Artist's concept of a dust storm on Titan.
Credits: IPGP/Labex UnivEarthS/University Paris Diderot – C. Epitalon & S. Rodriguez
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.
Infrared images of Saturn's moon Titan, captured by Cassini's the Visual and Infrared Mapping Spectrometer (VIMS) instrument. Credit: NASA/JPL-Caltech/Stéphane Le Mouélic, University of Nantes, Virginia Pasek, University of Arizona
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!
This true-color image of Titan, taken by the Cassini spacecraft, shows the moon's thick, hazy atmosphere. Credit: NASA
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.
A montage of some of the "ocean worlds" in our Solar System. From top to bottom, left to right, these include Europa, Enceladus, TItan and Ceres. Credit: NASA/JPL
For decades, scientists have believed that there could be life beneath the icy surface of Jupiter’s moon Europa. Since that time, multiple lines of evidence have emerged that suggest that it is not alone. Indeed, within the Solar System, there are many “ocean worlds” that could potentially host life, including Ceres, Ganymede, Enceladus, Titan, Dione, Triton, and maybe even Pluto.
But what if the elements for life as we know it are not abundant enough on these worlds? In a new study, two researchers from the Harvard Smithsonian Center of Astrophysics (CfA) sought to determine if there could in fact be a scarcity of bioessential elements on ocean worlds. Their conclusions could have wide-ranging implications for the existence of life in the Solar System and beyond, not to mention our ability to study it.
Artist’s depiction of a watery exoplanet orbiting a distant red dwarf star. Credit: CfA
In previous studies, questions on the habitability of moons and other planets have tended to focus on the existence of water. This has been true when it comes to the study of planets and moons within the Solar System, and especially true when it comes the study of extra-solar planets. When they have found new exoplanets, astronomers have paid close attention to whether or not the planet in question orbits within its star’s habitable zone.
This is key to determining whether or not the planet can support liquid water on its surface. In addition, astronomers have attempted to obtain spectra from around rocky exoplanets to determine if water loss is taking place from its atmosphere, as evidenced by the presence of hydrogen gas. Meanwhile, other studies have attempted to determine the presence of energy sources, since this is also essential to life as we know it.
In contrast, Dr. Lingam and Prof. Loeb considered how the existence of life on ocean planets could be dependent on the availability of limiting nutrients (LN). For some time, there has been considerable debate as to which nutrients would be essential to extra-terrestrial life, since these elements could vary from place to place and over timescales. As Lingam told Universe Today via email:
“The mostly commonly accepted list of elements necessary for life as we know it comprises of hydrogen, oxygen, carbon, nitrogen and sulphur. In addition, certain trace metals (e.g. iron and molybdenum) may also be valuable for life as we know it, but the list of bioessential trace metals is subject to a higher degree of uncertainty and variability.”
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
For their purposes, Dr. Lingam and Prof. Loeb created a model using Earth’s oceans to determine how the sources and sinks – i.e. the factors that add or deplete LN elements into oceans, respectively – could be similar to those on ocean worlds. On Earth, the sources of these nutrients include fluvial (from rivers), atmospheric and glacial sources, with energy being provided by sunlight.
Of these nutrients, they determined that the most important would be phosphorus, and examined how abundant this and other elements could be on ocean worlds, where conditions as vastly different. As Dr. Lingam explained, it is reasonable to assume that on these worlds, the potential existence of life would also come down to a balance between the net inflow (sources) and net outflow (sinks).
“If the sinks are much more dominant than the sources, it could indicate that the elements would be depleted relatively quickly. In other to estimate the magnitudes of the sources and sinks, we drew upon our knowledge of the Earth and coupled it with other basic parameters of these ocean worlds such as the pH of the ocean, the size of the world, etc. known from observations/theoretical models.”
While atmospheric sources would not be available to interior oceans, Dr. Lingam and Prof. Loeb considered the contribution played by hydrothermal vents. Already, there is abundant evidence that these exist on Europa, Enceladus, and other ocean worlds. They also considered abiotic sources, which consist of minerals leached from rocks by rain on Earth, but would consist of the weathering of rocks by these moons’ interior oceans.
Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL
Ultimately, what they found was that, unlike water and energy, limiting nutrients might be in limited supply when it comes to ocean worlds in our Solar System:
“We found that, as per the assumptions in our model, phosphorus, which is one of the bioessential elements, is depleted over fast timescales (by geological standards) on ocean worlds whose oceans are neutral or alkaline in nature, and which possess hydrothermal activity (i.e. hydrothermal vent systems at the ocean floor). Hence, our work suggests that life may exist in low concentrations globally in these ocean worlds (or be present only in local patches), and may therefore not be easily detectable.”
