The Dark Dunes Debate

Should Mars Express Resolve Seasonal Blotches?

Experiments planned on board Mars Express may provide clues to what is happening in the mysterious dark dunes and craters on Mars. Although hundreds of thousands of images have been part of previous mapping missions, as much as ninety-seven percent of the planet remains unexplored at high resolution.

Mars Express above the thin Martian upper atmosphere successfully achieved orbital insertion on Christmas
Credit: ESA

As part of its two to four year survey, the Mars Express orbiter passes within 250 kilometers [150 miles] of the surface at closest approach. Employing seven major instruments for its on-the-spot analyses, the orbiting Mars Express will be able to identify signs of water in liquid, solid, or vapor form on Mars. During each orbit, the overhead spacecraft will collect data from Mars between a half-hour to an hour, while spending the rest of its time broadcasting those results back to Earth.

Its onboard camera offers high-resolution stereo views of Mars. Its comprehensive maps will feature 10 meter resolution, but some particularly interesting regions will get a close-up view to 2 meters [about the size of small car, as seen from orbit]. So scientists are now deciding where are some of the more interesting places to start mapping. One spot may be the dunes and crater floors.

Images from earlier orbiting cameras continues to fuel such visual detective work, none perhaps more than the waxing and waning of dark ‘colony-like’ blotches recorded by the Mars Orbital Camera. The heated discussion has become known as "the dark dunes" debate.

A recent European Space Agency (ESA) meeting agreed that the seasonal variation in dark and light spots seen on Mars are certainly fascinating. They concluded that the dark dunes might well be worth a detailed look by the recently successfully achieved orbit of Mars Express, the European Space Agency’s Mars mission. Agustin Chicarro, ESA project scientist for Mars Express, called the meeting after the spots began fuelling controversy here on Earth in the summer of 2002. "As a geologist, I found the spots quite perplexing and very exciting. I wanted to tap a broad spectrum of expert opinion to decide whether they warrant closer examination by Mars Express," he said.

Mars 2001 Odyssey
Dark Gray Dunes, Bright Orange Dust (Released 26 December 2003). These dunes are at the southernmost edge of the Russel Crater dune field, located in the heavily cratered terrain of the southern highlands. Dust devil tracks off the eastern edge of the dune field reveal that the bright orange surface is material that can be stripped away, exposing an underlying darker surface. The gray surface to the west of the dunefield may be either a thin sand sheet or a surface preferentially stripped of the bright orange material, possibly revealing the same underlying surface that is exposed in the dust devil tracks. Bright orange areas between the dunes probably highlight areas that are protected from the wind, allowing dust to accumulate into bright orange patches.
Image Credit: NASA/JPL/Arizona State University

The dark dunes debate began when András Horváth, Tibor Gánti and Eörs Szathmáry from the Planetarium and the Institute for Advanced Study, Budapest, suggested that the spots could be colonies of Martian microbes which wax and wane with the season. The spots appear on dunes found on the floors of craters in the south and north polar regions.

The Hungarian team has examined the southern spots in detail. "They appear in late winter and by summer they have disappeared. They appear first at the margins of the dune fields and rarely appear on the ridges of dunes," Szathmary told the meeting.

Their location (which is independent of the elevation of the land) and shape (which is circular on flat surfaces but elongated on slopes) seems to be at odds with a physical explanation alone, say the Hungarian scientists who have proposed a biological explanation instead. A pocket of water, which would normally evaporate instantly in the thin Martian atmosphere, is trapped around them by the overlying ice. As this ice layer thins, the microorganisms show through gray. When it has completely melted, they rapidly desiccate and turn black. This explains why many dark dune spots have a black center surrounded by a gray aureole, say the Hungarian scientists.

On the other side of the debate, Michael Malin and Kenneth Edgett, designers of the Mars Orbital Camera on board NASA’s Mars Global Surveyor spacecraft, which recorded the images of the spots, had previously suggested an explanation involving evaporation and re-freezing of predominantly carbon dioxide ice.

Their hypothesis is the dark dunes are not biological. The Mars Orbiter Camera has already taken more than 120,000 pictures of Mars. Many of the camera’s images have sharp enough resolution to show features as small as a school bus. An online gallery of pictures taken by the camera is available.

