Article Title: The Resilience of Life to Astrophysical Events
Summary:
The article considers a likelihood of total sterilization of an Earth-like planet from three astrophysical sources – supernovae, gamma-ray bursts, large asteroid impacts, and passing-by stars.
To assess such probabilities authors examine what cataclysmic events could lead to the annihilation of not just human life, but also extremophiles, through the boiling of all water in Earth’s oceans.
Tardigrades can survive for a few minutes at temperatures as low as - 272 C or as high as 150 C. They withstand pressures from virtually 0 atm in up to 1200 atm at the bottom of the ocean, and resistant to radiation levels up to 6000 Gy.
Authors point out that although land life is somewhat fragile, ocean biosphere is extremely rich and sea species are protected by waters from huge doses of radiation, life might survive next to the volcanic vents, as well as ocean eventually will restore the atmosphere.
The tardigrade is capable of withstanding over 6000 Gy and 100 C. If our oceans are more than a few meters deep, this exceeds the threshold energy at which the oceans would boil before radiation would kill the tardigrade. Authors, therefore, consider temperature increase as the primary source of sterilization.
To raise the ocean’s temperature by T, we require an asteroid of mass over 1.7 x 10^18 kg.
The largest observed asteroids in the Solar System are Vesta and Pallas, with masses of 2.7 x 10^20 kg and 2.2 x 10^20 kg respectively. There are only 17 other known asteroids of sufficient mass, and a few dwarf planets, the most massive ones being Eris and Pluto. The figure below is a model for the impact rate of asteroids as a function of the mass.
Supernova
For an increase of 100 C in the ocean temperatures, authors point out, we would need a GRB within about 13.8 pc; again, this is an upper limit. Furthermore, although there is a dependence on the mass of the planet, this dependence is quite weak. None of the stars in the Alpha Centauri system are large enough to go supernova. The nearest potential supernova is the IK Pegasi system, approximately 45 pc away, which is three orders of magnitude farther than the estimated sterilization radius. To assess the relative risk faced by any planet in our galaxy, it is approximated in the paper the odds of a close enough supernova happening over a time span of 109 years. This is normalized to the global supernova rate in the Milky Way. Nonetheless, this rate seems almost insignificant even close to the galactic core, reaching only around 1% of planets being sterilized. This number is extremely small and we can conclude that such an event is unlikely.
The analysis has focussed on providing an absolute upper bound for the rate of complete sterilization of an Earth-like planet during its evolution, by considering the required events that would lead to the death of the hardiest species on Earth. With such assumptions, authors establish the probability of total sterilization of an Earth-like planet to be less than 10^(-7) per billion years.
Interestingly, the paper points out that we do not yet fully understand the mechanism in which life emerges, so our next step may be to search for tardigrades on Europa and Enceladus.
Comments