This naturally has implications for missions destined for Europa and other moons in the outer Solar System. These include the NASA Europa Clipper mission, which is currently scheduled to launch between 2022 and 2025. Through a series of flybys of Europa, this probe will attempt to measure biomarkers in the plume activity coming from the moon’s surface.
Similar missions have been proposed for Enceladus, and NASA is also considering a “Dragonfly” mission to explore Titan’s atmosphere, surface and methane lakes. However, if Dr. Lingam and Prof. Loeb’s study is correct, then the chances of these missions finding any signs of life on an ocean world in the Solar System are rather slim. Nevertheless, as Lingam indicated, they still believe that such missions should be mounted.
Artist’s concept of a Europa Clipper mission. Credit: NASA/JPL
“Although our model predicts that future space missions to these worlds might have low chances of success in terms of detecting extraterrestrial life, we believe that such missions are still worthy of being pursued,” he said. “This is because they will offer an excellent opportunity to: (i) test and/or falsify the key predictions of our model, and (ii) collect more data and improve our understanding of ocean worlds and their biogeochemical cycles.”
In addition, as Prof. Loeb indicated via email, this study was focused on “life as we know it”. If a mission to these worlds did find sources of extra-terrestrial life, then it would indicate that life can arise from conditions and elements that we are not familiar with. As such, the exploration of Europa and other ocean worlds is not only advisable, but necessary.
“Our paper shows that elements that are essential for the ‘chemistry-of-life-as-we-know-it’, such as phosphorous, are depleted in subsurface oceans,” he said. “As a result, life would be challenging in the oceans suspected to exist under the surface ice of Europa or Enceladus. If future missions confirm the depleted level of phosphorous but nevertheless find life in these oceans, then we would know of a new chemical path for life other than the one on Earth.”
In the end, scientists are forced to take the “low-hanging fruit” approach when it comes to searching for life in the Universe . Until such time that we find life beyond Earth, all of our educated guesses will be based on life as it exists here. I can’t imagine a better reason to get out there and explore the Universe than this!
Thanks to Cassini and other spacecraft, we’ve learned a tremendous amount about the icy worlds in the Solar System, from Jupiter’s Europa to Saturn’s Enceladus, to Pluto’s Charon. Geysers, food for bacteria, potential oceans under the ice and more. What new things have we learned about these places?
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The site of the Cassini probe's crash in Saturn's atmosphere, circled in white. Credit: NASA/JPL-Caltech/Space Science Institute
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.
Ligeia Mare, shown in here in data obtained by NASA's Cassini spacecraft, is the second largest known body of liquid on Saturn's moon Titan. It is filled with liquid hydrocarbons, such as ethane and methane, and is one of the many seas and lakes that bejewel Titan's north polar region. Credit: NASA/JPL-Caltech/ASI/Cornell
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.
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!
In this illustration, the Dragonfly helicopter drone is descending to the surface of Titan. Image: NASA
The only thing cooler than sending a helicopter drone to explore Titan is sending a nuclear powered one to do the job. Called the “Dragonfly” spacecraft, this helicopter drone mission has been selected as one of two finalists for NASA’s robotic exploration missions planned for the mid 2020’s. NASA selected the Dragonfly mission from 12 proposals they were considering under their New Horizons program.
Titan is Saturn’s largest moon, and is a primary target in the search for life in our Solar System. Titan has liquid hydrocarbon lakes on its surface, a carbon-rich chemistry, and sub-surface oceans. Titan also cycles methane the way Earth cycles water.
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
Dragonfly would fulfill its mission by hopping around on the surface of Titan. Once an initial landing site is selected on Titan, Dragonfly will land there with the assistance of a ‘chute. Dragonfly will spend periods of time on the ground, where it will charge its batteries with its radioisotope thermoelectric generator. Once charged, it would then fly for hours at time, travelling tens of kilometers during each flight. Titan’s dense atmosphere and low gravity (compared to Earth) allows for this type of mission.
During these individual flights, potential landing sites would be identified for further scientific work. Dragonfly will return to its initial landing site, and only visit other sites once they have been verified as safe.
Dragonfly is being developed at the Johns Hopkins Applied Physics Laboratory (JHAPL.) It has a preliminary design weight of 450 kg. It’s a double quad-copter design, with four sets of dual rotors.
