Much talk of Mars these days seems to focus almost exclusively upon the multi-faceted Curiosity rover, as it unravels the mysterious geological past of Gale Crater, or upon the Opportunity rover, which has endured 37 times longer than intended in one of the harshest environments ever encountered and last month broke the record for the longest distance driven by a U.S. vehicle on another world, passing a cumulative total of 22 miles. Yet high above both of these rovers’ heads, the Mars Reconnaissance Orbiter (MRO or “Mr. O”) has circled the planet for more than seven years since March 2006. Its role has encompassed that of both a science-gathering platform and a telecommunications relay satellite for other missions. As a science platform, it has been described as “a microscope in orbit” around the Red Planet … and that tool has enabled it to make a number of surprising new discoveries.
Or rather two tools have been responsible for making important new discoveries in the understanding of mysterious surface “gullies” and unexpected temperature swings during the course of each Martian day. Aboard MRO the High Resolution Imaging Science Experiment (HiRISE) has the capability to acquire imagery of Mars at resolutions of 0.3 meters per pixel (about 1 foot), resolving objects less than a meter across. Famously, it has imaged both Opportunity and Curiosity as they trundle across the red-hued terrain and photographed the Mars Phoenix Lander parachuting toward the surface ahead of touchdown in May 2008. The second MRO tool which has enabled the recent research is the Mars Climate Sounder (MCS), a visible and near-infrared spectrometer with the ability to measure temperature, pressure, water vapour, and dust levels to enable planetary scientists to assemble daily global weather maps.
This week, it was announced by NASA that frozen chunks of carbon dioxide—dry ice—may glide down some Martian sand dunes on gaseous “cushions,” akin to miniature hovercraft, plowing long furrows as they go. The data from HiRISE offers a possible explanation for enigmatic surface features known as “linear gullies,” which typically measure a few yards across and exhibit raised banks along their sides and pits at their downhill ends. “In debris flows, you have water carrying sediment downhill,” explained planetary scientist Serina Diniega of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., “and the material eroded from the top is carried to the bottom and deposited in a fan-shaped apron. In the linear gullies, you’re not transporting material. You’re carving out a groove, pushing material to the sides.”
The gullies are situated on dunes which spend the harsh Martian winter covered with carbon dioxide frost, and comparisons of images recorded at different stages of the year indicate that the grooves form in early spring. Several images have indicated the presence of bright objects—possibly fragments of dry ice which have broken away from points higher on the slope—in the gullies. According to the new hypotheses, the pits at the downhill ends may result from blocks of dry ice completely sublimating away into gas after they have ceased traveling. “Linear gullies don’t look like gullies on Earth or other gullies on Mars,” noted Diniega, “and this process wouldn’t happen on Earth. You don’t get blocks of dry ice on Earth … unless you go buy them!”
Candice Hansen of the Planetary Science Institute in Tucson, Ariz., co-author of the study, did exactly that: bought some slabs of dry ice at a supermarket and slid them down sand dunes on Earth. Her work found that gaseous carbon dioxide from the thawing ice helped to maintain a lubricating layer underneath the slab and served to push sand aside into small levees as the slabs glided down even low-angle slopes. Calculations which further took into account the conditions of Martian surface temperature and atmospheric pressure have led to increased confidence that the dry ice would act in a similar fashion in the early Martian spring. “MRO is showing that Mars is a very active planet,” said Hansen. “Some of the processes we see on Mars are like processes on Earth, but this one is in the category of uniquely Martian.”
In tandem with the linear gullies research by the Mars Reconnaissance Orbiter, the spacecraft’s MCS instrument has recently revealed that atmospheric temperatures on the Red Planet regularly rise and fall not once per day, but twice. “We see a temperature maximum in the middle of the day,” said Armin Kleinboehl of JPL, “but we also see a temperature maximum a little after midnight.” Temperatures swing by as much as 58 degrees Fahrenheit (32 Kelvin) and the pattern seems to be a global, year-round phenomenon, known as “atmospheric tides.” Unlike ocean tides here on Earth, they are driven by variations in daytime-nighttime heating. Our planet’s atmospheric tides produce little temperature difference in the lower atmosphere, but in Mars’ less dense atmosphere their effects tend to dominate temperature variations on a global scale. The research will be shortly published by the journal Geophysical Research Letters.
Atmospheric tides which rise and fall twice per day are known as “semi-diurnal tides” and, although first seen in Mars back in the 1970s, they were until recently thought only to appear in dusty seasons as sunlight warmed particulate matter in the atmosphere. “We were surprised to find this strong, twice-a-day structure in the temperatures of the non-dusty Martian atmosphere,” Kleinboehl admitted. “While the diurnal tide as a dominant temperature response to the day-night cycle of solar heating on Mars has been known for decades, the discovery of a persistent semi-diurnal response, even outside of major dust storms, was quite unexpected and caused us to wonder what drove this response.”
Kleinboehl’s team found the answer to lie within Mars’ water-ice clouds, which persist for most of the year. In the planet’s equatorial region, relatively transparent clouds at altitudes between 6 and 19 miles absorb infrared light emitted from the surface during daytime, and this process serves to heat the middle atmosphere each day. The observed semi-diurnal pattern, with its maximum temperature swings occurring away from the tropics, was also unexpected, but Mars climate models have successfully replicated it when the radiative effects of the water-ice clouds are included.
“We think of Mars as a cold and dry world, with little water,” said Kleinboehl, “but there is actually more water vapor in the Martian atmosphere than in the upper layers of Earth’s atmosphere. Water-ice clouds have been known to form in regions of cold temperatures, but the feedback of these clouds on the Martian temperature structure had not been appreciated.” He drew parallels in the importance of understanding cloud physics on Earth to understand the fundamental functions of our planet’s atmosphere.
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As Mr. Spock might say, “Fascinating.” We really need to experience these things in person.
I have been looking for Beagle 2 (the UK built Lander which went missing on Christmas Day 2003) in the available HiRISE images of the expected landing ellipse.
But, good as it is, the HiRISE resolution is not quite good enough to provide a ‘confirmation’ of my finding of objects which look like the different elements of Beagle 2 ie Lander, Front Shell, Back Cover and Drogue parachute, Airbags and Main parachute.
However I am hoping that with the imminent acquisition of a 5th image of the area, I can get some Image Processing specialist friends of mine to produce a super-res composite image, which should eiither confirm or reject my finding.
Stuart Hurst (ex-Beagle 2 Chief Engineer)
Stuart, As soon as I heard the name of the lander: “Beagle”, I immediately thought, “Uh oh, with sincere respect to Darwin and the HMS Beagle, the person who named this lander obviously does not live with a real live Beagle. It’s no surprise that the Beagle went missing, a Beagle lives to follow its nose, wherever that may take them, and they are oblivious to “return” or “power down” signals. As a person who lives with a couple beagles rescued from a shelter, I wish you good luck in acquiring an image of your beagle as they travel very fast and are quite good at evading obstacles such as human pursuers. Their ears must be solar panels because they seem to have limitless energy, and their “four paw drive” provides them with amazing maneuverability. I know how disheartening it can be to lose a Beagle, but as did one of ours, I’m sure your Beagle will be found eventually (hopefully sans the sheepish, guilty look and offensive odor). 🙂