Alfred McEwen wrote:Impacts from MSL Tungsten Blocks and Cruise Stage (ESP_029245_1755)
MSL (Curiosity) in cruise configuration jettisoned two 75-kilogram tungsten blocks just before atmospheric entry, used as cruise balance masses. A CTX image was acquired at the predicted impact site for these blocks, revealing four large impact markings that appear very recent.
HiRISE has covered these impact sites with two images, showing the impacts in greater detail, and also revealing a large number of new small impact sites. A mosaic was made from portions of the two map-projected HiRISE images at 0.5 m/pixel, with white boxes around the four large impacts and a number of smaller impacts. Other images provide full-resolution (0.25 m/pixel) views of the four large impacts (two of which are close together). There is a strewn field of impacts at least 8 kilometers long. A third HiRISE image, ESP_028889_1755 also shows many small fresh-looking impacts, and if it is part of the same strewn field then it was about 12 kilometers long.
We were expecting to see just two impacts sites here--from the tungsten blocks--and it is highly unlikely that these dense blocks broke apart in the atmosphere. The only other source of impacts at nearly the same time and place is the cruise stage itself, which was more likely to break apart in the atmosphere. The impacts were highly oblique, as shown by the asymmetric morphologies of the individual impacts and the elongation of the strewn field.
The large impacts created craters with diameters ranging from 3 - 5 meters diameter, about what was expected from the 75-kilogram tungsten blocks. Which two of the four impacts came from the tungsten blocks? The two central impacts that are close to each other and of similar size probably originated from the tungsten blocks. The other two impacts, which have more asymmetric ejecta, may be from the cruise stage, which broke apart into two main pieces. The many smaller impacts may have been formed by secondaries from the large impacts and additional pieces of the cruise stage.
Although hundreds of new impact sites have been imaged on Mars, we do not know the initial size, velocity, density, strength, or impact angle of the objects. However, for the MSL hardware we do have such information, so study of this impact field will provide data on impact processes and Mars surface and atmospheric properties.
The MSL descent stage later released six 25-kilogram tungsten blocks from a much lower altitude, whose impact markings have been imaged by CTX.
Ingrid Daubar wrote:Columnar Jointing on Mars and Earth (ESP_029286_1885)
HiRISE first discovered columnar jointing on Mars in 2009 (Milazzo et al. 2009). This jointing is an exciting discovery because it forms when molten lava is cooled quickly by liquid water resulting in columns of polygonal-shaped rock. Finding evidence like this for large amounts of liquid water on Mars is important for understanding the planet's climate history and habitability.
This image shows columnar jointing at a new site, previously unreported. These distinctly jointed layers are located on the northern wall of an 18-kilometer (approximately 10 miles) crater inside Isidis Planitia.
During a recent field trip near Moscow, Idaho, HiRISE scientists visited a site with similar columnar jointing. The sizes of the columns are only slightly different (the columns in the HiRISE image are a few yards across, while the ones we saw on Earth are only a few feet across). Smaller columns may also be present at this site, but they would be below even HiRISE's ability to resolve. The columns on Mars most likely formed in the same way as the columns we see on the Earth.
If you look at the enhanced color version, you can almost imagine those people walking along the base of that cliff, studying the geology of Mars in person.
Nicole Baugh wrote:Colorful Fractured Bedrock in Ritchey Crater (ESP_029542_1510)
HiRISE has imaged Ritchey Crater several times before (see ESP_011635_1510, ESP_025797_1515 and ESP_026931_1510 for some examples), but its interesting features extend beyond the range of a single--or even several--HiRISE footprints. HiRISE now has complete coverage over the central uplift in this 80-kilometer diameter crater.
Central peaks form in large impact craters during the impact event, when the crater floor rebounds after the initial compression. These central uplifts make attractive targets for HiRISE both because they tend to be relatively dust-free, and because as a consequence of their formation, they tend to bring up to the surface rocks that come from greater depths.
In the case of Ritchey Crater, these exposed rocks appear to be chunks of fractured bedrock, possibly of plutonic origin. (Meaning igneous rock that solidifies below the surface). Visible in enhanced colors here are some of these bedrock blocks. The dark, ridgelike lines that cut across the colorful blocks may be dikes, formed when molten rock forced its way into pre-existing linear fractures in the surrounding rock. Alternatively, the ridges could be the result of minerals precipitating out of groundwater flowing through the fractures.
Serina Diniega wrote:Apostrophe Dunes (ESP_029660_1350)
While most barchan dunes (also known as crescent dunes) on the Earth and Mars are near-symmetric in shape with two horns pointing down-wind (for example: PSP_007676_1385 or ESP_026745_2540), these dunes only have one pointy horn. Their other "horn" is very rounded and the steep slipface (the smooth slope between the horn and the rounded side that only forms in the sheltered downwind side of the dune) is offset from the middle.
The lack of symmetry within these dunes tells us that there is more than one wind direction acting here. The dominant wind within this region is along the direction of the horns (both the rounded and pointed one), or towards the bottom-left of the image. Another wind then alters the basic dune shape. Most likely, a more recent or weaker wind is coming in from the right, thus causing one horn to elongate and the slipface to be offset.
Credit: NASA/JPL/University of Arizona
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