“Titan is a fascinating ocean world,” said APL’s Elizabeth Turtle, principal investigator for Dragonfly. “It’s the only moon in the solar system with a dense atmosphere, weather, clouds, rain, and liquid lakes and seas—and those liquids are ethane and methane. There’s so much amazing science and discovery to be done on Titan, and the entire Dragonfly team and our partners are thrilled to begin the next phase of concept development.”
The science objectives of the Dragonfly mission center around prebiotic organic chemistry and habitability on Titan. It will likely have four instruments:
Being chosen as a finalist has the team behind Dragonfly excited for the project. “This brings us one step closer to launching a bold and very exciting space exploration mission to Titan,” said APL Director Ralph Semmel. “We are grateful for the opportunity to further develop our New Frontiers proposals and excited about the impact these NASA missions will have for the world.”
Exploring Titan holds a daunting set of challenges. But as we’ve seen in recent years, NASA and its partners have the capability to meet those challenges. The JHAPL team behind Dragonfly also designed and built the New Horizons mission to Pluto and the Kuiper Belt object 2014 MU69. Their track record of success has everyone excited about the Dragonfly mission.
The Dragonfly mission, and the other finalist—the Comet Astrobiology Exploration Sample Return being developed by Cornell University and the Goddard Space Flight Center—will each receive funding through the end of 2018 to work on the concepts. In the Spring of 2019, NASA will select one of them and will fund its continued development.
Dragonfly is part of NASA’s New Frontiers program. New Frontiers missions are planetary science missions with a cap of approximately $850 million. New Frontiers missions include the Juno mission to Jupiter, the Osiris-REx asteroid sample-return missions, and the aforementioned New Horizons mission to Pluto.
The moons of Europa and Enceladus, as imaged by the Galileo and Cassini spacecraft. Credit: NASA/ESA/JPL-Caltech/SETI Institute
In the hunt for extra-terrestrial life, scientists tend to take what is known as the “low-hanging fruit approach”. This consists of looking for conditions similar to what we experience here on Earth, which include at oxygen, organic molecules, and plenty of liquid water. Interestingly enough, some of the places where these ingredients are present in abundance include the interiors of icy moons like Europa, Ganymede, Enceladus and Titan.
Whereas there is only one terrestrial planet in our Solar System that is capable of supporting life (Earth), there are multiple “Ocean Worlds” like these moons. Taking this a step further, a team of researchers from the Harvard Smithsonian Center for Astrophysics (CfA) conducted a study that showed how potentially-habitable icy moons with interior oceans are far more likely than terrestrial planets in the Universe.
Cutaway showing the interior of Saturn’s moon Enceladus. Credit: ESA
To begin, Lingam and Loeb address the tendency to confuse habitable zones (HZs) with habitability, or to treat the two concepts as interchangeable. For instance, planets that are located within an HZ are not necessarily capable of supporting life – in this respect, Mars and Venus are perfect examples. Whereas Mars is too cold and it’s atmosphere too thin to support life, Venus suffered a runaway greenhouse effect that caused it to become a hot, hellish place.
On the other hand, bodies that are located beyond HZs have been found to be capable of having liquid water and the necessary ingredients to give rise to life. In this case, the moons of Europa, Ganymede, Enceladus, Dione, Titan, and several others serve as perfect examples. Thanks to the prevalence of water and geothermal heating caused by tidal forces, these moons all have interior oceans that could very well support life.
As Lingam, a post-doctoral researcher at the ITC and CfA and the lead author on the study, told Universe Today via email:
“The conventional notion of planetary habitability is the habitable zone (HZ), namely the concept that the “planet” must be situated at the right distance from the star such that it may be capable of having liquid water on its surface. However, this definition assumes that life is: (a) surface-based, (b) on a planet orbiting a star, and (c) based on liquid water (as the solvent) and carbon compounds. In contrast, our work relaxes assumptions (a) and (b), although we still retain (c).”
As such, Lingam and Loeb widen their consideration of habitability to include worlds that could have subsurface biospheres. Such environments go beyond icy moons such as Europa and Enceladus and could include many other types deep subterranean environments. On top of that, it has also been speculated that life could exist in Titan’s methane lakes (i.e. methanogenic organisms). However, Lingam and Loeb chose to focus on icy moons instead.
A “true color” image of the surface of Jupiter’s moon Europa as seen by the Galileo spacecraft. Image credit: NASA/JPL-Caltech/SETI Institute
“Even though we consider life in subsurface oceans under ice/rock envelopes, life could also exist in hydrated rocks (i.e. with water) beneath the surface; the latter is sometimes referred to as subterranean life,” said Lingam. “We did not delve into the second possibility since many of the conclusions (but not all of them) for subsurface oceans are also applicable to these worlds. Similarly, as noted above, we do not consider lifeforms based on exotic chemistries and solvents, since it is not easy to predict their properties.”
