HEMATITE IN THE THERMAL INFRARED: COMPARISON OF MEASUREMENTS MADE BY TES, MINI-TES AND IN THE LABORATORY

A main objective of sending the MER Opportunity rover to Meridiani Planum was to investigate the occurrence of gray, crystalline hematite detected by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) instrument. Among the instruments on the rover payload is the Mini-TES instrument, which serves as a complement to the orbital TES instrument and mid-infrared laboratory studies of hematite. The characteristic mid-infrared hematite spectrum contains strong absorptions at ~300, 450, and 540 cm^-1. An additional minor absorption is sometimes present at 390 cm^-1. The exact position, shape and relative strengths of these absorptions vary depending on the hematite formation process. The gray hematite at Meridiani Planum is present in the form of spherules that have been interpreted as concretions weathering out of a sulfate rich outcrop. The spherules are also abundant on the plains of Meridiani and are most likely present as a lag. The shapes of the hematite features in Mini-TES spectra have not varied substantially over the course of the Opportunity mission, implying that the composition of the hematite spherules is relatively uniform. Although the TES hematite spectrum is cut off by the presence of a fundamental atmospheric CO₂ absorption centered at 667 cm^-1, spectra from the TES and Mini-TES instruments are broadly similar. The TES and Mini-TES hematite spectra are matched best by a laboratory spectrum of a hematite sample that was produced by dehydroxylating goethite at 300°C. However, there are some small differences between the Mini-TES hematite spectra and hematite spectra from MGS-TES and laboratory measurements that may yield some new insight into the hematite formation process. The findings of the Opportunity rover have confirmed previous suggestions that the hematite at Meridiani Planum was deposited in a water-related process.