They found that Enceladus has a rocky core and an icy
crust. "Before, we knew almost nothing about the core beyond its likely
existence. Now we know roughly how big it is, and also that it has a
surprisingly low density," says team member Francis Nimmo
at the University of California, Santa Cruz. "That might be due to open
fractures, or low-density hydrated minerals like clays. Either answer
suggests that the rock has been in substantial contact with water, for
instance allowing minerals to dissolve, and explaining the salty ice
grains we see coming out of the surface."
crust. "Before, we knew almost nothing about the core beyond its likely
existence. Now we know roughly how big it is, and also that it has a
surprisingly low density," says team member Francis Nimmo
at the University of California, Santa Cruz. "That might be due to open
fractures, or low-density hydrated minerals like clays. Either answer
suggests that the rock has been in substantial contact with water, for
instance allowing minerals to dissolve, and explaining the salty ice
grains we see coming out of the surface."
The team also found that the southern
hemisphere has a stronger gravitational pull than its topography would
suggest. That could be explained by a localised sea, sitting beneath 35
kilometres of ice and up to 8 kilometres deep. It would contain about as
much water as Lake Superior in North America.
hemisphere has a stronger gravitational pull than its topography would
suggest. That could be explained by a localised sea, sitting beneath 35
kilometres of ice and up to 8 kilometres deep. It would contain about as
much water as Lake Superior in North America.
Destination, Enceladus?
If there might be life there, when can we go?
Cassini winds down in three years and there are no firm plans for
future craft to return to Saturn. However, Cassini team member Carolyn Porco at the Space Science Institute in Boulder, Colorado, has written a paper (soon to appear in the journal Astrobiology)
arguing for a mission to collect samples from Enceladus and return them
to Earth. She says the new results bode well for such an effort. "The
next mission there can immediately get down to the business of searching
for signs of life or its precursor chemistry. It's a big juncture!"
Cassini winds down in three years and there are no firm plans for
future craft to return to Saturn. However, Cassini team member Carolyn Porco at the Space Science Institute in Boulder, Colorado, has written a paper (soon to appear in the journal Astrobiology)
arguing for a mission to collect samples from Enceladus and return them
to Earth. She says the new results bode well for such an effort. "The
next mission there can immediately get down to the business of searching
for signs of life or its precursor chemistry. It's a big juncture!"
The subsurface-sea idea is just the simplest possible interpretation of the gravity data, cautions William McKinnon
at Washington University in St Louis, who was not involved in the work.
If the sea exists, there is the question of how long it has been liquid
and whether it might eventually freeze – or spray itself away. The
former is important as life would require the sustained presence of
water to gain a toehold.
at Washington University in St Louis, who was not involved in the work.
If the sea exists, there is the question of how long it has been liquid
and whether it might eventually freeze – or spray itself away. The
former is important as life would require the sustained presence of
water to gain a toehold.
As for the possibility of the sea freezing
completely, it is true that Enceladus is losing a lot of heat to space,
but astronomers suspect that this is an unusual episode. "We are
looking at Enceladus at a wonderful special time, where it's very active
and there's a lot of heat," McKinnon says.
completely, it is true that Enceladus is losing a lot of heat to space,
but astronomers suspect that this is an unusual episode. "We are
looking at Enceladus at a wonderful special time, where it's very active
and there's a lot of heat," McKinnon says.
Could the plumes deplete the sea
completely? Probably not. Even if they continue at the current rate, the
moon would only have lost 30 per cent of its water by mass when the sun
becomes a red giant in 6 billion years. "A lot of things can happen in 6
billion years, and it may shut off long before then, although the idea
of this thing blowing all of its ice away and becoming a little rocky
moon is kind of nice," Lunine says. "Some future extraterrestrials
visiting our solar system will be able to look at the naked rocky core
of what was once an ice moon."
completely? Probably not. Even if they continue at the current rate, the
moon would only have lost 30 per cent of its water by mass when the sun
becomes a red giant in 6 billion years. "A lot of things can happen in 6
billion years, and it may shut off long before then, although the idea
of this thing blowing all of its ice away and becoming a little rocky
moon is kind of nice," Lunine says. "Some future extraterrestrials
visiting our solar system will be able to look at the naked rocky core
of what was once an ice moon."
