Martian Field Trips on Earth
Geotimes staff

For nearly 40 years, several U.S. satellites and rovers and cameras and spectrometers have orbited or even landed on Mars: the Mariner missions, Viking missions, Mars Pathfinder, Mars Global Surveyor and 2001 Mars Odyssey. This month and next month, NASA is scheduled to launch the Mars Exploration Rovers, robotic field geologists that will land at two points on the planet. Following them will be Mars Express, scheduled for a June launch, and, planned for 2005, the Mars Reconnaissance Orbiter, Smart Lander and Long-range Rover and Scout Missions — and those are just the planned missions that already have names.

At the same time, many researchers look to Earth for examples of or contrasts to what we’re seeing of the Red Planet. The following stories are snapshots of Earth analogs: places on Earth that look similar to images we’ve seen from Mars. The researchers featured here are following these similarities and studying these spots on Earth in detail to see whether they offer clues or contrasts to the martian processes, past and present.

We offer these research snapshots with some caution. Earth can be an excellent guide to other planets, but not a lone perspective. “There are so many different things to be considered,” says Benton Clark, who has been studying the Red Planet since the first Viking images came back in 1976. “I’m always worried about people making their theories too comprehensive in the face of not enough data.”

The data will continue to pour in, and researchers hope to strengthen our understanding of the Red Planet as information travels from Mars to Earth.

The Grand Canyon: Carving a past for martian climate
Egypt: Rivers of sand
Great sand dunes: Windswept waves
Australia: Acid waters
Atacama desert: An arid testing ground

The Grand Canyon: Carving a past for martian climate

Nanedi Vallis sinuously stretches through the cratered plains of Mars’ Xanthe Terra region. About a kilometer deep and 2.5 kilometers wide, the origin of the canyon is unknown. Some researchers are looking to Arizona’s Grand Canyon here on Earth to understand Nanedi Vallis’ mysterious formation and, in turn, Mars’ ancient climate.

“I think that if you were standing at the surface of Nanedi Vallis, it would look much like the Grand Canyon — steep walls, fairly deep and quite broad,” says Jim Kasting, professor of geosciences and meteorology at Penn State. On Earth, it took millions of years for the Colorado River to carve out the 1.5-kilometer-deep Grand Canyon. Nanedi’s small channel, Kasting says, suggests it too formed from a small river cutting it out over millions of years. That slow formation would have required an atmosphere warm enough for running water, he says.

Visualizing a warm early Mars, however, is no easy task. “Cold early Mars is the leading theory right now, because no one has a good explanation for how early Mars could have been warm; what you see on the surface does not look like an earthlike climate,” Kasting says. Martian river channels do not have smaller branching channels, which on Earth represent a sustained water cycle. And researchers have yet to see evidence on Mars’ surface of carbonates, which on Earth signal past large amounts of the greenhouse gas carbon dioxide.

Kasting still contends that to create Nanedi Vallis over millions of years would have required a warm early Mars. He thinks that a greenhouse effect, like on Earth, could have been responsible. “In some ways the planets should have been more alike back then than they are now, because Mars when it was young was still geologically active.”

Named for the word for ‘planet’ in Sesotho, the national language of Lesotho, Africa, Nanedi Vallis is 2.5 kilometers wide and about a kilometer deep. Researchers are looking to Arizona’s Grand Canyon to understand the climatic environment that shaped Nanedi Vallis. NASA/JPL/Malin Space Science Systems.

His research team has modeled an ancient martian atmosphere that includes a combination of carbon dioxide and methane. The climate model yielded about 30 degrees of greenhouse warming, bringing the surface temperatures up near the freezing point of water, not quite warm enough for sustained liquid water.

Although the presence of methane on early Mars is speculative, research now in press by Chris McKay of the NASA Ames Research Center and Monika Kress of the University of Washington suggests that impacts could have provided a non-biological source of the gas.

The model did not include the effect of carbon dioxide clouds, which would have additionally warmed the atmosphere, says Kasting, who presented his team’s findings at the American Geophysical Union meeting in December. And if the sun were brighter, which is possible but contradicts the current leading theory, the climate also would have been warmer, he adds.

