Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p43a1384s&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P43A-1384
Mathematics
Logic
5420 Impact Phenomena, Cratering (6022, 8136), 5464 Remote Sensing, 5470 Surface Materials And Properties, 5480 Volcanism (6063, 8148, 8450), 6225 Mars
Scientific paper
The Syrtis Major volcanic region on Mars exhibits a number of craters with compositionally distinct ejecta (CDE), showing high concentrations of high-calcium pyroxene (HCP) compared to the general background. The examination of these craters provides insight into the true composition of Syrtis Major and possible temporal changes in spectral signatures of exposed materials that resulted in only the young craters displaying CDE. OMEGA observations have suggested that the Hesperian aged Syrtis Major region has a mafic composition with a pyroxene distribution slightly enriched in HCP (40/60 LCP/HCP) in contrast to the surrounding Noachian aged crust enriched in low-calcium pyroxene (LCP) (60/40 LCP/HCP). This indicates a fundamental difference between the petrology of early crustal formation and Hesperian volcanics. Observations from the CRISM instrument on the MRO spacecraft using MGM data processing indicates that just below a thin surface covering of material slightly enriched in HCP is a volcanic edifice highly enriched in HCP (10/90 LCP/HCP). We are examining these features with TES observations to expand the analysis into the thermal infrared region. This underlying material is only exposed as CDE from young impacts dated to have occurred since the Hesperian-Amazonian boundary (~2Ga). This indicates that the Hesperian volcanism in this region underwent a much greater compositional change, as seen in the pyroxene mineralogy, from the Noachian to the Hesperian than previously thought. Also, crater ejecta exposed since the Hesperian-Amazonian boundary appear less enriched in HCP in near infrared observations than older crater ejecta. While the exact nature of this difference is still unknown, three possibilities have been considered; layering, mixing, and coatings. A compositionally distinct layer could exist throughout Syrtis Major that the craters are tapping, although this layer would need to have been emplaced throughout much of the feature, near surface, late in the volcano's life and without noticeable effect to the surface expression. Given these considerations, the layered model seems unlikely. It is also possible that mixing occurred with surrounding LCP rich material, although the uniform values across Syrtis Major and a sharp mineralogical boundary around its edge make regional scale mixing unlikely. Finally, coatings may have formed near the end of the Hesperian, muting the HCP signature of older deposits. Similar CDE features have been detected in other regions, including Hesperia Planium, Meridiani Planium, and to the south of Valles Marineris, indicating a global effect. Martian global events that could have coincided with the crater type transition include the formation of outflow channels, possibly the last major water release event, and the cessation of Hesperian volcanism, leading to a decrease in volcanic gases in the atmosphere. Laboratory studies have shown that basaltic rock with an initial HCP signal and thin hydrated coatings can experience absorption flattening and addition of 1.9μm water absorption that could be modeled as additional LCP content. Coatings result from either leaching out non Si cations, or the dissolution of the basalt followed by the precipitation of a silica rich coating.
Murchie Scott L.
Mustard John F.
Skok John R.
Wyatt Michael Bruce
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