This item was submitted in May 2001 as the Research Article in the "Life in the Universe" Course run by Jodrell Bank, University of Manchester.
Are our nearest living neighbours on one of Jupiter's Moons?
Mankind has for centuries debated on whether it is possible for life to exist somewhere in the universe other than on Earth. Although our eyes are now turned to the possibility of planets similar to the earth existing elsewhere in the cosmos, discoveries made over the past few decades make it likely that we may find life in some form much nearer home, within our own solar system. Current research indicates that perhaps the most likely place to find living organisms within our solar system may be on Jupiter's moon Europa.
Composition of Europa.
Europa is a satellite of Jupiter just slightly smaller than our own moon. The photographs and measurements taken by the Galileo and Voyager spacecraft show that Europa seems to be covered with ice, and spectroscopic analysis indicates that this ice has a significant pure water content (various NASA announcements). The density of Europa has been measured as being far greater than that of pure water, and therefore the whole satellite must have some sort of heavier core overladen with water or ice.
Either the moon has a solid core with a large area of water topped by a thick layer of ice or the core is overlaid with a convecting layer of molten or slushy ice topped by a surface of brittle ice. These are the only two scenarios that would seem to explain the surface markings photographed by NASA spacecraft. The Galileo images appear to show evidence of "near surface" areas of melting ice, plus movements of large blocks of icy crusts very similar to those observed on the earth's ocean ice sheets.
Possible interior models of Europa
Although Europa is cold, with a surface temperature of only 110 degrees Kelvin (110 degrees centigrade above absolute zero) the moon may be internally heated from two possible heat sources. It may be partially warmed, as is the earth, by the decay of radioisotopes in the core. Much more likely is the heat provided by the "tidal heating" provided by the gravitational pull of its parent body Jupiter (Barry 2001).
As well as ice movement occurring due to core heat sources, it also occurs because of Jupiter's pull on Europa's sub surface ocean. Scientists have discovered that the satellite surface is covered in unique cycloidal (curved) cracks and ridges, running through the entire lunar surface. Each arc segment in a cycloidal crack seems to form in 85 hours - the time it takes Europa to complete an orbit of Jupiter. This hints at the near certainty of a totally liquid ocean beneath the icy crust.
As Europa has a slightly non elliptical orbit around Jupiter (plus its gravitational interaction with it's companion moons Gannymede, Callisto and Io) its fabric is likely to be subject to considerable geological and tectonic activities due to both the radioactive and the tidal heating. The sister satellite Io exhibits this type of heating in the form of continuous volcanic activity.
The Surface of Europa.
Photographs of Europa show a heavily cracked surface as if the ice has been continuously moving, perhaps partially re-melting then re-freezing again (Pappalardo 1999). For a moon so near to Jupiter, there are surprisingly few impact craters. This indicates that the ice surface has either been recently renewed, or is in a state of constant renewal, such as would occur if the water were to "well up" before flowing over the surface ice before re-freezing. Some photographs
Ice floes. Dark & bright areas of Europa
indicate that perhaps "hot spots" melt specific areas of the ice sheet, before again re-freezing.
Because Io, Jupiter's innermost satellite, supports highly active volcanic systems driven by internal tidal friction, the suspicion must be that perhaps similar - although less intense activity - may exist in the sub surface of Europa which would explain the apparent "new" nature of the moons icy surface.
How can life exist on Europa?
Although initially it appears as if Europa is a most unlikely place to find any form of life, over the last decade people are beginning to think the seemingly impossible. Recently, planet scientist Richard Green from University of Arizona said "I'd bet there's life on Europa - I wouldn't bet there's life on Mars (April 2000 Wired magazine). Until recently we thought that "life" was impossible in some of the most inhospitable parts of our own planet and that without light for photosynthesis, liquid water and oxygen nothing could survive. Since then, we have discovered places on earth where life is sustained in the dark, and without oxygen using instead the heat and thermal energy from volcanic fluids and water. Does this sound a bit like Europa?
What type of life might be possible on Europa?
If life of some sort were to exist under the ice of Europa due to thermic activity, it is likely be of a fairly basic form such as thermophillic (or heat loving) bacteria such as occurs on earth. Many scientists now speculate that life may have originally arisen in such circumstances here on earth. The ice in some areas of Europa is thought to be fairly thin, and in these places, the liquid water beneath may be close enough to the surface for photosynthesis to occur. Due to this, it may even be possible for algae or other similar forms of life to exist fairly near the surface.
On Europa we believe that the ice is torn apart by some sort of thermic or seismic activity. The resultant crustal movements, may allow material from below the surface to well up and fill the gaps in the ice floes, as can be seen in colouration exhibited in the ice cracks on some Galileo photographs (web site 1).
