The Sub-keV Atom Reflecting Analyzer (SARA) Experiment Aboard Chandrayaan-1 Mission: Instrument and Observations

TL;DR

The SARA experiment on Chandrayaan-1 used energetic neutral atom imaging to study solar wind interactions with the Moon, revealing that about 20% of solar wind ions are reflected as ENAs, challenging previous assumptions.

Contribution

This paper presents the design, implementation, and observational results of the SARA instrument, a novel ENA imaging system on Chandrayaan-1 for lunar surface interaction studies.

Findings

Approximately 20% of solar wind ions are backscattered as ENAs from the lunar surface.

SARA provided high-quality data on lunar surface interactions with solar wind.

Results challenge previous assumptions of near-complete absorption of solar wind by the Moon.

Abstract

SARA experiment aboard the first Indian lunar mission Chandrayaan-1 had the objective to explore the solar wind-lunar interaction using energetic neutral atoms (ENA) from the lunar surface as diagnostic tool. SARA consisted of an ENA imaging mass analyzer CENA (Chandrayaan-1 Energetic Neutral Analyzer) and an ion mass analyser SWIM (Solar Wind Monitor), along with a digital processing unit (DPU) which commands and controls the sensors and provides the interface to the spacecraft. Both sensors have provided excellent observational data. CENA has observed ENAs from the lunar surface and found that ~20% of the incident solar wind ions get backscattered as ENAs from the lunar surface. This is contrary to the previous assumptions of almost complete absorption of solar wind by the lunar surface. The observation is relevant for other airless bodies in the solar system.