Cassini Data Has Revealed a Towering Hexagonal Storm at Saturn’s Northern Pole

A new study based on data from the Cassini mission is revealing something surprising in the atmosphere of Saturn. We’ve known about the storm at the gas giant’s north pole for decades, but now it appears that this massive hexagonal storm could be a towering behemoth hundreds of kilometers in height that has its base deep in Saturn’s atmosphere.

This grey-scale image of Saturn’s northern polar vortex was captured by the Cassini spacecraft. This image was captured from a distance of about 1.2 million km. A portion of Saturn’s rings are barely visible in the top right. Image: NASA/JPL-Caltech/Space Science Institute.

Continue reading “Cassini Data Has Revealed a Towering Hexagonal Storm at Saturn’s Northern Pole”

Titan Looks Cool in Infrared

The Cassini spacecraft ended its mission on September 15th, 2017, when it crashed into Saturn’s atmosphere, thus preventing any possible contamination of the system’s moons. Nevertheless, the wealth of data the probe collected during the thirteen years it spent orbiting Saturn (of the gas giant, its rings, and its many moons) continues to be analyzed by scientists – with amazing results!

Case in point, the Cassini team recently released a series of colorful images that show what Titan looks like in infrared. The images were constructing using 13 years of data that was accumulated by the spacecraft’s Visual and Infrared Mapping Spectrometer (VIMS) instrument. These images represent some of the clearest, most seamless-looking global views of the icy moon’s surface produced so far.

Infrared images provide a unique opportunity when studying Titan, which is difficult to observe in the visible spectrum because of its dense and hazy atmosphere. This is primarily the result of small particles called aerosols in Titan’s upper atmosphere, which strongly scatter visible light. However, where the scattering and absorption of light is much weaker, this allows for infrared “windows” that make it possible to catch glimpses of Titan’s surface.

Comparison between how Titan appears in visible light (center), and in infrared. Credit: NASA/JPL-Caltech/Stéphane Le Mouélic, University of Nantes, Virginia Pasek, University of Arizona

It is because of this that the VIMS was so valuable, allowing scientists to provide clear images of Titan’s surface. This latest collection of images are especially unique because of the smoothness and clarity they offer. In previous infrared images captured by the Cassini spacecraft of Titan (see below), there were great variations in imaging resolution and lighting conditions, which resulted in obvious seams between different areas of the surface.

This is due to the fact that the VIMS obtained data over many different flybys with different observing geometries and atmospheric conditions. As a result, very prominent seams appear in mosaic images that are quite difficult to remove. But, through laborious and detailed analyses of the data, along with time consuming hand processing of the mosaics, Cassini’s imaging team was able to mostly remove the seams.

The process used to reduce the prominence of seams is known as the “band-ratio” technique. This process involves combining three color channels (red, green and blue), using a ratio between the brightness of Titan’s surface at two different wavelengths. The technique also emphasizes subtle spectral variations in the materials on Titan’s surface, as evidenced by the bright patches of brown, blue and purple (which may be evidence of different compositions).

The three mosaics shown here were composed with data from Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) taken during the three flybys of Titan. Credit: NASA/JPL/University of Arizona

In addition to offering the clearest and most-seamless glimpse of Titan yet, these unique images also highlight the moon’s complex geography and composition. They also showcase the power of the VIMS instrument, which has paved the way for future infrared instruments that could capture images of Titan at much higher resolution and reveal features that Cassini was not able to see.

In the coming years, NASA hopes to send additional missions to Titan to explore its surface and methane lakes for signs of biosignatures. An infrared instrument, which can see through Titan’s dense atmosphere, provide high-resolution images of the surface and help determine its composition, will prove very useful in this regard!

Further Reading: NASA

Cassini’s “Grande Finale” Earns an Emmy Nomination!

An artist's illustration of the Cassini probe's Grand Finale. Image: NASA/JPL/CalTech

In 1997, the NASA/ESA Cassini-Huygens mission launched from Earth and began its long journey towards the Saturn system. In 2004, the Cassini orbiter arrived around Saturn and would spend the next thirteen years studying the gas giant, its rings, and its system of Moons. On September 15th, 2017, the mission ended when the probe entered Saturn’s upper atmosphere and burned up.

This was known as Cassini’s “Grand Finale“, which began with the probe plunging into the unexplored region that lies between Saturn’s atmosphere and its rings and culminated with live coverage of it entering the atmosphere. In honor of the mission and NASA’s outstanding coverage of its final months, NASA was recently nominated for an Emmy Award by The Academy of Television Arts & Sciences.

The award is in the category of Outstanding Original Interactive Program, which recognizes the JPL’s multi-month digital campaign that celebrated the mission’s science and engineering accomplishments – which included news, web, education, television and social media efforts. It is also a nod to the agency’s success in communicating why the spacecraft concluded its mission in the skies of Saturn.

Essentially, the spacecraft was intentionally destroyed in Saturn’s atmosphere to prevent the possibility of it contaminating any of Saturn’s moons. Throughout the thirteen years it spent studying the Saturn system, Cassini found compelling evidence for the possible existence of life on Titan and in Enceladus’ interior ocean. In addition, scientists have speculated that there may be interior oceans within Rhea and Dione.

In this respect, Cassini ended its mission the same way the Galileo probe did in 2003. After spending 8 years studying Jupiter and its system the moons, the probe crashed into the gas giant’s upper atmosphere in order to prevent any possible contamination of Europa or Ganymede, which are also thought to have an interior oceans that could support life.

The “Grand Finale” campaign began on April 26th, 2017, and continued until the craft entered Saturn’s atmosphere on Sept. 15th, 2017, with the spacecraft sending back science to the very last second. The campaign utilized several different forms of media, was interactive, and was very comprehensive, providing regular updates and vital information about the mission.

