[meteorite-list] How Rosetta's Comet Got Its Shape

[Ron Baalke]

http://www.esa.int/Our_Activities/Space_Science/Rosetta/How_Rosetta_s_comet_got_its_shape

How Rosetta's Comet Got Its Shape
European Space Agency
28 September 2015

Two comets collided at low speed in the early Solar System to give rise 
to the distinctive "rubber duck" shape of Comet 67P/Churyumov-Gerasimenko, 
say Rosetta scientists.

The origin of the comet's double-lobed form has been a key question since 
Rosetta first revealed its surprising shape in July 2014.

Two leading ideas emerged: did two comets merge or did localised erosion 
of a single object form the "neck"?

Now, scientists have an unambiguous answer to the conundrum. By using 
high-resolution images taken between 6 August 2014 and 17 March 2015 to 
study the layers of material seen all over the nucleus, they have shown 
that the shape arose from a low-speed collision between two fully fledged, 
separately formed comets.

Layers on the comet's surface

"It is clear from the images that both lobes have an outer envelope of 
material organised in distinct layers, and we think these extend for several 
hundred metres below the surface," says Matteo Massironi, lead author 
from the University of Padova, Italy, and an associate scientist of the 
OSIRIS team.

"You can imagine the layering a bit like an onion, except in this case 
we are considering two separate onions of differing size that have grown 
independently before fusing together."

The results of the study are reported in the journal Nature and were presented 
today at the European Planetary Science Congress in Nantes, France.

To reach their conclusion, Matteo and his colleagues first used images 
to identify over 100 terraces seen on the surface of the comet, and parallel 
layers of material clearly seen in exposed cliff walls and pits. A 3D 
shape model was then used to determine the directions in which they were 
sloping and to visualise how they extend into the subsurface.
      
The comet's two lobes

It soon became clear that the features were coherently oriented all around 
the comet's lobes and in some places extended to a depth of about 650 
m.

"This was the first clue that the two lobes are independent, reinforced 
by the observation that the layers are inclined in opposite directions 
close to the comet's neck," says Matteo.

"To be sure, we also looked at the relationship between the local gravity 
and the orientations of the individual features all around the reconstructed 
comet surface."

Broadly speaking, layers of material should form at right angles to the 
gravity of an object. The team used models to compute the strength and 
direction of the gravity at the location of each layer.

In one case, they modelled the comet as a single body with a centre of 
mass close to the neck. In the other, they worked with two separate comets, 
each with its own centre of mass.

The team found that orientation of a given layer and the direction of 
the local gravity are closer to perpendicular in the model with two separate 
objects, rather than in the one with a single combined nucleus.

"This points to the layered envelopes in the comet's head and body forming 
independently before the two objects merged later," concludes Matteo. 
"It must have been a low-speed collision in order to preserve such ordered 
strata to the depths our data imply."

"In addition, the striking structural similarities between the two lobes 
imply that despite their initially independent origins, they must have 
formed through a similar accretion process," adds co-author Bjorn Davidsson 
of Uppsala University, Sweden.

"Layering has also been observed on the surface of other comets during 
previous flyby missions, suggesting that they also underwent a similar 
formation history."

Finally, the team note that even though erosion is not the root cause 
of the comet's double-lobed shape, it nevertheless does play an important 
role in the comet's evolution today.

Local variations seen in the structure of the surface likely result from 
different rates of sublimation - when ice turns directly into a gas - 
of frozen gases embedded within the individual layers, which are not necessarily 
distributed evenly throughout the comet.

"How the comet got its curious shape has been a major question since we 
first saw it. Now, thanks to this detailed study, we can say with certainty 
that it is a 'contact binary'," says Holger Sierks, OSIRIS principal investigator 
at the Max Planck Institute for Solar System Research in Gottingen.

"This result adds to our growing knowledge of the comet - how it formed 
and its evolution," says Rosetta project scientist Matt Taylor.

"Rosetta will continue to observe the comet for another year, to get the 
maximum amount of information on this celestial body and its place in 
the history of our Solar System."

Notes to Editors

"The two independent and primitive envelopes of the bilobate nucleus of 
comet 67P/C-G," by M. Massironi et al., is published as Advanced Online 
Publication on www.nature.com today.

Dr Massironi presented the study today at the European Planetary Science 
Congress in Nantes, France, in a dedicated press briefing.

For further information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer at esa.int

Matteo Massironi
University of Padova, Italy
Email: matteo.massironi at unipd.it

Holger Sierks
OSIRIS Principal Investigator
Max Planck Institute for Solar System Research
Tel: +49 551 384 979 242
Email: sierks at mps.mpg.de

Matt Taylor
ESA Rosetta Project Scientist
Email: matt.taylor at esa.int