Thursday, March 11, 2010

Crisium's Region of Interest

The region of interest located in Mare Crisium is a compelling Exploration site for many reasons. First, this site was visited by several Soviet landers - Luna 23 and Luna 24 both touched down in Crisium. Luna 24 succeeded in returning a 170 gram sample in 1976. Though the amount was small, it provided a wealth of information and an interesting mystery. The Luna 24 basalt has a titanium dioxide content of about 1%, placing it among the lowest abundances of any lunar basalt sampled. The titanium content of basalts on the Moon varies widely, from almost none up to nearly 15%; a much wider range than typically seen on Earth. Because samples were only returned from a few limited locations on the Moon, we use remote sensing data to fill in the gaps of our knowledge (read this PSRD article for more details!). Basalts that are rich in titanium absorb more light in ultraviolet and visible wavelengths than those with less titanium, and many people have used this relationship to estimate titanium contents for mare basalts across the Moon. However, in the case of Mare Crisium, the remote sensing estimates put the titanium abundance at two to four times higher than what is seen in the Luna 24 samples. Plus, the way the light is reflected from the samples (the reflectance spectrum) looks different from what spacecraft observe for Mare Crisium. Other landing sites for which we have samples and that we have observed with spacecraft do not show this difference. So what is happening in Mare Crisium, and why should we care?

Boulders on a wrinkle ridge in Mare Crisium may help us understand the geology of this Constellation region of interest. The scene is 460 meters across; image number M119469420LE. Credit: NASA/Goddard/Arizona State University/The Ohio State University

Scientists love a good mystery, but it's also important because titanium is both a valuable resource that could be utilized when people return to the Moon, and titanium abundances can tell us about the lunar interior. Basalts formed by partially melting the lunar mantle billions of years ago, and the wide range in titanium contents can tell us about the wide range of compositions and processes in the lunar mantle. Most of the high titanium basalts appear to be concentrated on the lunar nearside. But why? A straightforward interpretation of the lunar magma ocean theory, where the Moon was partially or completely molten just after its formation, suggests that titanium should be globally distributed, but that's clearly not the case. Human exploration of this region will produce valuable sampling and fieldwork to address this question.

The geology of this site is also compelling. The Constellation site is located near the rim of the Crisium impact basin (see image below), and samples and field work would give insight into the processes that occurred during the formation of the basin, as well as the age of the basin. Was this impact basin part of the so-called lunar cataclysm? This site also contains beautiful wrinkle ridges, sites of compressive stresses that resulted in faulting and wrinkling of the mare basalt surface. In the image below, you can even see a wrinkle-ridge ring, where a buried crater localized the stresses.

A WAC monochrome image centered on the Crisium Constellation region of interest. The highlands area in the south is the rim of the Crisium impact basin and wrinkle ridges and a wrinkle-ridge ring can be seen in the mare to the north. Arrow shows the location of the NAC frame above. Scene is 62 km across; image number M117107778ME Credit: NASA/GSFC/Arizona State University

In the WAC monochrome image above, the arrow indicates the location of the NAC frame at the beginning of this post, which shows a portion of a wrinkle ridge. A fascinating feature of this ridge when seen in high resolution is that its face is littered with boulders. Perhaps these boulders are generated by breakup of the of the mare basalt, and they're visible now because of the faulting and folding that caused this ridge. (Judge for yourself in the full-resolution NAC frame here.) Lunar scientists (but not engineers!) love boulders because they usually come from below the surficial regolith layer, and can indicate buried rock units of different compositions. Some have suggested that perhaps Luna 24 sampled a basalt unit that was buried by a subsequent lava flow of a different composition, and only exposed on the surface where impact craters excavated the material from depth. This scenario would explain why spacecraft don't see the sampled material widespread on the surface. Visiting the Crisium region of interest could help scientists unravel this interesting puzzle.

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