Writing in a Dec. 6, 2002, Science article, Teresa Segura of NASA Ames and colleagues proposed that an early Mars environment with millions of years of sustained warm temperatures and rain are not necessary for forming rivers. They suggest that an individual large impact, about 100 to 250 kilometers wide, would have vaporized both the object and the martian soil, melting and precipitating tens of meters of water. “You can get significant erosion from just a handful of these large impact events. The brief periods of torrential rains and flash flooding, coupled to warm temperatures, may be enough to carve rivers,” Segura says. For something as large as Nanedi Vallis, Segura says, multiple impact events would have been necessary to create the whole feature. Globally,

Mars would be above the freezing point of water with flowing water for years to millennia following each impact. The rest of the time, “we think Mars was pretty much as it is today, with a slightly higher surface pressure, but probably similar temperatures, especially when considering a fainter sun,” Segura says.

If, as Kasting contends, Nanedi Vallis formed slowly like the Grand Canyon, then it would take 10,000 large impacts to create the right environment for its formation under Segura’s impact scenario. Kasting adds that while the giant impacts could have contributed to the martian paleoclimate, they likely would have scarred the landscape, making it difficult for Nanedi to look as it does today.

The 25-year-long debate between cold and warm early Mars is not likely to end soon, Kasting says. “The challenge for any theory is to explain what we see in geomorphological evidence.” Ultimately, Kasting would like to gain insight on how wide the habitable zone is around the sun and, by implication, other stars. “Early Mars is a test of this idea, because it looks to many of us like early Mars was habitable; it did have flowing liquid water at its surface, which is the first requirement for life. Life could have evolved there, but we don’t have a good climate theory for why.”

Lisa M. Pinsker

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Egypt: Rivers of sand

At his office in the in the Smithsonian Institution’s National Air and Space Museum, Ted Maxwell pulls out a map of Egypt and begins pointing out areas in the southwestern desert.

“It rained here for an hour or two in 1927. Over here there was a shower in 1968. And it rained here in 1998. I was there, I saw it rain,” he says. The meteorological records for this region identify the land as hyperarid. Yet river channels zigzag below the flat, sandy surface. The only visible signs of these Cenozoic river beds are the scattered rocks and pebbles where the bedrock of the channel banks pokes through the red and yellow mottled plain. Only a few fracture zones allow the presence of oases where the near-surface aquifer can support scattered acacia mounds.

Andrew Johnston surveys the floor of the Kiseiba depression in southwest Egypt, 100 kilometers west of the Nile. Photo by Ted Maxwell.

Shortly after the Viking Landers in 1976 brought back images of rocky outcrops on an otherwise flat, dusty surface of Mars, Maxwell, working with Farouk El-Baz, at the time also with the Smithsonian, began to investigate the relationship between the Red Planet and Egypt. In 1978, traveling with a team of American and Egyptian specialists, El-Baz and Maxwell explored Gilf Kebir and Uweinat in Egypt to verify interpretations of Earth-orbital photographs and investigate the similarities between Earth and Mars. On Mars, several areas of dark and light wind streaks were identical to those seen in southern Egypt, leading El-Baz and Maxwell to interpret the light streaks as depositional, and the dark zones as erosional surfaces.

It wasn’t until 1981 when radar images taken during the second flight of the Space Shuttle Columbia gave the first indication that paleochannels exist in the Egyptian desert. Jack McCauley and Carol Breed, then at the U.S. Geological Survey, identified the now-defunct drainage pathways branching beneath the southeastern part of the Gilf Kebir Plateau region. Their find dramatically changed Maxwell’s opinion on the use of Egypt as an analog for the dusty deserts on Mars. Rather than comparing features on the planets and looking for similar explanations, today Maxwell finds analogs useful for examining how different paleoclimates may have created similar geologic surfaces.

Recent missions have shown that streaks on Mars have a complex relationship with the planet’s past and current environment. In 2002, Kenneth Edgett of Malin Space Science Systems, Inc. reported large, dark wind streaks in the Mars Orbiter Camera views of western Arabia Terra. “The relations between dunes, wind streak, and subjacent terrain imply that dark-toned grains finer than those that comprise the dunes are lifted into suspension and carried out of the craters to be deposited on the adjacent terrain,” he wrote in the Journal of Geophysical Research.

Bright and dark windstreaks in this image indicate past movement of fine sediment across the landscape from upper left toward lower right. Two impact craters are seen in the lower portion of the image. This picture — showing part of the Medusae Fossae region near the martian equator — was taken in early April 1999 and covers an area only 1 kilometer (0.62 miles) wide. NASA/JPL/MSSS.

Egypt’s streaky deserts bring into question the formation of features on both planets. Yes, the process of wind erosion plays a significant role in the desert morphology. But many features were not the result of the current hyperarid climate. Instead, ancient rivers, remnants of a wetter climate, were the hidden cause. Only wind had filled in the river channels with sand — leaving the coarser material of rocks and pebbles along the banks of the river that now mark the desert scene.

