Crater edge in Terra Sirenum has been imaged by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. Gully watching thanks to repeat sweeps over the same landscape by orbiting spacecraft could catch gullies in action, if they are active today.
Credit: NASA/JPL/University of Arizona
Thanks to a flotilla of Mars orbiters, there's been a steady flow of information streaming in from that puzzling world. Scientists are piecing together a far more coherent view of "real time" versus "geological time" in dealing with the whole of Mars today.
One of the more perplexing finds on Mars are features that look like the product of groundwater seeping to the surface. These gullies could be formed by flowing water--perhaps fed by a groundwater supply. Deposits of soil and rocks transported by these flows have been found too.
More importantly, gully features appear to be young. So young, in fact, they might be forming today. Clearly, gullies may well be areas of astrobiologial interest--a niche for life that could be present and accounted for on Mars.
It was back in June 2000 when Mars gullies became big news--new landforms that had never been seen before as revealed in images taken from the Mars Global Surveyor. Spacecraft have now been sending back views of the planet and its ever-changing face over the course of the past five Martian years.
Mars gullies remain controversial as to how they are created. But now gully watching scientists are nearing a watershed moment in unraveling the story behind the formations.
Mars, by gully
Gullies are high on a "change detection" target list for NASA's newly positioned Mars Reconnaissance Orbiter (MRO). Onboard that Mars-circling craft is the ultra-powerful High Resolution Imaging Science Experiment (HiRISE) camera.
"HiRISE can do an excellent job of change detection due to the high resolution" and other attributes of the imaging system, said Alfred McEwen, Director of the Planetary Image Research Lab at the University of Arizona in Tucson, Arizona. He is MRO's HiRISE principal investigator.
"We are planning a major effort for change detection over many terrains on Mars," McEwen told SPACE.com, such as polar layered deposits, dunes and gullies.
MRO's point-and-shoot skills also enable re-imaging of terrains at the exact same season of different Mars years to match illumination angles.
"Viewing angles will differ, but we plan to acquire stereo images and produce digital elevation models of any site that shows evidence for change ... so we can correct for any viewing effects and make precise measurements of changes," McEwen noted.
"We will of course see changes in color and albedo [variations in the amount of sunlight reflected by the martian surface] but interpretation of such surficial changes can be controversial," McEwen said. "So our hope is to detect and measure actual changes in the topography."
Therefore, MRO can constrain the current rates of change of features, McEwen added, like the age of younger features such as gullies--even if the spacecraft and Mars investigators doesn't actually spot new gully erosion.
The true nature of the Mars gullies remains a work in progress, said Linda Martel, a geologist and an educational outreach staffer at the Hawaii Institute of Geophysics and Planetology at the University of Hawaii.
"Most researchers are still favoring two models of formation for gullies on Martian slopes--erosion caused by groundwater discharge or by the melting of near-surface ice or snow," Martel advised.
Mapping the locations and orientations of gullies continues, with Mars scientists looking at temperature and pressure conditions in the subsurface that allow liquid water to exist where gully recesses are found. Also, they are evaluating atmospheric conditions and exposure of the surface to martian sunshine.
"They are coming up with very convincing reasons why either model makes sense," Martel said. "But when researchers say that gullies are recent they aren't talking about today, right now."
For one, Martel said, it's impossible to carve the larger--about 4 miles (7 kilometers) long--gullies by flowing pure water across the surface under today's temperature and pressure conditions on Mars. An on-going research focus, she said, is appreciating what the gullies say about changes in the stability of water or changes in atmospheric pressure, temperature, humidity or exposure to sunshine on Mars.
Evidence that water is carving out martian gullies doesn't, well, hold water in some circles. The Moon has gullies that look strikingly familiar to the ravines on Mars--and water certainly didn't form gullies on that bone-dry world.
Mars gullies are made up of a deep channel with a collapsed region at its upper end--an "alcove"--and at the other end an area of accumulated debris--an "apron" that appears to have been transported down the slope.
