Measurements by the six probes that have visited the Jovian
system suggest very strongly that Europa has a moderately dense iron nickel core, with
a rocky mantle round it. Above this is
water. It has been established nearly
conclusively that the surface of Europa is water ice with many other
substances. There is also a very high
probability that under this ice there is liquid water. It is quite uncertain how thick the ice is,
and how deep the water underneath is.
Possibly the ice is between 10 Kilometres and 30
kilometres thick, with perhaps 100 Kilometres of liquid water under it. The thicknesses of the different layers are
quite uncertain, but evidence for this basic structure is mounting.
The maximum depth of the Earth’s oceans is roughly 11
kilometres, but most of our oceans and seas are much shallower than this. Although Europa is a little smaller than
Earth’s own Moon, it probably has more water on it than the Earth. Some estimates suggest that the moon Europa has
twice as much water as the planet Earth.
If this structure is correct, there is no solid
connection between the solid core and the solid crust of Europa, and they are
only connected by liquid. This raises
the possibility that the core and the crust could be rotating at different
speeds. Measurements by two of the
probes that have visited the Jovian system have been interpreted by a few
people as indicating that this is happening, and that the core might be
rotating so that it turns completely in relation to the surface every 10,000
years or so.
Tidally
Locked
Many moons become tidally locked to their planet. This means that only one side of the moon
ever faces the planet. The Earth’s own
Moon is tidally locked to the Earth so we only ever see one side of it. The other side used to be referred to as the
dark side of the Moon because we had no idea what it was like until a
spacecraft flew round it and took pictures.
In fact in many cases the tidal locking is not complete. Our Moon has a wobble that allows us to see
rather more than half of its surface.
The four large moons of Jupiter are all tidally locked
to Jupiter to varying extents. Europa is
not completely locked either.
Heating
The main sources of heat for Europa and the Earth are
similar, but are quite different in relative importance.
Solar
The biggest source of heat for the Earth is the Sun’s
rays hitting our planet. This is also a
source of heat for Europa, but the concentration of the Sun’s energy is only
about a thirtieth of that hitting the Earth, so it is much less important for
Europa.
Radioactive
Decay
The second biggest source of heat for the Earth is the
energy released by the decay of radioactive elements, particularly Potassium,
Thorium and Uranium. This also occurs in
Europa, but the amount is probably less than that in the Earth, because there
is probably a higher concentration of heavy elements like Uranium in the Earth.
Tidal
Heating
Tidal Heating occurs in the Earth. We have tides raised by our Moon, the Sun,
and to tiny extents by other astronomical bodies. These tides occur in both our seas and in the
land. The ones in the land are much
smaller, but they are measurable.
Heating must occur in both the land and the sea from this tidal energy,
but it is not as important on the Earth as solar energy.
In Europa it is the biggest source of heat. These tides are partly because the orbit of
Europa is not circular, but is elliptical.
Europa is elongated by the gravitational pull of Jupiter because the
face nearer Jupiter is subjected to a greater force simply because it is
closer. There are other types of tides
on Europa as well from its interactions with Jupiter, the Sun and with the other
moons.
Magnetic
Europa has an induced magnetic field because of the
very powerful magnetic field of Jupiter.
This provides one of the types of evidence that Europa has a liquid salt
water ocean. It would also produce heat.
Greenhouse
Effect
This is not a source of heat, but is an atmospheric
effect that can slow down the loss of heat from the surface of a planet or
moon. On the Earth, it is of extreme
importance. Without this effect the
Earth would be too cold for many of the types of life we have here.
Europa has an Oxygen atmosphere, but the air pressure
is only about a billionth of that of the Earth.
With so little atmosphere there is no significant greenhouse effect.
Surface
Europa has the brightest surface of any moon. That is, it reflects light very well. This is a consequence of water ice being the
main thing on the surface. It also has a
very smooth surface. Although the
surface is basically white, there are many streaks of colour.
These may be from reactions between sulphur dioxide
and water. Experiments suggest that
reactions will happen even under the cold conditions on Europa.
Although these reactions are a plausible explanation
for the streaks of colour, they remind me (on a gigantic scale) of the discolorations
produced by some types of micro-organisms on the Earth, in aquatic
environments.
Radiation
The surface of Europa is bombarded by about 540 rems
of radiation. This is around the fatal
level for Humans, but there are other terrestrial organisms that can thrive in
the presence of this high level of radiation.
Most notable of these is Deinococcus radiodurans.
Ice will
stop radiation fairly effectively and an organism would not need to be very
deep in the ice to be protected.
Temperature
The temperature of the surface of Europa varies
between about minus 220 degrees C and minus 
150 degrees C (between minus 370
degrees F and minus 300 degrees F). This
compares with the coldest temperature ever recorded on Earth (At Vostok in the Australian Antarctic Territory) of minus 89 degrees C. So the surface of Europa is quite cold. Life as we know it requires liquid water, so
there are probably no living things on the Europan surface.
The liquid ocean
of Europa must be at
least the melting point of salty water under the pressure conditions inside
Europa.
Light
The Europan surface gets about one thirtieth of the
concentration of the Sun’s light that the Earth gets. This is well within the amount needed for
photosynthesis, but there is not likely to be enough light penetrating to a
depth where the ice is warm enough for life.
However, we do not really know about the temperature
distribution of the ice. The enormous
tidal flexing of the ice may mean that it does not simply get warmer steadily
until it is warm enough to be liquid water.
It is possible that there are major convection currents within the ice
bringing warm ice to the top. This could
be aided by the tidal flexing.
Atmosphere
The Europan atmosphere appears to be of Oxygen with a
pressure of about a billionth of the Earth’s sea level pressure. The oxygen probably comes from the breakdown
of water molecules by radiation. The
hydrogen part of the water would escape much faster than the Oxygen.
Life on
Europa
The Earth is the only place that we know has life on
it, but we can speculate about life in other places. Probably the most likely place other than the
Earth to have life is Mars. Personally I
think that there is a very high probability that Mars has living things. After Mars, perhaps the most likely abode of
life is Europa.
Life near the surface of Europa is possible, but a
much better habitat for life is the liquid ocean. Although there would probably not be enough
of the Sun’s light penetrating for photosynthesis, life is this ocean is
completely possible. The water may
contain a higher concentration of dissolved Oxygen that the Earth’s deep
oceans.
In our oceans there are organisms using inorganic
chemical energy. It is reasonable to
expect that this type of energy is also available on Europa.
Sources
http://www.nasa.gov/home
uropa
Mythology
