As the Cassini mission embarks on its final dive this Friday into Saturn, it will continue taking photos all the way down (or as far as it remains operations.)
We’ve grown accustomed to seeing remarkable images for the mission and the planet, but clearly the show is not over, and perhaps far from it.
This is what NASA wrote describing the image above:
This view shows a wave structure in Saturn’s rings known as the Janus 2:1 spiral density wave. Resulting from the same process that creates spiral galaxies, spiral density waves in Saturn’s rings are much more tightly wound. In this case, every second wave crest is actually the same spiral arm which has encircled the entire planet multiple times.
This is the only major density wave visible in Saturn’s B ring. Most of the B ring is characterized by structures that dominate the areas where density waves might otherwise occur, but this innermost portion of the B ring is different.
For reasons researchers do not entirely understand, damping of waves by larger ring structures is very weak at this location, so this wave is seen ringing for hundreds of bright wave crests, unlike density waves in Saturn’s A ring.
The image gives the illusion that the ring plane is tilted away from the camera toward upper-left, but this is not the case. Because of the mechanics of how this kind of wave propagates, the wavelength decreases with distance from the resonance. Thus, the upper-left of the image is just as close to the camera as the lower-right, while the wavelength of the density wave is simply shorter.
This wave is remarkable because Janus, the moon that generates it, is in a strange orbital configuration. Janus and Epimetheus (see PIA12602) share practically the same orbit and trade places every four years. Every time one of those orbit swaps takes place, the ring at this location responds, spawning a new crest in the wave.
The distance between any pair of crests corresponds to four years’ worth of the wave propagating downstream from the resonance, which means the wave seen here encodes many decades’ worth of the orbital history of Janus and Epimetheus.
According to this interpretation, the part of the wave at the very upper-left of this image corresponds to the positions of Janus and Epimetheus around the time of the Voyager flybys in 1980 and 1981, which is the time at which Janus and Epimetheus were first proven to be two distinct objects (they were first observed in 1966).
Epimetheus also generates waves at this location, but they are swamped by the waves from Janus, since Janus is the larger of the two moons.
This image is from a few months ago, but it certainly puts you there above the deep, deep clouds of Saturn. False color was used to make the patterns more discernible.
Saturn has some remarkable features in its atmosphere. When the Voyager missions traveled to the planet in the early 1980s, it imaged a hexagon-shaped cloud formation near the north pole. Twenty-five years later, infrared images taken by Cassini revealed the storm was still spinning, powered by jet streams that push it to speeds of about 220 mph (100 meters per second). At 15,000 miles across, the long-lasting storm could easily contain an Earth or two.
Cassini is now on its last full orbit, to be following by its partial finale. The final 22 orbits leading to the plunge into the clouds looked like this:
And here is a Jet Propulsion Lab video recapping the Cassini mission and describing its Friday rendezvous:
One of the great successes of the Curiosity mission to Mars is that the rover landed at what turned out to be a goldmine of a location.
The mission has once and for all determined that the planet was habitable at least during its early days, that it contains the organic building blocks of life, and that liquid water ran and formed lakes. And this leaves out the more basic Mars science that some day will some day produce new headline results.
The process of anointing a successor destination for NASA’s 2020 rover mission to Mars has been going on for several years now, and the field was narrowed to three possibilities earlier this year.
Because some of the primary goals of the 2020 mission differ from those of the Curiosity mission, the potential landing sites are unlike Gale Crater and all share certain features that are, not surprising, promising in terms of the new goals. What’s new is the requirement that the 2020 mission will search for biosignatures of life in the ancient rocks and to identify, pick up and store rocks samples for later return to Earth.
Given those (and other) science goals, the leaders of the Mars 2020 mission — and the large community of scientists eager to become a formal or informal part of the mission — have been looking for sites where water was clearly present in the distant past and where conditions seem best for actually preserving fossil microbial biosignatures that may have been present.
This is quite a dramatic change, and will be the first NASA mission sent to look for life — albeit fossilized and ancient life — since the Viking missions of four decades ago.
“What we’re down to now is three sites featuring different kinds of ancient water settings,” said Kenneth Williford of NASA’s Jet Propulsion Lab. He’s deputy project scientist for the 2020 mission and a specialist in identifying fossil remnants of lifeforms in ancient Earth rocks.
“On the list we have a site that was clearly a river delta, one that had a large concentration of subsurface water, and another that may be the site of a possible hot spring. All offer great possibilities, and we have a year to decide which is most promising.”
As assessed by both the Mars 2020 team and the associated community of scientists, the two favorites are Jezero Crater and Northeast Syrtis Major. The third, Columbia Hills, is where the rover Spirit spent five years exploring. Its inclusion in the final three is somewhat controversial since the nature and even presence of the fossil hot spring that would make it a desirable landing site remains a matter of some dispute.
