CRaTER: Lunar Proton Albedo Anomalies

Figure 1. Top: Lunar albedo proton yield map (cylindrical projection) with anomalous yield regions labeled “A” through “E”. Regions A (Mare Serenitatis) and B (Oceanus Procellarum) are both centered near the boundaries of mare regions. Regions C, D and E are all in the highlands on the far side of the Moon. Bottom: Visible global composite image from the Lunar Reconnaissance Orbiter Camera (LROC).

LUNAR PROTON ALBEDO ANOMALIES:
SOIL, SURVEYORS, AND STATISTICS

J.K. Wilson, N. Schwadron and H. E. Spence, et al.
Space Science Center
University of New Hampshire, Durham

Introduction: Since the launch of the Lunar Reconnaissance Orbiter (LRO) in 2009, the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) has been mapping albedo protons (~100 MeV) coming from the Moon [1,2].

These protons are produced by nuclear spallation, a consequence of galactic cosmic ray (GCR) bombardment of the lunar regolith. Just as spalled neutrons and gamma rays reveal elemental abundances in the lunar regolith [3-6], albedo protons may be a complimentary method for mapping compositional variations across the Moon’s surface.

Albedo Proton Yield: The CRaTER instrument simultaneously detects albedo protons from the Moon and GCRs arriving from the zenith direction. We divide the number of albedo protons observed over each point on the Moon by the number of GCRs detected over the same location to produce a map of the yield of albedo protons.

We presently find that the lunar maria have an average proton yield which is 0.9% ± 0.3% higher than the average yield in the highlands; this is consistent with some neutron data that shows a similar yield dichotomy due to differences in the average atomic weight between mare regolith and highland regolith [7].

Map Features: There are cases where two or more adjacent pixels (15° × 15°) in the map have significantly anomalous yields above or below the mean.

These include two high-yielding regions in the maria, and three low-yielding regions in the far-side highlands. Some of the regions could be artifacts of Poisson noise, but for completeness we consider possible effects from compositional anomalies in the lunar regolith, including pyroclastic flows, antipodes of fresh craters, and so-called "red spots" which are associated with volcanic domes. We also consider man-made landers and crash sites that may have brought elements not normally found in the lunar regolith.

References: [1] Wilson, J. K. et al. (2012) JGR, 117, E00H23. [2] Spence, H. E. et al. (2012) Space Weather, 11, 643-650. [3] Feldman W. C. et al. (1998) Science, 281, 1496-1500. [4] Gasnault, O. et al. (2001) GRL, 28, 3797-3800. [5] Maurice, S. et al. (2004) JGR, 109, E07S04. [6] Mitrofanov, I. G. et al. (2010) Science, 330, 483-486. [7] Litvak, M. L. et al. (2012) JGR, 117, E00H22.

CRaTER on LRO shows lighter materials may better mitigate cosmic ray health risks

As CRaTER, flying with LRO, closes out a fourth year in lunar orbit, long duration exposure to cosmic rays while traveling within and beyond Earth's magnetic field shows materials lighter than traditional aluminum and titanium alloyed hulls may reduce the probability of Radiation Exposure Induced Death (REID) [NASA/GSFC/UHN/SwRI].

University of New Hampshire - Durham –- Space scientists from the University of New Hampshire (UNH) and the Southwest Research Institute (SwRI) report that data gathered by NASA’s Lunar Reconnaissance Orbiter (LRO) show lighter materials like plastics provide effective shielding against the radiation hazards faced by astronauts during extended space travel. The finding could help reduce health risks to humans on future missions into deep space.

Aluminum has always been the primary material in spacecraft construction, but it provides relatively little protection against high-energy cosmic rays and can add so much mass to spacecraft that they become cost-prohibitive to launch.

The scientists have published their findings online in the American Geophysical Union journal Space Weather. Titled “Measurements of Galactic Cosmic Ray Shielding with the CRaTER Instrument,” the work is based on observations made by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on board the LRO spacecraft. Lead author of the paper is Cary Zeitlin (zeitlin@boulder.swri.edu) of the SwRI Earth, Oceans, and Space Department at UNH. Co-author Nathan Schwadron of the UNH Institute for the Study of Earth, Oceans, and Space is the principal investigator for CRaTER.

“This is the first study using observations from space to confirm what has been thought for some time—that plastics and other lightweight materials are pound-for-pound more effective for shielding against cosmic radiation than aluminum," Zeitlin said. "Shielding can’t entirely solve the radiation exposure problem in deep space, but there are clear differences in effectiveness of different materials.”

The plastic-aluminum comparison was made in earlier ground-based tests using beams of heavy particles to simulate cosmic rays. “The shielding effectiveness of the plastic in space is very much in line with what we discovered from the beam experiments, so we’ve gained a lot of confidence in the conclusions we drew from that work,” says Zeitlin. “Anything with high hydrogen content, including water, would work well.”

The space-based results were a product of CRaTER’s ability to accurately gauge the radiation dose of cosmic rays after passing through a material known as “tissue-equivalent plastic,” which simulates human muscle tissue. 

Prior to CRaTER and recent measurements by the Radiation Assessment Detector (RAD) on the Mars rover Curiosity, the effects of thick shielding on cosmic rays had only been simulated in computer models and in particle accelerators, with little observational data from deep space.

The CRaTER observations have validated the models and the ground-based measurements, meaning that lightweight shielding materials could safely be used for long missions, provided their structural properties can be made adequate to withstand the rigors of spaceflight.

Since LRO’s launch in June 2009, the CRaTER instrument has been measuring energetic charged particles— often very heavy and spectacularly energetic particles traveling at nearly the speed of light and cause detrimental health effects—from galactic cosmic rays and solar particle events (SPE's). 

Fortunately, Earth’s thick atmosphere and strong magnetic field provide adequate shielding against these dangerous high-energy particles.

To view the Space Weather article (behind academic pay wall), visit http://onlinelibrary.wiley.com/doi/10.1002/swe.20043/abstract

For more on the CRaTER instrument, visit http://crater.sr.unh.edu/ and for the LRO mission visit http://lunar.gsfc.nasa.gov/mission.html.

CRaTER team at UNH

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