This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Thursday, August 4, 2011

Water on Mars.....Again?

Streaks on Horowitz (impact) Crater, Mars
The dark features are dubbed "recurring slope lineae" (RSL)
The tails of the arrows are about 20 meters in length.
The author of a new article** claiming liquid water on the surface of Mars, Al McEwen, a friend and colleague, quipped that he'd surely be hearing from his colleagues about "So you've discovered water on Mars for the thousandth time?"

An excellent animation of these features can be found in Richard Kerr's ScienceNow blurb "Is Mars Weeping Salty Tears?"

The features of interest, the dark lines in the photo, are narrow (0.5-5 meters) marks on steep (25-40 degree) slopes. Repeated images taken by the Mars Reconnaissance Orbiter HiRise instrument shows that these features appear when the warm season starts, grow downslope (toward the XXX in this photo), attaining lengths of 100 m.  When the warm season (250-300 K) passes, the marks fade; the orange streaks in the photo near the arrows may be older faded RSL's.

The RSL's only occur in the southern hemisphere, and between latitudes 32-48 S. They tend to form on equator-facing slopes. There are seven confirmed locations, and a couple dozen candidate sites.  They extend downslope from rocky areas (toward the upper right in the adjacent photo), attaining lengths of hundreds of meters. More than 1000 lineae may be present in a particular site.

Individual lineae may split or merge. They terminate on steep slopes, leading to the conclusion that the volume of material to form them is limited. Growth rates vary from 0-20 m/day on average. There can be no activity on some for weeks.

McEwen et al. consider a number of mechanisms, including CO2 sublimation, and conclude that the seasonal behavior strongly suggests surface expression of briny water.  The association with bedrock outcrops could indicate control of the subsurface migration of the fluids by the bedrock structure, or that the bedrock contains hygroscopic salt-rich lenses to provide the salts to lower the melting temperature of ice.  The text of the article examines other hypotheses in detail.

**McEwen et al., Seasonal Flows on Warm Martian Slopes, Science, 333(6043) 740-743, 2011. There may be hundreds to a thousand at one site.

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