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Within the coming years spectroscopy will likely be employed to identify molecules that are indicative of life in the atmosphere of exoplanets1. In the context of the search for extraterrestrial life, it is useful to establish the necessary conditions for life to be present for such observations. Broadly speaking this relies upon two ingredients. The first is an unknown quantity–the fraction of planets on which life begins. The causes of the emergence of life on Earth are not understood, and thus we do not have a complete theory for predicting where life may begin elsewhere. The second is the probability that life has persisted from its inception to observation. In this work we will show that this is highly likely, as events which could lead to life being completely eradicated are rare. To establish this we break from the usual study in the literature2,3,4,5,6 of the possible paths to ending human life, and broaden the analysis to consider those astrophysical events which could rather remove all life by analysing the most resilient of species–tardigrades.

Tardigrades can survive for a few minutes at temperatures as low as -272?°C or as high as 150?°C, and -20?°C for decades7, 8. They withstand pressures from virtually 0?atm in space9 up to 1200?atm at the bottom of the Marianas Trench10. They are also resistant to radiation levels ~5000–6200?Gy11. For complete sterilisation we must establish the necessary event to kill all such creatures.

We consider three types of astrophysical events which could constitute a threat to the continuation of our chosen life forms: large asteroid impact, supernovae (SNe), and gamma-ray bursts (GRBs). GRBs and SNe can be deadly due to the lethal doses of radiation and in particular the shock wave associated with the burst. Radiation can cause the depletion of the ozone layer, removing the shield that protects us from cosmic radiation2, 12, 13.

The effects of gamma-ray bursts (GRBs) on humans and land-based life could be disastrous as the eradication of the ozone layer would leave us exposed to deadly levels of radiation2. However, in such circumstances life could continue below the ground. Significantly, several marine species would not be adversely affected, as the large body of water would provide shielding. Even the complete loss of the atmosphere would not have an effect on species living at the ocean’s floor. The impact of a large asteroid could lead to an “impact winter”, in which the surface of the planet receives less sunlight and temperatures drop. This would prove catastrophic for life dependent on sunlight, but around volcanic vents in the deep ocean life would be unaffected. Similarly, an increase in pressure, or acidity spread across the entirety of the (deep) ocean is an unlikely scenario for extinction. The physical processes by which ocean pressure could significantly increase involve increasing planetary mass; such impacts would first lead to extreme heating. Even following extreme events, spreading acidity through the entire ocean is unlikely. The removal of the atmosphere would also lead to mass extinction. However, following such an event the remaining ocean water would form a new atmosphere below which oceans could still form. The energy requirements for total sterilisation of the planet through atmospheric removal are significantly greater than those for boiling the oceans, so the threat of atmospheric removal is contained within that of oceans boiling. We are therefore led to consider death due to heat or radiation.



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