Effects of sterilizing doses of gamma radiation on Mars analog rocks and minerals

Abstract

Abstract. Rock and soil samples from the planet Mars are due to be returned to Earth within a decade. Martian samples initially will be tested for evidence of life and biological hazard under strict biological containment. Wider distribution of samples for organic and inorganic analysis may occur only if neither evidence of life nor hazard is detected, or if the samples are first sterilized. We subjected a range of Mars analog rocks and minerals to high doses of gamma radiation in order to determine the effects of gamma sterilization on the samples' isotopic, chemical, and physical properties. Gamma photons from 6øCo (1.17 and 1.33 MeV) in doses as high as 3 x 107 rads did not induce radioactivity in the samples and produced no measurable changes in their isotopic and chemical compositions. This level of irradiation also produced no measurable changes in the crystallographic structure of any sample, the surface areas of soil analogs, or the fluid inclusion homogenization temperature of quartz. The only detectable effects of irradiation were dose-dependent changes in the visible and near-infrared spectral region (e.g., discoloration and darkening of quartz and halite and an increase in albedo of carbonates) and increases in the thermoluminescence of quartz and plagioclase. If samples returned from Mars require biological sterilization, gamma irradiation provides a feasible option. BackgroundMartian rock and soil, collected by robotic spacecraft, will be returned to terrestrial laboratories early in the next century. The return of documented samples, carefully collected and preserved, will be a major step in the search for evidence of Martian life. Martian conditions, including the lack of organic matehal, subfreezing temperatures, high flux of solar ultraviolet radiation, and strongly oxidizing chemical species in the soil, severely limit the survival of organisms at or near the surface [Klein, 1998] inCenters for Disease Control and Prevention, Atlanta, Georgia.•University of Portsmouth, Portsmouth, England.Copyright 1999 by the American Geophysical Union. Paper number 1999JE001064.0148-0227/99/1999JE001064509.00 specialized organisms at low concentrations in the returned samples is conceivable [Clark, 1998].Current planetary protection strategies call for the samples to be immediately placed into biological containment and tested for signs of present or past life and biological hazard [DeVincenzi et al., 1998]. It is recommended that "Controlled distribution of unsterilized materials from Mars should occur only if rigorous analyses determine that the materials do not constitute a biological hazard. If any portion of the sample is removed from containment prior to completion of these analyses it should first be sterilized" [Space Studies Board, 1997]. While sterilization of Mars samples may not be required, an acceptable method must be available before the samples are returned to Earth.A variety of sterilization techniques have been used or proposed for spacecraft missions to Mars. These include dry heating to t...

Effects of gamma-sterilization on the physico-chemical properties of natural sediments

“…Though autoclaving tends to be more commonly used, some studies have found sterilization by exposure to γ-irradiation emitted by cobalt-60 to be less destructive and more effective (Bowen and Rovira, 1961;McLaren, 1969;Allen et al, 1999;;Lotrario et al, 1995;Trevors, 1996;McNamara et al, 2003;Herbert et al, 2005). Small doses of γ-irradiation are capable of sterilizing large soil samples without increasing soil temperature or pressure.…”

“…For determining abiotic reaction rates in mine tailings, γ-radiation was suggested as the most effective sterilization method (Herbert et al, 2005). Sterilization by γ-irradiation was recently popularized when it was suggested as a suitable technique for sterilization of Mars samples (Allen et al, 1999). Experiments showed that a dose of 30 Mrad of γirradiation caused no measurable changes in isotopic signature, chemical composition, crystallographic structure, or homogenization temperatures of fluid inclusions in rocks and minerals analogous to those found on Mars.…”

Planetary protection for a Europa surface sample return: The Ice Clipper mission

“…The radiation levels would be in excess of the sterilizing dose of even the most radiation resistant organisms. The radiation dose received by the samples is in excess of the radiation does (10 -100 MRads) proposed as a method to sterilize samples returned from Mars (Allen et al 1999). In that respect any biological hazard is expected to be nil.…”

Long‐Term Reactive Solute Transport in an Aquitard Using a Centrifuge Model

“…Gamma sterilization was selected over chemical sterilization to better preserve the isotopic, chemical, and physical characteristics of the samples. The only change observed in montmorillonite clay after radiation exposure was the reversible loss of interlayer water (Allen et al 1999). Subcores were inserted into Teflon core holders (52 mm length, 33 mm inner diameter) using a mechanical cutting and loading device.…”