OMAT : What is the Optical Maturity index?

Hello. This post might not be for you big science guys. It is rather done for my own understanding of the OMAT index than to teach you something new. I tried here to summarize some of the reading I did this summer. I might also use this blog post to further explain to my undergraduate friends and colleagues the tiny bits of science I have learned during this summer internship.

Why was the Optical Maturity index developped?

The OMAT index was developped by Paul Lucey and a team of scientists at the Hawaii Institute of Geophysics and Planetology. In a paper first published in August 2000, the team proposed a new way to estimate soil maturity. While other maturity indices existed prior to OMAT, they were only modestly correlated between one another and a well established index to estimate maturity was non existant. Hence the need to develop a new method of determining maturity.

The newly developped index allowed the mapping of maturity of the lunar soils. From multispectral images taken by the Clementine spacecraft, it was possible to create a mosaic where it was possible to see the spatial distribution of mature soils on the Moon. On the image below, brighter values correspond to fresher material, while darker values are attributed to more mature material.

Figure 1 : Clementine image of the Moon coupled with OMAT index

Image credit : Paul G. Lucey et al. (2000)

What is maturity?

Maturity is the degree of maturation of lunar soils, and maturation is caused by the exposure of lunar soils to space. Maturation consists of changes in grain size, in the production of glassy impact melts, production of agglutinates, reduction of native iron to submicrometer particles, in the vapor deposition on the coating of grains and in variations caused by exposure to solar wind and cosmic ray. These changes in physical properties of soils, among others not listed, cause variations in their optical properties. Mature soils tend to be dark red and to have a reduced spectral contrast when compared to immature soils.

While maturity is correlated to the duration of exposure to space, it is not synonymous.

What does the OMAT index allow us to see?

The maturation of lunar soils tend to cause rays and ejecta of craters to fade out and to not be visible to the human eye. However, these features can still be observed with the Optical Maturity parameter. The images below show the differences between what can be observed. Figure 2 shows the surface reflectance crater Petavius B at 750 nm wavelenght - which is the upper limit of visible light. Figure 3 shows the same area, coupled with OMAT index. On this image, we can spot bright areas at the north-west of the crater that are not present in Figure 2. These bright spots correspond to fresher material.

Figure 2 : Surface reflectance of Petavius B crater at 750 nm

Figure 3 : OMAT image of Petavius B crater

It has been noted by scientists that young craters have higher OMAT values at their rim. These values decrease over distance from the rim. The values also decrease over time and are reportedly nearly indisguishable from lunar background values with very mature craters. (Grier et al. 2015)

OMAT has also been used to adress the problem of counting small craters. It was difficult for scientists to determine which craters were primary impacts and which ones were secondaries. OMAT was used to mitigate uncertainty in identifying primary and secondary impacts.

Conclusion

When the Optical Maturity was first developped in 2000, a new tool was accessible for lunar mapping. New possibilities in lunar studies were created, as new ways of dating the relative age of craters were discovered. It has since been used numerous times in various papers.

Sources :

+ Grier et al. Past and present use of the optical maturity parameter (OMAT) on the Moon. Workshop on Issues in Crater Studies and the Dating of Planetary Surfaces. 2015.

+ Lucey et al. Imaging of lunar surface maturity. Journal of Geophysical Research. 2000.