ASYMMETRIC GAUSSIAN FITTING FOR VNIR SPECTRA

Overlapping Gaussian shapes can be effectively fitted using Asymmetric Gaussian shapes. This extra information provided by the asymmetry parameter can be effectively used to differentiate between otherwise standard looking spectra.

Asymmetric spectral bands can develop from several situations, three of which we list here and show how asymmetric shapes can assist:

1. Proximity. Nearby bands possess nearby centroids and relatively wide half widths.

2. Saturated bands. Large grain sizes or high optical depth situations [1] lead to band saturation, which creates an asymmetry.

3. Resolution. Lack of resolution of two narrow bands creates an "apparently asymmetric" shape

The asymmetric band technique has been used in the past to map asymmetry of absorption bands in VNIR spectra [2] and laboratory spectra [3]. We have started work on distinction of anhydrous carbonates using their asymmetry in the 2.5 to 4 micron region [4].

Janice Bishop

This work relies entirely upon the good graces of Janice Bishop, this year's GSA Planetary Geology Gilbert award recipient. As a post doc student at the SETI Institute for 10 years with Janice, I can testify to her unswerving dedication to Science and Spectroscopy.

My own post post doc period as a researcher was not only collaborative with Janice, but she supported my research with her interest and her ASD spectrometer and spectroscopy lab. Each summer we would have a new summer student come by at the lab and that is how I met many of the luckiest summer students to be immediately involved in cutting-edge Mars related science.

Janice and her husband and two kids would regularly go on field trips with the ASD spectrometer and I remember Janice's poster one year at LPSC displaying an almost life sized image of baby Marcus. Janice continues to inspire the next generation of scientists in her family, and in her students. Long may she succeed in this endeavour!

References

[1] Ladenburg, R., and F. Reiche (1913) “Über Selektive Absorption.” Annalen Der Physik 347 181–209.

[2] Brown, Adrian J., et al. (2010) “Hydrothermal Formation of Clay-Carbonate Alteration Assemblages in the Nili Fossae Region of Mars.” Earth and Planetary Science Letters 174.

[3] Bishop, Janice L., et al.. (2013) “Coordinated Spectral and XRD Analyses of Magnesite-Nontronite-Forsterite Mixtures and Implications for Carbonates on Mars.” JGR Planets 635.

[4] Brown, A.J., S.J. King, and J.L. Bishop. (2021) “Distinguishing Anhydrous Carbonates Using Spectral Centroid and Asymmetry Near 2.5 and 4 Microns.” LPSC, Abs #2708. Houston, TX: LPI