Saturday, August 8

If Pluto has a subsurface ocean, it may be old and deep

A suspected subsurface ocean
on Pluto might be old and deep.

New analyses of images from NASA’s
New Horizons spacecraft suggest that the dwarf planet has had an underground
ocean since shortly after Pluto formed 4.5 billion years ago, and that the ocean
may surround and interact with the rocky core.

If so, oceans could be
common at the solar system’s edge — and may even be able to support life. That
possibly “transforms the way we think about the Kuiper Belt,” the region of icy objects beyond the orbit of Neptune (SN: 3/27/19), says planetary scientist Adeene
Denton of Purdue University in West Lafayette, Ind.

On its pass through the
Kuiper Belt in 2015, New Horizons revealed that despite the dwarf planet’s location
nearly 6 billion kilometers from the sun, Pluto showed signs of hosting an ocean of liquid water beneath an icy shell (SN: 9/23/16).

How much liquid may lie
beneath Pluto’s ground, how long it’s been there, and how much the water may
have partially frozen over time is hard to tell from the surface. The new
research, which had been scheduled for presentation the week of March 16 at the
canceled Lunar and Planetary Science Conference in The Woodlands, Texas, has dug into those questions.

Pluto's surface
Dark lines (circled) on the surface of Pluto, shown in this image from NASA’s New Horizons spacecraft, could mark ripples created when a large object probably slammed into the opposite side of the dwarf planet.New Horizons Team

“If there’s an ocean today,
it raises the question of, when did that ocean get there?” says planetary scientist
Carver Bierson of the University of California, Santa Cruz.

Bierson considered two
possible histories for Pluto’s potential ocean. If the dwarf planet had a “cold
start,” any subsurface water would first have been frozen before melting under heat
from decaying radioactive elements in the dwarf planet’s core, only to
partially freeze again over time. In that scenario, Bierson expected to see
cracks and ripples across Pluto’s icy shell from the orb’s contraction as the
ice melted and then expansion as water refroze. Contracting would make the ice
crumple into mountainlike features, while expanding would stretch the ice and
create faults and graben.

Bierson’s second scenario
envisioned a “warm start” for Pluto, where the ocean would have been liquid for
nearly all of Pluto’s 4.5-billion-year existence. In that case, the surface
would show only cracks from the sea expanding as it partially froze. And that’s
exactly what Bierson and colleagues found in New Horizon’s images, suggesting that
Pluto’s liquid ocean is nearly as old as the dwarf planet itself. 

“That means maybe Pluto did
start off warm,” Bierson says. “Maybe it started with a liquid ocean really
early on.”

In a separate study, Denton
and colleagues considered the impact that formed Sputnik Planitia, the left
lobe of Pluto’s distinctive heart-shaped basin. Because of how New Horizons
flew past Pluto, scientists’ view of half the dwarf planet is fuzzy. But the
team was able to see lines on Pluto’s surface on the exact opposite side of
the globe from Sputnik Planitia, the researchers
reported in October 2019 at arXiv.org. Those
lines might be the imprints of shock waves from a massive impact that formed the
enormous basin, Denton says.

“If the impact is large
enough … the planet itself can act like a lens, and focus the wave energy at
the exact opposite point on the planet from the impact,” she says.

Pluto’s internal structure
would have controlled how those shock waves shuddered through the dwarf planet.
Looking at the cracks in the surface ice could give clues to the thickness of
the proposed ocean or the core’s chemical makeup. So Denton and her colleagues ran
computer simulations of an impact to look for clues.

“We got the fun answer,” she
says. To explain the lines seen on the dwarf planet, not only would Pluto need
a large ocean, 150 kilometers or more in thickness, but the core must contain
minerals, such as serpentine, that form through interactions between rock and
water. Astrobiologists think that water-rock interactions could provide energy and nutrients for life (SN: 5/19/15). The possibility of a somewhat soggy
core could let life get a toehold at the fringes of the solar system, Denton
says.

“It’s certainly not exactly
a smoking gun,” she says. “But it’s exciting.”

The possibility that Pluto has a habitable ocean raises the odds that other Kuiper Belt objects do too, says planetary scientist James Tuttle Keane of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who is a member of the New Horizons team but was not involved in either study. 

“This lays out one of the
coolest hypotheses that a future Pluto mission could test,” he says. “If Pluto
can have an ocean and potentially be habitable, it’s very likely that other
bodies in the Kuiper Belt also are ocean worlds and also are potentially
habitable.”