[meteorite-list] Charon: An Ice Machine in the Ultimate Deep Freeze

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 18 Jul 2007 08:40:07 -0700 (PDT)
Message-ID: <200707181540.IAA17355_at_zagami.jpl.nasa.gov>

http://www.gemini.edu/index.php?option=content&task=view&id=244

Charon: An Ice Machine in the Ultimate Deep Freeze
Tuesday, 17 July 2007

Press Release
Gemini Observatory

      Media Contact:

    * Peter Michaud
      Gemini Observatory, Hilo HI, USA
      (808) 974-2510 (desk)
      (808) 937-0845 (cell)
      pmichaud at gemini.edu
    * Carol Hughes
      Arizona State University, Tempe, AZ
      (480) 965-6375 (desk)
      (480) 254-3753 (cell)
      carol.hughes at asu.edu


      Science Contact:

    * Jason Cook
      Southwest Research Institute
      Boulder CO, USA
      (720) 240-0160 (desk)
      (303) 546-9670 (main)
      Jason.Cook3 at asu.edu
    * Steven Desch
      Arizona State University
      School of Earth and Space Exploration
      Tempe, AZ
      Desk: (480) 965-7742
      steve.desch at asu.edu

Frigid geysers spewing material up through cracks in the crust of
Pluto's companion Charon and recoating parts of its surface in ice
crystals could be making this distant world into the equivalent of an
outer solar system ice machine.

Evidence for these ice deposits comes from high-resolution spectra
obtained using the Gemini Observatory's Adaptive Optics system, ALTAIR
coupled with the near-infrared instrument NIRI. The observations, made
with the Frederick C. Gillett Gemini North telescope on Hawaii's Mauna
Kea, show the fingerprints of ammonia hydrates and water crystals spread
in patches across Charon, and have been described as the best evidence
yet for the existence of these compounds on worlds such as Charon. The
observations suggest that liquid water mixed with ammonia from deep
within Charon is pushing out to the ultra-cold surface. This action
could be occurring on timescales as short as a few hours or days, and at
levels that would recoat Charon to a depth of one millimeter every
100,000 years. This discovery could have profound implications for other
similar-type worlds in the Kuiper Belt, which is the region of the solar
system that extends out beyond the orbit of Neptune and contains a
number of small bodies, the largest of which include Pluto and Charon.

"There are a number of mechanisms that could explain the presence of
crystalline water ice on the surface of Charon," said Jason Cook, the
PhD student at Arizona State University who led the team of planetary
scientists studying the surface of Charon. "Our spectra point
consistently to cryovolcanism, which brings liquid water to the surface,
where it freezes into ice crystals. That implies that Charon's interior
possesses liquid water."

About Pluto and Charon

Charon is the companion world to Pluto (or one would say a moon of
Pluto, except that Pluto is no longer considered to be a planet). It is
one the largest members of the Kuiper Belt, a group of objects that
orbit the Sun in the solar system beyond Neptune. The pair has an
exceptionally low mass ratio (8:1; the ratio of Earth and the Moon's
masses is 81:1), and is tidally locked in synchronous orbit. Pluto
rotates on its axis once every 6.3 days, and Charon takes the same
amount of time to make one orbit with respect to Pluto. As a result,
they each show the other the same face at all times.

Because Charon is a somewhat typical Kuiper Belt Object for its size,
the NIRI observations imply that other KBOs of similar size and
composition (containing water ice and ammonia hydrates on their
surfaces) could also harbor reservoirs of water deep beneath their
surfaces. If so, the team speculates that there could be more liquid
water in the Kuiper Belt than on Earth. "If Charon contains a liquid
ocean," said Jason Cook, "then all Kuiper Belt objects greater than 500
kilometers across have oceans."

This idea flows from work being done by Steve Desch, who has been
developing internal models of the thermal evolution of KBOs that take
into account the antifreeze properties of ammonia as seen on Charon. "We
find that bodies the size of Charon end up with about half of their rock
content in a central core," he said. The other half remains in a crust
that is a mixture of rock and ice.

To reach this conclusion, Cook and his collaborators studied a number of
other mechanisms that could explain the presence of water ice crystals
on Charon. The crystals are not likely to be "primordial ice" made at
the time the solar system formed because such ice would become amorphous
(that is, it would lose its crystalline appearance) in a few tens of
thousands of years, due to solar ultraviolet radiation and cosmic ray
bombardment. Processes that create fresh, icy patches on other worlds,
such as impact "gardening" by meteorites and convection of subsurface
materials to the surface, are not supported by the chemical fingerprints
of the water and ammonia hydrates on Charon's surface. The only
mechanism that explained the data was cryovolcanism, the eruption of
liquids and gases in an ultra-cold environment.

The key to understanding cryovolcanism on Charon, according to Cook, is
to look at Charon's physical makeup. "Charon's surface is almost
entirely water ice," he said. "So it must have a vast amount of water
under the surface, and much of that should be frozen as well. Only deep
inside Charon could water be a liquid. Yet, there is fresh ice on the
surface, meaning that some liquid water must somehow reach the surface.
The ammonia sitting on the surface provides the clue. It's the ammonia
that helps keep some material liquid. It makes it all feasible. Without
ammonia the water could not get out there."

