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2005 Annual Science Report

Virtual Planetary Laboratory (JPL/CalTech) Reporting  |  JUL 2004 – JUN 2005

The Astrophysical Environment and Planetary Habitability

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

We have examined the effects of transient astronomical radiation events, such as stellar flares and Galactic cosmic rays, on planetary atmospheres and biospheres over time. Work this year focused on identification and quantification of sources of cosmic-ray (CR) variations over a large range of timescales, including parent star CR variations, passage of planetary systems through interstellar density structure, and variations in the star formation rate during the parent star’s Galactic orbit.

We calculated the statistics of amplitudes, timing, and durations of CR flux and spectrum variations on terrestrial-like planets with different parent star types, ages and Galactic locations. We modeled the variation in screening of lower-energy CRs as the astrosphere (analogue of heliosphere) expands and contracts during passage through the interstellar medium (ISM). We use a simple pressure balance model for the astrosphere size, a convection-diffusion model for CR transport, and an ISM model based on 3D MHD simulations that realistically captures the density, velocity, and magnetic field variations that control the astrosphere size. Planetary magnetospheric screening is included using empirical terrestrial magnetic field variation statistics, with the planetary field intensity a parameter. Statistics for cosmic ray fluxes were accumulated using ~106 planetary system trajectories through model clouds of different mean densities.

Scalo, in collaboration with P. Padoan (UCSD), discovered a new source of CR flux variation, intrinsic to CR propagation through the ISM which should vary by one to two orders of magnitude in the cool ISM. The effect of this flux variation on planetary systems passing through the ISM was compared to the astrospherically-induced variations.

A modestly enhanced CR flux would contribute significantly to the exogenous and total DNA damage and mutation rate, and may also modify atmospheric chemistry. Population genetics models are used with stochastic mutation rates and selection coefficients to examine the effects on genomic diversity and specific problems like mutation fixation and extinction timescales and the rate of evolution. The importance of CR variations on population genetics compared to UV variations due to flaring in young and low-mass stars is being compared.

    John Scalo
    Project Investigator
    Objective 4.3
    Effects of extraterrestrial events upon the biosphere

    Objective 6.2
    Adaptation and evolution of life beyond Earth

    Objective 7.2
    Biosignatures to be sought in nearby planetary systems