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Radiation Environments at Solar System Destinations: Exploration Constraints and Scientific Discovery

Harlan Spence
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Abstract Text: 

We examine the energetic particle ionizing radiation environments at airless planetary surfaces throughout the solar system. Energetic charged particles fill interplanetary space and bathe the environments of planetary objects with a ceaseless source of sometimes powerful but always ever-present ionizing radiation. In turn, these charged particles interact with planetary bodies in various ways, depending upon the properties of the body as well as upon the nature of the charged particles themselves. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaisance Orbiter (LRO), launched in 2009, measures and constrains the ways by which the lunar surface is influenced by these energetic particles. In this paper, we briefly review some of these mechanisms and how they operate at the Moon, and then compare and contrast the radiation environments at other atmospherereless planetary objects within our solar system. In particular, we explore two primary sources of ionizing radiation, galactic cosmic rays (GCR) and solar energetic particles (SEP), in the environments of planetary objects that have weak or absent atmospheres and intrinsic magnetic fields. We use scaling relationships with heliocentric distance to estimate their intensity, which then serves as a basis for estimating and ultimately validating the relative importance of various energetic particle and planetary surface physical interactions. Finally, we focus on one element of this for lunar explorations/science which speaks to the discoveries we continue to make and the mysteries these discoveries often reveal. In May and November 2015, the CRaTER instrument on LRO collected 35 hours of data with its field of view pointed at the lunar horizon, resulting in detections of grazing-angle albedo protons from the Moon. We find that the flux of grazing-angle albedo protons varies with local time of day, with the dawn horizon yielding twice the flux of protons as the evening horizon. This discrepancy may result from forward-scattering knock-on collisions of cosmic rays with protons (hydrogen) which are more abundant in the lunar regolith at sunrise than at sunset.

M. Darby Dyar, Sridhar Mahadevan
SSERVI Identifier: 

Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."