Starship Asterisk*

Ingrid Daubar wrote:

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A Circular Crack (ESP_029362_1795)

This circular crack is very odd-looking. When you zoom in to HiRISE scale, the crack looks a lot like a graben.

Canyonlands, Utah is a great place to see grabens on Earth. Grabens form when solid rock is pulled apart. Two cracks form at distinctive angles, and the material between the cracks collapses downward, forming a straight-walled canyon. Well, they're usually straight-walled, but this one is circular, which is unusual.

The first thing anyone thinks of when they see circular features on Mars is a crater. So one hypothesis about how this formed is that an ancient crater was buried by some material, maybe lava or even multiple layers of wet sediments. Because the center of the crater was deeper, more material settled there, and the high-standing rim was only thinly covered. The mantling material solidified and shrank, creating extensional forces.

The heavy central fill pulled downward, cracking the material at the rim where it was thinnest. That's just one guess, though - what else do you think could it be?

This is a stereo pair with ESP_029217_1795.

Ginny Gulick wrote:

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Layers, Dunes and Cliffs in Hydrae Chasma (ESP_029516_1730)

Hydrae Chasma is a deep, circular depression approximately 50 kilometers across, situated between Juventae Chasma to the north and the large canyon system Valles Marineris to the south. The Chasma has steep walls flanked by numerous landslides and a massive scarp along its southern boundary where the surface has collapsed into this depression.

This closeup is of an isolated flat-topped mountain (known as a mesa) rising out of a sea of dunes located in the center of Hydrae Chasma. Darker-toned dunes, likely composed of basaltic sands, form an apron along the base of the mesa's northern margin. The western side of the mesa is gently sloping and is composed of a highly fractured light-toned rubbly base. It is overlaid by alternating light and dark layered cliff-forming units and is covered by a sediment cap containing still more dunes.

The layered sequences are present only in the interior deposits and not in the walls of the Chasma. Similar deposits are located on the floors of Valles Marineris and other chasmata and may be the sediment remnants of ancient lakes formed within these canyon systems.

Ginny Gulick wrote:

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Monitoring a New Impact Site in Fortuna Fossae (ESP_029583_1825)

This impact site is located on the floor of a large fracture within Fortuna Fossae. This site formed sometime between September 2005 and May 2008 and consists of five distinct craters each displaying individual dark-toned ejecta patterns.

The resulting craters indicate that the impactor broke up into five parts prior to its collision with the surface. Craters continue to form on Mars today and repeat imaging of these recent impacts--especially in the color portion--provides information about how impact features change with time.

Serina Diniega wrote:

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A Defrosting Mess (ESP_029614_1105)

This image is from the high latitudes in the Southern hemisphere, about half-way through southern spring.

Just like on the Earth, the frost layer that accumulates over the winter will disappear as summer approaches and Mars heats up. However, most of this frost is not made of water ice and snow -- on Mars, most of the frost/ice is made of carbon dioxide (also known as "dry ice").

This material will not melt, but instead will go directly from solid into vapor (a process called sublimation) as it heats up (above approximately 147 K, which is -195 F, or -126 C). In doing so, it'll create all sorts of interesting landforms.

In this image, we can see sublimation spots (small spots where the frost/ice has sublimated away, exposing the darker ground). We also see small fans, which form when jets of gaseous carbon dioxide erupt through a weak spot in the surface ice, ejecting dark surface material that then gets smeared across the surface by the wind (so the different directions of the fans show us how the wind varies across this landscape).

Large darker "flows" are also visible; these are avalanches of material that extend downslope (and scientists are still debating if flows are dry or wet features). In the next few weeks, more of these features will appear and grow, until Mars heats up enough for all of the frost and ice (and sublimation features) to disappear.