Add 'sparking' in PSRs to the space weathering zoo

University of New Hampshire (UNH) scientists propose the addition of "sparking"  to cosmic rays and micrometeor bombardment as part of the relentless space weather gardening always underway within the Moon's permanently shadowed regions. This illustration shows a PSR undergoing subsurface sparking, to a depth of about 1mm, which ejects vaporized material [UNH/SVS].

Durham (NH) –- The Moon appears to be a tranquil place, but modeling done by University of New Hampshire and NASA scientists suggests that, over the eons, periodic storms of solar energetic particles may have significantly altered the properties of regolith in the Moon’s coldest craters through the process of "sparking" —a finding that could change our understanding of the evolution of planetary surfaces in the solar system.

The study, published recently in the Journal of Geophysical Research-Planets, proposes that high-energy particles from uncommon, large solar storms penetrate the Moon’s frigid, polar regions and electrically charges the regolith. The charging may create sparking, or an electrostatic breakdown, and this “breakdown weathering” process has possibly changed the nature of the Moon’s regolith within its permanently shadowed regions, or "PSR's," which may be more active than previously thought.

“Decoding the history recorded within these cold, dark craters requires understanding what processes affect their regolith,” says Andrew Jordan of the UNH Institute for the Study of Earth, Oceans, and Space, lead author of the paper. 

“To that end, we built a computer model to estimate how high-energy particles detected by the CRaTER (Cosmic Ray Telescope for the Effects of Radiation) instrument, on board LRO can create significant electric fields in the top layer of lunar regolith,” Jordan wrote.

The scientists also used data from the Electron, Proton, and Alpha Monitor (EPAM) on the Advanced Composition Explorer (ACE).

CRaTER, which is led by scientists from UNH, and EPAM both detect high-energy particles, including solar energetic particles (SEPs). SEPs, after being created by solar storms, stream through space and bombard the Moon. These particles can build up electric charges faster than the regolith can dissipate them and may cause sparking, particularly in the polar cold of permanently shadowed regions—unique lunar sites as cold as minus 240 degrees Celsius and known to contain water ice. 

The record cold at Hermite (108 km; 86.16°N, 266.68°E), straddling the 85 parallel and 270th meridian, host significant zones in permanent shadow, including permanently shadowed regions (PSRs) along it's southern wall and floor (left) the host the lowest temperatures yet recorded in the solar system, 24°K. LROC Quickmap, 250 meter resolution, orthographic projection of the Moon's north pole and vicinity [NASA/GSFC/Arizona State University].

Says Jordan, “Sparking is a process in which electrons, released from the regolith grains by strong electric fields, race through the material so quickly that they vaporize little channels.” Repeated sparking with each large solar storm could gradually grow these channels large enough to fragment the grains, disintegrating the regolith into smaller particles of distinct minerals, Jordan and colleagues hypothesize.

The next phase of this research will involve investigating whether other instruments aboard LRO could detect evidence for sparking in lunar regolith, as well as improving the model to better understand the process and its consequences.

“If breakdown weathering occurs on the moon, then it has important implications for our understanding of the evolution of planetary surfaces in the solar system, especially in extremely cold regions that are exposed to harsh radiation from space,” says coauthor Timothy Stubbs of the NASA Goddard Space Flight Center.