Evidence supports 'hot start' scenario and early ocean formation on
Pluto

Date:
June 22, 2020
Source:
University of California - Santa Cruz
Summary:
A new study suggests that Pluto and other large Kuiper belt objects
started out with liquid oceans which have been slowly freezing
over time.



FULL STORY ========================================================================== [Pluto illustration | Credit: (c) revers_jr / stock.adobe.com] Pluto illustration (stock image).

Credit: (c) revers_jr / stock.adobe.com [Pluto illustration | Credit:
(c) revers_jr / stock.adobe.com] Pluto illustration (stock image).

Credit: (c) revers_jr / stock.adobe.com Close 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.

In addition to Bierson and Nimmo, the paper was coauthored by Alan Stern
at the Southwest Research Institute, the principal investigator of the
New Horizons mission.


========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Cruz. Original written by Tim
Stephens. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Carver J. Bierson, Francis Nimmo, S. Alan Stern. Evidence for a
hot start
and early ocean formation on Pluto. Nature Geoscience, 2020; DOI:
10.1038/s41561-020-0595-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200622132959.htm

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