Starship Asterisk*

Candy Hansen wrote:

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Spring Fans (ESP_029577_0925) (video)

At high latitudes every winter carbon dioxide condenses from Mars' atmosphere onto the surface forming a seasonal polar cap. In the spring, the Sun shines through this semi-translucent layer of dry ice and heats the ground below.

The ice sublimates (goes directly from ice to gas) on the underside of the seasonal ice layer and the gas is trapped. When the pressure is high enough the ice cracks and ruptures allowing the gas to escape. When the conditions are optimal this gas may condense locally near the source, forming a bright fan.

The dark fans are fine bits of surface material that get carried along by the escaping gas up to above the surface ice. Fine particles are also carried downwind and deposited in dark fans on top of the ice, where they may slowly sink into the ice. The rows of dark fans outline the original crack in the ice that allowed the gas to escape.

Alfred McEwen wrote:

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Beautiful Butterfly Crater (ESP_029854_1890) (HiWish Granted Again)

Emily Lakdawalla of the Planetary Society wrote a nice blog entry about her HiWish image here.

She requested stereo coverage, which is also available, and here's a resulting anaglyph cutout.

If you're interested in suggesting a potential target for us to image, consider setting up a HiWish account here.

This is a stereo pair with ESP_029999_1890.

Alfred McEwen wrote:

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Banded Bedrock in Terra Sabaea (ESP_030184_1585) (video)

Terra Sabaea is the region of ancient highlands north of the Hellas impact basin.

The enhanced color image shows bands of bedrock with different colors. Such colorful bedrock is typical of ancient Mars, when water played a more active role in altering minerals, and multiple geologic processes were very active (impact, volcanism, fluvial, tectonic).

The ridges or bright or dark lines that cut across the layers mark faults, places where the crust fractured and accommodated motions. A field geologist could spend years mapping the geology of the terrain covered by this image.

Alfred McEwen wrote:

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Stone Circles (ESP_030222_1220) (video)

This image covers a region southeast of the giant Hellas impact basin, which has distinctive properties in THEMIS temperature images.

The daytime temperatures are relatively cold while nighttime temperatures are relatively warm. This tells us that the surface material conducts heat efficiently, like rocks rather than fine-grained materials.

This image shows lots of boulders, and in places they form crude circular patterns. This could be due to the effects of many small impact craters (which we can no longer see, except perhaps for the boulder patterns). An alternative idea is that active processes on Mars can move large boulders (up to several meters in diameter) little by little over time.

There is almost certainly ice in the ground at this latitude (58 degrees south latitude), which expands and contracts with temperature changes. Those temperature stresses combined with the removal of fine-grained materials by the wind might be able to organize the boulders. Another idea is that in a recent past, climate the ice could seasonally melt, then re-freeze, which leads to stone circles in terrestrial permafrost (ground that is largely frozen throughout the year).