Characterization of hydrated silicate-bearing outcrops in Tyrrhena Terra, Mars: Implications to the alteration history of Mars

Research areas:
  • D. Loizeau
  • J. Carter
  • S. Bouley
  • Nicolas Mangold
  • F. Poulet
  • J. -P. Bibring
  • F. Costard
  • Y. Langevin
  • B. Gondet
  • S. L. Murchie
The Tyrrhena Terra region of Mars is studied with the imaging
spectrometers OMEGA (Observatoire pour la Mineralogie, l'Eau, les Glaces
et l'Activite) onboard Mars Express and CRISM (Compact Reconnaissance
Infrared Spectrometer for Mars) onboard Mars Reconnaissance Orbiter,
through the observation of tens of craters that impacted into this part
of the martian highlands. The 175 detections of hydrated silicates are
reported, mainly associated with ejecta blankets, crater walls and rims,
and central up-lifts. Sizes of craters where hydrated silicates are
detected are highly variable, diameters range from less than 1 km to 42
km. We report the presence of zeolites and phyllosilicates like
prehnite, Mg-chlorite, Mg-rich smectites and mixed-layer
chlorites-smectites and chlorite-vermiculite from comparison of
hyperspectral infrared observations with laboratory spectra. These
minerals are associated with fresh craters post-dating any aqueous
activity. They likely represent ancient hydrated terrains excavated by
the crater-forming impacts, and hence reveal the composition of the
altered Noachian crust, although crater-related hydrothermal activity
may have played a minor role for the largest craters (>20 km in
diameter). Most detected minerals formed over relatively high
temperatures (100-300 degrees C), likely due to aqueous alteration of
the Noachian crust by regional low grade metamorphism from the Noachian
thermal gradient and/or by extended hydrothermal systems associated with
Noachian volcanism and ancient large impact craters. This is in contrast
with some other phyllosilicate-bearing regions like Mawrth Vallis where
smectites, kaolinites and hydrated silica were mainly identified,
pointing to a predominance of surface/shallow sub-surface alteration;
and where excavation by impacts played only a minor role. Smooth plains
containing hydrated silicates are observed at the boundary between the
Noachian altered crust, dissected by fluvial valleys, and the Hesperian
unaltered volcanic plains. These plains may correspond to alluvial
deposition of eroded material. The highlands of Tyrrhena Terra are
therefore particularly well suited for investigating the diversity of
hydrated minerals in ancient martian terrains. (c) 2012 Elsevier Inc.
All rights reserved.