On Monday 26 September, NASA announced that it has discovered ‘surprising evidence of activity that may be related to the presence of a subsurface ocean on Europa’.
This evidence is in the form of UV images captured by the Hubble Space telescope, which suggest that there might be water jets that are emanating from ‘cryogeysers’ on Europa’s surface. These are in the same place where previous infrared spectroscopy detected excess hydrogen and oxygen, possibly formed as water molecules break up when they are fired into space and then fall back towards the moon (where this oxygen may have built up to give Europa its thin oxygen atmosphere). Only now has evidence for the presence of cryogeysers on Europa been put forward from a second seemingly independent source.
The action of ice plate tectonics above a liquid water ocean has been speculated before as an explanation for Europa’s smooth surface. A large body of water such as a subsurface ocean or lake is also without doubt essential for cellular life. However, it could be the presence of jets themselves which is most important when speculating if life could have developed on Europa. This is because the presence of jets raises the possibility of places with an adequate thermodynamic gradient to drive the building of complex biological molecules from simpler raw materials. For example the jets might be the product of warmer water rising from above systems of hydrothermal vents at a lower boundary, between Europa’s rocky core and its liquid ocean. Heat for such activity might largely be provided by tidal heating due to Europa’s elliptical orbit around the gas giant Jupiter (in effect the moon is constantly deforming as it moves through its orbit).
A similar cryogeyser system on Saturn’s moon Enceladus has been photographed by the Cassini probe, and seafloor ‘white smoker’ hydrothermal vents are currently believed to represent the most likely place where life on Earth may have started, especially because the alkaline nature of the solution ejected into acidic seawater would create a similar proton gradient to that measured across mitochondrial inner membranes in cells (which power the production of ATP and so all metabolism). Could this be the origin of new life?
- Europa moon ‘spewing water jets’ (Jonathon Amos, BBC news article 26 September 2016)
- Forces of Nature (Brian Cox and Andrew Cohen, Chapter 3).
- A Very Short Introduction to Astrobiology (David. C. Catling)