As NASA indicated on their Cassini website:

“The multi-faceted campaign included regular updates on Twitter, Facebook, Snapchat, Instagram and the Cassini mission website; multiple live social, web and TV broadcasts during which reporter and public questions were answered; a dramatic short film to communicate the mission’s story and preview its endgame; multiple 360-degree videos, including NASA’s first 360-degree livestream of a mission event from inside JPL mission control; an interactive press kit; a steady drumbeat of articles to keep fans updated with news and features about the people behind the mission; state-standards aligned educational materials; a celebration of art by amateur space enthusiasts; and software to provide real-time tracking of the spacecraft, down to its final transmission to Earth.”

The short film, titled “For Your Consideration: The NASA Cassini Grand Finale“, showcases the missions many accomplishments, pays tribute to all those who made it happen and who helped inform the public and communicate the importance of the mission.

The Primetime Emmys will be awarded be on September 17th in Los Angeles. The Creative Arts Emmys, which includes interactive awards, will be presented during a separate ceremony on Saturday, Sept. 15th, at the Microsoft Theatre in Los Angeles. Other contenders include Back to the Moon, a Google Spotlight Stories App; Blade Runner 2049: Memory Lab, Coco VR, and Spiderman Homecoming, three Oculus VR experiences.

And be sure to check out the videos, FYC: NASA Cassini Grand Finale, below:

Further Reading: NASA

Complex Organics Molecules are Bubbling up From Inside Enceladus

The Cassini orbiter revealed many fascinating things about the Saturn system before its mission ended in September of 2017. In addition to revealing much about Saturn’s rings and the surface and atmosphere of Titan (Saturn’s largest moon), it was also responsible for the discovery of water plumes coming from Enceladus‘ southern polar region. The discovery of these plumes triggered a widespread debate about the possible existence of life in the moon’s interior.

This was based in part on evidence that the plumes extended all the way to the moon’s core/mantle boundary and contained elements essential to life. Thanks to a new study led by researchers from of the University of Heidelberg, Germany, it has now been confirmed that the plumes contain complex organic molecules. This is the first time that complex organics have been detected on a body other than Earth, and bolsters the case for the moon supporting life.

The study, titled “Macromolecular organic compounds from the depths of Enceladus“, recently appeared in the journal Nature. The study was led by Frank Postberg and Nozair Khawaja of the Institute for Earth Sciences at the University of Heidelberg, and included members from the Leibniz Institute of Surface Modification (IOM), the Southwest Research Institute (SwRI), NASA’s Jet Propulsion Laboratory, and multiple universities.

The “tiger stripes” of Enceladus, as pictured by the Cassini space probe. Credit: NASA/JPL/ESA

The existence of a liquid water ocean in Enceladus’ interior has been the subject of scientific debate since 2005, when Cassini first observed plumes containing water vapor spewing from the moon’s south polar surface through cracks in the surface (nicknamed “Tiger Stripes”). According to measurements made by the Cassini-Huygens probe, these emissions are composed mostly of water vapor and contain molecular nitrogen, carbon dioxide, methane and other hydrocarbons.

The combined analysis of imaging, mass spectrometry, and magnetospheric data also indicated that the observed southern polar plumes emanate from pressurized subsurface chambers. This was confirmed by the Cassini mission in 2014 when the probe conducted gravity measurements that indicated the existence of a south polar subsurface ocean of liquid water with a thickness of around 10 km.

Shortly before the probe plunged into Saturn’s atmosphere, the probe also obtained data that indicated that the interior ocean has existed for some time. Thanks to previous readings that indicated the presence of hydrothermal activity in the interior and simulations that modeled the interior, scientists concluded that if the core were porous enough, this activity could have provided enough heat to maintain an interior ocean for billions of years.

However, all the previous studies of Cassini data were only able to identify simple organic compounds in the plume material, with molecular masses mostly below 50 atomic mass units. For the sake of their study, the team observed evidence of complex macromolecular organic material in the plumes’ icy grains that had masses above 200 atomic mass units.

Hydrothermal activity in Enceladus’ core and the rise of organic-rich bubbles. Credit and Copyright: ESA; F. Postberg et al (2018)

This constitutes the first-ever detection of complex organics on an extraterrestrial body. As Dr. Khawaja explained in a recent ESA press release:

“We found large molecular fragments that show structures typical for very complex organic molecules. These huge molecules contain a complex network often built from hundreds of atoms of carbon, hydrogen, oxygen and likely nitrogen that form ring-shaped and chain-like substructures.”

The molecules that were detected were the result of the ejected ice grains hitting the dust-analyzing instrument aboard Cassini at speeds of about 30,000 km/hour. However, the team believes that these were mere fragments of larger molecules contained beneath Enceladus’ icy surface. As they state in their study, the data suggests that there is a thin organic-rich film on top of the ocean.

These large molecules would be the result of by complex chemical processes, which could be those related to life. Alternately, they may be derived from primordial material similar to what has been found in some meteorites or (as the team suspects) that is generated by hydrothermal activity. As Dr. Postberg explained:

“In my opinion the fragments we found are of hydrothermal origin, having been processed inside the hydrothermally active core of Enceladus: in the high pressures and warm temperatures we expect there, it is possible that complex organic molecules can arise.”

Artist rendering showing an interior cross-section of the crust of Enceladus, which shows how hydrothermal activity may be causing the plumes of water at the moon’s surface. Credits: NASA-GSFC/SVS, NASA/JPL-Caltech/Southwest Research Institute

As noted, recent simulations have shown the moon could be generating enough heat through hydrothermal activity for its interior ocean to have existed for billions of years. This study follows up on that scenario by showing how organic material could be injected into the ocean by hydrothermal vents. This is similar to what happens on Earth, a process that scientists believe may have played a vital role in the origins of life on our planet.

On Earth, organic substances are able to accumulate on the walls of rising air bubbles created by hydrothermal vents, which then rise to the surface and are dispersed by sea spray and the bubbles bursting. Scientists believe a similar process is happening on Enceladus, where bubbles of gas rising through the ocean could be bringing organic materiel up from the core-mantle boundary to the icy surface.

