The Cassini mission to Saturn ended in September 2017, but the data it gathered during its 13 year mission is still yielding scientific results. On the heels of a newly-released global image of Saturn’s moon Titan comes another discovery: Rainfall at Titan’s north pole.
Climate models developed by scientists during Cassini’s mission concluded that rain should fall in the north during Titan’s summer. But scientists hadn’t seen any clouds. Now, a team of scientists have published a paper centered on Cassini images that show light reflecting off a wet surface. They make the case that the reflecting light, called a Bright Ephemeral Flare (BEF) is sunlight reflecting from newly-fallen rain.
Saturn’s moon Titan is a very strange place. It’s surrounded by a dense, opaque atmosphere, the only moon in the solar system with an atmosphere to speak of. It has lakes of liquid methane on its surface, maybe some cryovolcanoes, and some scientists speculate that it could support a form of life. Very weird life.
But we still don’t know a lot about it, because we haven’t really seen much of the surface. Until now.
It has been almost forty years since the Voyager 1 and 2 missions visited the Saturn system. As the probes flew by the gas giant, they were able to capture some stunning, high-resolution images of the planet’s atmosphere, its many moons, and its iconic ring system. In addition, the probes also revealed that Saturn was slowly losing its rings, at a rate that would see them gone in about 100 million years.
More recently, the Cassini orbiter visited the Saturn system and spent over 12 years studying the planet, its moons and its ring system. And according to new research based on Cassini’s data, it appears that Saturn is losing its rings at the maximum rate predicted by the Voyager missions. According to the study, Saturn’s rings are being gobbled up by the gas giant at a rate that means they could be gone in less 100 million years.
Ever since the Cassini orbiter entered the Saturn system in July of 2004, scientists and the general public have been treated to a steady stream of data about this ringed giant and its many fascinating moons. In particular, a great deal of attention was focused on Saturn’s largest moon Titan, which has many surprising Earth-like characteristics.
These include its nitrogen-rich atmosphere, the presence of liquid bodies on its surface, a dynamic climate, organic molecules, and active prebiotic chemistry. And in the latest revelation to come from the Cassini orbiter, it appears that Titan also experiences periodic dust storms. This puts it in a class that has so far been reserved for only Earth and Mars.
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.
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.
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).
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!
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.
“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 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:
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 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.
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.”
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!
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.”
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.”
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.”
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.
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 Cassinimission, 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.
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.
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.”
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: