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Yet another possible landing site for Curiosity is highlighted today: youtube video of Mawrth vallis landing site.
As I wrote before Mawrth = Mars in Welsh. Pronunciation of the Welsh word Marwth is hard for a non-Welsh person. It sounds to me most like ‘Mar-th-f’. Try for yourself.

Mawrth vallis is north of the Martian equator at 22.3°N, 343.5°E with an elevation approximately two kilometers below datum. Where all the other landing site contenders are south of it. Mawrth vallis is older than the other landing sites. It is one of the oldest valleys on Mars.  It was formed in and subsequently covered by layered rocks, from beneath which it is now being exhumed. The rocks surrounding the valley have been observed by the Omega spectrometer aboard the European Space Agency’s Mars Express spacecraft. This instrument found that the rocks include minerals with water bound within their structure.

This in turn made the team using the mineral-mapping Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on NASA’s Mars Reconnaissance Orbiter (MRO) very keen on having images taken by Michael Malin and his group using the Context Camera on MRO to have these images to go with their spectrographic measurements. All in all the age and the rock-consistency makes this vally very attractive for further and more close up research as we want to know if early Mars had the habitability needed for life to form and Mawrth vallis gets us closest to these early times.

Mawrth Vallis is believed to have been exposed to a lot of water. Although water is needed, we think, in forming life, too much of it is a bad thing as the abundance of water at this valley might actually have destroyed any remaining evidence of life ever being present here.

The image on the right (by MSSS / NASA JPL) is centered near 25.6 degrees north, 19.4 degrees west. This area was discussed during an Oct. 16, 2006, news briefing, click here for related imagery from CRISM and the High Resolution Imaging Science Experiment camera.

Hypothesis:

Mawrth Vallis records the geologic processes during early Martian history, when water was abundant and altered the rocks to form a wide variety of clay minerals. Because it is such an old portion of the crust that has been subjected to so much water, Mawrth would help us understand habitability on early Mars. Mawrth Vallis may be representative of global conditions on Mars.

Pros:

• * Mawrth exposes the oldest rocks of the four candidate sites, as Early as the Noachian epoch of Mars, so exploring these rocks would allow MSL to study evidence for processes that were active on early Mars.  The landing ellipse appears to be mineralogically representative of other Noachian crustal sections in Arabia Terra, thus allowing an understanding of what possibly were widespread processes on early Mars.
• * The rock record at Mawrth may be among the oldest preserved on Mars, which is to say from a time that is not recorded in Earth’s rocks, but which was crucial for the evolution of life.
• * Mawrth has the strongest spectral signatures for clays on Mars that we know of, and the layers of clay minerals seen at Mawrth extend over a broad region of Mars. In Mawrth Vallis a very large percentage of all the Mars rocks that were formed in water can be found.
• * MSL would land on the primary target: the diverse hydrated minerals, and the changes in spectrally dominant minerals are close together, so MSL wouldn’t have to drive far.
• * The ellipse shows a very great diversity of mineralogical history of Mars. Reading Mars history here through its rock records promisses to be easy and representative for much of Mars. Besides which the rocks are well preserved.
• * Phyllosilicates are good at trapping organic molecules when conditions are right. Both of the major clay mineral types should be investigated for organics.
• * A good list of prioritized targets within the ellipse has been identified and targets outside the ellipse, including sulfates, are also well-defined.

Uncertainties:

• * The presence of phyllosilicates does not guarantee good preservation of organics (despite the obsession with clay minerals throughout this landing site selection process).
• * We don’t know much about how the rocks at Mawrth were deposited, or what environments they might record. It’s not clear whether the mineral stratigraphy matches with the physical stratigraphy, or was overprinted later by alteration. We would probably need to land at Mawrth to find out.
• * The amount of water, where it came from (atmosphere or ground) and how long it was present are all unknown.
• * The relationship between the rocks at Mawrth and the huge Oyama crater just to the west is not fully known. We aren’t sure if the dark capping unit in the ellipse is Oyama ejecta.
• * The importance of impacts in forming the rocks is not clear, but given the age of the rocks they probably played a significant role in depositing the rocks and damaging them.

Cons:

• * There was no consensus that the environment in which the rocks formed could be understood based on rock texture and composition changes since Mawrth is so beat up by impacts, lacks context and may even lack traceable sedimentary layers.
• * Oddly enough, Mawrth may have been exposed to too much water, which can be bad for preserving organic molecules.

Note: Mawrth Vallis is on top of the list of possible landing sites for the European ExoMars rover. As that program has suffered recent funding setbacks it is hard to say whether this valley will get a visit from a rover soon.