Maxwell and other scientists continue to urge for radar instrumentation on both Mars orbital and landed missions. The 2003 Mars Express and 2005 Mars Reconnaissance Orbiter missions will carry sounding radars to probe the martian crust in the 100-meter to 5-kilometer depth range, says John Grant of the Center for Earth and Planetary Studies at the Smithsonian Institution’s National Air and Space Museum. “In light of the extensive surficial deposits that blanket much of Mars, radar may hold the key to unraveling some of the planet’s outstanding mysteries,” he says, indicating for example the evolution of the hydrologic cycle.

Interpreting whether a wetter climate helped form the desert features of Mars is difficult. “I used to say, ‘this looks like this so they formed the same way,’ but now I know you can’t say that for certain,” Maxwell says. “It is better to ask what happens under severe climate conditions and think of how different climates in the past affect what is seen now.” On Mars the lack of small tributaries remains a puzzle to wet-weather climate modelers. Perhaps a similar cover-up is occurring on Mars, Maxwell suggests. But before he takes that thought any further, he’s waiting to see what answers might lay buried beneath the surface.

Christina Reed

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Great sand dunes: Windswept waves

The surface of much of Mars looks like an ocean frozen in time. Waves stretch out to the horizon. The distances between crests range from 10 to 60 meters. But these are not waves made of water. Rather, they are made of solid, roughly sand-sized particles. Winds give them shape.

While this terrain may sound reminiscent of the many sand dunes that dot coastal regions on Earth, there are, in fact, very few places here that sport waves of similar dimensions. Terrestrial dunes tend to be much larger. And the sand ripples that undulate across many deserts tend to be much smaller.

Dark and light bands delineate waves made of roughly sand-sized particles in this Mars Orbiter Camera image (near 20.14 degrees north, 20.78 degrees west). NASA/JPL/MSSS.

However, at Great Sand Dunes National Monument in Colorado and Edwards Air Force Base in California, ripples grow up to 10 meters, from crest to crest, matching the smallest waves found on Mars. Geomorphologists from the Center for Earth and Planetary Studies (CEPS) at the Smithsonian Institution’s National Air and Space Museum have traveled to these sites to study how the unique waves form and, by analogy, to understand the processes behind the martian waves.

Many ripples on Earth consist of only sand-sized particles. However, a thin veneer of coarse pebbles consistently coats the anomalously large ripples in the California and Colorado field sites, says James Zimbelman, a CEPS geologist. Indeed, the wavelengths of the ripples at those sites appears to increase in proportion to the size of the particles sitting on the surface, as Zimbelman and colleagues showed in an abstract for the 2002 Lunar and Planetary Science Conference (LPSC) in Houston.

The exact role of the pebbles is not yet known. However, larger grains can form a protective cap, says Mary Bourke, a geomorphologist at the University of Oxford. Wind cannot easily penetrate this cap, protecting sand grains beneath from being blown away.

Unusually large, wind-shaped ripples in the Great Sand Dunes National Monument in Colorado may hold clues to the processes that formed the martian features. James Zimbelman/Sharon Wilson.

The need for a pebble veneer limits the distribution of large ripples on Earth, Zimbelman says. Sand particles often hop downwind, bouncing off the desert surface, and can travel long distances. In contrast, pebbles travel much more slowly; they roll along the surface, too heavy to bounce. Pebbles rarely stray far from their source — a stream bed or a mountainside, for example. Large ripples, therefore, must sit near sediment sources; Great Sand Dunes National Monument lies at the base of the Sangre de Cristo Mountains.

In addition, Zimbelman says, fluvial transport tends to separate out small grains from larger ones, limiting their co-occurrence — the essential ingredient for large ripples.

On Mars, however, rivers do not dominate the landscape (although they may have been active a long time ago), limiting fluvial segregation. Also, geomorphological sources for pebbles abound, Zimbelman says. “In the Mars Orbiter Camera images, the large ripple-like bedforms are always around something; there tend to be hills or knobs in close proximity. Coarse particles are likely readily available.”

But the majority of the waves on Mars are much larger than even the largest ripples that the CEPS research crew has found on Earth. Zimbelman suggests the difference may lie in the atmosphere and in gravity.

