Thursday, February 25, 2010

Mojave Crater: The Rosetta Stone of Martian Craters?

This Digital Terrain Model (DTM) covers the northwestern portion of the approximately 60 kilometer diameter Mojave Crater, centered at 327.0 degrees East, 7.5 degrees North in Xanthe Terra on Mars.

The perspective views subimage 1 [the black-and-white image above] and subimage 2 shows the entire HiRISE image covering portions of the crater’s northwestern wall-terraces, rim and ejecta blanket (see subimage 3 for context). Subimage 1 is viewed from the southeast and highlights the crater interior. Subimage 2 is viewed from the northwest and highlights the crater rim and ejecta blanket. The vertical exaggeration for these images are set to 3x (meaning that features appear to be 3x taller than they are wide).

One of the most interesting features in Mojave are the “pitted ponds” that appear to be “backed-up” behind massive wall-terrace blocks of bedrock. Pitted materials are currently recognized in hundreds of fresh and well-preserved Martian craters, and are thought to represent impact melt bodies that were captured behind the wall-terraces as they faulted off the rim and into the crater interior. With the exception of the dense population of pits, these “ponds” are quite similar to impact melt ponds observed on the wall-terraces of lunar craters (see the Kaguya image gallery).

The DTM shows terrain spanning -4803.65 meters to -2744.87 meters (-3 miles to -1.7 miles) with respect to the Martian datum (just over 2 kilometers [1.3 miles] from the lowest to highest points; see color altimetry image [above]). Based on observations and modeling of fresh craters, a crater the size of Mojave should be approximately 2,595 meters (1.62 miles) deep in its most pristine state. The DTM here does not show the lowest point in the crater, but still demonstrates nicely that Mojave has minimal infilling or erosion, and is very close to its pristine state – estimates from other datasets indicate that Mojave is approximately 2,604 meters (1.63 miles) deep. A survey of Mojave’s features indicates that there are very few overprinting craters on them.

A statistical analysis of Mojave’s overprinting craters further indicates the youthfulness of Mojave, as models indicate the crater may be as “young” as approximately 10 million years old – well within the Amazonian Era, and indeed a very young Martian crater for its size.

Mojave no doubt gives us a glimpse of what a very large complex crater should look like on Mars; and perhaps in a sense, it is a “Rosetta Stone” of craters, given that it’s so “fresh” and most others – especially this size – have been affected by erosion, sedimentary infilling and overprinting by other geologic processes. Such craters like Mojave, especially when accounting for size, location and target properties, are one-of-a-kind ... , but give tremendous insight into the impact process (e.g., ejecta, melt-generation and deposition, etc.).

Mojave’s fans and channels are most intriguing, and hint that impacts such as Mojave may have unleashed water/water-ice from the subsurface to flow across the surface and, perhaps, condense as rain or snow for only a brief period of Martian time. This further suggests that early climate on Mars could have been heavily influenced by its most intense bombardment when many Mojave-sized craters (and far larger) were more common, approximately 3.9 billion years ago.
Credit: NASA/JPL/University of Arizona/USGS

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