Hubble Space Telescope (HST) confirms the discovery of the permanent Lunar OH atmosphere by the CHACE experiment in Chandrayaan-1

Reported by: Tirtha Pratim Das, Space Physics Laboratory, VSSC

Observations from the Hubble Space Telescope (HST) have confirmed the discovery of the water and hydroxyl atmosphere in the lunar environment made by the CHACE payload in Chandrayaan 1/MIP mission. When it is said ‘water atmosphere’ or ‘hydroxyl atmosphere’ in scientific jargon, it really means the presence of water molecules or hydroxyl molecules making their independent (non-interacting) respective atmospheres in the lunar environment without any inter-particle collision.

Moon’s atmosphere is so tenuous that there is hardly any collision between the gaseous species. Such an atmosphere where collisions are very rare is called exosphere. In the absence of any collision, it is said that different types of gaseous species simultaneously occupy the same volume of space without any interaction among themselves. It is described as the interaction-less coexistence of independent atmospheres (like the Argon atmosphere, Helium atmosphere, etc.) which are dictated by individual source and sink processes.

The CHACE (Chandra’s Altitudinal Composition Explorer) experiment onboard the Moon Impact Probe (MIP) in Chandrayaan 1 mission made the first successful measurement on the lunar day-side atmosphere on 14th November, 2008. It also revealed the presence of the lunar H2O and OH (due to dissociation of H2O to OH) atmospheres (say, exospheres). It meant the presence of H2O and OH molecules in gaseous form in the lunar exosphere which condense at the higher lunar latitudes to form polar water-ice. The analysis of CHACE data revealed the presence of H2O and OH molecules in significant amount at all the latitudes maximising near 700S (CHACE observations were more in the Southern hemisphere of the moon).

If such a significant H2O and OH exosphere is there at moon as discovered by CHACE, it must be amenable to detection by other means as well. The Hubble Space Telescope (HST), from Low Earth Orbit, has detected a persistent emission of 283 nm emission from the moon before, during and after the impact of the LCROSS module of the LRO mission. This emission was from the OH, which, researchers believe (Storrs et.al., 2011), may indicate the confirmation of a permanent lunar OH exosphere as reported by Sridharan et al (2010a), rather than OH produced by photolysis of water liberated by the impact event. This observation was published during the American Astronomical Society (AAS) Meeting in January, 2011 and the CHACE observations were referred.

References:

1. R. Sridharan, S.M. Ahmed, Tirtha Pratim Das, P. Sreelatha, P. Pradeepkumar, Neha Naik and Gogulapati Supriya (2010a): “`Direct’ evidence for water (H2O) in the sunlit lunar ambience fromCHACE on MIP of Chandrayaan I”, Planetary and Space Science Volume 58, Issue 6, May 2010, Pages 947-950

2. Storrs, Alex; Guillou, L.; O’Hara, A. (2011): The Impact Of The Lcross Satellite With The Moon As Seen By The Hubble Space Telescope; American Astronomical Society, AAS Meeting #217, #156.07