The location and shape of the spots is at odds with a physical explanation. (A) Spots develop on the dark dunes rather than on the stony soil nearby. (B) Spots are also ellipsoid in shape or even (C) fan-shaped.
Credit: NASA/JPL/Malin Space Systems/MOC

The meeting considered these and other possibilities.

Russell Crater and Mudflows

Another fasinating place has been suggested by a pair of German scientists, who have examined close-up Martian dune images and found what appears to resemble terrestrial mudflows. On Earth, particularly in alpine and arctic regions, such a debris trail is left behind when fine-grained soil mixes with liquid water from intense rainfall or sudden melting of surface ice and frost.

The researchers–Dennis Reiss and Ralf Jaumann–published their findings in a recent edition of the Geophysical Research Letters. The pair are scientists at the Institute of Space Sensor Technology and Planetary Exploration, German Aerospace Center, in Berlin. The article’s title, "Recent debris flows on Mars: Seasonal observations of the Russell Crater dune field", suggests the intriguing location for some of the best satellite images of what the researchers conclude may be recent, liquid water on Mars.

While a full explanation is ambiguous, the scientists note that the two key ingredients–seasonal thawing, sandy soil with slopes–seem consistent with what may be a brief and relatively recent summer noon of liquid water in Russell Crater. Based on the lack of small craters in the dune fields, the team estimates its surface age at 100 to 10,000 years–a recent soil pattern. The researchers conclude that on the dunes of Russell Crater: "liquid water may be stable over a limited period around noon/early afternoon in late spring/early summer under current climatic conditions," and likely within the last thousand years.

Closeup View

Experiments on board Mars Express could help to determine whether the same had happened on Mars. Several instruments on the Mars Express orbiter can observe selected areas of the Martian surface at very high resolution.

"If the dark dune spots are selected as targets for analysis, many outstanding questions about the spots could be answered," said Chicarro.

Russell Crater
Sand dunes in Russell Crater, right showing the raised embankment and sudden dam-like terminus Credit: NASA/JPL/Malin Space Science Systems

OMEGA, the infrared mapping spectrometer, for example, could determine the mineral composition of the spots, allowing some hypotheses to be eliminated. PFS, the planetary Fourier spectrometer, could measure the amount of carbon dioxide and water ice present, the temperature of the spots compared with their surroundings and the pressure of the local atmosphere.

MARSIS, the radar sounder, could determine the thickness of the ice and the HRSC, the camera, could take high-resolution, 3D, full-color images of the spots. The radar uses a large 40 meter antenna to collect the sound waves that bounce off any density pockets below the rusty-red soil. If pockets of water are found to a depth of 2 kilometers [1.2 miles], then theories of active hydrology on Mars will be borne out. Subterranean aquifers are considered one possible way in which liquid water could exist in the frigid, hostile conditions, where the atmosphere is about one percent of the Earth’s pressure.

Images and data from orbit may eliminate some hypotheses, but proof of life on Mars will require landers and possibly humans to see the evidence firsthand. A future Mars lander could carry a Raman spectrometer capable of detecting the sorts of pigments used by microbes on Earth to harness solar energy for photosynthesis and to protect them from UV, Wynn-Williams told the meeting. Opportunities to fly this and other innovative instruments to Mars could be provided by Aurora, ESA‘s program of planetary exploration currently under discussion.

Malcolm Fridlund, project scientist for Darwin, an ESA mission to search for life on extrasolar planets, however, ended the meeting on a philosophical note which expressed an understandable sentiment.

"I find it hard to believe," he said "that Martian life, the last vestiges of a fertile time 3.5 billion years ago, has hung on by a thread for all this time until humans have developed the technology to find it."

Imaging by Mars Express should last at least one Martian year (687 days), with further operations possibly extending into 2008.

Related Web Pages

Raman Reveals Relics
J. William Schopf’s Cradle of Life
Red Rovers: Returning to Mars
Mars Exploration Website
Two Mars Rover Sites Get Science Stamp of Approval
Evidence for Snow on Mars – and Perhaps an Abode for Life?
Mars Odyssey web site
Ancient Fossils – or Just Plain Rocks?