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

[Ron Baalke]

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."