[Paper Review] Chandrayaan-3 Alternate Landing Site: Pre-Landing Characterisation

K. Durga Prasad, D. Misra+6 more|arXiv (Cornell University)|Aug 21, 2023

Planetary Science and ExplorationPhysics and Astronomy3 citations

TL;DR

This study conducts a comprehensive pre-landing characterization of Chandrayaan-3's alternate landing site (ALS) at 69.497764°S, 17.330409°W using high-resolution Chandrayaan-2 OHRC DEMs and ortho-images, along with data from Chandrayaan-1 and LRO. It finds the ALS is hazard-free over 75% of its area, compositionally similar to highland regolith (Fe ~4.8 wt.%, Mg ~5 wt.%, Ca ~11 wt.%), and exhibits distinct thermophysical behavior with lower daytime and higher nighttime temperatures than the primary site, indicating unique surface properties.

ABSTRACT

India's third Moon mission Chandrayaan 3 will deploy a lander and a rover at a high latitude location of the Moon enabling us to carry out first ever in-situ science investigations of such a pristine location that will potentially improve our understanding on primary crust formation and subsequent modification processes. The primary landing site (PLS), is situated at 69.367621 degS, 32.348126 degE. As a contingency, an alternate landing site (ALS) was also selected at nearly the same latitude but nearly 450 km west to PLS. In this work, a detailed study of the geomorphology, composition, and temperature characteristics of ALS has been carried out using the best-ever high resolution Chandrayaan 2 OHRC DEMs and Ortho images, datasets obtained from Chandrayaan 1 and on-going Lunar Reconnaissance Orbiter. For understanding the thermophysical behaviour, we used a well-established thermophysical model. We found that the Chandrayaan 3 ALS is characterised by a smooth topography with an elevated central part. The ALS is a scientifically interesting site with a high possibility of sampling ejecta materials from Tycho and Moretus. Based on the spectral and elemental analysis of the site, Fe is found to be near approx. 4.8 wt.%, with Mg approx. 5 wt.%, and Ca approx. 11 wt.%. Compositionally, ALS is similar to PLS with a highland soil composition. Spatial and diurnal variability of around 40 K and 175 K has been observed in the surface temperatures at ALS. Although belonging to similar location like PLS, ALS showed reduced daytime temperatures and enhanced night-time temperatures compared to PLS, indicating a terrain of distinctive thermophysical characteristics. Like PLS, ALS is also seems to be an interesting site for science investigations and Chandrayaan 3 is expected to provide new insights into the understanding of lunar science even if it happens to land in the alternate landing site.

Motivation & Objective

• To evaluate the scientific and engineering suitability of Chandrayaan-3's alternate landing site (ALS) as a contingency option.
• To assess geomorphological features, surface composition, and thermophysical properties of the ALS for mission planning and science interpretation.
• To compare the ALS with the primary landing site (PLS) in terms of topography, hazard potential, and thermal behavior.
• To support in-situ science operations by providing a detailed pre-landing characterization for rover path planning and instrument data interpretation.
• To determine if the ALS can yield valuable scientific insights comparable to the PLS, especially regarding lunar crust formation and ejecta composition.

Proposed method

• Utilized high-resolution (25 cm) OHRC-derived digital elevation models (DEMs) and ortho-images for detailed geomorphological analysis of the ALS.
• Integrated multi-mission datasets from Chandrayaan-1 (M3), LRO (Diviner, LOLA, WAC), and SELENE for compositional and topographic context.
• Applied a 3D thermophysical model (PRL 3D Thermophysical Model) with finite element method to simulate surface and subsurface temperature variations over a 200 m × 200 m area.
• Incorporated a 9 cm insulating surface layer and solar irradiation inputs in the model, with thermal conductivity and specific heat as analytic functions.
• Generated a hazard map based on slope and boulder density from OHRC data to assess landing safety and operational feasibility.
• Conducted spectral analysis using M3 data to derive elemental abundances (Fe, Mg, Ca) and infer surface composition.

Experimental results

Research questions

• RQ1 What is the geomorphological structure and topographic variation of the Chandrayaan-3 alternate landing site (ALS)?
• RQ2 How does the surface composition of the ALS compare to the primary landing site (PLS), and what is its mineralogical signature?
• RQ3 What are the spatial and diurnal variations in surface temperature at the ALS, and how do they differ from those at the PLS?
• RQ4 To what extent is the ALS safe for landing and rover operations, based on hazard assessment?
• RQ5 What is the potential for in-situ sampling of Tycho and Moretus crater ejecta at the ALS, and what scientific insights could be gained?

Key findings

• The ALS is characterized by a smooth, relatively elevated central region with an average elevation of 216 m and is dominated by ejecta from the Eratosthenian-aged Moretus-A crater and the Copernican-aged Tycho crater.
• The hazard map derived from OHRC data indicates that approximately 75% of the ALS is hazard-free, supporting safe landing and rover deployment.
• Spectral analysis reveals a surface composition of ~4.8 wt.% Fe, ~5 wt.% Mg, and ~11 wt.% Ca, consistent with typical lunar highland soil.
• Surface temperatures at the ALS show a diurnal range of ~40 K (spatial) and ~175 K (diurnal), with lower daytime and higher nighttime temperatures compared to the PLS, indicating distinct thermophysical properties.
• Model simulations show significant local-scale thermal variability (up to ~40 K) across a 200 m × 200 m area, likely due to subtle topographic or compositional heterogeneities.
• Despite being at a similar high latitude, the ALS exhibits thermophysical behavior distinct from the PLS, suggesting differences in regolith properties or thermal inertia.