[meteorite-list] The Dual Personality of Comet 67P/Churyumov-Gerasimenko

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 17 Jul 2014 12:09:47 -0700 (PDT)
Message-ID: <201407171909.s6HJ9l8v023071_at_zagami.jpl.nasa.gov>

http://blogs.esa.int/rosetta/2014/07/17/the-dual-personality-of-comet-67pc-g/

The dual personality of comet 67P/C-G
Rosetta Blog
July 17, 2014

[Image]
Comet 67P/C-G imaged on 14 July 2014 from a distance of approximately
12 000 km.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

This week's images of comet 67P/Churyumov-Gerasimenko reveal an extraordinarily
irregular shape. We had hints of that in last week's images and in the
unscheduled previews that were seen a few days ago, and in that short
time it has become clear that this is no ordinary comet. Like its name,
it seems that comet 67P/C-G is in two parts.

What the spacecraft is actually seeing is the pixelated image shown at
right, which was taken by Rosetta's OSIRIS narrow angle camera on 14 July
from a distance of 12 000 km.

A second image and a movie show the comet after the image has been processed.
The technique used, called "sub-sampling by interpolation", only acts
to remove the pixelisation and make a smoother image, and it is important
to note that the comet's surface features won't be as smooth as the processing
implies. The surface texture has yet to be resolved simply because we
are still too far away; any apparent brighter or darker regions may turn
out to be false interpretations at this early stage.

But the movie, which uses a sequence of 36 interpolated images each separated
by 20 minutes, certainly provides a truly stunning 360-degree preview
of the overall complex shape of the comet. Regardless of surface texture,
we can certainly see an irregular shaped world shining through. Indeed,
some people have already likened the shape to a duck, with a distinct
body and head.

Although less obvious in the "real" image, the movie of interpolated images
supports the presence of two definite components. One segment seems to
be rather elongated, while the other appears more bulbous.

Dual objects like this - known as "contact binaries" in comet and asteroid
terminology - are not uncommon.

Indeed, comet 8P/Tuttle is thought to be such a contact binary; radio
imaging by the ground-based Arecibo telescope in Puerto Rico in 2008 suggested
that it comprises two sphere-like objects. Meanwhile, the bone-shaped
comet 103P/Hartley 2, imaged during NASA's EPOXI flyby in 2011, revealed
a comet with two distinct halves separated by a smooth region. In addition,
observations of asteroid 25143 Itokawa by JAXA's Hayabusa mission, combined
with ground-based data, suggest an asteroid comprising two sections of
highly contrasting densities.

[Animation]
Rotating view of comet 67P/C-G on 14 July 2014.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Is Rosetta en-route to rendezvous with a similar breed of comet? The scientific
rewards of studying such a comet would be high, as a number of possibilities
exist as to how they form.

One popular theory is that such an object could arise when two comets
- even two compositionally distinct comets - melded together under a low
velocity collision during the Solar System's formation billions of years
ago, when small building blocks of rocky and icy debris coalesced to eventually
create planets. Perhaps comet 67P/C-G will provide a unique record of
the physical processes of accretion.

Or maybe it is the other way around - that is, a single comet could be
tugged into a curious shape by the strong gravitational pull of a large
object like Jupiter or the Sun; after all, comets are rubble piles with
weak internal strength as directly witnessed in the fragmentation of comet
Shoemaker-Levy 9 and the subsequent impacts into Jupiter, 20 years ago
this week. Perhaps the two parts of comet 67P/C-G will one day separate
completely.

[Image]
Comet 67P/C-G on 14 July 2014 - processed view. Credits: ESA/Rosetta/MPS
for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

On the other hand, perhaps comet 67P/C-G may have once been a much rounder
object that became highly asymmetric thanks to ice evaporation. This could
have happened when the comet first entered the Solar System from the Kuiper
Belt, or on subsequent orbits around the Sun.

One could also speculate that the striking dichotomy of the comet's morphology
is the result of a near catastrophic impact event that ripped out one
side of the comet. Similarly, it is not unreasonable to think that a large
outburst event may have weakened one side of the comet so much that it
simply gave away, crumbling into space.

But, while the interpolated images are certainly brilliant, we need to
be closer still to see a better three-dimensional view - not to mention
to perform a spectroscopic analysis to determine the comet's composition
- in order to draw robust scientific conclusions about this exciting comet.

Rosetta Mission Manager Fred Jansen comments: "We currently see images
that suggest a rather complex cometary shape, but there is still a lot
that we need to learn before jumping to conclusions. Not only in terms
of what this means for comet science in general, but also regarding our
planning for science observations, and the operational aspects of the
mission such as orbiting and landing.

"We will need to perform detailed analyses and modelling of the shape
of the comet to determine how best we can fly around such a uniquely shaped
body, taking into account flight control and astrodynamics, the science
requirements of the mission, and the landing-related elements like landing
site analysis and lander-to-orbiter visibility. But, with fewer than 10
000 km to go before the 6 August rendezvous, our open questions will soon
be answered."
Received on Thu 17 Jul 2014 03:09:47 PM PDT