Saturday, 26 January 2019

Could microbes from Earth have reached other planets in the Solar System?

The theory of panspermia postulates that life could be carried from planet to planet by microbes within asteroids; with some variants on this idea going as far as to suggest that life may not have originated on Earth, but on an early, wet Mars or even outside the Solar System. While ideas about the extra-terrestrial origins of life on Earth are impossible to verify at the current time, there is certainly good evidence that a proportion of asteroidal material has originated on planets, thought to have been ejected into space due to powerful bolide impacts, and a number of meteorites found on Earth are considered to be of Martian and Lunar origin.

In a paper published in the American Journal of Astronomy and Astrophysics on 27 November 2018, Martin Beech of Campion College and the Department Physics at the University of Regina, Ian Coulson of the Department Geology at the University of Regina, and Mark Comte, also of the Department Physics at the University of Regina, discuss the possibility of microbes from Earth having reached other planets in the Solar System, as a means of assessing the overall likelihood of microbes moving from one planet to another.

Beech et al. first attempted to assess the amount of material to have left the Earth due to impacts by extra-terrestrial bodies. In order to do this, they arbitrarily chose to look at continental impact craters less more than 5 km across and less than 550 million years old (i.e. Cambrian or younger). This is a less than complete data set, as two thirds of the Earth’s surface is covered by ocean and the Earth is about 4.5 billion years old, with even the most conservative estimates suggesting that life of some sort has been around for more than two billion years, but it is unlikely that many significant impacts that fit this criteria have yet to be discovered, though Beech et al note that determining the age and original size of impact craters is not a precise science, to the results of this search cannot be thought to be completely accurate.

A total of 96 impact criteria that fit Beech et al.’s criteria were found. These range in age from the 8 km diameter Neugrund Crater in Estonia, which is estimated to be about 535 million years old, and the 6 km Foelsche Crater in Australia's Northern Territory, which is thought to be over 545 million years old, to the 14 km Zhamanshin Crater in Kazakhstan, with an estimated age of 900 000 years and the 10.5 km Bosumtwi Crater in Ghana, which is thought to be about 1 070 000 years old.

The Bosumtwi Crater in Ghana, 10.5 km in diameter and estimated to be 1 070 000 years old. Wikiwand.

The largest crater in the study is the 65-million-year old Chicxulub Crater in Mexico, at 150 km in diameter, followed by the 35-million-yeat-old Popigai Crater in  Siberia, at 90 km, and the 214-year-old Manicouagan Crater in Quebec, at 85 km. The smallest are the 500-million-year-old Gardnos Crater in Norway, the 500-milion-year-old Mizarai Crater in Lithuania and the 250-million-year-old Gow Lake Crater in Saskatchwan, all at 5 km in diameter.

Beech et al. calculate that in past 550 million years around 10 000 000 000 000 kg of material has been ejected from the Earth into the Inner Solar System, potentially reaching as far as Saturn. Of this material 67% came from a single impact, the event that generated the Chicxulub Crater in Mexico at the End of the Cretaceous.

An atist's impression of the Chicxulub Crater shortly after it formed. Detlev Van Ravenswaay/Science.

Rocks thrown from the Earth are likely to contain sediments within them, which could potentially be carried to other planets within the Solar System, though whether they could survive there is less certain. Mars is widely thought to have had a warmer, wetter, past (though not in the last 550 million years ago), and it is possible that habitable conditions (for microbes) persist beneath the surface, so potentially microbes from Earth could survive on Mars should they reach it. Venus, in contrast, has a quite uninhabitable surface, but there are zones within the atmosphere where microbes could possibly survive. The surface of the Moon is quite sterile today, and is likely to have been so for more than 550 million years, but the early Moon may well have harboured an atmosphere, and possibly even oceans, and it is possible that such an early Moon could have been colonised by microbes from Earth. Neither Jupiter nor Saturn is likely to be capable of supporting Earthly microbes, but both planets have moons which are thought to harbour oceans (Europa, Enceladus and Titan) and which could potentially have been colonised by microbes from Earth.

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