Here is this week's answer article, sorry its been a bit delayed!
This week’s image comes from the icy moon of Europa. This
satellite of Jupiter has long captured the imagination of both scientists and
the public due to the fact that it most likely has an ocean of liquid water
deep below its icy shell. This image, which was captured by NASA’s Gallileo spacecraft,
covers a large swathe of the small moon’s surface. In the full image below you
can see variations in colour across the area around the grid-like cracks. The
whiter and bluer regions are predominantly covered by purer ice, while the reddish
and brownish areas are likely to be a mixture of other materials such as salts
which have been brought to the surface by activity on this small world.
The most striking feature of this image is the presence of a
grid of highly regular, rectilinear cracks. The presence this grid is actually a
major piece of evidence for the presence of a subsurface ocean. It is believed
that these cracks are left over from occasions when the icy crust broke up into
several “rafts”, floating on the ocean below. These blocks of ice would have been
much larger than the icebergs we are familiar with in terrestrial oceans, and
would not have been adrift on an open sea. Instead a better analogy would be
the tectonic plates responsible for the distinctive pattern of continents and
oceans on the Earth. These are large sections of crust which float on, and
indeed merge into the fluid mantle below. However there are no proper gaps
between them, or sections of exposed magma. The cracks we are seeing in this
image are the boundaries between these plates, and show just how many areas
have exhibited movement over geological time.
These ice rafts seem to have fused into a single icy shell,
and it is possible that the ocean below is frozen as well. There isn’t very
much heat from the sun that far out in the solar system, and the internal heat
from Europa’s core would be expected to have dispersed long ago. However the
scientific consensus is that Europa is far from dead and frozen through. We
believe that there is still a liquid ocean below the ice, and that this small
moon is very active. One major piece of evidence for this is cryovolcanism. Plumes
similar to those found on Enceladus have been seen emanating from Europa, and
suggest a ready source of liquid water. Another line of evidence can be seen in
any image of the moon. The surface of Europa is extremely smooth, with very few
craters. This suggests that it has been resurfaced fairly recently (in
geological terms at least) this lends weight to the hypothesis that there is
liquid water below the ice.
This subsurface ocean is likely to be kept liquid by the
heat from tidal flexing. The gravitational pull of Jupiter has a very strong
effect on the small bodies which orbit it, and the flexing of Europa’s crust
introduces a lot of energy into the system. This likely counteracts the very
cold conditions which would be expected in this part of the solar system and
provides a strong geothermal gradient between the core of Europa and the liquid
layer. It has been estimated that there is substantially more water on Europa
than there is on Earth, even if much of it is frozen that is a substantial sea.
It is even possible that this ocean could host the necessary chemical for the
development of life. A lot of work related to Europa is dedicated to examining
the likely chemical composition based on what limited data we currently have.
More missions to the Jupiter system are planned, and a variety of methods for
sampling the subsurface ocean have been proposed including having a satellite
fly through a plumes or sending a lander to drill into the ice. It is hoped
that the Europa Clipper mission, which will launch in the 2020s will include a
lander and shed more light on this fascinating moon.
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