When these bubbles burst at the surface, it helps disperse some of the organics which then become part of the salty spray coming through the tiger cracks. This spray then freezes into icy particles as it reaches space, sending organic material and ice throughout the Saturn System, where it has now been detected. If this study is correct, then another fundamental ingredient for life is present in Enceladus’ interior, making the case for life there that much stronger.

This is just the latest in a long-line of discoveries made by Cassini, many of which point to the potential existence of life on or in some of Saturn’s moons. In addition to confirming the first organic molecules in an “ocean world” of our Solar System, Cassini also found compelling evidence of a rich probiotic environment and organic chemistry on Titan.

In the future, multiple missions are expected to return to these moons to gather more evidence of potential life, picking up where the venerable Cassini left off. So long Cassini, and thanks for blazing a trail!

Further Reading: ESA, Nature

There’s Sand on Titan, Where Does it Come From?

This true-color image of Titan, taken by the Cassini spacecraft, shows the moon's thick, hazy atmosphere. Image: By NASA - http://photojournal.jpl.nasa.gov/catalog/PIA14602, Public Domain, https://commons.wikimedia.org/w/index.php?curid=44822294

Even though the Cassini orbiter ended its mission on of September 15th, 2017, the data it gathered on Saturn and its largest moon, Titan, continues to astound and amaze. During the thirteen years that it spent orbiting Saturn and conducting flybys of its moons, the probe gathered a wealth of data on Titan’s atmosphere, surface, methane lakes, and rich organic environment that scientists continue to pore over.

For instance, there is the matter of the mysterious “sand dunes” on Titan, which appear to be organic in nature and whose structure and origins remain have remained a mystery. To address these mysteries, a team of scientists from John Hopkins University (JHU) and the research company Nanomechanics recently conducted a study of Titan’s dunes and concluded that they likely formed in Titan’s equatorial regions.

Their study, “Where does Titan Sand Come From: Insight from Mechanical Properties of Titan Sand Candidates“, recently appeared online and has been submitted to the Journal of Geophysical Research: Planets. The study was led by Xinting Yu, a graduate student with the Department of Earth and Planetary Sciences (EPS) at JHU, and included EPS Assistant Professors Sarah Horst (Yu’s advisor) Chao He, and Patricia McGuiggan, with support provided by Bryan Crawford of Nanomechanics Inc.

To break it down, Titan’s sand dunes were originally spotted by Cassini’s radar instruments in the Shangri-La region near the equator. The images the probe obtained showed long, linear dark streaks that looked like wind-swept dunes similar to those found on Earth. Since their discovery, scientists have theorized that they are comprised of grains of hydrocarbons that have settled on the surface from Titan’s atmosphere.

In the past, scientists have conjectured that they form in the northern regions around Titan’s methane lakes and are distributed to the equatorial region by the moon’s winds. But where these grains actually came from, and how they came to be distributed in these dune-like formations, has remained a mystery. However, as Yu explained to Universe Today via email, that is only part of what makes these dunes mysterious:

“First, nobody expected to see any sand dunes on Titan before the Cassini-Huygens mission, because global circulation models predicted the wind speeds on Titan are too weak to blow the materials to form dunes. However, through Cassini we saw vast linear dune fields that covers almost 30% of the equatorial regions of Titan!

“Second, we are not sure how Titan sands are formed.Dune materials on Titan are completely different from those on Earth. On Earth, dune materials are mainly silicate sand fragments weathered from silicate rocks. While on Titan, dune materials are complex organics formed by photochemistry in the atmosphere, falling to the ground. Studies show that the dune particles are pretty big (at least 100 microns), while the photochemistry formed organic particles are still pretty small near the surface (only around 1 micron). So we are not sure how the small organic particles are transformed into the big sand dune particles (you need a million small organic particles to form one single sand particle!)

“Third, we also don’t know where the organic particles in the atmosphere are processed to become bigger to form the dune particles. Some scientists think these particles can be processed everywhere to form the dune particles, while some other researchers believe their formation need to be involved with Titan’s liquids (methane and ethane), which are currently located only in the polar regions.”

Dunes on Titan seen in Cassini’s radar (top) that are similar to Namibian sand dunes on Earth. The features that appear to be clouds in the top picture are actually topographic features. Credit: NASA

To shed light on this, Yu and her colleagues conducted a series of experiments to simulate materials being transported on both terrestrial and icy bodies. This consisted of using several natural Earth sands, such as silicate beach sand, carbonate sand and white gyspum sand. To simulate the kinds materials found on Titan, they used laboratory-produced tholins, which are molecules of methane that have been subjected to UV radiation.

The production of tholins was specifically conducted to recreate the kinds of organic aerosols and photochemistry conditions that are common on Titan. This was done using the Planetary HAZE Research (PHAZER) experimental system at Johns Hopkins University – for which the Principal Investigator is Sarah Horst. The last step consisted of using a nanoidentification technique (overseen by Bryan Crawford of Nanometrics Inc.) to study the mechanical properties of the simulated sands and tholins.

This consisted of placing the sand simulants and tholins into a wind tunnel to determine their mobility and see if they could be distributed in the same patterns. As Yu explained:

“The motivation behind the study is to try to answer the third mystery. If the dune materials are processed through liquids, which are located in the polar regions of Titan, they need to be strong enough to be transported from the poles to the equatorial regions of Titan, where most of the dunes are located. However, the tholins we produced in the lab are in extremely low amounts: the thickness of the tholin film we produced is only around 1 micron, about 1/10-1/100 of the thickness of human hair. To deal with this, we used a very intriguing and precise nanoscale technique called nanoindentation to perform the measurements. Even though the produced indents and cracks are all in nanometer scales, we can still precisely determine mechanical properties like Young’s modulus (indicator of stiffness), nanoindentation hardness (hardness), and fracture toughness (indicator of brittleness) of the thin film.”

