High-resolution surface temperatures of the Moon derived from Imaging Infra-Red Spectrometer (IIRS) on-board Chandrayaan-2: A case study on the Schrödinger Basin

Imaging Infra-Red Spectrometer (IIRS), sent on-board Chandrayaan-2, has been mapping the lunar surface since 2019 with high spatial (80 m/pixel) as well as spectral resolutions (0.7 to 5 µm) [1-3]. This study uses two IIRS images, portions of which encompass the floor of Schrödinger Basin. Surface temperatures were derived from the two overlapping IIRS strips, which were then used to study the thermal properties of a part of the basin’s floor. Additionally, surface temperatures were also acquired by processing corresponding Diviner data, by matching IIRS pixels according to their Ground Control Points (GCPs) along with their local times of acquisition. It has been deduced that, for a particular period of time, the temperatures derived from IIRS are comparable with the corresponding Diviner data. Furthermore, factors contributing towards the mean temperature differences between the IIRS and Diviner datasets have been identified.

In order to use the temperatures obtained from IIRS, a case study on the ~320-km-wide-Schrödinger Basin was performed. In this regard, a volcanic vent (Schrödinger G), situated at 75° S and 139° E was studied, which is known to have deposited pyroclastic material on the basin floor [4-5]. Our analysis on the pyroclastic material-covered surface around the vent reveals several interesting findings: (1) Using the temperature data from IIRS, we were able to identify two distinct morphological units within the same pyroclastic deposit. The two units surrounding the vent exhibit different surface temperatures (an average relative difference of ~25-30K). (2) We have also used Clementine UVVIS colour-ratio data to identify differences related to soil mineralogy across the two deposits. The two units display contrasting tonal signatures on the UVVIS FCC image, thereby confirming the presence of two mineralogically distinct surface units. (3) In addition, using images from LRO-NAC, we have also performed a crater size frequency distribution (CSFD) measurement on the two units to estimate their relative ages. Our analysis reveals that the vent has experienced multiple eruptions, with the oldest ~3.7 Ga ago, while the latest being ~1.8 Ga ago, interspersed with different eruptions in between. Furthermore, CSFD measurements reveal that one of the units is significantly younger than the other. Based on the above results, we attribute the deposition of materials with different textures on either side of the vent to the several episodes of eruptions.

In conclusion, due to IIRS’s relatively higher spatial resolution, it was possible to identify and establish significant differences within a single pyroclastic deposit on the basis of derived temperature data, which have been validated from spectroscopic data as well. This study thus highlights the importance of using high-resolution IIRS data for such studies in future.

References: [1] Chowdhury et al. 2020, Current Science, 118, 368–375. [2] Bose et al. 2024, Advances in Space Research, 73, 2720-2752. [3] Verma et al. 2022, Icarus, 383, 115075. [4] Kramer et al. 2013, Icarus, 223, 131-148. [5] Kring et al. 2021, Advances in Space Research, 2021, 4691-4701.