Thawing ‘Dry Ice’ on Mars Proves the Planet is Still Active


Researchers using NASA’s Mars Reconnaissance Orbiter (MRO) see seasonal changes on far-northern Martian sand dunes caused by warming of a winter blanket of frozen carbon dioxide.

Seasonal Changes on Far-Northern Mars: The High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter snapped this series of false-color pictures of sand dunes in the north polar region of Mars. The area covered in each of the five panels is about 0.8 mile (1.3 kilometers) wide. The progression begins at left (Panel A) in early spring, when the ground is covered by a seasonal layer of carbon dioxide ice (dry ice) about 2 feet thick. As spring progresses the ice cracks (Panel B), releasing dark sand from the dune below. When pressurized gas trapped below the ice layer is released, it carries along sand and dust to the top of the ice layer, where it is dropped in fan-shaped deposits downhill and downwind (panels C and D). The final panel shows more and more of the dark dunes as the overlying layer of seasonal ice evaporates back into the atmosphere. Image Credit: NASA/JPL-Caltech/Univ. of Arizona

Earth has no naturally frozen carbon dioxide, though pieces of manufactured carbon-dioxide ice, called “dry ice,” sublime directly from solid to gas on Earth, just as the vast blankets of dry ice do on Mars. A driving factor in the springtime changes where seasonal coverings of dry ice form on Mars is that thawing occurs at the underside of the ice sheet, where it is in contact with dark ground being warmed by early-spring sunshine through translucent ice. The trapped gas builds up pressure and breaks out in various ways.

Transient grooves form on dunes when gas trapped under the ice blanket finds an escape point and whooshes out, carrying out sand with it. The expelled sand forms dark fans or streaks on top of the ice layer at first, but this evidence disappears with the seasonal ice, and summer winds erase most of the grooves in the dunes before the next winter. The grooves are smaller features than the gullies that earlier research linked to carbon-dioxide sublimation on steeper dune slopes.

Similar activity has been documented and explained previously where seasonal sheets of frozen carbon dioxide form and thaw near Mars’ south pole. Details of the different northern seasonal changes are newly reported in a set of three papers for the journal Icarus.

Mars Reconnaissance Orbiter captures the springtime thaw of seasonal carbon dioxide ice on Mars. Video Credit: NASA/JPL-Caltech/Univ. of Arizona

The findings reinforce growing appreciation that Mars today, however different from its former self, is still a dynamic world, and however similar to Earth in some respects, displays some quite unearthly processes.

“It’s an amazingly dynamic process,” said Candice Hansen of the Planetary Science Institute, Tucson. She is lead author of the first of the three new reports. “We had this old paradigm that all the action on Mars was billions of years ago. Thanks to the ability to monitor changes with the Mars Reconnaissance Orbiter, one of the new paradigms is that Mars has many active processes today.”

With three Martian years (six Earth years) of data in hand from the Mars Reconnaissance Orbiter’s High Resolution Imaging Science Experiment (HiRISE) camera, the researchers report on the sequence and variety of seasonal changes. The spring changes include outbursts of gas carrying sand, polygonal cracking of the winter ice blanketing the dunes, sandfalls down the faces of the dunes, and dark fans of sand propelled out onto the ice.

Spider-shaped features in the south polar region of Mars are carved by vaporizing dry ice in a dynamic seasonal process. This image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter includes several of the distinctive features in an area 1.2 kilometers (three-fourths of a mile) wide. Image Credit: NASA/JPL-Caltech/University of Arizona

“It is a challenge to catch when and how those changes happen, they are so fast,” said Ganna Portyankina of the University of Bern in Switzerland, lead author of the second report. “That’s why only now we start to see the bigger picture that both hemispheres actually tell us similar stories.”

The process of outrushing gas that carves grooves into the northern dunes resembles the process creating spider-shaped features in far southern Mars, as seen in the image above, but the spiders have not been seen in the north. The seasonal dry-ice sheets overlie different types of terrain in the two hemispheres. In the south, diverse terrains include the flat, erodible ground where the spiders form, but in the north, a broad band of sand dunes encircles the permanent north polar ice cap.

Another difference is in brightening on parts of the ice-covered dunes. This brightening in the north results from the presence of water-ice frost, while in the south, similar brightening is caused by fresh carbon dioxide. The third paper of the Icarus set, by Antoine Pommerol of the University of Bern and co-authors, reports distribution of the water frost using the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). The light water frost is blown around by spring winds.

