View Full Version : A Meteoroid Smashed Into the Side of a Crater on Mars and Then Started a Landslide

2018-Jun-15, 07:10 PM
The Mars Reconnaissance Orbiter (MRO) recently captured an image of an impact crater on Mars that triggered an avalanche that left behind a dark streak.
The post A Meteoroid Smashed Into the Side of a Crater on Mars and Then Started a Landslide (https://www.universetoday.com/139473/a-meteoroid-smashed-into-the-side-of-a-crater-on-mars-and-then-started-a-landslide/) appeared first on Universe Today (https://www.universetoday.com).

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2018-Jun-17, 11:25 PM
There may be reasons to be skeptical of this explanation. Dry dust flows occur when there is an overburden. Often the underlying surface has a darker albedo, but bright streaks are monitored as well. What has been monitored since MGS is the appearance of new dark slope streaks, but not bright slope streaks. If this particular image were a dry flow. the shock of impact would have the flow be the width of the crater.Instead a dark streak starts from a central rock inside the crater. Also note that the streak is considerably darker than the ejecta around the crater. The ejecta should give an indication of how dark underlying dust would be to constrain how dark the streak could get. It appears that the impact may have pierced the surface enough to let exit either a fluid or heavy gas that chemically etched the streak. Over time dark streaks seem to fade. Bright, dark, and fading dark streaks tend to show no topographical relief. Scars or "dead" slope streaks tend to show a minimally perceptible relief in HiRise images. If some Dark slope streaks have a life cycle of etching to a thin depth, chemical reaction with an atmospheric component to cause the fading, the final product of a scar could be due to the compound being more subject to wind erosion leaving the scar behind.

RSL are different from slope streaks, they are thinner in general and seem to disappear and re-appear seasonally where slope streaks are long lasting. Even if slope streaks get buried by a global dust storms, later wind erosion and/or dry dust flows recover the original streak and its contours and length for a subset of dark streaks. How RSL form is still an open question. An intriguing constraint is that they seem to form in the same place where they were the prior reason. A possibly important find is the presence of water loving salts such as perchlorates. Considerations have been for seasonal brine flows but this could require a great abundance of aquifers at each of the sites. Atmospheric water is too low to provide all the water needed for a brine flow. Dry flows have been considered but they do not address the seasonal constraints. The appearance of RSL as a growing down-slope process comes from usually up to 4 snapshots taken by HiRise which shows a seasonal downhill progression. The impression of flow may be a false illusion. One possible scenario not typically presented goes thus: A thermal event quite some time ago, it could be geothermal including volcanic, or impact heat, warms and melts HEND water beneath the regolith at a depth where it is protected from immediate boiling. In flowing towards to the egress points where RSL begin, it dissolves and becomes saturated with perchlorate salts. When it exits into the atmosphere and starts to flow down the slope, the streak can be thinner than a typical slope streak as a trickle stream. Upon exposure the water quickly begins to evaporate. It may boil, but some protection is afforded by being a brine solution. if at saturation the loss of water will cause the the salt to precipitate out with a higher density of salt near the point of origin with a declining gradient of salt as the water tapers out near the end of the RSL. Then the impression of seasonal flow is due to salt density varying from top to bottom. It is more like litmus paper. When there is the small water vapor afforded by the warmer season, all the salts will try to grab the vapor, but the greater density of salts near the origin get more water and darker albedo than the lower slope. As more vapors impact upon the entire streak one will get the illusion of flow down slope when the possibility is rather that gradients are being exposed. When the dry cold season comes the little water is given up and the RSL fades until the next warm season that brings a little water vapor. Most of the RSL tend to form as the same place with the same contour and length each year rather than being formed in new places each year. This may indicate that the salts are not much moved each year and is consistent with the observation that many RSL sites are low dust sites so as to not to bury the salts when the vapor returns.