Radar image of sand dunes on Titan. Credit: NASA/JPL–Caltech/ASI/ESA and USGS/ESA

In the end, the team determined that the organic molecules found on Titan are much softer and more brittle when compared to even the softest sands on Earth. Simply put, the tholins they produced did not appear to have the strength to travel the immense distance that lies between Titan’s northern methane lakes and the equatorial region. From this, they concluded that the organic sands on Titan are likely formed near where they are located.

“And their formation may not involve liquids on Titan, since that would require a huge transportation distance of over 2000 kilometers from the Titan’s poles to the equator,” Yu added. “The soft and brittle organic particles would be grinded to dust before they reach the equator. Our study used a completely different method and reinforced some of results inferred from Cassini observations.”

In the end, this study represents a new direction for researchers when it comes to the study of Titan and other bodies in the Solar System. As Yu explained, in the past, researchers were mostly constrained with Cassini data and modelling to answer questions about Titan’s sand dunes. However, Yu and her colleagues were able to use laboratory-produced analogs to address these questions, despite the fact that the Cassini mission is now at an end.

What’s more, this most recent study is sure to be of immense value as scientists continue to pore over Cassini’s data in anticipation of future missions to Titan. These missions aim to study Titan’s sand dunes, methane lakes and rich organic chemistry in more detail. As Yu explained:

“[O]ur results can not only help understand the origin of Titan’s dunes and sands, but also it will provide crucial information for potential future landing missions on Titan, such as Dragonfly (one of two finalists (out of twelve proposals) selected for further concept development by NASA’s New Frontiers program). The material properties of the organics on Titan can actually provide amazing clues to solve some of the mysteries on Titan.

“In a study we published last year in JGR-planets (2017, 122, 2610–2622), we found out that the interparticle forces between tholin particles are much larger than common sand on Earth, which means the organics on Titan are much more cohesive (or stickier) than silicate sands on Earth. This implies that we need a larger wind speed to blow the sand particles on Titan, which could help the modeling researchers to answer the first mystery. It also suggests that Titan sands could be formed by simple coagulation of organic particles in the atmosphere, since they are much easier to stick together. This could help understand the second mystery of Titan’s sand dunes.”

Artist’s concept of the dragonfly being deployed to Titan and commencing its exploration mission. Credit: APL/Michael Carroll

In addition, this study has implications for the study of bodies other than Titan. “We have found organics on many other solar system bodies, especially icy bodies in the outer solar system, such as Pluto, Neptune’s moon Triton, and comet 67P,” said Yu. “And some of the organics are photochemically produced similarly to Titan. And we do found wind blown features (called aeolian features) on those bodies as well, so our results could be applied to these planetary bodies as well.”

In the coming decade, multiple missions are expected to explore the moons of the outer Solar System and reveal things about their rich environments that could help shed light on the origins of life here on Earth. In addition, the James Webb Space Telescope (now expected to be deployed in 2021) will also use its advanced suit of instruments to study the planets of the Solar System in the hopes of address these burning questions.

Further Reading: arXiv

Scientists Find that Earth Bacteria Could Thrive on Enceladus

For decades, ever since the Pioneer and Voyager missions passed through the outer Solar System, scientists have speculated that life might exist within icy bodies like Jupiter’s moon Europa. However, thanks the Cassini mission, scientists now believe that other moons in the outer Solar System – such as Saturn’s moon Enceladus – could possibly harbor life as well.

For instance, Cassini observed plume activity coming from Enceladus’ southern polar region that indicated the presence of hydrothermal activity inside. What’s more, these plumes contained organic molecules and hydrated minerals, which are potential indications of life. To see if life could thrive inside this moon, a team of scientists conducted a test where strains of Earth bacteria were subjected to conditions similar to what is found inside Enceladus.

The study which details their findings recently appeared in the journal Nature Communications under the title “Biological methane production under putative Enceladus-like conditions“. The study was led by Ruth-Sophie Taubner from the University of Vienna, and included members from the Johannes Kepler University Linz, Ecotechnology Austria, the University of Bremen, and the University of Hamburg.

Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL

For the sake of their study, the team chose to work with three strains of methanogenic archaea known as methanothermococcus okinawensis. This type of microorganism thrives in low-oxygen environments and consumes chemical products known to exist on Enceladus – such as methane (CH4), carbon dioxide (CO2 ) and molecular hydrogen (H2) – and emit methane as a metabolic byproduct. As they state:

“To investigate growth of methanogens under Enceladus-like conditions, three thermophilic and methanogenic strains, Methanothermococcus okinawensis (65 °C), Methanothermobacter marburgensis (65 °C), and Methanococcus villosus (80 °C), all able to fix carbon and gain energy through the reduction of CO2 with H2 to form CH4, were investigated regarding growth and biological CH4 production under different headspace gas compositions…”

These strains were selected because of their ability to grow in a temperature range that is characteristic of the vicinity around hydrothermal vents, in a chemically defined medium, and at low partial pressures of molecular hydrogen. This is consistent with what has been observed in Enceladus’ plumes and what is believed to exist within the moon’s interior.

These types of archaea can still be found on Earth today, lingering in deep-see fissures and around hydrothermal vents. In particular, the strain of M. okinawensis has been determined to exist in only one location around the deep-sea hydrothermal vent field at Iheya Ridge in the Okinawa Trough near Japan. Since this vent is located at a depth of 972 m (3189 ft) below sea level, this suggests that this strain has a tolerance toward high pressure.

Hydrothermal vents on Earth’s ocean floor. Credit: NOAA

For many years, scientists have suspected that Earth’s hydrothermal vents played a vital role in the emergence of life, and that similar vents could exist within the interior of moons like Europa, Ganymede, Titan, Enceladus, and other bodies in the outer Solar System. As a result, the research team believed that methanogenic archaea could also exist within these bodies.

