment for life.”

But who knows?  As he pointed out, Lake Vostok and other subglacial Antarctic lake, are known to be home to single-cell organisms that not only survive in their very salty world, but use the salt as part of their essential metabolism.

 

 

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A New Frontier for Exoplanet Hunting

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The spectrum from the newly-assembled EXtreme PREcision Spectrometer (EXPRES)  shines on Yale astronomy professor Debra Fischer, who is principal investigator of the project. The stated goal of EXPRES is to find many Earth-size planets via the radial velocity method — something that has never been done. (Ryan Blackman/Yale)

The first exoplanets were all found using the radial velocity method of measuring the “wobble” of a star — movement caused by the gravitational pull of an orbiting planet.

Radial velocity has been great for detecting large exoplanets relatively close to our solar system, for assessing their mass and for finding out how long it takes for the planet to orbit its host star.

But so far the technique has not been able to identify and confirm many Earth-sized planets, a primary goal of much planet hunting.  The wobble caused by the presence of a planet that size has been too faint to be detected by current radial velocity instruments and techniques.

However, a new generation of instruments is coming on line with the goal of bringing the radial velocity technique into the small planet search.  To do that, the new instruments, together with their telescopes. must be able to detect a sun wobble of 10 to 20 centimeters per second.  That’s quite an improvement on the current detection limit of about one meter per second.

At least three of these ultra high precision spectrographs (or sometimes called spectrometers) are now being developed or deployed.  The European Southern Observatory’s ESPRESSO instrument has begun work in Chile; Pennsylvania State University’s NEID spectrograph (with NASA funding) is in development for installation at the Kitt Peak National Observatory in Arizona; and the just-deployed EXPRES spectrograph put together by a team led by Yale University astronomers (with National Science Foundation funding) is in place at the Lowell Observatory outside of Flagstaff, Arizona.

The principal investigator of EXPRES, Debra Fischer, attended the recent University of Cambridge Exoplanets2 conference with some of her team, and there I had the opportunity to talk with them. We discussed the decade-long history of the instrument, how and why Fischer thinks it can break that 1-meter-per-second barrier, and what it took to get it into attached and working.

 

This animation shows how astronomers use very precise spectrographs to find exoplanets. As the planet orbits its gravitational pull causes the parent star to move back and forth. This tiny radial motion shifts the observed spectrum of the star by a correspondingly small amount because of the Doppler shift. With super-sensitive spectrographs the shifts can be measured and used to infer details of a planet’s mass and orbit. ESO/L. Calçada)

One of the earliest and most difficult obstacles to the development of EXPRES, Fischer told me, was that many in the astronomy community did not believe it could work.

Their view is that precision below that one meter per second of host star movement cannot be measured accurately.  Stars have flares, sunspots and a generally constant churning, and many argue that the turbulent nature of stars creates too much “noise” for a precise measurement below that one-meter-per-second level.

Yet European scientists were moving ahead with their ESPRESSO ultra high precision instrument aiming for that 10-centimeter-per-second mark, and they had a proven record of accomplishing what they set out to do with spectrographs.

In addition to the definite competiti0n going on, Fisher also felt that radial velocity astronomers needed to make that leap to measuring small planets “to stay in the game” over the long haul.

She arrived at Yale in 2009 and led an effort to build a spectrograph so stable and precise that it could find an Earth-like planet.  To make clear that goal, the instrument is at the center of a project called “100 Earths.”

Building on experience gained from developing two earlier spectrographs, Fischer and colleagues began the difficult and complicated process of getting backers for EXPRES, of finding a telescope observatory that would house it (The Discovery Channel Telescope at Lowell) and in the end adapting the instrument to the telescope.

And now comes the actual hard part:  finding those Earth-like planets.

As Fischer described it:  “We know from {the Kepler Telescope mission} that most stars have small rocky planets orbiting them.  But Kepler looked at stars very far away, and we’ll be looking at stars much, much closer to us.”

Nonetheless, those small planets will still be extremely difficult to detect due to all that activity on the host suns.