In addition to being limited by the availability of pebbles, ripples tend to scale with the distance that individual sand grains hop in the wind, Zimbelman says — the larger the hop, the longer the wavelength of the ripple. Lower gravity on Mars means that similarly powerful winds will blow a grain further on Mars than Earth. However, Mars’ atmosphere is also less dense than Earth’s. That density difference means that winds must blow much more strongly on Mars to induce an individual grain to hop up off the surface. But once the grain is in the air, the strong winds blow it farther.

Graduate student Sharon Wilson and geomorphologist Steve Williams, both working at the National Air and Space Museum, plan to test the reliability of the terrestrial analog for the martian waves. They have analyzed ripples from both Great Sand Dunes National Monument and Edwards Air Force Base and found that the height to wavelength ratio is roughly 1:15, as reported in an abstract in March for the 2003 LPSC meeting. If the martian waves formed by mechanisms similar to the terrestrial ones, they should have a similar ratio, Wilson says. She has already measured the wavelengths of hundreds of ripple-like features from Mars Orbiter Camera images. The next step is to determine the height of those waves, based on the shadows that they cast.

Greg Peterson

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Australia: Acid waters

In 1979, Mars researcher Benton Clark suggested that, assuming the past Mars was very cold, pure water would not have been stable in liquid form on the planet. He also suggested that acidic water would.

Several scientists have followed Clark in saying that water on Mars might have been acidic. “It’s sort of popped up from time to time,” says Martha Schaefer, a planetary geologist at the Center for Space Research in Houston.

Benison’s work started in the United States with her research on fluid inclusions — microscopic pools of preserved liquid water — contained in halite that was in turn layered and trapped within red sandstones and shales in North Dakota and Kansas. Within these fluid inclusions were significant amounts of bisulfate, a marker of sulfuric acid. The lake water and groundwater that precipitated the halite and gypsum 270 million years ago had been acidic, with pH levels as low as below zero, she says. In researching the Permian waters, she needed a modern comparison. That search brought her to the modern acidic lakes in western Australia.

She was surprised to find both the Permian and the modern acidic waters, because most saline waters on Earth are alkaline, she says.

Aerodrome Lake is an acid salt lake in western Australia. Kathy Benison, Central Michigan University.

“I don’t really know what the cause of the acidity is,” Benison adds. Volcanic activity or the presence of large ore deposits are the usual culprits for acidic waters, she says, but aren’t factors in the area of her Permian rocks. “I think it might have something to do with long-term weathering and climate trends.” Weathering, she suggests, might over time eliminate the usual buffers, such as calcite and feldspar.

In 1997, when Mars Pathfinder returned images of the martian surface, Benison remembers thinking, “Hey, those look like my Permian rocks.” Recently, when LaClair said she wanted to study Mars, they both saw an opportunity.

LaClair made a solution that was 39 percent sulfuric acid and 61 percent water — the combination Clark suggested could be stable as liquid on a cold Mars. She conducted a small-scale geomorphology experiment, first pouring dilute water over sand, then pouring the acid solution over sand, and finally comparing the features each created. She found some features unique to the acid solution that also appear on Mars, Benison says.

She adds that, while their work is new, it suggests that discussions of water on Mars should not assume that dilute water alone carved the features we see today. “We don’t think we have enough information to say specifics on Mars; but we think [this idea of acid water] should be considered,” she says. “When you look at surface water and groundwater on Earth, there are all different kinds of chemistries.”

The idea intrigued M. Darby Dyar of Mount Holyoke, who heard Benison’s talk on the subject during the fall 2002 meeting of the Geological Society of America. Dyar and Schaefer are analyzing samples from the modern Australian lakes and Permian lake deposits. They will compare the terrestrial properties with any new mineralogical clues waiting in the data the Mars Exploration Rovers send back. For example, the presence of the mineral jarosite, found in some of the Australian samples, would be a good clue, Schaefer and Benison say.

“This time we actually have a specific Earth analog [for acid lakes],” Schaefer says. “It gives us a chance to study one place that’s still active so that we can see what kind of weathering is going on.”

The toe of this landslide at Mars’ Ganges Chasma sports more impact crater features than the valley below, says Deidre LaClair, a student at Central Michigan University. This contrast is counterintuitive, she says, because the landslide is probably the younger feature. “You’d expect older features to have more impact craters.” The craters, LaClair suggests, could also be places where bubbles of trapped air or gases popped and left an imprint. She observed crater-like features form this way during an experiment in which she ran acidic water over sand to see what types of features resulted. NASA/JPL; Thermal Emission Imaging System aboard 2001 Mars Odyssey.

