[meteorite-list] An Ammonia-Water Slurry May Swirl Below Pluto's Icy Surface

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 11 Nov 2015 12:09:19 -0800 (PST)
Message-ID: <201511112009.tABK9JOc013898_at_zagami.jpl.nasa.gov>

http://www.purdue.edu/newsroom/releases/2015/Q4/an-ammonia-water-slurry-may-swirl-below-plutos-icy-surface.html

An ammonia-water slurry may swirl below Pluto's icy surface
Purdue University
November 9, 2015

WEST LAFAYETTE, Ind. - Researchers propose an ammonia-water slurry as
the basis for Pluto's newly discovered geologic activity and possible
volcanism, and offer a new method to predict planetary vigor.

The findings were presented at the annual meeting of the Division for
Planetary Sciences of the American Astronomical Society on Monday (Nov.
9) in National Harbor, Maryland.

Graduate student Alex Trowbridge, under the guidance of Jay Melosh, a
distinguished professor of earth, atmospheric and planetary sciences,
and professor of physics and aerospace engineering, performed the research
in response to data reported by NASA's New Horizons mission to the Pluto
system and the Kuiper Belt that revealed a surprising amount of geologic
activity on the dwarf planet's surface.

"We wanted to know why this small, icy cold dwarf planet is so active
and to find a way to predict such activity for planets and other planetary
bodies for which we have little information," Melosh said. "The New Horizons
mission has already provided an astounding amount of new information,
and its surprises remind us how little we know about the far reaches of
our solar system and the depths of outer space beyond it. This is why
these missions are so important."

A geologically active surface that buries craters and raises mountains
means the mantle that lies below is moving.

The mantle is the layer between the crust and core of a planet or planetary
body. In the Earth's mantle a very slow convection occurs in which hot
material rises up and cooler material sinks. This movement occurs over
millions of years and is responsible for the movement of the tectonic
plates. Mantle convection also leads to geologic activity that rejuvenates
the surface, erasing impact craters and bringing fresh material to the
surface. Images of the surface of Pluto show young surfaces that indicate
it is geologically active.

Mantle convection is like a pot of oatmeal on a hot stove, Melosh said.
If the heat is too low, it just sits there, but, if the heat is turned
up, it starts overturning and hot oatmeal is pushed to the top.

"We expect a piping hot pot of oatmeal to roil and churn, but we don't
expect a block of ice to have much movement," he said. "It had been thought
that pure water ice sat below the surface of Pluto, which has an average
temperature of more than 300 degrees below zero, but there is evidence
that suggests the presence of ammonia. That little bit of antifreeze makes
all the difference."

Trowbridge, Melosh and Andy Freed, a Purdue professor of earth, atmospheric
and planetary sciences, used a convection model to analyze the possibility
for geologic activity with a mantle of pure water ice and one that was
5 percent ammonia.

The team used the model to deduce the Rayleigh number, a mathematical
term associated with the level of buoyancy-driven flow, or convection.

"We found that a mantle containing a small amount of ammonia - which has
been seen on the surface of bodies in the outer solar system and plausibly
condensed in the planets in this region ? lowers the temperature required
to achieve a Rayleigh number where convection occurs," Trowbridge said.
"The ammonia lowers the viscosity of water ice by a factor of 100,000.
This would allow for the geologically active and vigorous Pluto seen in
the New Horizon images."

The team also created a new model to predict mantle convection and, in
turn, whether a planet or object is geologically active.

Previously the set of equations used to compute convective vigor required
knowledge of the average temperature of the mantle, which is not known
for many planets and planetary bodies, including Pluto, Trowbridge said.

The team added the understanding of heat transport into the mix, and crafted
an equation which, when added into the standard system of equations, gave
the ability to solve for convective vigor when all that is known is size,
heat generation and the mantle material, Melosh said.

"It is a complicated and interdependent set of mathematical equations,
as often is the case, but convective vigor and heat transport have been
known to be entwined for some time," Melosh said. "In hindsight it is
a solution that was hiding in plain sight all along. We hope that we now
have a universal model for planetary vigor that can be applied to planets,
satellites and other bodies where such a prediction was previously impossible."

The team plans to continue to test the model and apply it to other planetary
bodies encountered on the New Horizons mission.

Abstracts submitted to the organizing committee of the meeting of the
Division for Planetary Sciences of American Astronomical Society do not
go through a peer review process. The team next plans to submit a paper
detailing the research for publication in a peer-reviewed journal.

Writer: Elizabeth K. Gardner, 765-494-2081, ekgardner at purdue.edu

Sources: Jay Melosh, 765-494-3290, jmelosh at purdue.edu

Alexander Trowbridge, atrowbr at purdue.edu

Andy Freed 765-496-3738, freed at purdue.edu

Note to Journalists: Alexander Trowbridge made his presentation on Monday
(Nov. 9) in a session on Pluto's surface properties at the annual meeting
of the division for planetary sciences of the American Astronomical Society.
Jay Melosh is unable to attend the meeting, but is available for interview
by phone or via email Nov. 9-11.
Received on Wed 11 Nov 2015 03:09:19 PM PST