Monday, 17 November 2014

Does dirty snowball keep life’s ingredients on ice?

Mankind has semi-successfully landed on a comet for the first time last week. By semi-successfully, I mean to say, the European Space Agency’s (ESA) Philae lander (pictured) touched down not once, not twice, but thrice on the surface of catchily-named Comet 67P/Churyumov-Gerasimenko (hereon referred to as 67P). Philae’s mission, as part of the overall Rosetta mission, was to examine the comet’s magnetic characteristics, internal structure, and various chemical compositions (on the surface, in the soil and what have you).

[caption id="attachment_86" align="alignleft" width="300"]Philae-ing through the sky One giant bounce: Philae Lander bounced from comet's surface to land again elsewhere. Source: ESA/ATG medialab[/caption]

Among the chemicals being sought after are organic molecules, containing carbon, oxygen, and hydrogen – the key elements for the building blocks of life. Carbon, oxygen, and hydrogen atoms are among the most common elements in the universe, so their presence on 67P is expected. The big question is whether these atoms exist in the form of organic molecules, and could comets have brought these molecules, as well as water in the form of ice, to Earth in the early solar system? We might have to wait a while to get the full story from Philae, as its current position in the shadow of a cliff means it has to power down until its solar batteries can be recharged.

Philae is not equipped to look for life itself on 67P, after all, it is unlikely anything could actually live on a rock without an atmosphere. However, that does not mean a hardy living organism couldn’t merely survive there. Take the wee, hardy tardigrade for example. It can endure high and low temperatures. It can survive the loss of most of its bodily water. It can even shake off exposure to the vacuum of space and keep on truckin’. Bacterial spores are another example of living beings that can survive great extremes. But these creatures can only withstand extremes, they cannot live and breed under these conditions in the long-term. Scientists, like DNA co-discoverer Francis Crick, though that life could have been seeded on Earth via comets containing DNA or even dormant bacteria frozen in the ice or buried in the soil. This hypothesis is called ‘panspermia’.

Panspermia, as an idea, has provided much fuel for science fiction writers and interesting speculation among the scientific community. Evolutionary biologists tend to pooh-pooh the idea of life arriving on Earth from the stars. They argue that the early Earth could easily have allowed life to spring up from self-replicating molecules and flourish in a warm, organic molecule-filled primordial soup. Conversely, some proponents of panspermia say the early Earth was a far too volatile place, with large asteroids sterilising the surface of our newly-formed world at intervals that would prevent life from evolving there.

Panspermia enthusiasts hypothesise that life formed elsewhere in the universe or solar system (like Mars) and somehow got sent to earth on meteorites. This isn’t as far-fetched as it sounds. Asteroid collisions into planet surfaces tend to eject some of the crust back into space, such is the force of impact. It’s possible some chunks of rock from a distant world, containing alien bacteria, found a new home on our warm, watery Earth.

On a less radical level than panspermia, some scientists hypothesise that life did evolve on Earth but, because of early asteroid impacts, got ejected into space in rocks while the surface of the Earth burned. The Earth cooled again over hundreds of years and later came across one or more of these bacteria-harbouring rocks which fell again as meteorites, seeding the Earth with the life it had previously evolved. This process could happen any number of times and can account for the development of life despite Earth’s violent past with early asteroids.

There are is yet another school of thought regarding celestial bodies influencing life on Earth which proposes a far less radical supposition than the others – the building blocks for life originally came from space. Organic molecules, the precursors of amino acids and DNA, may have come to Earth on asteroids and comets, providing the recipe for life that was cooked in Earth’s primordial soup. Asteroids and comets are already thought of as being vehicles that brought water to the early Earth, so why not other molecules?

Philae and Rosetta, sent out ten years ago, have already begun trying to answer these questions by analysing the light from 67P. Data from soil analysis is eagerly awaited from the Philae lander and we’ll soon be closer to understanding the role of asteroids and comets in Earth’s early history.

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