The oceans of Pluto - NRC Publications Archive

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The oceans of Pluto
Tapping, Ken
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Skygazing: Astronomy through the seasons, 2016-07-05
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THE OCEANS OF PLUTO
Ken Tapping, 5th July, 2016
Just about a year ago the New Horizons
spacecraft flew past Pluto. It showed us a world
like nothing we had expected. Out there in the
cold, dark outer reaches of the Solar System, we
expected Pluto to be a deep-frozen ball of rock
and ice, unchanged over billions of years, apart
from a long history of being cratered by collisions
with other objects. We had made temperature
measurements. During the Plutonian winter,
temperatures fall to about –240 C, and in summer,
they rise to a not-very-balmy –220 C. Our main
interest in Pluto was due to our idea that it would
have changed little since the birth of the Solar
System, and would be a preserved sample of the
primordial material from which the Sun and other
bodies formed. We were wrong.
As the spacecraft approached Pluto, and the
images got more and more detailed, it became
clear that Pluto was very different from what we
expected. Instead of an icy, rocky ancient surface
covered with craters we saw great areas with no
craters, and other areas that were cratered, but
relatively few of them. Pluto’s surface is not
ancient; it is geologically young, and still changing.
One area that really stood out was a large, pinkish
plain, which has been provisionally named
“Sputnik Planum”, the “Plain of Sputnik”, after the
first manmade satellite to orbit the Earth. It was
covered with huge polygon-shaped tiles, separated
by grooves. At the edge of the plain, where it met
the surrounding mountainous terrain, there were
features that looked like the broken and refrozen
masses of ice we see on the seashore or on
Canadian lakes in winter, where the tide or wind
piles up ice on the shore. The tiled appearance
looked very like the surfaces we see in the
Canadian Arctic, where the ground overlying
permafrost has repeatedly melted and refrozen. It
also resembles the frozen tops of convection cells,
like the ones we see in a pot of heating cooking oil.
This leads to a surprising conclusion; the surface
of Pluto is not permanently frozen. It is melting,
rearranging and freezing again. This raises two
questions: what material is melting and re-freezing,
and where is the heat coming from?
In the Canadian Arctic the terrain is moulded by
the melting and refreezing of water. However, on
Pluto the temperatures are so low that water is a
permanently solid rock mineral. The best
candidate is nitrogen. There is a lot of it on Pluto
and at the temperatures on that world, it would be
a solid. The melting point of nitrogen is –210 C. It
is possible that on particularly warm days the
nitrogen would melt, or at least become a soft,
semi-molten slurry. That would explain the glacierlike features we see. However, it is likely that the
Sputnik Plain is not a plain at all; it is the frozen
cover of an ocean. In addition, the complete lack of
craters on the Sputnik Plain suggests that the
cover is not permanently frozen; it melts
sometimes. However, even the Plutonian summers
are not really warm enough, and to have a molten
ocean below that icy cover requires some heat to
be applied from below, at the seabed.
The most likely candidate is heat released in its
core by the decay of radioactive elements
collected when Pluto formed. Since these
elements are usually dense, they tend to
accumulate in the core, where they produce heat.
Such elements contribute to our world having a
molten core. Heat from the bottom of Pluto’s ocean
melts the nitrogen into a liquid or slurry, which
convects to the surface, forming those huge
polygons. Due to its great distance from us, the
spacecraft signals are very weak and the
information is being sent back very slowly, so it is
likely that more surprises will be coming.
Jupiter is descending in the west, and Mars and
Saturn lie in the southern sky. Mars is the bright
one; Saturn is fainter and to Mars’ left. The Moon
will reach First Quarter on the 11th.
Ken Tapping is an astronomer with the National
Research Council's Dominion Radio Astrophysical
Observatory, Penticton, BC, V2A 6J9.
Tel (250) 497-2300, Fax (250) 497-2355
E-mail: [email protected]