Extremely high reflection of solar wind protons as neutral hydrogen atoms from regolith in space

TL;DR

This study reveals that up to 20% of solar wind protons are reflected as neutral hydrogen atoms from lunar regolith, challenging previous assumptions about solar wind absorption by airless bodies.

Contribution

It provides the first direct measurements of high reflection rates of solar wind protons from lunar surface regolith using space-based instrumentation.

Findings

Up to 20% of solar wind protons are reflected as neutral hydrogen.

Reflection rates are significantly higher than previously assumed.

Implications for space weathering and surface-exosphere interactions.

Abstract

We report on measurements of extremely high reflection rates of solar wind particles from regolith-covered lunar surfaces. Measurements by the Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft in orbit around the Moon show that up to 20% of the impinging solar wind protons are reflected from the lunar surface back to space as neutral hydrogen atoms. This finding, generally applicable to regolith-covered atmosphereless bodies, invalidates the widely accepted assumption that regolith almost completely absorbs the impinging solar wind.