Ultimately, Lingam and Loeb chose to focus on worlds that would orbit stars and likely contain subsurface life humanity would be capable of recognizing. They then went about assessing the likelihood that such bodies are habitable, what advantages and challenges life will have to deal with in these environments, and the likelihood of such worlds existing beyond our Solar System (compared to potentially-habitable terrestrial planets).
For starters, “Ocean Worlds” have several advantages when it comes to supporting life. Within the Jovian system (Jupiter and its moons) radiation is a major problem, which is the result of charged particles becoming trapped in the gas giants powerful magnetic field. Between that and the moon’s tenuous atmospheres, life would have a very hard time surviving on the surface, but life dwelling beneath the ice would fare far better.
“One major advantage that icy worlds have is that the subsurface oceans are mostly sealed off from the surface,” said Lingam. “Hence, UV radiation and cosmic rays (energetic particles), which are typically detrimental to surface-based life in high doses, are unlikely to affect putative life in these subsurface oceans.”
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
“On the negative side,’ he continued, “the absence of sunlight as a plentiful energy source could lead to a biosphere that has far less organisms (per unit volume) than Earth. In addition, most organisms in these biospheres are likely to be microbial, and the probability of complex life evolving may be low compared to Earth. Another issue is the potential availability of nutrients (e.g. phosphorus) necessary for life; we suggest that these nutrients might be available only in lower concentrations than Earth on these worlds.”
In the end, Lingam and Loeb determined that a wide range of worlds with ice shells of moderate thickness may exist in a wide range of habitats throughout the cosmos. Based on how statistically likely such worlds are, they concluded that “Ocean Worlds” like Europa, Enceladus, and others like them are about 1000 times more common than rocky planets that exist within the HZs of stars.
These findings have some drastic implications for the search for extra-terrestrial and extra-solar life. It also has significant implications for how life may be distributed through the Universe. As Lingam summarized:
“We conclude that life on these worlds will undoubtedly face noteworthy challenges. However, on the other hand, there is no definitive factor that prevents life (especially microbial life) from evolving on these planets and moons. In terms of panspermia, we considered the possibility that a free-floating planet containing subsurface exolife could be temporarily “captured” by a star, and that it may perhaps seed other planets (orbiting that star) with life. As there are many variables involved, not all of them can be quantified accurately.”
A new instrument called the Search for Extra-Terrestrial Genomes (STEG) is being developed to find evidence of life on other worlds. Credit: NASA/Jenny Mottor
Professor Leob – the Frank B. Baird Jr. Professor of Science at Harvard University, the director of the ITC, and the study’s co-author – added that finding examples of this life presents its own share of challenges. As he told Universe Today via email:
“It is very difficult to detect sub-surface life remotely (from a large distance) using telescopes. One could search for excess heat but that can result from natural sources, such as volcanos. The most reliable way to find sub-surface life is to land on such a planet or moon and drill through the surface ice sheet. This is the approach contemplated for a future NASA mission to Europa in the solar system.”
Exploring the implications for panspermia further, Lingam and Loeb also considered what might happen if a planet like Earth were ever ejected from the Solar System. As they note in their study, previous research has indicated how planets with thick atmospheres or subsurface oceans could still support life while floating in interstellar space. As Loeb explained, they also considered what would happen if this ever happened with Earth someday:
“An interesting question is what would happen to the Earth if it was ejected from the solar system into cold space without being warmed by the Sun. We have found that the oceans would freeze down to a depth of 4.4 kilometers but pockets of liquid water would survive in the deepest regions of the Earth’s ocean, such as the Mariana Trench, and life could survive in these remaining sub-surface lakes. This implies that sub-surface life could be transferred between planetary systems.”
The Drake Equation, a mathematical formula for the probability of finding life or advanced civilizations in the universe. Credit: University of Rochester
This study also serves as a reminder that as humanity explores more of the Solar System (largely for the sake of finding extra-terrestrial life) what we find also has implications in the hunt for life in the rest of the Universe. This is one of the benefits of the “low-hanging fruit” approach. What we don’t know is informed but what we do, and what we find helps inform our expectations of what else we might find.
And of course, it’s a very vast Universe out there. What we may find is likely to go far beyond what we are currently capable of recognizing!
Google Maps now lets users explore the Solar System. Credit: NASA/Google
Chances are, at one time or another, we’ve all used Google Maps to find the shortest route from point A to point B. But if you are like some people, you’ve used this mapping tool to have a look at geographical features or places you hope to visit someday. In an age where digital technology is allowing for telecommuting and even telepresence, it’s nice to take virtual tours of the places we may never get to see in person.