But Jezero Crater is well understood — from the perspective, that is, of what can be determined from orbiting satellites — and offers many scientific riches.
Several times the home to substantial lakes, with rivers both coming in to the area and going out, from above Jezero looks very much like a river delta on Earth.
“The delta was clearly part of a standing body of water,” Williford told me during the recent Astrobiology Science Conference. “Life on Earth loves shallow water environments, and the fine grain sediments are good at preserving ancient life.”
“But there are also many interesting niches for would be good for ancient microbial mats and stromatolites,” he said, referring to the mounds and columns that can be found in shallow water on Earth and that were originally formed by the growth of layer upon layer of cyanobacteria.
“There are signs of carbonates in the shallow edges of what would have been the lake. That possibility really excites me.”
Jezero has some features similar to those found in the Yellowknife Bay section of Gale Crater. A series of lakes existed there some 3.5 billion to 4 billion years ago, and conditions on the ground were determined through geochemistry to have been sufficiently benign to support life (if it ever began.)
Many layers of sedimentary rock are also visible, with some seemingly exposed in a manner similar to those in Yellowknife Bay. Most likely in past, finer grains of sediments further out in the delta were scoured and swept away by winds, and substantial walls of firmer sedimentary rocks were left for geologists to examine, interpret and delight.
Jezero is relatively close to the northern plains where some scientists infer that there was — indeed, had to be — a large ocean. While searching for signs of that ocean is not a core objective of the Mars 2020 mission, Williford said that it was entirely possible that landing at Jezero could provide information that would make a northern ocean more or less plausible.
And in terms of sample return, Williford said that Jezero offered a lot. The delta sediments, he said, concentrate extremely ancient igneous materials from the large watershed that includes Jezero and the larger Northeast Syrtis region.
The Northeast Syrtis Major region lies on the eastern edge of Syrtis Major, a huge shield volcano, and near the northwestern rim of Isidis Planitia, a giant impact basin. This region exposes early Noachian era bedrock, more than 4 billion years old, and contains many minerals that can only be formed in water.
But unlike Jezero, the rock appears to be igneous rather than sedimentary, and the water present for long periods was a subsurface acquifer.
“Almost everything we know of the history of life on Earth comes from sedimentary rocks, but there are no clear sedimentary deposits at Syrtris,” Williford said. “So we’ll work to understand another exploration model. We’ll go to find fracture networks in the rocks and find chemical gradients where microbes could make a living.”
The region has major appeal because it is known to have features from that early Noachian era of Mars alongside features from the later Hesperian times — making it possible to study the transformations that occurred during that transition period. There is also evidence that the area had deep subsurface water as well as later running water, creating layers of clays from very different times. This is considered a great opportunity to both understand Martian history and to search for biosignatures.
Northeast Syrtis Major also had longtime volcanic activity that once warmed the region. Underground heat sources make hot springs flow and surface ice melt. Microbes could have flourished there in liquid water that was in contact with minerals. The layered terrain offers a detailed record of the interactions that occurred between water and minerals — essential to provide the molecular building blocks of life — over successive periods of early Mars history.
The remaining signatures of an ancient magnetic field at Northeast Syrtis Major is especially intriguing to scientists working to understand the fundamentals of Martian history. The planet had a much more extensive magnetic field 4 billion years ago, but the internal dynamo that powered it is inferred to have failed and left Mars with less and less protection from solar winds and cosmic rays. The result was the loss of much of the planet’s atmosphere and gradually a cooling and desiccating of the surface.
“The opportunity to sample rocks emplaced while the Mars dynamo was active is highly desirable from the standpoint of both astrobiology and planetary evolution,” Williford said.
Both the Jerezo and Northeast Syrtis Major sites have substantial deposits of carbonates, which Mars scientists find especially important and interesting. These carbon compounds can both preserve ancient fossils and can potentially provide insight into the early Martian world, when conditions were warmer and wetter. Climate scientists look to greenhouse gases such as carbon dioxide and methane to warm the planet, yet the carbonates that would have been formed by these gases in later stages have not been nearly as ubiquitous as expected.
The third short-listed site, Columbia Hills, comes with some controversy. It was not given particularly active support by the scientists meeting in February for the third Mars 2020 site selection, but the Mars 2020 steering committee and mission science leaders decided to include it.
The Mars rover Spirit explored the area from early 2004 until late 2009, when it got stuck in the sand. During its time on Mars, Spirit send back images of silica deposits that had distinctive shapes, sometimes called “cauliflowers.” Two Mars scientists, Steven Ruff and James Farmer of Arizona State University, proposed that those rocks detected near the Home Plate section of Gusev Crater in Columbia Hills could have been formed inside hot springs.
They pointed to similarly shaped rocks found near the El Tatio geyser in Atacama Desert in Chile, in Yellowstone National Park in Wyoming and in the Taupo Volcanic Zone in New Zealand. The rocks from Wyoming and New Zealand have been determined to have been shaped by microbes, but those in the desiccated Atacama have not so far, although the two scientists suspect that they will be.
Clearly, to land nearby an ancient hot springs areas with rock formations potentially identical to microbial-formed rocks found on Earth has substantial appeal. And as Williford explained it, during the February meeting there was long debate about whether the shapes on Mars were or were not sculpted by life, and whether they are remnants of an ancient hot sprins — which is an ideal breeding grounds for life on Earth.
“In terms of sample return, there is a clear appeal to focusing on these features that just might be like those at El Tatio,” Williford said.” The advocates “argued in a compelling way, and there are other volcanic features and igneous rocks that would be a real interest at Columbia Hills.”
But he also said that some members of the Spirit team disagreed with the interpretation of the “cauliflowers” and the roughly 200 community scientists meeting the site selection meeting did not rank Columbia Hills especially high. Nonetheless, the committee decided to keep it in the running, pending further study.
The landing site decision will ultimately be made by the head of the NASA science mission directorate, Thomas Zurbuchen, based on the results of the site selection process. So far the engineering teams have determined all the three sites to be safe for landing.
The final decision is expected to come within one year, leaving two years before launch. This time is set aside, Williford said, to give the Mars 2020 team time to focus on detailed scientific mission planning and mapping efforts on the selected site. But the two years also leave time to switch to a different landing site if important new discoveries or problems come to light.
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 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.
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.
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.”
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.”
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.
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.
Though on holiday, I wanted to share these images and a bit of the Juno at Jupiter news.
Because telescopes have never been able to see clearly down through the thick clouds of Jupiters– the ones that together form the planet’s glorious stripes– it has remained a mystery how deep they may be.
Based on the Juno spacecraft’s August pass, we now know via its microwave radiometer that the stripes reflect dynamics that occur deep into the planet.
Scott Bolton, leader of the Juno mission reported the team’s conclusions during a press conference at the 2016 meeting of the American Astronomical Society’s Division for Planetary Sciences.
“The structure of the zones and belts still exists deep down,” Bolton said. “So whatever is making those colors, whatever is making those stripes, is still existing pretty far down into Jupiter. That came as a surprise to many of the scientists. We didn’t know if this was [just] skin-deep.”
The new images penetrate to depths of about 200 to 250 miles below the surface cloud layer, Bolton said. While the bands seen on the cloud tops are not identical to the bands identified further down, there is a strong resemblance. “They’re evolving. They’re not staying the same,” Bolton said.
The findings have intriguing implications for exoplanet research. Bolton said that the hint at “the deep dynamics and the chemistry of Jupiter’s atmosphere. And this is the first time we’ve seen any giant planet atmosphere underneath its layers. So we’re learning about atmospheric dynamics at a very basic level.”
These early Juno findings came as it was also reported that the spacecraft had two malfunction that caused it to go into safe mode, just as it was approaching Jupiter for an October 19 flyby.
Right now, Juno makes one orbit every 53 days. Juno was scheduled to fire its engines on Oct. 19 and reduce its orbit to every 14 days. But because of a problem with the engine valves, the Juno team delayed that engine firing for now.
Then, NASA officials said, a second problem, apparently related to the “software-performance monitor,” caused the probe’s onboard computer to reboot. Officials said the problem was not related to that earlier propulsion issue.
“At the time safe mode was entered, the spacecraft was more than 13 hours from its closest approach to Jupiter,” said Rick Nybakken, Juno project manager from NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “We were still quite a ways from the planet’s more intense radiation belts and magnetic fields. The spacecraft is healthy and we are working our standard recovery procedure.”
In safe mode, all unneeded subsystems were shut down and instructions were relayed by controllers. Juno did not collect any data during the flyby, which was to take place as it passed 3,000 miles above Jupiter’s clouds.
The root causes of the problem have not been made public and apparently remain unresolved. But Bolton that no long-term problems were anticipated, and that the team expected the spacecraft to be ready to turn on all science instruments at the next close flyby, on December 11.
Just as Juno was approaching Jupiter this summer, researchers at the University of California, Berkeley, reported that whirling ammonia flows below the sutface clouds help form the planet’s distinctive features.
Researchers used the upgraded Very Large Array radio telescope in New Mexico to probe 60 miles below the top of the clouds. They reported a correlation between the colorful whirls and spots on the visible surface and the movement of gas below, which is driven by Jupiter’s internal heat source.
The Juno findings certainly suggest that the correlation goes much deeper.
This blog is being hosted by Knowinnovation Inc. and is supported by the Lunar and Planetary Institute (LPI). LPI is operated by the Universities Space Research Association (USRA) under a cooperative agreement with NASA. The purpose of this blog is to communicate the work of the Nexus for Exoplanet Systems Science (NExSS). Any opinions, findings, and conclusions or recommendations expressed on this blog or its comments are those of the author(s) and do not necessarily reflect the views of NASA.