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

In the Solar System, space weathering results in most surfaces of atmosphereless bodies being covered by regolith, a layer of loose, heterogeneous material of small grain size (Clark et al., 2002). In the absence of an atmosphere, plasma interacts directly with the surface, e.g. solar wind with the lunar regolith, and it has been tacitly assumed that the plasma is almost completely absorbed (<1% reflected) in the surface material (e.g. Crider et al. 2002, Schmitt et al., 2000, Feldman et al., 2000, Behrisch & Wittmaack, 1991).

Regolith reaches hydrogen saturation on a geologically very short timescale of at most 10 4 years (Johnson & Baragiola, 1991). Once saturated, for every impinging proton a hydrogen atom is removed from the surface by sputtering, a scatter or desorption process.

Here we present measurements that invalidate the widely-accepted assumption that regolith almost completely absorbs the impinging solar wind. A large fraction of up to 20% is reflected as energetic neutral hydrogen atoms back to space.

The Sub-keV Atom Reflecting Analyzer (SARA) instrument (Barabash et al., 2009) onboard the Indian Chandrayaan-1 spacecraft (Goswami & Annadurai, 2009) orbiting the Moon in a 100-km polar orbit measures the neutral atom flux from the lunar surface and simultaneously monitors the impinging flux of solar wind protons. The SARA instrument consists of two sensors: the Solar Wind Monitor (SWIM) (McCann et al., 2007) measures solar wind ions and the Chandrayaan-1 Energetic Neutrals Analyzer (CENA) (Kazama et al., 2007) measures energetic neutral atoms (10eV-3keV) arriving from the direction of the lunar surface. Both sensors provide angular and mass resolution: SWIM has a fieldof-view of 7.5° x 180° divided into 16 angular pixels covering directions from nadir to zenith on the sun facing side of the spacecraft. The orbital motion of Chandrayaan-1 is used to scan the SWIM field-of -view across the full solar wind angular distribution. The center of the CENA field-of-view is nadir-pointing and in total 160° cross-track times 7° along-track in size, divided into 7 angular pixels. Mass resolution m/dm of both sensors is 2-3, depending on sensor, angular pixel and mass. The geometric factor for SWIM is 5 ⋅10 -5 cm 2 sr eV/eV for 500 eV protons. The geometric factor of CENA is 5 ⋅10 -8 cm 2 sr eV/eV for 500eV neutral hydrogen. Both values are for a single angular pixel at the center of the field-of-view.

During nominal solar wind conditions SARA observed that up to 16-20% of the proton flux impinging on the lunar surface is reflected back to space as energetic neutral hydrogen atoms. SARA persistently detects high intensity fluxes of energetic neutral hydrogen atoms propagating from the lunar surface while the latter is illuminated by the solar wind. The reflected neutral atom flux changes consistently with the change in solar wind incident angle (Figure 1), with the reported numbers reached at the lunar equator.

No significant energetic neutral atom flux is observed on the night side. As the intensity of the impinging solar wind proton flux changed orbit by orbit, the intensity of the reflected neutral atom flux changed correspondingly and consistently (Figure 2). The reflected neutral hydrogen that was observed has an energy spectrum with a distinct upper energy of 300 eV, roughly half of the measured central energy (540 eV) of the impinging protons. Upon reflection from the surface, solar wind particles experience an average energy loss of more than 50%.

Ions with solar wind energies interacting with surfaces are likely to be neutralized e.g. by resonant or Auger neutralization, before being absorbed or reflected (e.g. Niehus et al., 1993). We observe that up to 20% of solar wind protons are reflected from the lunar surface back to space as energetic neutral hydrogen atoms with energies larger than 25 eV (Figure 2). This high reflection invalidates the previous assumption that regolith almost completely absorbs the impinging solar wind. On a saturated surface the sum of all loss processes equals the solar wind precipitation, the high reflection yield reduces thus the amount of hydrogen available for release by other processes, such as sputtering or desorption, both mechanisms predominantly producing neutrals with energies of only a few eV (Betz & Wien, 1994). Since the energy of the reflected energetic neutral hydrogen is much higher than the escape energy, escaping atoms propagate on ballistic trajectories. They can be used for detailed remote imaging of the solar wind-surface interaction (Bhardwaj et al., 2005, Futaana et al., 2006). The average energy loss of more than 50% when interacting with the surface is likely due to several mechanisms. A simple classical binary collision model (Niehus et al, 1993) gives an energy loss between 10% and 20%. This indicates the pr

…(Full text truncated)…