Cryovolcanism in the outer solar system is a fairly common occurrence.
Enceladus (a moon of Saturn) and Europa (orbiting Jupiter) both show
evidence of water ice oozing or spewing out from beneath the surfaces.
So-called "tiger stripes" on Enceladus were first reported in 2006 by
planetary scientist Carolyn Porco of the Space Science Institute in
Boulder, Colorado. They may be created by geysers that send water ice
out through surface cracks. Markings seen on the Uranian moon Ariel in
Voyager 2 fly-by images also suggest active cryovolcanism of some kind.
Enceladus and Europa are tidally squeezed by the gravitational forces of
their giant planets and in some cases by large nearby moons. This forces
water out through cracks. Ariel may have been affected by tidal
squeezing in the past. By contrast, Kuiper Belt Objects (KBOs) such as
Charon, Quaoar, Orcus, and others are not tidally squeezed. Yet, they
seem to show evidence of cryovolcanism.

Figure 2. The spectrum of Charon obtained by NIRI at Gemini North. It is
centered at 2.2 microns for the sub-Pluto (top) and anti-Pluto (bottom)
hemispheres of Charon. The solid line denotes the best fit for a model
of a surface with ammonia hydrate and water ices. The dashed lines are
data that indicate the position of the ammonia hydrate feature. The
sub-Pluto and anti-Pluto ammonia hydrate minima are located at 2.2131
and 2.1995, respectively. (The error bars represent 1 sigma.). (Spectrum
by Jason Cook.)

In the case of Charon, it is thought that heat from internal
radioactivity creates a pool of melted water mixed with ammonia inside
the ice shell. "As some of the subsurface water cools and approaches the
freezing point, it expands into the cracks in the ice shell above it,"
said Cook. "Due to the expansion, even a small vertical crack of a half
a kilometer at the base of the ice shell will allow material to
propagate to the outer surface of Charon in a matter of hours, making
that the conduit for the water."

As the water sprays out through the crack, it freezes and immediately
"snows" back down to the surface, creating bright ice patches that can
be distinguished in near-infrared light. "I half expect that if we ever
get to actually SEE a plume going off on Charon, we'd be seeing the
process that makes tiger stripes similar to what we see on Enceladus and
other frozen worlds," Cook said. "The real proof will come from the
deep-space NASA probe New Horizons, which will arrive at this system in
2015 and send back images that can verify what we've seen in our Gemini
results."

The team's intent was to find evidence of methane, carbon dioxide,
ammonia, and a form of ammonia called ammonia hydrate on the surface of
Charon, which has also been reported on Quaoar and suspected on at least
one other KBO. According to Assistant Professor Steven Desch, Cook's
colleague and thesis advisor at Arizona State University, ammonia
hydrates help keep liquid water from freezing solid, making it easier
for water to escape from the inside before it turns to ice. "It is
literally an antifreeze," he said, "and like the antifreeze we're
familiar with here on Earth, it depresses the melting point of water."

Cook and his colleagues concentrated their observations on Charon's
anti-Pluto and sub-Pluto hemispheres, where patches of water ice exist
at temperatures between 40-50 Kelvin (-387 to -369 F). They used NIRI to
observe at 2.21 and 1.65 microns, wavelengths that reveal the presence
of ammonia hydrates and water ice, respectively.

Prior to this work, all observations of Charon made by Cook and others
showed crystalline water ice on its surface. Cook's finding is the only
one to date to take two hemispheres and try to estimate the temperatures
from the spectra. He notes that more observations are needed to show how
temperatures might change with rotation.

"We were using the rotation of Charon to see if these ices are
distributed evenly across its surface or are found preferentially at
certain longitudes," Cook said. "Based on measurements from 2001, we
knew the ices were there, but we didn?t have precise locations. What we
needed to do was to get high resolution, time-resolved spectra. NIRI
coupled with ALTAIR was the best way to do that. And using an 8-meter
telescope was a no-brainer. It all came together so well."

According to Desch, the spectra obtained by Cook and the team are the
best evidence yet for the existence of ammonia hydrates on KBOs. "It had
been tentatively identified on Charon before by other groups," he said,
"but the lack of spectral resolution hindered its identification. This
clinches it. These spectra are also better than those of other KBOs.
I've talked to seasoned observers who are convinced for the first time
that ammonia hydrates exist on KBOs."

The next step is to get better spectra of other Kuiper Belt Objects such
as Quaoar and Orcus. "Those larger than 500 kilometers across show us
crystalline water ice," Cook said. "But, there's a whole set of
intermediate-sized objects, between 200 and 500 kilometers across that
we want to sample to test our ideas. As I think about at these other
KBOs, I want to look for the ammonia hydrate. I think it has to be out
there."

These results were published in the paper "Near-infrared Spectroscopy of
Charon: Possible Evidence for Cryovolcanism on Kuiper Belt Objects"
<http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v663n2/70488/brief/7
0488.abstract.html> in volume 663 of the Astrophysical Journal, by
Jason Cook while at Arizona State University, along with his graduate
advisor Steven J. Desch (Arizona State University), the team also
included Ted L. Roush (NASA Ames Research Center), and Chad Trujillo and
Tom Geballe (Gemini Observatory).
Received on Wed 18 Jul 2007 11:40:07 AM PDT