After subjecting the strains to Enceladus-like temperature, pressure and chemical conditions in a laboratory environment, they found that one of the three strains was able to flourish and produce methane. The strain even managed to survive after the team introduced harsh chemicals that are present on Enceladus, and which are known to inhibit the growth of microbes. As they conclude in their study:

“In this study, we show that the methanogenic strain M. okinawensis is able to propagate and/or to produce CH4 under putative Enceladus-like conditions. M. okinawensis was cultivated under high-pressure (up to 50 bar) conditions in defined growth medium and gas phase, including several potential inhibitors that were detected in Enceladus’ plume.”

From this, they determined that some of the methane found in Enceladus’ plumes were likely produced by the presence of methanogenic microbes. As Simon Rittmann, a microbiologist at the University of Vienna and lead author of the study, explained in an interview with The Verge. “It’s likely this organism could be living on other planetary bodies,” he said. “And it could be really interesting to investigate in future missions.”

Artist impression of an interior cross-section of the crust of Enceladus, which shows how hydrothermal activity may be causing the plumes of water at the moon’s surface. Credits: NASA-GSFC/SVS, NASA/JPL-Caltech/Southwest Research Institute

In the coming decades, NASA and other space agencies plan to send multiple mission to the Jupiter and Saturn systems to investigate their “ocean worlds” for potential signs of life. In the case of Enceladus, this will most likely involve a lander that will set down around the southern polar region and collect samples from the surface to determine the presence of biosignatures.

Alternately, an orbiter mission may be developed that will fly through Enceladus’ plumes and collect bioreadings directly from the moon’s ejecta, thus picking up where Cassini left off. Whatever form the mission takes, the discoveries are expected to be a major breakthrough. At long last, we may finally have proof that Earth is not the only place in the Solar System where live can exist.

Be sure to check out John Michael Godier’s video titled “Encedalus and the Conditions for Life” as well:

Further Reading: The Verge, Nature

This was Exactly Where Cassini Crashed into Saturn

On September 15th, 2017, after nearly 20 years in service, the Cassini spacecraft ended its mission by plunging into the atmosphere of Saturn. During the 13 years it spent in the Saturn system, this probe revealed a great deal about the gas giant, its rings, and its systems of moons. As such, it was a bittersweet moment for the mission team when the probe concluded its Grand Finale and began descending into Saturn’s atmosphere.

Even though the mission has concluded, scientists are still busy poring over the data sent back by the probe. These include a mosaic of the final images snapped by Cassini’s cameras, which show the location of where it would enter Saturn’s atmosphere just hours later. The exact spot (shown above) is indicated by a white oval, which was on Saturn’s night side at the time, but would later come around to be facing the Sun.

From the beginning, the Cassini mission was a game-changer. After reaching the Saturn system on July 1st, 2004, the probe began a series of orbits around Saturn that allowed it conduct close flybys of several of its moons. Foremost among these were Saturn’s largest moon Titan and its icy moon Enceladus, both of which proved to be a treasure trove of scientific data.

Artist’s impression of the Cassini spacecraft orbiting Saturn. Credit: NASA/JPL-Caltech/Space Science Institute

On Titan, Cassini revealed evidence of methane lakes and seas, the existence of a methanogenic cycle (similar to Earth’s hydrological cycle), and the presence of organic molecules and prebiotic chemistry. On Enceladus, Cassini examined the mysterious plumes emanating from its southern pole, revealing that they extended all the way to the moon’s interior ocean and contained organic molecules and hydrated minerals.

These findings have inspired a number of proposals for future robotic missions to explore Titan and Enceladus more closely. So far, proposals range from exploring Titan’s surface and atmosphere using lightweight aerial platforms, balloons and landers, or a dual quadcopter. Other proposals include exploring its seas using a paddleboat or a even a submarine. And alongside Europa, there are scientists clamoring for a mission to Enceladus and other “Ocean Worlds” to explore its plumes and maybe even its interior ocean.

Beyond that, Cassini also revealed a great deal about Saturn’s atmosphere, which included the persistent hexagonal storm that exists around the planet’s north pole. During its Grand Finale, where it made 22 orbits between Saturn and its rings, the probe also revealed a great deal about the three-dimensional structure and dynamic behavior of the planet’s famous system of rings.

This montage of images, made from data obtained by Cassini’s visual and infrared mapping spectrometer, shows the location on Saturn where the NASA spacecraft entered Saturn’s atmosphere on Sept. 15, 2017. Credits: NASA/JPL-Caltech/University of Arizona

It is only fitting then that the Cassini probe would also capture images of the very spot where its mission would end. The images were taken by Cassini’s wide-angle camera on Sept. 14th, 2017, when the probe was at a distance of about 634,000 km (394,000 mi) from Saturn. They were taken using red, green and blue spectral filters, which were then combined to show the scene in near-natural color.

The resulting image is not dissimilar from another mosaic that was released on September 15th, 2017, to mark the end of the Cassini mission. This mosaic was created using data obtained by Cassini’s visual and infrared mapping spectrometer, which also showed the exact location where the spacecraft would enter the atmosphere – 9.4 degrees north latitude by 53 degrees west longitude.

The main difference, of course, is that this latest mosaic benefits from the addition of color, which provides a better sense of orientation. And for those who are missing the Cassini mission and its regular flow of scientific discoveries, its much more emotionally fitting! While we may never be able to find the wreckage buried inside Saturn’s atmosphere, it is good to know where its last known location was.

Further Reading: NASA

Just Like Earth, Titan Has a “Sea Level” for its Lakes and Seas

Thanks to the Cassini mission, we have learned some truly amazing things about Saturn and its largest moon, Titan. This includes information on its dense atmosphere, its geological features, its methane lakes, methane cycle, and organic chemistry. And even though Cassini recently ended its mission by crashing into Saturn’s atmosphere, scientists are still pouring over all of the data it obtained during its 13 years in the Saturn system.

And now, using Cassini data, two teams led by researchers from Cornell University have released two new studies that reveal even more interesting things about Titan. In one, the team created a complete topographic map of Titan using Cassini’s entire data set. In the second, the team revealed that Titan’s seas have a common elevation, much like how we have a “sea level” here on Earth.

