Past Habitability of Mars: Interpretations of Mars Global Surveyor TES
Abstract
A primary question in habitability studies of Mars is whether significant surface water was present in the past. If large water deposits were present, then carbonate deposits likely would have formed. Thus, much debate about the past climate and habitability centers on whether carbonate deposits have been detected. Here we discuss relevant infrared spectral evidence; implications for astrobiology; and the unifying research needed to address astrobiology concerns. Under specific conditions, carbonates exhibit spectral features near 6.5, 11, and 33 microns. Several past studies proposed detection of carbonates on Mars (reviewed Roush et al. [1993]). The detections have not widely been perceived as conclusive. Two recent orbited instruments (1996 Global Surveyor TES; 2001 Odyssey THEMIS) were intended to resolve the question. However, no 11 or 33 micron band detections have been reported. Complexities in atmospheric unmixing significantly delayed and complicated analysis of the 6.5 micron region. Bandfield et al. [2003] recently proposed detection of a 6.5 micron carbonate feature in the surface dust. However, TES did not detect the 11 and 33 micron bands, placing the interpretation again in the debated category. Christensen et al. [2001] concluded that TES detected no carbonate to the 10 percent per pixel level, and Bandfield et al. [2003] in selected dark regions to 5 percent. The 2003 rover Mini-TES is comparable to TES, and is similarly intended to resolve the minerals present. One interpretation of those results is that the types of carbonate deposits expected from large bodies of water are not present. One inference is that no large, standing bodies of water on Mars existed. However, discussion of a key issue to the debate has been missing: What impacts whether a mineral deposit is detectable by these instruments? We will demonstrate that rough surface texture can cause large mineral deposits, including rocks, to have subdued spectral signatures [Kirkland et al. 2001, 2002, 2003]. Subdued signatures can make a mineral undetectable ("infrared stealthy"). For example, the Mars "White Rock" deposit exhibits no detectable TES bands [Ruff et al. 2001]. Yet White Rock certainly is made of something. Grain scale roughness can be a primary cause of infrared stealthy behavior. The astrobiology community should be alerted to the following: 1. Accurate interpretations of these data sets require knowledge of the expected target texture. The textures are poorly known for the formation and weathering conditions on Mars. 2. An assessment of texture and dectectability will require collaboration between scientists who research astrobiology, geology of Mars, and the physics of spectral behavior. The unifying theme is identification of materials of interest to the astrobiology community on Mars by an infrared spectrometer, and conversely, defining when such materials will remain undetected. 3. Interpretations based on non-detection need to be revisited based on improved understanding of the true detection capabilities. 4. Interpretations founded on detection of a single spectral feature when more than one feature is expected should be viewed cautiously. References. Bandfield et al. Science 301, 1084 (2003); Christensen et al. JGR 106, 23823 (2001); Kirkland et al. Appl. Opt., 40, 4852 (2001); Kirkland et al. Remote Sens. Env., 80, 447 (2002); Kirkland et al., JGR in press, preprint www.lpi.usra.edu/science/kirkland (2003); Roush et al. Ch. 16 Remote Geochem. Analysis (1993); Ruff et al. JGR 106, 23921 (2001).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.B51G..04K
- Keywords:
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- 5410 Composition;
- 5460 Physical properties of materials;
- 5464 Remote sensing;
- 5470 Surface materials and properties