The accretion of
new material during Pluto’s formation may have generated enough heat to create
a liquid ocean that has persisted beneath an icy crust to the present day,
despite the dwarf planet’s orbit far from the sun in the cold outer reaches of
the solar system.
This “hot start” scenario,
presented in a paper published June 22 in Nature Geoscience, contrasts with
the traditional view of Pluto’s origins as a ball of frozen ice and rock in
which radioactive decay could have eventually generated enough heat to melt the
ice and form a subsurface ocean.
“For a long time people have thought about the thermal
evolution of Pluto and the ability of an ocean to survive to the present day,”
said coauthor Francis Nimmo, professor of Earth and planetary sciences at UC
Santa Cruz. “Now that we have images of Pluto’s surface from NASA’s New
Horizons mission, we can compare what we see with the predictions of different
thermal evolution models.”
Because water expands when it freezes and contracts
when it melts, the hot-start and cold-start scenarios have different
implications for the tectonics and resulting surface features of Pluto,
explained first author and UCSC graduate student Carver Bierson.
“If it started cold and the ice melted internally,
Pluto would have contracted and we should see compression features on its
surface, whereas if it started hot it should have expanded as the ocean froze
and we should see extension features on the surface,” Bierson said. “We see
lots of evidence of expansion, but we don’t see any evidence of compression, so
the observations are more consistent with Pluto starting with a liquid ocean.”
The thermal and tectonic evolution of a cold-start
Pluto is actually a bit complicated, because after an initial period of gradual
melting the subsurface ocean would begin to refreeze. So compression of the
surface would occur early on, followed by more recent extension. With a hot
start, extension would occur throughout Pluto’s history.
“The oldest surface features on Pluto are harder to
figure out, but it looks like there was both ancient and modern extension of
the surface,” Nimmo said.
The next question was whether enough energy was
available to give Pluto a hot start. The two main energy sources would be heat
released by the decay of radioactive elements in the rock and gravitational
energy released as new material bombarded the surface of the growing
protoplanet.
Bierson’s calculations showed that if all of the
gravitational energy was retained as heat, it would inevitably create an
initial liquid ocean. In practice, however, much of that energy would radiate
away from the surface, especially if the accretion of new material occurred
slowly.
“How Pluto was put together in the first place matters
a lot for its thermal evolution,” Nimmo said. “If it builds up too slowly, the
hot material at the surface radiates energy into space, but if it builds up
fast enough the heat gets trapped inside.”
The researchers calculated that if Pluto formed over a
period of less that 30,000 years, then it would have started out hot. If,
instead, accretion took place over a few million years, a hot start would only
be possible if large impactors buried their energy deep beneath the surface.
The new findings imply that other large Kuiper belt
objects probably also started out hot and could have had early oceans. These
oceans could persist to the present day in the largest objects, such as the
dwarf planets Eris and Makemake.
“Even in this cold environment so far from the sun,
all these worlds might have formed fast and hot, with liquid oceans,” Bierson
said.
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