The two studies recently appeared in the Geophysical Research Letters, titled “Titan’s Topography and Shape at the End of the Cassini Mission” and “Topographic Constraints on the Evolution and Connectivity of Titan’s Lacustrine Basins“. The studies were led by Professor Paul Corlies and Assistant Professor Alex Hayes of Cornell University, respectively, and included members from The Johns Hopkins University Applied Physics Laboratory, NASA’s Jet Propulsion Laboratory, the US Geological Survey (USGS), Stanford University, and the Sapienza Universita di Roma.

This true-color image of Titan, taken by the Cassini spacecraft, shows the moon's thick, hazy atmosphere. Image: By NASA - http://photojournal.jpl.nasa.gov/catalog/PIA14602, Public Domain, https://commons.wikimedia.org/w/index.php?curid=44822294
This true-color image of Titan, taken by the Cassini spacecraft, shows the moon’s thick, hazy atmosphere. Credit: NASA

In the first paper, the authors described how topographic data from multiple sources was combined to create a global map of Titan. Since only about 9% of Titan was observed with high-resolution topography (and 25-30% in lower resolution) the remainder of the moon was mapped with an interpolation algorithm. Combined with a global minimization process, this reduced errors that would arise from such things as spacecraft location.

The map revealed new features on Titan, as well as a global view of the highs and lows of the moon’s topography. For instance, the maps showed several new mountains which reach a maximum elevation of 700 meters (about 3000 ft). Using the map, scientists were also able to confirm that two locations in the equatorial regions are depressions that could be the result of ancient seas that have since dried up or cryovolcanic flows.

The map also suggests that Titan may be more oblate than previously thought, which could mean that the crust varies in thickness. The data set is available online, and the map which the team created from it is already proving its worth to the scientific community. As Professor Corlies explained in a Cornell press release:

“The main point of the work was to create a map for use by the scientific community… We’re measuring the elevation of a liquid surface on another body 10 astronomical units away from the sun to an accuracy of roughly 40 centimeters. Because we have such amazing accuracy we were able to see that between these two seas the elevation varied smoothly about 11 meters, relative to the center of mass of Titan, consistent with the expected change in the gravitational potential. We are measuring Titan’s geoid. This is the shape that the surface would take under the influence of gravity and rotation alone, which is the same shape that dominates Earth’s oceans.”

False-color mosaic of Titan’s northern lakes, made from infrared data collected by NASA’s Cassini spacecraft. Credit: NASA

Looking ahead, this map will play an important role when it comes tr scientists seeking to model Titan’s climate, study its shape and gravity, and its surface morphology. In addition, it will be especially helpful for those looking to test interior models of Titan, which is fundamental to determining if the moon could harbor life. Much like Europa and Enceladus, it is believed that Titan has a liquid water ocean and hydrothermal vents at its core-mantle boundary.

The second study, which also employed the new topographical map, was based on Cassini radar data that was obtained up to just a few months before the spacecraft burned up in Saturn’s atmosphere. Using this data, Assistant Professor Hayes and his team determined that Titan’s seas follow a constant elevation relative to Titan’s gravitational pull. Basically, they found that Titan has a sea level, much like Earth. As Hayes explained:

“We’re measuring the elevation of a liquid surface on another body 10 astronomical units away from the sun to an accuracy of roughly 40 centimeters. Because we have such amazing accuracy we were able to see that between these two seas the elevation varied smoothly about 11 meters, relative to the center of mass of Titan, consistent with the expected change in the gravitational potential. We are measuring Titan’s geoid. This is the shape that the surface would take under the influence of gravity and rotation alone, which is the same shape that dominates Earth’s oceans.”

This common elevation is important because liquid bodies on Titan appear to be connected by something resembling an aquifer system. Much like how water flows underground through porous rock and gravel on Earth, hydrocarbons do the same thing under Titan’s icy surface. This ensures that there is transference between large bodies of water, and that they share a common sea level.

Artist concept of Cassini’s last moments at Saturn. Credit: NASA/JPL.

“We don’t see any empty lakes that are below the local filled lakes because, if they did go below that level, they would be filled themselves,”  said Hayes. “This suggests that there’s flow in the subsurface and that they are communicating with each other. It’s also telling us that there is liquid hydrocarbon stored on the subsurface of Titan.”

Meanwhile, smaller lakes on Titan appear at elevations several hundred meters above Titan’s sea level. This is not dissimilar to what happens on Earth, where large lakes are often found at higher elevations. These are known as “Alpine Lakes”, and some well-known examples include Lake Titicaca in the Andes, Lakes Geneva in the Alps, and Paradise Lake in the Rockies.

Last, but not least, the study also revealed the vast majority of Titan’s lakes are found within sharp-edged depressions that are surrounded by high ridges, some of which are hundreds of meters high. Here too, there is a resemblance to features on Earth – such as the Florida Everglades – where underlying material dissolves and causes the surface to collapse, forming holes in the ground.

The shape of these lakes indicate that they may be expanding at a constant rate, a process known as uniform scarp retreat. In fact, the largest lake in the south – Ontario Lacus – resembles a series of smaller empty lakes that have coalesced to form a single feature. This process is apparently due to seasonal change, where autumn in the southern hemisphere leads to more evaporation.

While the Cassini mission is no longer exploring the Saturn system, the data it accumulated during its multi-year mission is still bearing fruit. Between these latest studies, and the many more that will follow, scientists are likely to reveal a great deal more about this mysterious moon and the forces that shape it!

Further Reading: NASA, Cornell University, Geophysical Research Letters

New Study Says Enceladus has had an Internal Ocean for Billions of Years

When the Cassini mission arrived in the Saturn system in 2004, it discovered something rather unexpected in Enceladus’ southern hemisphere. From hundreds of fissures located in the polar region, plumes of water and organic molecules were spotted periodically spewing forth. This was the first indication that Saturn’s moon may have an interior ocean caused by hydrothermal activity near the core-mantle boundary.

