HiRISE Science Team wrote:A Youthful Crater in the Cydonia Colles Region (ESP_025716_2200)
This observation shows a youthful crater with sharp rim and gullied slopes.
Just what makes a Martian crater youthful, in a geologic sense? Very old craters tend to have eroded rims and can have plenty of material that's filled in the floor. Gale Crater, where the Mars Science Laboratory will land this summer, is an example of an ancient, highly eroded crater. By contrast, the crater in this image appears to have experienced much less erosion.
Note that even though a crater might be called "youthful," it can still mean that the crater formed tens of thousands of years ago, if not more. For an example of a truly recent crater, see the 7 meter (about 23 feet) diameter crater in ESP_015989_1835, which we know formed sometime between 2005 and 2010.
Note: the above image is not map-projected, so North is down.
Alfred McEwen wrote:Monitoring Dune Gullies (ESP_026506_1300)
There are landforms called "gullies," consisting of an alcove, channel, and apron, on many large sand dunes on Mars.
Remarkably, we have learned that the gullies form primarily or entirely during seasons when there is carbon dioxide frost on the ground. To understand this better we image key locations multiple times throughout the Martian year.
This image, at 49.5 S latitude, was acquired very near the winter solstice, when shadows are very long in the middle afternoon when MRO passes overhead. Dark sand inside shadows is a challenging scene to image while flying overhead at 3.4 km/sec, but the HiRISE camera has the sensitivity needed to acquire useful images even at the most challenging time of the year.
The subimage shows one of these gullies, hidden in the shadow. Vertical stripes in the image are from electronic noise that is usually hidden by the image signal, but in this case the signal is extremely low.
Laszlo Kestay wrote:Streamlined Landforms near the Cerberus Fossae (ESP_026580_1940)
One of the earliest observations of Mars that indicated that water once flowed across its surface was the presence of large streamlined landforms. Such landforms are carved by flowing fluids that erode islands into teardrop shapes.
While wind can also produce streamlined landforms (called yardangs), many features on Mars were clearly produced by a liquid that was confined to the low areas inside channels. There is an ongoing debate about the roles of lava and water in carving these features.
Streamlined forms visible inside channels on the Moon and Mercury must have been carved by lava since there is no plausible way water could have flowed over those bodies. On Mars, the HiRISE team is seeing a pattern where we believe most channels were carved by water but then covered with lava.
Observations like this one help us test the idea that the lava is simply coating a water-carved surface.
Mike Mellon wrote:Sand Ripples at a Potential Landing Site (ESP_026726_1790)
Landing of the surface of Mars requires extensive planning and imaging reconnaissance. This terrain west of Aeolis Planum is being considered as landing site for a future Mars mission.
The surface is relatively flat with numerous small-scale ridges and mounds. Some of these ridges curve and form the rims of old impact craters, now in various stages of erosion and largely filled with soil. However, most ridges have the appearance of a wind-driven landform called ripples. These features are about 10 yards wide and 100 yards long, and meander slightly but are generally oriented south to north.
Mounds are less distinct, but may also be aeolian (wind derived) in nature. Most of the "freshest" looking crater floors, those seeming least eroded, contain smaller characteristic sand ripples.
All of the craters appear eroded and heavily filled with a soil mantle. Even many of the ripples features appear mantled and smothered by soil. Nevertheless the occurrence of these aeolian bedforms (landforms that are formed by wind blown sand) suggest these surface soils are comprised of loose fine-grained regolith (rocks and fine rock fragments), which the wind has been able to move and organize.
Very few rocks or boulders are seen and those that are seen are largely limited to ejecta around some of the fresher appearing and larger craters. Although still heavily eroded and mantled, these rocky craters indicate more cohesive material such as bedrock or partially-cemented regolith exists beneath the loose soil cover. The absence of widely distributed rocks ejected from craters may suggest this area has experienced extensive, possibly ongoing, accumulation of soil, or that the subsurface rocks are weak and easily eroded by wind and windblown sand.
This is a stereo pair with ESP_027003_1790.
Credit: NASA/JPL/University of Arizona
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