Source: Jet Propulsion Laboratory (JPL)

Ice on Lakes and Seas at Saturn’s Moon Titan


A new paper by scientists on NASA’s Cassini mission finds that blocks of hydrocarbon ice might decorate the surface of existing lakes and seas of liquid hydrocarbon on Saturn’s moon Titan. The presence of ice floes might explain some of the mixed readings Cassini has seen in the reflectivity of the surfaces of lakes on Titan.

Artist's concept of Titan lakes

This artist’s concept envisions what hydrocarbon ice forming on a liquid hydrocarbon sea of Saturn’s moon Titan might look like. A new model from scientists on NASA’s Cassini mission suggests that clumps of methane-and-ethane-rich ice – shown here as the lighter-colored clusters – could float under some conditions. Image Credit: NASA/JPL-Caltech/USGS

“One of the most intriguing questions about these lakes and seas is whether they might host an exotic form of life,” said Jonathan Lunine, a paper co-author and Cassini interdisciplinary Titan scientist at Cornell University, Ithaca, N.Y. “And the formation of floating hydrocarbon ice will provide an opportunity for interesting chemistry along the boundary between liquid and solid, a boundary that may have been important in the origin of terrestrial life.”

Titan is the only other body besides Earth in our Solar System with stable bodies of liquid on its surface. But while our planet’s cycle of precipitation and evaporation involves water, Titan’s cycle involves hydrocarbons like ethane and methane. Ethane and methane are organic molecules, which scientists think can be building blocks for the more complex chemistry from which life arose. Cassini has seen a vast network of these hydrocarbon seas cover Titan’s northern hemisphere, while a more sporadic set of lakes bejewels the southern hemisphere.

Lakes on Saturn's moon Titan

Lakes on Saturn’s moon Titan reflect radio waves in varying ways in this image from NASA’s Cassini spacecraft. Scientists think the variations in reflectivity, or brightness, have to do with the smoothness or texture of the surface. If a lake is fully liquid, it looks dark, but if it is only partially liquid, it looks brighter.
In this image taken from Titan’s high northern latitudes on May 22, 2012, the lakes on the left are full of liquid hydrocarbons and the lakes on the top right are only partially filled, or represent saturated ground (i.e., a mudflat). The lakes in this image are each about 35 to 45 kilometers (22 to 30 miles) across, or about the size of Lake Tahoe on the California-Nevada border. Some of the differences in reflectivity could also be explained by the presence of floating hydrocarbon ice. Image Credit: NASA/JPL-Caltech/ASI/Cornell

Up to this point, Cassini scientists assumed that Titan lakes would not have floating ice, because solid methane is denser than liquid methane and would sink. But the new model considers the interaction between the lakes and the atmosphere, resulting in different mixtures of compositions, pockets of nitrogen gas, and changes in temperature. The result, scientists found, is that winter ice will float in Titan’s methane-and-ethane-rich lakes and seas if the temperature is below the freezing point of methane — minus 297 degrees Fahrenheit (90.4 kelvins). The scientists realized all the varieties of ice they considered would float if they were composed of at least 5 percent “air,” which is an average composition for young sea ice on Earth. (“Air” on Titan has significantly more nitrogen than Earth air and almost no oxygen.)

If the temperature drops by just a few degrees, the ice will sink because of the relative proportions of nitrogen gas in the liquid versus the solid. Temperatures close to the freezing point of methane could lead to both floating and sinking ice – that is, a hydrocarbon ice crust above the liquid and blocks of hydrocarbon ice on the bottom of the lake bed. Scientists haven’t entirely figured out what color the ice would be, though they suspect it would be colorless, as it is on Earth, perhaps tinted reddish-brown from Titan’s atmosphere.

These images obtained by NASA's Cassini spacecraft show Titan's stable northern lake district.