According to a new study based on Cassini data, which it obtained before diving into Saturn’s atmosphere on September 15th, this activity may have been going on for some time. In fact, the study team concluded that if the moon’s core is porous enough, it could have generated enough heat to maintain an interior ocean for billions of years. This study is the most encouraging indication yet that the interior of Enceladus could support life.

The study, titled “Powering prolonged hydrothermal activity inside Enceladus“, recently appeared in the journal Nature Astronomy. The study was led by Gaël Choblet, a researcher with the Planetary and Geodynamic Laboratory at the University of Nantes, and included members from NASA’s Jet Propulsion Laboratory, Charles University, and the Institute of Earth Sciences and the Geo- and Cosmochemistry Laboratory at the University of Heidelberg.

Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL

Prior to the Cassini mission’s many flybys of Enceladus, scientists believed this moon’s surface was composed of solid ice. It was only after noticing the plume activity that they came to realize that it had water jets that extended all the way down to a warm-water ocean in its interior. From the data obtained by Cassini, scientists were even able to make educated guesses of where this internal ocean lay.

All told, Enceladus is a relatively small moon, measuring some 500 km (311 mi) in diameter. Based on gravity measurements performed by Cassini, its interior ocean is believed to lie beneath an icy outer surface at depths of 20 to 25 km (12.4 to 15.5 mi). However, this surface ice thins to about 1 to 5 km (0.6 to 3.1 mi) over the southern polar region, where the jets of water and icy particles jet through fissures.

Based on the way Enceladus orbits Saturn with a certain wobble (aka. libration), scientists have been able to make estimates of the ocean’s depth, which they place at 26 to 31 km (16 to 19 mi). All of this surrounds a core which is believed to be composed of silicate minerals and metal, but which is also porous. Despite all these findings, the source of the interior heat has remained something of an open question.

This mechanism would have to be active when the moon formed billions of years ago and is still active today (as evidenced by the current plume activity). As Dr. Choblet explained in an ESA press statement:

“Where Enceladus gets the sustained power to remain active has always been a bit of mystery, but we’ve now considered in greater detail how the structure and composition of the moon’s rocky core could play a key role in generating the necessary energy.”

Gravity measurements by NASA’s Cassini spacecraft and Deep Space Network suggest that Saturn’s moon Enceladus, which has jets of water vapor and ice gushing from its south pole, also harbors a large interior ocean beneath an ice shell, as this illustration depicts. Credit: NASA/JPL-Caltech

For years, scientists have speculated that tidal forces caused by Saturn’s gravitational influence are responsible for Enceladus’ internal heating. The way Saturn pushes and pulls the moon as it follows an elliptical path around the planet is also believed to be what causes Enceladus’ icy shell to deform, causing the fissures around the southern polar region. These same mechanisms are believed to be what is responsible for Europa’s interior warm-water ocean.

However, the energy produced by tidal friction in the ice is too weak to counterbalance the heat loss seen from the ocean. At the rate Enceladus’ ocean is losing energy to space, the entire moon would freeze solid within 30 million years. Similarly, the natural decay of radioactive elements within the core (which has been suggested for other moons as well) is also about 100 times too weak to explain Enceladus interior and plume activity.

To address this, Dr. Choblet and his team conducted simulations of Enceladus’ core to determine what kind of conditions could allow for tidal heating over billions of years. As they state in their study:

“In absence of direct constraints on the mechanical properties of Enceladus’ core, we consider a wide range of parameters to characterize the rate of tidal friction and the efficiency of water transport by porous flow. The unconsolidated core of Enceladus can be viewed as a highly granular/fragmented material, in which tidal deformation is likely to be associated with intergranular friction during fragment rearrangements.”
Artist rendering showing an interior cross-section of the crust of Enceladus, which shows how hydrothermal activity may be causing the plumes of water at the moon’s surface.Credits: NASA-GSFC/SVS, NASA/JPL-Caltech/Southwest Research Institute

What they found was that in order for the Cassini observations to be borne out, Enceladus’ core would need to be made of unconsolidated, easily deformable, porous rock. This core could be easily permeated by liquid water, which would seep into the core and gradually heated through tidal friction between sliding rock fragments. Once this water was sufficiently heated, it would rise upwards because of temperature differences with its surroundings.

This process ultimately transfers heat to the interior ocean in narrow plumes which rise to the meet Enceladus’ icy shell. Once there, it causes the surface ice to melt and forming fissures through which jets reach  into space, spewing water, ice particles and hydrated minerals that replenish Saturn’s E-Ring. All of this is consistent with the observations made by Cassini, and is sustainable from a geophysical point of view.

In other words, this study is able to show that action in Enceladus’ core could produce the necessary heating to maintain a global ocean and produce plume activity. Since this action is a result of the core’s structure and tidal interaction with Saturn, it is perfectly logical that it has been taking place for billions of years. So beyond providing the first coherent explanation for Enceladus’ plume activity, this study is also a strong indication of habitability.

As scientists have come to understand, life takes a long time to get going. On Earth, it is estimated that the first microorganisms arose after 500 million years, and hydrothermal vents are believed to have played a key role in that process. It took another 2.5 billion years for the first multi-cellular life to evolve, and land-based plants and animals have only been around for the past 500 million years.

Knowing that moons like Enceladus – which has the necessary chemistry to support for life – has also had the necessary energy for billions of years is therefore very encouraging. One can only imagine what we will find once future missions begin inspecting its plumes more closely!

Further Reading: ESA, Nature Astronomy

Forecast for Titan: Cold, with a Chance of Noxious Ice Clouds

During the 13 years and 76 days that the Cassini mission spent around Saturn, the orbiter and its lander (the Huygens probe) revealed a great deal about Saturn and its systems of moons. This is especially true of Titan, Saturn’s largest moon and one of the most mysterious objects in the Solar System. As a result of Cassini’s many flybys, scientists learned a great deal about Titan’s methane lakes, nitrogen-rich atmosphere, and surface features.

