Planetary Science Research Discoveries

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FEATURE ARTICLEposted December 17, 1999gamma rays

Zapping Mars Rocks with Gamma Rays
Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

Because we do not know what deadly microorganisms might be lurking inside samples returned from Mars, the samples will either have to be sterilized before release or kept in isolation until biological studies declare them safe. One way to execute microorganisms is with radiation, such as gamma rays. Although quite effective in snuffing out bacteria and viruses, gamma rays might also affect the mineralogical, chemical, and isotopic compositions of the zapped rocks and soils. Carl Allen (Lockheed Martin Space Operations, Houston) and a team of 18 other analysts tested the effect of gamma rays on rock and mineral samples like those we expect on Mars. Except for some darkening of some minerals, high doses of gamma rays had no significant effect on the rocks, making gamma radiation a feasible option for sterilizing samples returned from Mars.

Reference: Allen, Carlton C. and 18 others, 1999, Effects of sterilizing doses of gamma radiation on Mars analog rocks and minerals. Journal of Geophysical Research, v. 104, p. 27,043-27,066.

The Possibility of Tiny, Hostile Extraterrestrials

The Andromeda Strain by Michael Crichton is a scary and, some people fear, realistic story about a dangerous disease that fell to Earth from a satellite in space. If life exists on Mars, the samples we expect to return during the next decade might be teeming with microorganisms, some of which might be out to get us here on Earth. Although we have samples of Mars already (Martian meteorites) with no apparent ill effects, the fear and the possibility are still there, so the samples will be treated as if they were biohazards. The Space Studies Board, an arm of the National Academy of Sciences, studied the issue and stated in a report 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."
biohazard sign The trouble with biohazards is that they have to be studied inside a containment facility that ensures that the organisms cannot escape into the surrounding environment. This means that all studies of the non-biological nature of the samples will have to wait until the samples are deemed safe by a team of biologists, take place inside the containment facility, or use sterilized samples.
Waiting would be torture for scientists who have waited for years to study samples returned from Mars, and would deprive the public of the excitement of their discoveries. Doing all the work inside the special laboratory would limit the number of scientists involved to a small, elite (and possibly elitist) group. The samples should be studied by the best scientists with the best equipment all over the world. Besides, some equipment is too large to be housed inside the containment facility.

Sterilization of part of each sample is an interesting alternative, but it needs to be done without changing the nature of the samples. Dry or steam heating the samples can alter the structures of some minerals and result in loss of Martian gases. Exposure to ultraviolet light or plasmas would sterilize only the surfaces of the samples, leaving the interiors potentially crawling with Andromeda-strain-like organisms. Gamma rays might do the trick with minimal damage, but no studies have been done until now.

Effectiveness of Gamma Rays as Germ Killers

Radiation causes ionization inside tissues, which damages cells, including their DNA. A common measure of the amount of radiation is the rad (radiation absorbed dose). One rad is the amount of radiation delivered by about ten chest x-rays.

Studies have shown that some bacteria are more resistant to radiation than others. The record holder for radiation resistance is deinococcus radiodurans, which requires more than one million rads to decrease its concentration in a culture to 0.1% of its starting value. Increasing the dose by a factor of ten increases the effectiveness by about a factor of 8000. Common bacteria, such as Escherichia coli, require about 100,000 rads to knock off. Viruses are rendered inactive by about a million rads. Even the feared Ebola, Lassa, and Marburg viruses are deactivated with 700,000 rads. So, assuming Martian organisms have the same resistance to gamma radiation, doses of a million or more rads should sterilize a sample, though caution might call for much larger doses, perhaps 10 million rads.

The Experiments

To cover the range in which bacteria and viruses are killed, Carl Allen zapped samples with four different levels of radiation: 300,000 rads, 3,000,000 rads, 30,000,000 rads, and 100,000,000 rads. He used the High Dose Research Irradiator at the Centers for Disease Control and Prevention (located in Atlanta). This is a six-foot tall box lined with lead (see photo on left). Allen used the radioactive isotope cobalt-60 to supply gamma rays, which produced 31,500 rads per minute. (Lucky that box is lined with lead!) Samples were exposed inside the chamber for as short as 9 minutes to as long as 53 hours.

The samples used in the experiments represented a variety of geological materials expected to be found on Mars: rock (basalt, carbonaceous chondrite, chert), minerals (quartz, feldspar, olivine, pyroxene, clay minerals, halite, aragonite, gypsum), and a simulated Mars soil. Both before and after irradiation, these materials were analyzed by numerous techniques. These measured chemical compositions, the atomic structures of the minerals, and optical properties. The techniques and results are briefly outlined below. (Allen's team did not study the effect of gamma rays on organic compounds or on bacteria in their samples. However, such studies would be done on non-sterilized Martian samples inside the containment laboratory as part of the biological assessment.)

What It Means for Martian Samples

NASA is planning two sample-return missions to Mars during the next ten years. The returned samples will contain cores and soil collected by a rover, as well as core material collected by a drill on the lander. The goals of the missions are to search for evidence that the conditions for life existed on Mars (especially standing water), the raw ingredients for life (organic compounds), and for evidence for life (fossils, diagnostic organic compounds, minerals produced by organisms). Sample return mission will also provide rocks and soil for inorganic analysis. Landing sites will be chosen to maximize the chances for finding life and studying the history of the Martian climate.

rover picture of Mars showing Twin Peaks

Rocks and soil, like those at the Pathfinder landing site shown above, will be collected by future sample-return missions to Mars. Some might harbor microscopic organisms. These will be of intense scientific interest, but there is a small chance that they could threaten life on Earth if released into the terrestrial environment.

The samples will be studied in a special biological laboratory to search for Martian organisms. This will take months. However, some samples could be released soon after return to Earth for non-biological studies if the samples were sterilized. Carl Allen and his colleagues have shown that very high doses of gamma radiation, known to be quite effective at snuffing out terrestrial bacteria and viruses, will not harm most of the studies planned for the returned samples. If biologists conclude that gamma radiation is a safe way to sterilize Martian samples, then work on the chemical and mineralogical compositions and ages of the samples could begin within weeks after the samples were returned to earth. Allen notes that their studies were done with cobalt-60. Radioactive elements that emit gamma rays with much higher energy than those spewed out by cobalt-60 might cause more alteration than they observed in their experiments. Thus, their conclusions apply only to radiation like that emitted from cobalt-60. Fortunately, sterilization using cobalt-60 is a standard practice. Gamma sterilizers are available commercially and are a proven technology.


Allen, Carlton C. and 18 others, 1999, Effects of sterilizing doses of gamma radiation on Mars analog rocks and minerals. Journal of Geophysical Research, v. 104, p. 27,043-27,066. Athena (Mars Rover) homepage at Cornell University. Space Studies Board, 1997, Mars Sample Return Issues and Recommendations, National Research Council, Washington, DC. on-line from the National Academy of Sciences. Thermal Emission Spectrometer on Mars Global Surveyor: homepage at Arizona State University.

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