The lunar surface progressively darkens and reddens as a result of sputtering from solar wind particles and bombardment of micrometeoroids. The extent of exposure to these space weathering agents is frequently calculated as the location in a diagram of reflectance at 750 nm vs. 950 nm/750 nm color (R-C). Sim & Kim (2018) examined the R-C trends of pixels within ∼3,500 craters, and revealed that the length (L) and skewness (s) of R-C trends can be employed as a secondary age or maturity indicator. We broaden this research to general lunar surface areas (3,400 tiles of 0.25◦ × 0.25◦ size) in 218 mare basalt units, whose ages have been derived from the size-frequency distribution analysis by Hiesinger et al. (2011). We discover that L and s rise with age until ∼3.2 Gyr and reduce rather rapidly afterward, while the optical maturity, OMAT, reduces monotonically with time. We show that in some situations, when not only OMAT but also L and s are incorporated in the estimation utilizing 750 & 950 nm photometry, the age estimation becomes considerably more reliable. We also observed that OMAT and the lunar cratering chronology function (cumulative number of craters larger than a certain diameter as a function of time) have a relatively linear relationship.

Abstract
1. INTRODUCTION
2. DATA
3. SPECTRAL TREND IN R-C SPACE
4. CORRELATION BETWEEN R-C TREND ANDMATURITY PARAMETERS
5. DISCUSSION
ACKNOWLEDGMENTS
REFERENCES

• Sungsoo S. Kim(Humanitas College, Kyung Hee University,School of Space Research, Kyung Hee University)
• Chae Kyung Sim(Korea Astronomy and Space Science Institute) Corresponding Author