These data accompany the above referenced paper: "Searching for the
Source Regions of Martian Meteorites using MGS TES: Integrating Martian
Meteorites into the Global Distribution of Igneous Materials on Mars"
by V. E. Hamilton, P. R. Christensen, H. Y. McSween, Jr., and J. L. Bandfield
This is only a summary of the discussion found in the above article.
Please refer to the article for additional information.
In examining our deconvolution results, we found a large number of pixels
with detectable concentrations of the Nakhla end member, but these pixels did
not demonstrate any obvious spatial coherence. A plot of when these spectra
were acquired (in terms of TES orbit number, or OCK) versus latitude show that
these pixels primarily are restricted to data collected subsequent to OCK 7000,
and are correlated with sub-solar latitude. (Figure 1)
![noise plot](/~hamilton/SNC_images/nakhla_noise.gif)
Figure 1. Plot of detectable concentrations of the Nakhla end member in time (OCK)
vs. latitude. (Right-click on the images and choose 'view/open image...' to see at
full size.)
This pattern is similar to a pattern of noise in the TES data previously
identified by the TES team (Figure 2), and briefly mentioned by
Bandfield [2002], suggesting that our identifications of Nakhla-like
material are spurious. The pattern in the TES data is due to microphonic noise
induced by vibration of the aging MGS spacecraft. The manifestation of this
noise in the the TES spectral data is random in time, but affects all six TES
detectors simultaneously, confirming an instrumental/spacecraft source. There
are three spurious features that are introduced into the spectral data as a
result of this noise; the most important of these (for this study) is a feature
at ~1000 cm-1. Whether the feature is a positive (emission maximum)
or a negative (emission minimum) feature is random - averages of dozens or more
spectra average out the spurious features. The Nakhla spectrum was used in the
best fits to these spectra because it is dominated by the features of
clinopyroxene, one of which is a strong emission maximum near ~1000 cm-1
. It is important to note that clinopyroxene is not necessarily the only
mineral that could be erroneously identified in the post-OCK 7000 data; any
mineral with a strong emission minimum or maximum at these wave numbers could
be identified incorrectly (e.g., sulfate), unless averages of numerous spectra
are used, and/or results are verified by manual inspection. Note that this
discussion pertains to data acquired in the TES 10 cm-1 mode; the
noise has a stronger effect on data collected at 5 cm-1 sampling.
Note also that non-spectral data (e.g., thermal and visible bolometers) are
not affected by this noise, nor are spectral data at wavelengths >20 microns
affected.
The other two spurious features that are introduced into the TES data are
correlated with the motion of the high gain antenna (HGA) and the solar panels.
(In fact, a minor contribution from solar panel motion is observed in Figure 2
as the "streamer" of slightly higher values between 30 - 90 deg. N latitude
between OCKs 5000 - 8500.) Data that were acquired during periods of HGA and
solar panel motion are avoided easily by utilizing the appropriate quality
fields included in the TES database.
![noise plot 2](/~hamilton/SNC_images/tes_noise.gif)
Figure 2. Plot of TES microphonic noise in time (OCK) vs. latitude.
The scale is in units of 1.e-8 and represents a band strength index. Values
greater than ~9-10.e-8 are considered non-nominal. Data are provided courtesy
of Mike Smith (Goddard Space Flight Center) and Josh Bandfield (Arizona State
University).
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