Posted to uplink.space.com:

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exoscientist
(A)
08/24/03 07:22 PM
Keep in mind the limitations of the instruments. [re: serak_the_preparer]

To paraphrase an old saying:

"Scientists who don't remember the limitations of their instruments are doomed to keep making the same mistakes over and over again."

Experiments on actual samples *in the field* show that the signature of even large deposits of carbonate can be obscured simply by a rough surface or even by a dust covering only 300 microns thick, which is about the thickness of three strands of human hair:

A Field Study of Thermal Infrared Spectra of Carbonates, with Implications for Studies of Mars.
"Observations. SEBASS recorded spectra of areas
that had a range of percent coverage by calcrete vs.
soil, with typical coverage of ~50% calcrete, up to
100% calcrete coverage near spoils from past road
work. However, none of the regions that consisted
predominantly of calcrete or soil exhibited a strong
11.25 µm carbonate band. This observation is
significant, because it has generally been assumed that
a massive carbonate will exhibit a strong 11.25 µm
band, and detection limits of carbonates on Mars have
been based on this assumption. Only the asphalt road,
(which contained limestone in the aggregate), and the
arroyo, (which contained limestone, Fig. 3), showed a
distinct signature at 11.25 µm."
http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1876.pdf

Utilizing Thermal Infrared Spectra of Mars for Mission Planning.
Concepts and Approaches for Mars Exploration.
"Ruff et al. [5] conclude that TES spectra
recorded of White Rock exhibit the spectral behavior
of a blackbody (emissivity = 1) over the spectral range
that TES measures, and over which they have obtained
an acceptable atmospheric correction, 7.7 to 12.5 and
20 to 40 µm (1300 to 800, and 500 to 250 cm-1) [5].
This blackbody signature is consistent with
(1) finely particulate material, such as aeolian dust [5];
or (2) vesicular or rough surfaces, such as the rough
carbonate at Mormon Mesa. Differentiating between
the two possibilities, and determining the composition
of White Rock will require higher SNR measurements,
or the measurement of many additional spectra for
extensive averaging."
"Figure 5: "White Rock." Ruff et al. [5] conclude
that TES spectra of White Rock exhibit the signature
of a blackbody (emissivity = 1). This is consistent
either with a material that has no spectral features
across these regions [5], or a material that is rough and
pitted, such as the indurated carbonates at Mormon
Mesa [1]. This second possibility should be taken into
account in mission planning based on these data sets."
http://www.lpi.usra.edu/science/kirkland/Papers/marsplan_abs6231.pdf

CARBONATES ON MARS: PROBABLE OCCURRENCES, SPECTRAL SIGNATURES, AND
EXPLORATION STRATEGIES. B. J. Thomson and P. H. Schultz, 1Brown University, Department of Geological
Sciences, Box 1846, Providence, RI 02912 (Bradley_Thomson@brown.edu).
Sixth International Conference on Mars (2003) 3229.pdf
"Exploration strategies: The most readily accessible
concentrations of carbonates on Mars are likely
reprecipitated calcrete layers within dust deposits. The
potential for entombment and preservation of organic
material in chemical precipitates makes these layers a
high-priority target for exobiology [18]. Depending on
the degree of cementation, these calcretes may form
resistant hardpans that are exposed through differential
erosion. However, due to mantling by continued dust
deposition, exposed calcrete horizons may be difficult
to spectrally detect. In addition, terrestrial calcrete
hardpans are known to weather to boulders, cobbles,
and smaller fragments [19] that will increase macroscopic
roughness and reduce spectral contrast, thus
further complicating spectral detection."
http://www.lpi.usra.edu/meetings/sixthmars2003/pdf/3229.pdf

FIRST AIRBORNE THERMAL INFRARED HYPERSPECTRAL IMAGING OF A DRY LAKE: REAL-WORLD DATA AND TES/THEMIS MARS INTERPRETATIONS.
L. E. Kirkland1,2, E. R. Keim2, K. C. Herr2, P. M. Adams2, D. Burt3, J. M. McAfee4; 1Lunar and Planetary Institute, kirkland@lpi.usra.edu; 2The Aerospace Corp., Eric.R.Keim@aero.org, Kenneth.C.Herr@aero.org, Paul.M.Adams@aero.org; 3Arizona State U., dmburt@asu.edu; 4Los Alamos National Lab, mcafee_john_m@lanl.gov
Lunar and Planetary Science XXXIV (2003) 1887.pdf
"Discussion: Laboratory spectra used for TES interpretations show much greater spectral contrast than the field data, consistent with the findings in [2]. Fig. 5 compares the strongest SEBASS sulfate band with laboratory data used for TES interpretations [5]. A resulting interpretation would conclude that very little sulfate is present in a region covered by indurated sulfate. Likely causes of lower spectral contrast are surface roughness and coverage by dust. The true cause requires laboratory characterization. Implications for TES/THEMIS interpretations are that lake deposits are almost certainly much harder to detect than what is predicted using laboratory spectra of pure, smooth-surfaced samples (e.g., Fig. 5). As a result, interpretations based on non-detection have high uncertainties and should be approached with due caution."
http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1887.pdf

Bob Clark

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