But now, Google Maps is using its technology to enable the virtual exploration of something far grander: the Solar System! Thanks to images provided by the Cassini orbiter of the planets and moons it studied during its 20 year mission, Google is now allowing users to explore places like Venus, Mercury, Mars, Europa, Ganymede, Titan, and other far-off destinations that are impossible for us to visit right now.
Similar to how Google Earth uses satellite imagery to create 3D representations of our planet, this new Google Maps tool relies on the more than 500,000 images taken by Cassini as it made its way across the Solar System. This probe recently concluded its 20 year mission, 13 of which were spent orbiting Saturn and studying its system of moons, by crashing into the atmosphere of Saturn.
Artist rendition of the Cassini spacecraft over Saturn. Credit: NASA/JPL-Caltech/SSI/Kevin M. Gill.
After launching from Earth on October 15th, 1997, Cassini conducted a flyby of Venus in order to pick up a gravity-assist. It then flew by Earth, obtaining a second gravity-assist, while making its way towards the Asteroid Belt. Before reaching the Saturn System, where it would begin studying the gas giant and its moons, Cassini also conducted a flyby of Jupiter – snapping pictures of its moons, rings, and Great Red Spot.
When it reached Saturn in July of 2004, Cassini went to work studying the planet and its larger moons – particularly Titan and Enceladus. During the next 13 years and 76 days, the probe would provide breathtaking images and sensor data on Saturn’s rings, atmosphere and polar storms and reveal things about Titan’s surface that were never before seen (such as its methane lakes, hydrological cycle, and surface features).
It’s flybys of Enceladus also revealed some startling things about this icy moon. Aside from detecting a tenuous atmosphere of ionized water vapor and Enceladus’ mysterious “Tiger Stripes“, the probe also detected jets of water and organic molecules erupting from the moon’s southern polar region. These jets, it was later determined, were indicative of a warm water ocean deep in the moon’s interior, and possibly even life!
Interestingly enough, the original Cassini mission was only planned to last for four years once it reached Saturn – from June 2004 to May 2008. But by the end of this run, the mission was extended with the Cassini Equinox Mission, which was intended to run until September of 2010. It was extended a second time with the Cassini Solstice Mission, which lasted until September 15th, 2017, when the probe was crashed into Saturn’s atmosphere.
Artist’s impression of the Cassini orbiter entering Saturn’s atmosphere. Credit: NASA/JPL
Thanks to all the images taken by this long-lived mission, Google Maps is now able to offer exploratory tours of 16 celestial bodies in the Solar System – 12 of which are new to the site. These include Earth, the Moon, Mercury, Venus, Mars, Pluto, Ceres, Io, Europa, Ganymede, Mimas, Enceladus, Dione, Rhea, Titan, Iapetus and (available as of July 2017) the International Space Station.
This latest development also builds on several extensions Google has released over the years. These include Google Moon, which was released on July 20th, 2005, to coincide with the 36th anniversary of the Apollo 11 Moon Landing. Then there was Google Sky (introduced in 2007), which used photographs taken by the Hubble Space Telescope to create a virtual map of the visible universe.
Then there was Google Mars, the result of a collaborative effort between Google and NASA scientists at the Mars Space Flight Facility released in 2011, one year before the Curiosity rover landed on the Red Planet. This tool relied on data collected by the Mars Global Surveyor and the Mars Odyssey missions to create high-resolution 3D terrain maps that included elevations.
In an age of high-speed internet and telecommunications, using the internet to virtually explore the many planets and bodies of the Solar System just makes sense. Especially when you consider that even the most ambitious plans to conduct tourism to Mars or the Moon (looking at you, Elon Musk and Richard Branson!) are not likely to bear fruit for many years, and cost an arm and a leg to boot!
In the future, similar technology could lead to all kinds of virtual exploration. This concept, which is often referred to as “telexploration”, would involve robotic missions traveling to other planets and even star systems. The information they gather would then be sent back to Earth to create virtual experiences, which would allow scientists and space-exploration enthusiasts to feel like they were seeing it firsthand.
In truth, this mapping tool is just the latest gift to be bestowed by the late Cassini mission. NASA scientists expect to be sifting through the volumes of data collected by the orbiter for years to come. Thanks to improvements made in software applications and the realms of virtual and augmented reality, this data (and that of present and future missions) is likely to be put to good use, enabling breathtaking and educational tours of our Universe!
