HiRISE Science Team wrote:Pitted Materials in Bakhuysen Crater (ESP_020309_1570)
Large impact craters often have pits on their floors. Some of these (with raised rims) are later impacts, but some are thought to originate immediately after the crater forms when slurries of molten and broken rocks occupy the crater floor.
Bakhuysen Crater, located in Noachis Terra, is thought to be the largest crater that possesses (and has preserved) these pitted materials.
Cathy Weitz wrote:Hematite in Capri Chasma (ESP_023331_1670)
Coarsely crystalline gray hematite is an iron oxide (Fe2O3) initially discovered from orbit by the instrument TES (Thermal Emission Spectrometer). TES has detected gray hematite in this area of Capri Chasma, one of several large depressions that make up the Valles Marineris canyon system.
This HiRISE image shows light-toned units beneath darker mantles. At the Opportunity landing site in Meridiani Planum, the same gray hematite is found in millimeter size globules that have weathered out of the sulfate outcrop and become concentrated along upper soils.
It is likely that the same scenario is taking place here in Capri with the hematite grains forming in the light-toned sulfates and then eroding out and concentrating in the darker mantle soils.
Shane Byrne wrote:Caves and Craters (ESP_023531_1840)
Earlier this year, the CTX camera team saw a crater containing a dark spot on the dusty slopes of the Pavonis Mons volcano. We took a closer look at this feature with HiRISE and found this unusual geologic feature.
The dark spot turned out to be a "skylight," an opening to an underground cavern, that is 35 meters (115 feet) across. Caves often form in volcanic regions like this when lava flows solidify on top, but keep flowing underneath their solid crust. These, now underground, rivers of lava can then drain away leaving the tube they flowed through empty. We can use the shadow cast on the floor of the pit to calculate that it is about 20 meters (65 feet) deep.
The origin of the larger hole that this pit is within is still obscure. You can see areas where material on the walls has slid into the pit. How much of the missing material has disappeared via the pit into the underground cavern?
Credit: NASA/JPL/University of Arizona
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