HiRISE Updates (2015 May 06)

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HiRISE Updates (2015 May 06)

Post by bystander » Wed May 06, 2015 6:19 pm

Shane Byrne wrote:

Slope Streaks on a Dusty Planet (ESP_040386_1915) (HiClip)

Mars is a dusty place and in some locations thick blankets of its characteristically red dust can slowly settle out of the atmosphere and accumulate on slopes. This dust is also a lot brighter than the dust-free terrain on Mars; so, if you scrape off the dust, you'll see a darker surface underneath.

This particular image shows one of these dusty areas. The dark streaks on the slopes are locations where the dust has slumped downhill revealing a less dusty surface underneath. In some cases, these slope streaks might be triggered by Marsquakes or nearby meteorite impacts. Scientists think they form quickly: more like an avalanche than dust slowly creeping downhill.

Look more closely and you'll notice that some streaks are darker than others. Dust is settling out of the atmosphere all the time and these dark streaks get slowly buried by fresh dust so that they fade back into their brighter redder surroundings. It's not certain how long this fading takes to happen, but it's probably close to a few decades.

Dust is an important player in the weather and climate on Mars. Images like this are used to monitor slow changes in these streaks over time to better understand how much dust is settling on the surface.
Shane Byrne wrote:

On the Beauty of Yardangs (ESP_040504_1920) (HiClip)

Some geological materials (like solid rock) are incredibly tough, but others (like piles of volcanic ash) are quite soft. Some materials are soft enough that they can be eroded by the wind alone and yield landscapes that look like what we see in this HiRISE image.

The long straight ridges seen here are called yardangs and they form on Mars (and Earth) when the wind strips away the inter-ridge material. This process is greatly aided when the wind is also blowing sand along. The sand grains do an effective job at stripping away loose material: these ridges are literally being sandblasted.

Yardangs are useful features to recognize because the tell us the direction the wind is blowing in. They take a long time to form so this direction is the dominant wind orientation averaged over a long period of time (which might be quite different that the winds on Mars today). These yardangs also tell us that the surface here is made up of loose weak material and this information, in conjunction with other data, can tell us what the material is composed of and what the history of this particular site on Mars has been.
Alfred McEwen wrote:

Warm-Season Flows in Cold-Season Ravines (ESP_040590_1335) (HiClip)

Ravines (or very large gullies) are actively forming on Mars during the coldest times of year, when carbon dioxide frost aids mass wasting.

However, some of these ravines also show activity in the warmest time of year, in the form of recurring slope lineae (RSL); dark, narrow flows in some alcoves that flow part way down the channels. Few topographic changes have been seen in association with RSL, and they appear to be seeps of water that seasonally extend down slopes, then fade when inactive, and recur each warm season. Could the RSL activity carve the ravines?

In some places the RSL extend to the ends of the fans and appear to match in scale, and perhaps gradually form the ravines. In other places, such as this image, the ravines are much larger than the RSL, so presently-observed RSL flow did not produce the larger landforms, but maybe the flow was greater in the past or maybe the RSL just follow the topography created by other processes.

The largest ravines are on pole-facing slopes in the middle latitudes, where RSL have never been seen to form, unless the ravine creates a small equator-facing slope.

Credit: NASA/JPL/University of Arizona

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