For perspective, a wider view of these lakes. These images obtained by NASA’s Cassini spacecraft show Titan’s stable northern lake district. Cassini’s radar instrument obtained the recent images on May 22, 2012. It observed a.o. regions containing lakes that were last observed about six years—nearly one Titan season–ago. 
The top image here shows part of the radar swath from May 22, 2012, about 220 by 47 miles (350 by 75 kilometers) in dimension. At the bottom, parts of this image are compared with those obtained in 2006.  In 2006, it was winter in the northern hemisphere and the lakes were in the dark. Although Titan spring began in 2009 and the Sun has now risen over the lakes, there is no apparent change in lake levels since the 2006 flybys, consistent with climate models that predict stability of liquid lakes over several years. This shows that the northern lakes are not transient weather events, in contrast to the temporary darkening of parts of the equator after a rainstorm in 2010.
Changes in lake levels may still be detected later in the mission as Cassini continues to observe these high northern latitudes into the beginning of summer in 2017. At that point, the Sun may cause evaporation. However, the lack of significant change over six years sets important constraints for climate models and the stability of liquids on Titan. Image Credit: NASA/JPL-Caltech/ASI

“We now know it’s possible to get methane-and-ethane-rich ice freezing over on Titan in thin blocks that congeal together as it gets colder — similar to what we see with Arctic sea ice at the onset of winter,” said Jason Hofgartner, first author on the paper and a Natural Sciences and Engineering Research Council of Canada scholar at Cornell. “We’ll want to take these conditions into consideration if we ever decide to explore the Titan surface some day.”

Cassini’s radar instrument will be able to test this model by watching what happens to the reflectivity of the surface of these lakes and seas. A hydrocarbon lake warming in the early spring thaw, as the northern lakes of Titan have begun to do, may become more reflective as ice rises to the surface. This would provide a rougher surface quality that reflects more radio energy back to Cassini, making it look brighter. As the weather turns warmer and the ice melts, the lake surface will be pure liquid, and will appear to the Cassini radar to darken.

“Cassini’s extended stay in the Saturn system gives us an unprecedented opportunity to watch the effects of seasonal change at Titan,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “We’ll have an opportunity to see if the theories are right.”

Source: Jet Propulsion Laboratory (JPL)

Titan’s Seasonal Changes Affect It More Than Thought


Detailed observations of Saturn’s moon Titan have now spanned 30 years, covering an entire solar orbit for this distant world. Dr Athena Coustenis from the Paris-Meudon Observatory in France has analysed data gathered over this time and has found that the changing seasons of Titan affect it more than previously thought. 

An artist’s impression of the Cassini spacecraft performing a fly-by of Titan, gathering data used in this research. Image Credit: NASA/JPL-Caltech

Explains Dr Coustenis, “As with Earth, conditions on Titan change with its seasons. We can see differences in atmospheric temperatures, chemical composition and circulation patterns, especially at the poles. For example, hydrocarbon lakes form around the north polar region during winter due to colder temperatures and condensation. Also, a haze layer surrounding Titan at the north pole is significantly reduced during the equinox because of the atmospheric circulation patterns. This is all very surprising because we didn’t expect to find any such rapid changes, especially in the deeper layers of the atmosphere.”

This real photograph of Titan shows two thin haze layers, now known to change with the seasons. Image Credit: NASA/JPL/Cassini

The main cause of these cycles is solar radiation. This is the dominant energy source for Titan’s atmosphere, breaking up the nitrogen and methane present to create more complex molecules, such as ethane, and acting as the driving force for chemical changes. Titan is inclined at around 27 degrees, similar to the Earth, meaning that the cause of seasons – sunlight reaching different areas with varying intensity due to the tilt – is the same for both worlds. Says Dr Coustenis, “It’s amazing to think that the Sun still dominates over other energy sources even as far out as Titan, over 1.5 billion kilometres from us.”

This impression of Titan’s surface is based on data from the Huygens mission, giving an idea the view from the ground. Image Credit: Cassini-Huygens DISR

To draw these conclusions data was analysed from several different missions, including Voyager 1 (1980), the Infrared Space Observatory (1997), and Cassini (2004 onwards), complemented by ground-based observations. Each season on Titan spans around 7.5 years, while it takes 29.5 years for Saturn to orbit the Sun, so data has now been gathered for an entire Titan year, encapsulating all seasons.

Different missions have gathered data on Titan over a full course of its seasons. Credit: Ralph Lorenz.

Dr Coustenis explains why it is important to investigate this distant moon: “Titan is the best opportunity we have to study conditions very similar to our own planet in terms of climate, meteorology and astrobiology and at the same time a unique world on its own, a paradise for exploring new geological, atmospheric and internal processes.”

Dr Coustenis has presented these results at the European Planetary Science Congress in Madrid on Friday 28th September.

Source: Europlanet