Clark adds that the mineralogy samples expected from the Mars Exploration Rovers could yield other clues — such as sulfur-coated rocks (sulfuric acid or sulfurous acid would react with the rocks and leave sulfur behind) or a deficiency of magnesium or calcium. Also, the Marvel mission — set to begin scanning the martian atmosphere in 2007 — might find atmospheric evidence for volcanic activity. Such evidence would be quite a find, Clark says, and would also pinpoint a possible source of any acid. “The idea is that this acidity comes from volcanoes, because that’s what we see on Earth.”

Kristina Bartlett

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Atacama desert: An arid testing ground

It’s another kind of picturesque: mile after mile of dry soil and aridity in any given direction. Hardly a hint of water exists in sight. The Atacama Desert, on the northern Chilean pacific coast, has experienced total rainfall of only a few millimeters per decade for the last 10 to 15 million years. To astrobiologists and planetary geochemists, this makes Atacama not only one of the oldest and driest deserts on Earth, but also one of the best analogs on Earth for the present- day dry conditions on Mars.

Of course, Mars has been dry for much longer than Atacama, perhaps billions of years longer. Mars may have started out with a higher percentage of volatiles than Earth; however, impacts and solar winds probably stripped away most of Mars’ early atmosphere. Today Mars’ atmosphere, mostly carbon dioxide, is thin — approaching 100 percent humidity, but containing very little water. If you were to precipitate the water in a column stretching from the surface of Mars to the top of its atmosphere, you would have little more than 10 microns of water. That’s less water than you’re likely to have on the palm of your hand at any given time.

The Yungay region of the Atacama desert in Chile provides an ideal analog for Mars. Photo by Richard Quinn.

Whether or not life can exist in such a dry place, has been one of the driving questions of NASA Mars exploration. In July and September of 1976, NASA’s two Viking Landers set down on the surface of Mars. The landers each held a Biological Package — at the time the most complex instrument ever assembled for spaceflight — designed to conduct three experiments. Outside of the biological package, the Vikings also contained molecular analysis instruments able to detect the organic molecules associated with life. All of these experiments yielded results, some even seemed positive at first, but none were conclusive. The most surprising results came from molecular analysis, which found no organics. The question of life on Mars remained open, but the search taught us much about the chemistry of the Martian surface.

For NASA Astrobiologist Kim Warren-Rhodes, Atacama holds the promise of showing what the “limits” of life really are. According to Warren-Rhodes, “In deserts, the controlling environmental factor for life is the availability of water. We collect meteorological and soil moisture data in order to understand the effect of the physical environment, especially liquid water, on the microorganisms so as to be able to quantitatively determine the limits of life as set by the availability of water. Understanding the limits of life on Earth in desert environments can help establish models for survival in the dry conditions on Mars.”

Life has its limits in harsh conditions, and so does life-seeking technology. In April, researchers from NASA and Carnegie Mellon University began using Atacama to help them develop a robot that will be better prepared to search for life in other worlds — even those that seem inhospitable. The robot they took with them was called Hyperion. Hyperion proved itself in 2001 in the Canadian Arctic, where it was able to track the sun as it explored its surroundings for a 24-hour period. The group is planning an annual visit to Atacama over the next three years. According to project leader David Wettergreen, a research scientist at Carnegie Mellon’s Robotics Institute, “Each time, an increasingly capable robot will use sensing and intelligence to find land forms or environmental conditions that could harbor life.” In 2005, the last year of the project, the group hopes to have assembled an autonomous robot with a full array of instruments. Nathalie Cabrol, a planetary scientist at NASA Ames and the SETI institute, calls the three year Atacama project a “stepping stone to Mars.”

Hyperion isn’t the first instrument to be tested at Atacama. Researchers from NASA Ames, JPL and SETI, recently used Atacama to test the Mars Oxidant Instrument (MOI). The MOI is designed to establish the presence of reactive chemical species in the Martian soil, dust and atmosphere. The MOI will test many of the hypotheses that have been set forth to explain Viking’s indication that Martian surface material was reactive. While testing MOI at Atacama, the researchers took the opportunity to use Atacama field data, along with data from Viking, to perform comparative studies. These studies took advantage of the fact that some areas of Atacama are so dry that they contain no organics and no culturable bacteria. The researchers hope these studies will help them understand the formation and properties of Martian oxidants. The experiments seem so far to have yielded results similar to the results obtained with Viking.

Matthew Shindell
Geotimes contributing writer

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