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Project Progress

We worked to characterize the intermittent aspects of stellar radiation fields, namely flares and stellar cosmic rays, and their effects on biosignature detection, habitability, and space exploration.

X-ray Flares and Astronaut Lunar EVA Risk: With current spacesuit designs, hard solar X-ray flares should expose astronauts on lunar extravehicular activities (EVAs) to hazardous acute biological doses every few months, yet have never been investigated as a health risk. Radiative transfer calculations and observed statistics of solar flares were used to show that acute doses of 0.1 Gray, associated with various health risks, should occur with a probability of about 3% per day of accumulated EVA inside the current spacesuit. The rapid onset and lack of reliable precursor events or prediction techniques, requires strategies for quick retreat from X-ray flares. We find that polymer shielding of reasonable areal density is untenable as a shield for high-energy photons. The most efficient protection is afforded by high-atomic number articulated or detachable shields, with an areal density at least 7 kg per square meter for an aluminum shield.

[D. S. Smith and J. M. Scalo, X-ray flares and astronaut risk during extravehicular activity, submitted.]

X-ray Flares and Martian Surface Habitability and EVA Risk: We also studied the sterilization of organisms on the Martian surface due to strong solar X-ray flares. Using a Monte-Carlo code for the radiation transport, biological doses were calculated for a parameterized flare spectrum by integrating over the opacity of water. Averaging over the distribution of spectral slopes, a mean time between solar flares producing a given biological dose of ionizing radiation on Mars was calculated. A comparison with lethal doses for a variety of terrestrial organisms gives mean times between lethal flares that vary between decades for human mutations to 0.5 Myr for D. radiodurans. Such doses require very large total flare energies, which are observed on other stars. Any extant Martian life may occupy a subsurface habitat, with soil columns required to provide 1/e shield estimated to be 4-8 g cm-2. The risk for Martian astronaut EVAs is marginal, but underscores the conflicting solutions for lightweight protection from energetic particles and X-rays.

[D. S. Smith and J. Scalo, Solar X-ray flare hazards on the surface of Mars, Planetary and Space Science, in press.]

Focusing on cosmic ray radiation and solar particle events, Schneider used the NASA HZETRN transport code to simulate the Martian surface high-energy radiation environment and derive dose equivalent estimates for all charged and neutral particle radiation fields. Spatial variations in surface radiation doses were also explored as a function of different surface compositions (polar water and CO2 ice, olivine, enstatite/jarosite and basaltic), and as a function of surface topography and therefore atmospheric column depth. This research informs the potential surface habitability of current and ancient Mars, and is relevant to future human exploration.

Cosmic-Ray Flux Variation in the Habitable Zone: The variation of cosmic-ray flux due to declining solar activity and especially passage of the Sun and other stars through the interstellar medium was studied using a simple model for the heliosphere, including charge exchange, and for cosmic ray diffusion. Major results are: 1. The clouds that cause the most change in cosmic-ray flux for any planetary system are the low-density diffuse neutral hydrogen clouds. The cosmic ray attenuation is already saturated by the time molecular cloud densities are reached. Considering also their low frequency, molecular clouds are unimportant compared to the low-density clouds like those in the immediate solar neighborhood. 2. Very large variations in the low-energy cosmic-ray flux should occur for solar mass stars, but lower-mass stars are better shielded, both by having larger astrospheres because of their higher average activity level, and because the habitable zone is so close to the star that cosmic rays are greatly attenuated in traversing such a large distance in the astrosphere. We are attempting to reconstruct the cosmic-ray history of the Earth during its past journey through the Local Bubble, using HST archives and new observations specially designed to make a catalogue of clouds along the Sun’s historical trajectory.

[J. M. Scalo and D. S. Smith. Descreening Densities for the Sun and Lower-Mass Stars. Astrobiology, submitted. ]

[J. M. Scalo, D. S. Smith, S. Redfield. Reconstruction of the Cosmic-Ray Flux History of the Sun Using Absorption Line Observations of Clouds along the Sun’s Past Trajectory. In preparation.]

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