Dark streaks on Mars may signal active water Dec 11, 2002 ( taken from SpaceFlightNow )

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 oflong-lived hydrothermal activity, magma-ground-ice interactions, andvolcanic activity. Some of the dark slope streaks are brand new--they haveformed after the Mars Global Surveyor spacecraft began detailed mapping ofthe planet in April 1999. Others have been observed to fade away on decadaltime scales. Their findings support the hypothesis that Mars remainshydrologically active and that water could be shaping the planet's landscapetoday.

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

While acknowledging that dry processes can create such features, the UAresearchers argue that some of the streaks' characteristics can be betterexplained by water seeps.

"There is no identifiable characteristic of a dark slope streak that candefinitively say whether it was formed by water-related processes or not.But there are certainly some features which strongly suggest the role ofwater," says Dr. Justin C. Ferris, National Research Council PostdoctoralFellow 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 andwater resources.

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

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

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

The dark slope streaks, while not identifiable by any one feature, do have anumber 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 notuniformly occur in regions of Mars with similar materials, topography, andslopes. Geologists would expect this only if dry processes were involved.Also, the streaks occur both on steep and very gentle slopes--something notexpected for features formed by landslides or avalanches.

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

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

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

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

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