Rapid Communication
‘Direct’ evidence for water (H2O) in the sunlit lunar ambience from CHACE on MIP of Chandrayaan I

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Abstract

Direct detection of water in its vapour phase in the tenuous lunar environment through in situ measurements carried out by the Chandra’s Altitudinal Composition Explorer (CHACE) payload, onboard the Moon Impact Probe (MIP) of Chandrayaan I mission vindicates the presence of water on the surface of the moon in form of ice at higher lunar latitudes inferred from IR absorption spectroscopy, (especially that of OH), by the Moon Mineralogy Mapper (M3) of Chandrayaan I. The quadrupole mass spectrometer based payload, CHACE, sampled the lunar neutral atmosphere every 4 s with a broad latitudinal (∼40°N to 90°S, with a resolution of ∼0.1°) and altitudinal (from 98 km up to impact on the lunar surface with a resolution of ∼0.25 km) coverage in the sunlit side of the moon for the first time. These two (CHACE and M3) complementary experiments are shown to collectively provide unambiguous signatures for the distribution of water in solid and gaseous phases in Earth’s moon.

Introduction

The presence of water, or its derivatives in the lunar surface had been a subject matter of intense debate and research in the recent times. More so, after the first indication of water in the form of frozen ice in the permanently shadowed polar regions by the Clementine mission of NASA (Nozette et al., 1996), the topic got revived and the basic question whether the moon is really ‘bone dry’, came up again. Once the speculation and indirect inferences have been made, finding suitable direct evidences has become of paramount importance. All the more so, when the whole facet of future lunar exploration may be decided based on the availability water. Finding water or its derivatives therefore had been one of the fundamental scientific objectives of the recent lunar missions, like, Change-1 of China, Kaguya of Japan, Chandrayaan-I of India and the most recent Lunar Reconnaissance Orbiter (LRO) of the USA. Of all the four, Chandrayaan-I and LRO had carried along with them the Moon Impact Probe (MIP) and LCROSS respectively, which were released from the mother space craft and made to impact the polar regions of the moon. While the former was designed to carry out measurements during its descent, the latter was intended to create a deep impact crater and the ejecta due to the impact were to be analyzed. At the time of revising this paper LCROSS/LRO combination had confirmed the presence of water ice deeply buried in the permanently shadowed regions of the South Pole with soon-to-come more exciting results.

The most sensational finding from the Moon Mineralogy Mapper (M3), an experiment from the USA on Chandrayaan-I had been the clear tell-tale signature of the presence of OH/ H2O in the top few millimetres of the lunar regolith, over an extended region preferably at higher latitudes (Pieters et al., 2009). The inferences were arrived at through absorption spectroscopy in the 2.8–3 micron IR band. Though M3 response was limited to 3 microns, just falling short of the peak of the absorption feature of water/ice, the signatures were unambiguous and were supported by the results from the Deep Impact Probe, while it was passing the moon on its way and also the Cassini space craft during the fly by when it focused on some of the Apollo landing sites for calibration purposes (Sunshine et al., 2009; Roger, 2009). In the light of these important findings and inferences, any direct measurement of H2O would add further credence to the whole of lunar science. The result reported here-in from the Chandra’s Altitudinal Composition Explorer (CHACE) on the Moon Impact Probe of India’s Chandrayaan-I just does that by providing valuable information directly on the relative concentration of H2O in the ‘sunlit’ lunar atmosphere through unique neutral composition measurements. The Moon Impact Probe, a micro satellite by itself, carried three experiments viz., Radar Altimeter, a Visible Imaging Camera, and an extremely sensitive neutral mass spectrometer (CHACE). The ‘first of its kind’ of results providing the altitudinal/latitudinal neutral composition of the tenuous lunar atmosphere during the lunar day have been obtained and the results pertaining to water (H2O) form the subject matter for this communication.

Section snippets

Data base

The basic data pertaining to this paper is from CHACE, a quadrupole mass spectrometer, with a mass range of 1–100 amu, with an electron multiplier in its detector stage and having a partial pressure sensitivity of <10−13 torr and nearly nine orders of magnitude dynamic range. It had a mass resolution of better than 1 amu all through its entire mass range and scanned the whole range in 4 s. On 14 November, 2008, as a part of the Moon Impact Probe (MIP), CHACE was switched on 20 min before its actual

Results

Fig. 2 depicts a sample spectrum obtained by CHACE at an altitude of ∼97 km just after it had been released from the main orbiter–Chandrayaan-1 highlighting the capability of the instrument and also the quality of data. The Lunar ALSEP cold cathode gauge and the LACE (Lunar Atmosphere composition Experiment) of the Apollo series which provide the only authentic measurements available till recently had indicated a possible upper limit of 10−9 torr for the dayside pressures (Alan Stern, 1999 and

Conclusion

The CHACE experiment on the Moon Impact Probe of Chandrayan-I, made a direct detection of H2O in the tenuous lunar ambience complementing the Moon Mineralogy Mapper in the same mission. Having established the complementary nature of the results from these two experiments, the inherent higher sensitivity of the CHACE enables one to extend the latitudinal coverage down to 20°S (Fig. 3). These results further substantiate those obtained from the Deep Impact Probe revealing extended regions of

Acknowledgements

Our sincere thanks are due to the MIP and the Chandrayaan-1 teams that did a meticulous job in the preparation of the space crafts. The constant encouragement from the Chairman of ISRO secretary Dept. of Space, Dr. G. Madhavan Nair is gratefully acknowledged. This work is supported by the Dept. of Space, Govt. of India.

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Current affiliation: Central University of Hyderabad, India.

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