Gwendolyn Bart, a University of Arizona lunar and planetary scientist, has made a comparison of small lunar landslides and martian gullies. Some lunar landslides have an alcove-channel-apron morphology--structure and form of a feature--similar to that of the martian gullies
"Because the Moon is devoid of geologically active water, we know the lunar features formed by dry landslides," Bart observed. "As a result, it is impossible to rule out dry landslides as the formation mechanism for martian gullies, based solely on the alcove-channel-apron morphology."
Bart said that more high resolution lunar data--like that expected from NASA's super-snooping Lunar Reconnaissance Orbiter to be launched in 2008--will give researchers the ability to identify more lunar features, and to see whether the Moon reproduces the great variety of gullies spotted on Mars.
"It is my opinion that there easily could be active melting or release of water from sub-surface aquifers," said Philip Christensen at Arizona State University (ASU) in Tempe, a leading Mars researcher. He is Principal Investigator for the 2001 Mars Odyssey's Thermal Emission Imaging System (THEMIS) instrument.
Christensen said that he believes some of the Mars gullies emanate from dust-covered surface snowpacks. Small, local changes in the conditions of these snowpacks--such as wind or landslides removing the dust cover--could allow these snows to begin to melt and release water today, he advised.
"The main argument against a lot of current activity is that these snowpacks likely formed during the previous climate cycles and are 50,000 to 300,000 years old," Christensen told SPACE.com. "If they had remained active all that time they would be completely gone by now."
However, Christensen added, if the snowpacks get covered by a protective layer of dust, then the snow can be stable for a long time. Remove the dust and the melting is reactivated, then gullies could be eroding in some places on Mars today, he said.
"Alternatively, if the water is coming from sub-surface aquifers or ground ice ... local heating or other local changes could also release water and these gullies could also be active," Christensen suggested.
In his view, Christensen said that all this gully-speak leads to some key questions regarding the red planet.
For one, what is the nature of near-surface snow and water in the mid-latitude regions?
"These regions are extremely interesting," Christensen remarked, "because they're cold enough for snow and ice to accumulate during climate cycle--but warm enough for melting and release of liquid water to occur at other times ... perhaps current times."
The equatorial regions appear very dry today, Christensen noted, and the poles are very cold throughout the year.
"The gullies indicate that there is snow or water near the surface in the mid-latitudes," Christensen pointed out. "This is where I think a lot of the action and excitement on Mars is occurring today."
Off limits, no wheel zones
So why not send a robot to get up-close and personal with a Mars gully?
NASA's Mars Science Laboratory (MSL), now being built for a 2009 sendoff to the red planet, won't be up to sterility snuff to explore a gully site due to planetary protection rules.
Gullies for MSL are off limits, no wheel zones. You don't want to infect possible martian microbes or water sources with hitchhiking bacteria brought from Earth.
"Gullies appear to be the best chance we will have--or perhaps could have--for seeking present-day life on the martian surface," suggested John Rummel, Senior Scientist for Astrobiology in the Science Mission Directorate at NASA Headquarters in Washington, D.C.
For astrobiologists, Rummel continued, there is no more compelling target than gullies for exploration on Mars. But there's need for the right rover that's outfitted with the right instruments before we go, he said.
Delving into Mars gullies, Rummel said, can be extended from orbit around the planet--or even from platforms flying through the martian atmosphere, like robotic airplanes or balloons.
"But the most important phase of the exploration of the gullies can only be done by visiting them and making measurements on site," Rummel said. "To do this requires an agile, sterile rover and finely tuned analytical instruments ... but with those tools, we have perhaps the best chance this century to discover whether or not Mars is alive."
This article is part of SPACE.com's weekly Mystery Monday series.
- Images: Visualizations of Mars
- The June 2000 News of Mars Gullies
- Mars Gullies Likely Formed By Underground Aquifers
- Mars Gullies Could Harbor Martian Biology
- All About Mars