[meteorite-list] Rosetta's First Peek at the Comet's Dark Side

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=4728

Rosetta's First Peek at the Comet's Dark Side
Jet Propulsion Laboratory
October 1, 2015

Since its arrival at comet 67P/Churyumov-Gerasimenko, the European Space 
Agency's Rosetta spacecraft has been surveying the surface and the environment 
of this curiously shaped body. But for a long time, a portion of the nucleus 
-- the dark, cold regions around the comet's south pole -- remained inaccessible 
to almost all instruments on the spacecraft.

Due to a combination of its double-lobed shape and the inclination of 
its rotation axis, Rosetta's comet has a very peculiar seasonal pattern 
over its 6.5-year-long orbit. Seasons are distributed very unevenly between 
the two hemispheres. Each hemisphere comprise parts of both comet lobes 
and the "neck."

For most of the comet's orbit, the northern hemisphere experiences a very 
long summer, lasting over 5.5 years, while the southern hemisphere undergoes 
a long, dark and cold winter. However, a few months before the comet reaches 
perihelion -- the closest point to the sun along its orbit -- the situation 
changes, and the southern hemisphere transitions to a brief and very hot 
summer.

When Rosetta arrived at 67P/C-G in August 2014, the comet was still experiencing 
its long summer in the northern hemisphere, and regions on the southern 
hemisphere received very little sunlight. Moreover, a large part of this 
hemisphere, close to the comet's south pole, was in polar night and had 
been in total darkness for almost five years.

With no direct illumination from the sun, these regions could not be imaged 
with Rosetta's OSIRIS (the Optical, Spectroscopic, and Infrared Remote 
Imaging System) science camera, or its Visible, InfraRed and Thermal Imaging 
Spectrometer (VIRTIS). For the first several months after Rosetta's arrival 
at the comet, only one instrument on the spacecraft could observe and 
characterize the cold southern pole of 67P/C-G: the Microwave Instrument 
for Rosetta Orbiter (MIRO).

In a paper accepted for publication in the journal Astronomy and Astrophysics, 
scientists report on the data collected by MIRO over these regions between 
August and October 2014.

"We observed the 'dark side' of the comet with MIRO on many occasions 
after Rosetta's arrival at 67P/C-G, and these unique data are telling 
us something very intriguing about the material just below its surface," 
said Mathieu Choukroun from NASA's Jet Propulsion Laboratory (JPL), Pasadena, 
California, lead author of the study.

Observing the comet's southern polar regions, Choukroun and colleagues 
found significant differences between the data collected with MIRO's millimeter 
and sub-millimeter wavelength channels. These differences might point 
to the presence of large amounts of ice within the first few tens of centimeters 
below the surface of these regions.

"Surprisingly, the thermal and electrical properties around the comet's 
south pole are quite different than what is found elsewhere on the nucleus," 
said Choukroun. "It appears that either the surface material or the material 
that's a few tens of centimeters below it is extremely transparent, and 
could consist mostly of water ice or carbon-dioxide ice."

The difference between the surface and subsurface composition of this 
part of the nucleus and that found elsewhere might originate in the comet's 
peculiar cycle of seasons. One of the possible explanations is that water 
and other gases that were released during the comet's previous perihelion, 
when the southern hemisphere was the most illuminated portion of the nucleus. 
The water condensed again and precipitated on the surface after the season 
changed and the southern hemisphere plunged again into its long and cold 
winter.

These are, however, preliminary results, because the analysis depends 
on the detailed shape of the nucleus. At the time the measurements were 
made, the shape of the dark, polar region was not known with great accuracy.

"We plan to revisit the MIRO data using an updated version of the shape 
model, to verify these early results and refine the interpretation of 
the measurements," added Choukroun.

Rosetta scientists will be testing these and other possible scenarios 
using data that were collected in the subsequent months, leading to the 
comet's perihelion, which took place on Aug. 13, 2015 and beyond.

In May 2015, the seasons changed on 67P/C-G and the brief, hot southern 
summer, which will last until early 2016, began. As the formerly dark 
southern polar regions started to receive more sunlight,  it has been 
possible to observe them with other instruments on Rosetta, and the combination 
of all data might eventually disclose the origin of their curious composition.

"In the past few months, Rosetta has flown over the southern polar regions 
on several occasions, starting to collect data from this part of the comet 
after summer began there," said Matt Taylor, ESA Rosetta project scientist. 
"At the beginning of the southern summer, we had a paucity of observations 
in these regions as Rosetta's trajectory focused on the northern hemisphere 
due to ongoing communication with the lander, Philae. However, closer 
to perihelion we were able to begin observing the south."

Rosetta is currently on an excursion out to about 930 miles (1,500 kilometers) 
from the nucleus to study the comet's environment at large. But the spacecraft 
will soon come closer to the comet, focusing on full orbits to compare 
the northern and southern hemispheres, as well as some slower passes in 
the south to maximize observations there. In addition, as activity will 
start to wane later this year, the team hopes to get closer to the nucleus 
and gain higher-resolution observations of the surface.

"First, we observed these dark regions with MIRO, the only instrument 
able to do so at the time, and we tried to interpret these unique data. 
Now, as these regions became warmer and brighter around perihelion, we 
can observe them with other instruments, too."

Mark Hofstadter, MIRO principal investigator at JPL, adds, "We hope that, 
by combining data from all these instruments, we will be able to confirm 
whether or not the south pole had a different composition and whether 
or not it is changing seasonally."

The MIRO instrument is a small, lightweight spectrometer that can map 
the abundance, temperature and velocity of cometary water vapor and other 
molecules that the nucleus releases. It can also measure the temperature 
up to about one inch (three centimeters) below the surface of the comet's 
nucleus. One reason the subsurface temperature is important is that the 
observed gases likely come from sublimating ices beneath the surface. 
By combining information on the gas and the subsurface, MIRO is able to 
study this process in detail.

Comets are time capsules containing primitive material left over from 
the epoch when the sun and its planets formed. Rosetta is the first spacecraft 
to witness at close proximity how a comet changes as it is subjected to 
the increasing intensity of the sun's radiation. Observations will help 
scientists learn more about the origin and evolution of our solar system 
and the role comets may have played in the formation of planets.

Rosetta is an ESA mission with contributions from its member states and 
NASA. Rosetta's Philae lander is provided by a consortium led by the German 
Aerospace Center, Cologne; Max Planck Institute for Solar System Research, 
Gottingen; French National Space Agency, Paris; and the Italian Space 
Agency, Rome. JPL, Pasadena, California, a division of the California 
Institute of Technology in Pasadena, manages the U.S. contribution of 
the Rosetta mission for NASA's Science Mission Directorate in Washington. 
JPL also built the MIRO and hosts its principal investigator, Mark Hofstadter. 
The Southwest Research Institute (San Antonio and Boulder, Colorado), 
developed the Rosetta orbiter's IES and Alice instruments and hosts their 
principal investigators, James Burch (IES) and Alan Stern (Alice).

For more information on the U.S. instruments aboard Rosetta, visit:

http://rosetta.jpl.nasa.gov

More information about Rosetta is available at:

http://www.esa.int/rosetta


Media Contact

DC Agle
Jet Propulsion Laboratory, Pasadena, California
818-393-9011
agle at jpl.nasa.gov 

Dwayne Brown / Laurie Cantillo
NASA Headquarters, Washington
202-358-1726 / 202-358-1077
dwayne.c.brown at nasa.gov / laura.l.cantillo at nasa.gov 

Markus Bauer
European Space Agency, Noordwijk, Netherlands
011-31-71-565-6799
markus.bauer at esa.int 

2015-307