Mysterious dark finger-like features on steep Martian slopes that lengthen downhill during the Red Planet's warm season could be caused by flowing water, scientists at Southwest Research Institute and the SETI Institute reported.
That possibility is based on the correlation of the streaks' growth with summer warming at Mars' southern mid-latitudes. The warmer temperatures would allow melting and flow of not just salty brines, but pure water.
Data from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) over the last four Martian years (one Martian year is about 1.9 Earth years) revealed that the majority of these dark features, called RSL (recurring slope lineae), only lengthen for about 100 sols, or Martian days, about 37 minutes longer than an Earth day, each year. Using data from temperature-sensing instruments aboard the MRO and NASA's Mars Odyssey and Mars Global Surveyor orbiters, the researchers were able to correlate the RSL lengthening with afternoon surface temperatures.
"We were expecting to find colder temperatures, requiring a lower freezing-point for water than 32 degrees F, which could occur with salty water," said Dr. David Stillman, a senior research scientist in SwRI's Space Science and Engineering Division. "Instead, we found that RSL lengthen when afternoon surface temperatures are greater than 32 degrees F. This suggests that these flows could be due to pure water," said Stillman, lead author of one report and co-author of another, both dealing with RSL studies. A previous explanation required these flows to have a significant brine concentration. The significance of this new finding is that the source of the water could simply be subsurface ice, which is abundant on Mars, and not the less common salt-rich ice compounds or yet-to-be-discovered briny groundwater.
To further support their hypothesis the team looked at the increase in RSL length and numbers after a global dust storm in 2007. The thick dust clouds block out most of the incoming light; therefore, such storms typically decrease the maximum surface temperature. However, dust also produces a significant greenhouse effect that raises the minimum surface temperature. Both effects were measured in the 1970s by the twin Viking landers in the northern hemisphere of Mars.
"We thought since the dust storm occurred when the southern mid-latitudes were receiving their maximum amount of sunlight, enough extra energy would be available to increase maximum daily surface temperatures there, even though they would still decrease in the northern hemisphere (as the Viking landers observed)," said co-author Timothy I. Michaels, of the SETI Institute's Carl Sagan Center.
The atmospheric modeling showed that dust storms normally produce a decrease in maximum surface temperatures on most of Mars, but not in the southern mid-latitudes during the southern summer. The low atmospheric pressure found on the Martian surface keeps pure water from remaining in a stable, liquid state over the majority of the surface. "Our hydrological modeling showed that the RSL flow is actually quasi-stable in the shallow subsurface," said Program Director Dr. Robert Grimm, also of SwRI's Space Science and Engineering Division. "Even a few inches of overlying soil can slow the evaporation rate enough that the underlying water-vapor pressure can rise above the point of liquid stability."
The team infers that surface darkening would then be caused by upward wicking of water into the surficial layer, or top half-inch of the surface. The RSL might flow in the shallow subsurface (less than 4 inches deep) during the warmest periods of the day, at surface temperatures that can exceed 80 degrees F, and refreeze each night. They probably are sourced from near-surface ice in bedrock units. However, how the ice gets recharged remains a mystery.
The research for this project was funded by NASA. The reports are "New Observations of Martian Southern Mid-latitude Recurring Slope Lineae (RSL) Imply Formation by Freshwater Subsurface Flows," by D.E. Stillman, T.I. Michaels, R.E. Grimm and K.P. Harrison and "Water Budgets of Martian Recurring Slope Lineae," by Robert E. Grimm, D.E. Stillman and Keith P. Harrison. Both reports are to be published in the journal Icarus (in press), http://dx.doi.org/10.1016/j.icarus.2014.01.017 and http://dx.doi.org/10.1016/j.icarus.2013.11.013.
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