Dark streaks on Mars may signal active water
Posted: December 10, 2002

Salty water driven by hot magma from Mars' deep interior may be forming some of the mysterious dark slope streaks visible near the Red Planet's equator, according to University of Arizona scientists.

They have determined the dark slope streaks generally occur in areas of long-lived hydrothermal activity, magma-ground-ice interactions, and volcanic activity. Some of the dark slope streaks are brand new--they have formed after the Mars Global Surveyor spacecraft began detailed mapping of the planet in April 1999. Others have been observed to fade away on decadal time scales. Their findings support the hypothesis that Mars remains hydrologically active and that water could be shaping the planet's landscape today.

Dark slope streaks were first detected using Viking Orbiter images during the early 1980s. At that time, Holly Ferguson and Baerbel Lucchitta of the U.S.G.S. in Flagstaff suggested that these features may be explained by wet debris flowing down the slopes. But all other explanations exclude a role for water and instead involve wind erosion, dust avalanching, or landslides.

While acknowledging that dry processes can create such features, the UA researchers argue that some of the streaks' characteristics can be better explained by water seeps.

"There is no identifiable characteristic of a dark slope streak that can definitively say whether it was formed by water-related processes or not. But there are certainly some features which strongly suggest the role of water," says Dr. Justin C. Ferris, National Research Council Postdoctoral Fellow at the U.S. Geological Survey in Denver, Colo.

Ferris, formerly at UA, has been working on this issue with James M. Dohm, Victor R. Baker, and Tom Maddock III of the UA department of hydrology and water resources.

"It's been said that martian geomorphology could also be called forensic geomorphology, because we are always looking for the 'culprit' behind the formation of certain features. However, too many scientists, in their quest for a culprit, forget that it could be a 'gang,' Ferris says. "Dry mass-wasting processes might be good explanations for a particular type of dark slope streaks occurring in certain areas, but it isn't for all the features we observe," he adds.

"Interestingly, most regions that contain dark slope streaks show evidence of ground ice or water and magma interactions," notes James Dohm, a UA planetary geologist and collaborator in the study.

"Published geologic maps of Mars have portrayed the planet as dynamic and hydrologically active throughout most of its history," says Dohm, who has been geologically mapping Mars for almost two decades. "The possibility of presently active hydrological activity as revealed in the Mars Global Surveyor and Odyssey missions is extremely exciting," he adds.

The dark slope streaks, while not identifiable by any one feature, do have a number of traits in common:

  • They often originate at or near the interface between two visibly different rock units and within topographic depressions.
  • They often occur on valley walls and occasionally continue on to valley floors.
  • They usually run down the slope and braided, finger-like features resembling deltas form at their ends.
  • Any one feature has a constant albedo, (that is, the fraction of sunlight it reflects) but albedos vary in a group of dark slope streaks.
  • Also, there are signs of erosion above the source of some of these streaks, which on Earth is common when water is eroding a gully or a valley.
What most greatly suggests water is involved is that these streaks do not uniformly occur in regions of Mars with similar materials, topography, and slopes. Geologists would expect this only if dry processes were involved. Also, the streaks occur both on steep and very gentle slopes--something not expected for features formed by landslides or avalanches.

Where is the culprit then? There are places on Earth where subsurface water breaks to the surface as springs. When water runs below ground at elevated temperatures for a long time, it can become enriched in dissolved salts and other minerals. This happens especially in arid regions.

"The same may be happening in Tharsis or Elysium, which we believe are the long-lived zones of magma-driven activity," Dohm says. "Large intrusions of magma at depth may provide the heat to drive a regional hydrothermal system."

This briny groundwater may be emerging through springs on the slopes of impact craters, in depressions, or along faults and fractures, Dohm adds. Since brines have a lower freezing point than pure water, they could exist at the martian surface at current low temperatures and pressures.

"Thus, the briny water could flow slowly down slope, leaving behind a ghostly image that we call a dark slope streak," Ferris says. "This hypothesis implies that there is current hydrological activity on the surface of Mars."

"And where you have a long-lived heat source and ample water, there is an exciting potential for subsurface life," Dohm adds.

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