Space Forum / Space Science / April 2005

http://www.gemini.edu/index.php?option=content&task=view&id=126

Sedna Reveals Pristine Surface in Gemini Near-infrared Spectra
Gemini Observatory
12 April 2005

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Sedna Discovery

Sedna was discovered on November 14, 2003 by a CalTech team, using the
48-inch Samuel Oschin Telescope at Palomar Observatory. The object lies
more than 8 billion miles from the Earth, making it the most distant
object ever observed in the solar system. Its official IAU provisional
designation was minor planet 2003 VB12, but as of September 28, 2004 it
officially became known as Sedna after the Inuit goddess of arctic sea
life.
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Recent spectroscopic studies of infrared light reflected from the
surface of Sedna reveal that it is probably unlike Pluto and Charon
since Sedna's surface does not display evidence for a large amount of
either water or methane ice. Due to Sedna's extreme distance from the
Sun, the frigid surface has probably been untouched for millions of
years by anything except cosmic rays and solar ultraviolet radiation.

Gemini Observatory astronomer Chad Trujillo led an effort by the same
California Institute of Technology research team responsible for
Sedna's
original discovery to obtain spectra of this distant planetoid using
the
Near Infrared Imager (NIRI) on Gemini North. Their aim was to better
understand the surface of this distant world and how it has evolved
since its formation. "It is likely that Sedna has experienced an
extremely isolated life in the outskirts of our solar system," said
Trujillo. "Out there beyond what we used to think was the edge of the
solar system, interactions or collisions between bodies are probably
very rare. Our observations confirm what you would expect from a
surface
that has been so far out in our solar system for such a long time and
exposed to space weathering."

 Figure 1

 The relative reflectance spectrum of (90377) Sedna (black circles)
and
 the spectrum of the nearby sky (grey circles). Gray curves are the
model
 3 sigma upper limits to the surface fraction of water ice (smooth
gray line)
 and methane ice (jagged gray line). Surface fractions that cause more
 absorption than the indicated lines are ruled out by our observations
at
 the 3 sigma level. The model shown is for 100-micron diameter
particles.
 Spectrum error bars are computed from the reproducibility of the
 spectral data in each spectral point.

The Sedna data lack the strong spectral lines that would indicate the
existence of substances like methane and water ice, but deeper studies
are needed to confirm how low the levels of these ices might be on this
planetoid. Sedna might be more like the minor planet Pholus (that lies
just inside the orbit of Saturn), which is similar in its redness in
visible light. This same "space weathering" may also affect Pluto and
Charon, but there may be other processes that replenish their water-
and
methane-rich surfaces, such as atmospheric effects, geological
processes
and collisions.

The data could reveal something of Sedna's evolutionary history in the
outer solar system. Astronomers think that objects like Sedna start out
with icy surfaces. Over time cosmic rays and solar ultraviolet
radiation
"bake and burn" the surfaces into black hydrocarbon-rich substances
similar to asphalt, which do not reveal themselves well in infrared
spectra. Such a history might explain why Sedna doesn't exhibit traces
of methane and water ice, whereas Pluto and Charon do.

"Like a sandblaster operating for several billion years, most of the
objects out as far as Pluto are constantly being resurfaced by impacts
and collisions which expose and supply fresh surface materials before
the black stuff can get baked on," said Michael Brown of California
Institute of Technology, who is the Principle Investigator of the team
that originally discovered Sedna. "Pluto and its moon Charon provide an
excellent example of this process, with Pluto displaying a strong
methane ice signature in its spectrum and Charon dominated by water
ice."

The team does not rule out the possibility that longer-duration
(deeper)
observations might reveal evidence of methane or water ice on Sedna.
However, the Gemini data indicate that if they do exist their extent is
limited.

The results of these observations will appear in The Astrophysical
Journal.

Gemini Observations of Sedna

The Gemini observations of Sedna were possible due to the observatory's
powerful ability to collect infrared radiation reflected from the
planetoid (which lies over 12 light-hours from Earth). The research
team
used NIRI (Near Infrared Imager) on Gemini North to obtain the spectra
over a period of about two hours on the night of December 27, 2003.

The infrared light captured by Gemini is really "recycled" starlight
from the Sun, which at Sedna's distance would appear more like a very
bright nighttime star. Just as some of the sunlight striking this
object
is reflected back as optical light (which allowed its discovery), some
of the infrared light is also reflected back. Materials on the surface
imprint a characteristic fingerprint on the re-emitted light, which can
reveal details about the composition of the planetoid's surface.