Dark Streaks on Martian Slopes May Signal Active Water
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.
Ferris will be talking about the martian dark slope streaks at 1:30 p.m.
Monday, Dec. 9 at the American Geophysical Union meeting in San Francisco.
“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.
Contact Information
Justin C. Ferris
303-236-5039
ferris@hwr.arizona.edu
jcferris@usgs.gov
James M. Dohm
520-626-8454
jmd@hwr.arizona.edu