 

EXPRES in its vacuum-sealed chamber at the Lowell Observatory. will help detect Earth-sized planets in neighboring solar systems. (Ryan Blackman/Yale)

 

 

The 4.3 meter Discovery Channel Telescope in the Lowell Observatory in Arizona.  The photons collected by the telescope are delivered via optical fiber to the EXPRES instrument. (Boston University)

Spectrographs such as EXPRES are instruments astronomers use to study light emitted by planets, stars, and galaxies.

They are connected to either a ground-based or orbital telescope and they stretch out or split a beam of light into a spectrum of frequencies.  That spectrum is then analyzed to determine an object’s speed, direction, chemical composition, or mass.  With planets, the work involves determining (via the Doppler shift seen in the spectrum) whether and how much a sun is moving to and away from Earth due to the pull of a planet.

As Fisher and EXPRES postdoctoral fellow John Brewer explained it, the signal (noise) coming from the turbulence of the star is detectably different from the signal made by the wobble of a star due to the presence of an orbiting planet.

While these differences — imprinted in the spectrum captured by the spectrograph — have been known for some time, current spectrographs haven’t had sufficient resolving power to actually detect the difference.

If all works as planned for the EXPRES, Espresso and NEID spectrographs, they will have that necessary resolving power and so can, in effect, filter out the noise from the sun and identify what can only come from a planet-caused wobble.  If they succeed, they provide a major new pathway to  for astronomers to search for Earth-sized worlds.

“This is my dream machine, the one I always wanted to build,” Fischer said. “I had a belief that if we went to higher resolution, we could disentangle (the stellar noise from the planet-caused wobble.)

“I could still be wrong, but I definitely think that trying was the right choice to make.”

This image shows spectral data from the first light last December of the ESPRESSO instrument on ESO’s Very Large Telescope in Chile. The light from a star has been dispersed into its component colors. This view has been colorised to indicate how the wavelengths change across the image, but these are not exactly the colors that would be seen visually. (ESO/ESPRESSO)

While Fischer and others have very high hopes for EXPRES, it is not the sort of  big ticket project that is common in astronomy.  Instead, it was developed and built primarily with a $6 million grant from National Science Foundation.

It was completed on schedule by the Yale team, though the actual delivering of EXPRES to Arizona and connecting it to the telescope turned out to be a combination of hair-raising and edifying.

Twice, she said, she drove from New Haven to Flagstaff with parts of the instrument; each trip in a Penske rental truck and with her son Ben helping out.

And then when the instrumentation was in process late last year, Fischer and her team learned that funds for the scientists and engineers working on that process had come to an end.

Francesco Pepe of the University of Geneva. He is the principal scientist for the ESPRESSO instrument and gave essential aid to the EXPRES team when they needed it most.

She was desperate, and sent a long-shot email to Francesco Pepe of University of Geneva, the lead scientist and wizard builder of several European spectrographs, including ESPRESSO. In theory, he and his instrument — which went into operation late last year at the ESO Very Large Telescope in Chile — will be competing with EXPRES for discoveries and acknowledgement.

Nonetheless, Pepe heard Fischer out and understood the predicament she was in.  ESPRESSO had been installed and so he was able to contact an associate who freed up two instrumentation specialists who flew to Flagstaff to finish the work.  It was, Fischer said, an act of collegial generosity and scientific largesse that she will never forget.

Fischer is at the Lowell observatory now, using the Arizona monsoon as a time to clean up many details before the team returns to full-time observing.  She write about her days in an EXPRES blog.  Earlier, in March after the instrumentation had been completed and observing had commenced, she wrote this:

“Years of work went into EXPRES and as I look at this instrument, I am surprised that I ever had the audacity to start this project. The moment of truth starts now. It will take us a few more months of collecting and analyzing data to know if we made the right design decisions and I feel both humbled and hopeful. I’m proud of the fact that our design decisions were driven by evidence gleaned from many years of experience. But did I forget anything?”

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