The infilling material may be warmer, softer ice from below the surface, or water from a subsurface ocean. It is possible for material from the core to be vented to the surface via the cracks or gaps in the surface ice. This shows on Galileo images as reddish brown colouring near some of the cracks. Although life is unlikely to exist on the surface of Europa due to the low temperature, future research may prove that even this is possible under certain circumstances especially as recent observations have shown that Europa has a tenuous atmosphere composed mainly of oxygen. Traces of sulphuric acid have also been discovered on Europa and even though one may imagine that its presence might lessen the chances of life, it should not be forgotten that sulphur and sulphuric acid act as oxidants (energy sources) for some living organisms.
On Earth, all living organisms existed originally only in the oceans and it was not until the end of the Silurian period some 440 million years ago that living creatures ventured onto dry land. Primordial life has had plenty of time to develop under the ice of Europa and it has even been suggested (Hoagland 1980) that the equivalent of Plesiosaurus could be swimming under Europa's ice! However later research (Gaidos 1999) has shown that perhaps Europa lacks the heat sources needed to provide the energy required for the development of complex forms of mulicelular life forms.
How do we find this "life"?
It will be extremely difficult to discover whether life forms exist on Europa for not only has a craft to get to the moon but a lander will also have to be sent to the surface. Estimates for the ice thickness vary from a few kilometers to 200 kilometers. The task of searching beneath the ice poses problems, which are as yet, unanswerable. Initially, an orbiter will be needed to gather more data about the moon, perhaps equipped with laser altimetry equipment (to measure small changes in the moon's shape as it orbits Jupiter). Alternatively, a radar imager could also be used to look through the ice to determine how the crust changes with depth. As the orbiter flies overhead, it might measure how much Europa is "pulling" on the spacecraft at different locations, the amount of pull at each place being dependent on the mass between the centre and the surface of the moon. Using this data from different locations would provide a 3D image of the interior of the moon. The orbiter could also carry an infrared spectrometer to provide information about the chemical composition of Europa's surface, including the presence of organic molecules.
Part of the Europa surface.
The decision, on which instruments will be carried on an orbiter is difficult, as the craft will be limited to a very small payload of perhaps only 20 kilograms (Chyba 1998). The low payload is due to the propulsion problems involved in firstly getting a craft into Jupiter orbit, then braking itself into a low circular orbit of Europa. The craft would also require heavy shielding in order to survive the very high radiation levels from the radiation belts around Jupiter.
The orbiter missions would be followed by craft landing on Europa. These would be used to probe through the ice crust and sample the possible liquid beneath, to determine amongst other things, the temperature, pH levels etc. at different depths. The problem with any search beneath the surface ice would be to avoid contamination of Europa by outside influences. Many scientists feel that full under ice exploration of Europa is decades away (Moonaw 2001).
As well as research taking place for future space missions to Europa, investigations into the possibilities of life on Europa and in other inhospitable habitats continue apace here on earth. We already know that simple organisms can live and flourish in the darkness of the deep oceans near to undersea volcanic vents, in the blistering heat of hot springs and now we even read (Parkes 1998) of lowly bacteria existing to a great depth in the earth's rocky crust.
The "test bed" for future Europa exploration is now taking place under the earth's ice sheets by NASA with a great deal of work being done at Lake Vostok in Antarctica. This lake is a body of water the size of Lake Ontario deep under the ice - which acts as a shield and protects the lake from the cold on the surface. Cores taken from the ice above the lake have shown the presence of bacteria, fungi, spores, diatoms plus other organisms which have not yet been recognised. Investigation of the lake itself is delayed because as yet, methods of exploration without contaminating the water have not yet been perfected.
Location of Lake Vostok
It is thought that Europa and Lake Vostok could be very alike in their condition and anything learned from Vostok will undoubtedly have an effect on future Europa exploration.
Is life possible on Europa? Well, we're hardly likely to find a humanoid in an aqualung swimming along under the ice crust, but there now appears to be more than a fighting chance that some form of living organisms could survive on Europa if a liquid ocean really does exist below the ice. Our nearest living "relatives" really could be living on one of Jupiter's moons.
D A Lane May 2001.
All photo's courtesy of NASA
Barry P L. NASA Science News April 2001
Papalardo,Head, Greely Scientific American Oct 1999
Hoagland R C "The Europa Enigma" Jan 1980 Star & Sky.
Gaidos E "Science Journal" June 1999
Chyba C Paper given to US Geophysical Union Meeting, San Francisco 1998
Moomaw B, Cameron Park, Space Daily, March 7 2001
Parkes John, New Scientist 7 March 1996
Ralph Greenberg "An ocean on Europa" .
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