Diversity and Distribution of Amorphous Silicate Phases on Mars

Amorphous silicates represent a window into the geologic processes and environments of a planet. Beginning in the early 1980s, the presence of amorphous silicate phases in the form of palagonite in Martian bright regions was suggested based on visible/near infrared spectra. With the advent of the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor spacecraft in the late 1990s, evidence emerged for an amorphous phase different from palagonite in some Martian dark regions. The nature of this phase has been a source of debate for the past decade. Although an earlier hypothesis that it actually represents a crystalline phyllosilicate phase appears to be ruled out, the question of whether we are observing a primary or secondary silicate phase remains. Primary silicate glass can be expected amongst the massive volcanic outpourings on Mars, but its persistence over time is less certain. Observations from TES combined with those of the Thermal Emission Imaging System on the Mars Odyssey orbiter demonstrate clear evidence in at least one location for primary volcanic glass in a small flow of basaltic andesite. This is necessary but insufficient evidence that larger regions of Mars contain primary volcanic glass. Observations by the Miniature Thermal Emission Spectrometers (Mini-TES) on the Mars Exploration Rovers provide further evidence for amorphous silicates in the rocks at two locations. At Meridiani Planum, these phases likely are secondary in nature and associated with the aqueous alteration of basaltic bedrock that also yielded sulfate and hematite phases. In Gusev Crater, Mini-TES provides evidence for a range of amorphous phases from basaltic glass to nearly pure opaline silica. The discovery of apparently ancient opaline silica is unassailable evidence that amorphous silicate phases are resistant to devitrification and recrystallization in the dry Martian environment.