Even though Cassini plunged into Saturn’s atmosphere on September 15th, 2017, scientists are still pouring over the things it revealed. For instance, before it ended its mission, Cassini captured an image of a strange cloud floating high above Titan’s south pole, one which is composed of toxic, hybrid ice particles. This discovery is another indication of the complex organic chemistry occurring in Titan’s atmosphere and on it’s surface.

Since this cloud was invisible to the naked eye, it was only observable thanks to Cassini’s Composite Infrared Spectrometer (CIRS). This instrument spotted the cloud at an altitude of about 160 to 210 km (100 to 130 mi), far above the methane rain clouds of Titan’s troposphere. It also covered a large area near the south pole, between 75° and 85° south latitude.

Artist concept of Cassini’s last moments at Saturn. Credit: NASA/JPL.

Using the chemical fingerprint obtained by the CIRS instrument, NASA researchers also conducted laboratory experiments to reconstruct the chemical composition of the cloud. These experiments determined that the cloud was composed of the organic molecules hydrogen cyanide and benzene. These two chemicals appeared to have condensed together to form ice particles, rather than being layered on top of each other.

For those who have spent more than the past decade studying Titan’s atmosphere, this was a rather interesting and unexpected find. As Carrie Anderson, a CIRS co-investigator at NASA’s Goddard Space Flight Center, said in a recent NASA press statement:

“This cloud represents a new chemical formula of ice in Titan’s atmosphere. What’s interesting is that this noxious ice is made of two molecules that condensed together out of a rich mixture of gases at the south pole.”

The presence of this cloud around Titan’s southern pole is also another example of the moon’s global circulation patterns. This involves currents of warm gases being sent from the hemisphere that is experiencing summer to the hemisphere experience winter. This pattern reverse direction when the seasons change, which leads to a buildup of clouds around whichever pole is experiencing winter.

Artist’s impression of Saturn’s moon Titan shows the change in observed atmospheric effects before, during and after equinox in 2009. Credit: NASA

When the Cassini orbiter arrived at Saturn in 20o4, Titan’s northern hemisphere was experiencing winter – which began in 2004. This was evidenced by the buildup of clouds around its north pole, which Cassini spotted during its first encounter with the moon later than same year. Similarly, the same phenomena was taking place around the south pole near the end of Cassini’s mission.

This was consistent with seasonal changes on Titan, which take place roughly every seven Earth years – a year on Titan lasts about 29.5 Earth years. Typically, the clouds that form in Titan’s atmosphere are structured in layers, where different types of gas will condense into icy clouds at different altitudes. Which ones condense is dependent on how much vapor is present and temperatures – which become steadily colder closer to the surface.

However, at times, different types of clouds can form over a range of altitudes, or co-condense with other types of clouds. This certainly appeared to be the case when it came to the large cloud of hydrogen cyanide and benzene that was spotted above the south pole. Evidence of this cloud was derived from three sets of Titan observations made with the CIRS instrument, which took place between July and November of 2015.

The CIRS instrument works by separating infrared light into its constituent colors, and then measures the strengths of these signals at the different wavelengths to determine the presence of chemical signatures. Previously, it was used to identify the presence of hydrogen cyanide ice clouds over the south pole, as well as other toxic chemicals in the moon’s stratosphere.

Artist’s impression of the Cassini orbiter’s Composite Infrared Spectrometer (CIRS). Credit: NASA-JPL

As F. Michael Flasar, the CIRS principal investigator at Goddard, said:

“CIRS acts as a remote-sensing thermometer and as a chemical probe, picking out the heat radiation emitted by individual gases in an atmosphere. And the instrument does it all remotely, while passing by a planet or moon.”

However, when examining the observation data for chemical “fingerprints”, Anderson and her colleagues noticed that the spectral signatures of the icy cloud did not match those of any individual chemical. To address this, the team began conducting laboratory experiments where mixtures of gases were condensed in a chamber that simulated conditions in Titan’s stratosphere.

After testing different pairs of chemicals, they finally found one which matched the infrared signature observed by CIRS. At first, they tried letting one gas condense before the other, but found that the best results were obtained when both gases were introduced and allowed to condense at the same time. To be fair, this was not the first time that Anderson and her colleagues had discovered co-condensed ice in CIRS data.

For example, similar observations were made near the north pole in 2005, about two years after the northern hemisphere experienced its winter solstice. At that time, the icy clouds were detected at a much lower altitude (below 150 km, or 93 mi) and showed chemical fingerprints of hydrogen cyanicide and caynoacetylene – one of the more complex organic molecules in Titan’s atmosphere.

Artist’s impression of the Cassini orbiter entering Saturn’s atmosphere. Credit: NASA/JPL

This difference between this and the latest detection of a hybrid cloud, according to Anderson, comes down to differences in seasonal variations between the north and south poles. Whereas the northern polar cloud observed in 2005 was spotted about two years after the northern winter solstice, the southern cloud Anderson and her team recently examined was spotted two years before the southern winter solstice.

In short, it is possible that the mixture of the gases was slightly different in the two case, and/or that the northern cloud had a chance to warm slightly, thus altering its composition somewhat.  As Anderson explained, these observations were made possible thanks to the many years that the Cassini mission spent around Saturn:

“One of the advantages of Cassini was that we were able to flyby Titan again and again over the course of the thirteen-year mission to see changes over time. This is a big part of the value of a long-term mission.”

Additional studies will certainly be needed to determine the structure of these icy clouds of mixed composition, and Anderson and her team already have some ideas on how they would look. For their money, the researchers expect these clouds to be lumpy and disorderly, rather than well-defined crystals like the single-chemical clouds.

In the coming years, NASA scientists are sure to be spending a great deal of time and energy sorting through all the data obtained by the Cassini mission over the course of its 13-year mission. Who knows what else they will detect before they have exhausted the orbiter’s vast collections of data?

Future Reading: NASA