[meteorite-list] Comet Sinkholes Generates Jets (Rosetta)

[Ron Baalke]

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

Comet Sinkholes Generates Jets
Active pits on comet
European Space Agency
1 July 2015

A number of the dust jets emerging from Rosetta's comet can be traced 
back to active pits that were likely formed by a sudden collapse of the 
surface. These "sinkholes" are providing a glimpse at the chaotic 
and diverse interior of the comet.

Rosetta has been monitoring Comet 67P/Churyumov-Gerasimenko's activity 
for over a year, watching how its halo of dust and gas grows as the comet 
moves closer to the Sun along its orbit.

>From a distance of a few hundred kilometres, Rosetta observes an intricate 
pattern of the dust jets emitted from the nucleus as they stream out into 
space. But now, thanks to high-resolution images from the OSIRIS camera 
from distances of just 10-30 km from the comet centre last year, at 
least some of these dust jets can be traced back to specific locations 
on the surface, the first time this has ever been seen.

Active regions in Seth

In a study reported today in the science journal Nature, 18 quasi-circular 
pits have been identified in the northern hemisphere of the comet, some 
of which are the source of continuing activity.

The pits are a few tens to a few hundreds of metres in diameter and extend 
up to 210 m below the surface to a smooth dust-covered floor. Material 
is seen to be streaming from the most active pits.

'We see jets arising from the fractured areas of the walls inside the 
pits. These fractures mean that volatiles trapped under the surface can 
be warmed more easily and subsequently escape into space," says Jean-Baptiste 
Vincent from the Max Planck Institute for Solar System Research, lead 
author of the study.

Scientists analysing the images think that the pits are formed when the 
ceiling of a subsurface cavity becomes too thin to support its own weight 
and collapses as a sinkhole. This exposes the fractured interior of the 
comet, allowing otherwise hidden material to sublimate, thus continuing 
to erode the pit over time.

Active pits

"Although we think the collapse that produces a pit is sudden, the cavity 
in the porous subsurface could have growing over much longer timescales," 
says co-author Sebastien Besse, of ESA's ESTEC technical centre in the 
Netherlands.

The authors suggest three possible ways the voids are formed.  

One idea is that they have existed since the comet itself formed, as a 
result of very low-speed collisions between primordial building blocks 
tens to hundreds of metres in size. The collapse of the roof above such 
a void could then be triggered through weakening of the surface, perhaps 
by sublimation or via seismic shaking or impact from boulders ejected 
from elsewhere on the comet.

Another possibility is the direct sublimation of pockets of volatile ices 
like carbon dioxide and carbon monoxide below the surface, heated by the 
warmth of sunlight penetrating an insulating top layer of dust.

Alternatively, sublimation could be driven by the energy liberated by 
water ice changing its physical state from amorphous to crystalline then 
sublimating the more volatile surrounding carbon dioxide and carbon monoxide 
ices.
                                                                      
Comet pit formation

If either of the latter two processes is the driving force, then the fact 
that the pits are not seen everywhere may indicate an uneven distribution 
of ices inside the comet.

"Regardless of the processes creating the cavities, these features show 
us that there are large structural and/or compositional differences within 
the first few hundred metres of the comet's surface and the cavities 
are revealing relatively unprocessed materials that might not otherwise 
be visible," adds Sebastien.

The authors note that the internal features revealed on the pit walls 
vary quite significantly from pit to pit, and include fractured material 
and terraces, horizontal layers and vertical striations, and/or globular 
structures nicknamed "goosebumps".

"We think that we might be able to use the pits to characterise the 
relative ages of the comet's surface: the more pits there are in a region, 
the younger and less processed the surface there is," explains Jean-Baptiste.

"This is confirmed by recent observations of the southern hemisphere: 
this is more highly processed because it receives significantly more energy 
than the northern hemisphere, and does not seem to display similar pit 
structures."
      
The evolution of comet pits

The active pits are particularly steep-sided, whereas pits without any 
observed activity are shallower and may instead indicate regions that 
were active in the past. The team suggests that the active pits are the 
youngest, while middle-aged pits exhibit boulders on their floors that 
have fallen from the sides. Meanwhile, the oldest pits have degraded rims 
and are filled with dust.

"We are continuing to analyse our observations to see if this theory 
holds true, and if this 'time series' is related to the internal thermal 
evolution of the comet, for example," adds Sebastien.

"But we think that most of the active pits must have been present for 
several orbits around the Sun already, or else we would have expected 
to see a number of outbursts as their collapses were triggered this time 
around."

Rosetta did witness one outburst during its approach to the comet in April 
2014, which is thought to have generated between 1000 kg and 100 000 kg 
of material. The authors state that a pit collapse could have been the 
driver for this outburst, but only a small fraction of the total volume 
of a typical pit could have been liberated at the time.

For example, given the measured average comet density of 470 kg per cubic 
metre, the rapid evacuation of a typical large pit 140 m wide and 140 
m deep would result in the release of around a billion kilograms of material, 
several orders of magnitude greater than was observed in April 2014.

'We are very interested to see how these active pits evolve and maybe 
we'll even witness the formation of a new pit," says Matt Taylor, 
ESA's Rosetta project scientist.

"Being able to observe changes in the comet, in particular linking activity 
to features on the surface, is a key capability of Rosetta and will help 
us to understand how the comet's interior and surface have evolved since 
its formation.

"And with the extension of the mission until September 2016, we can 
do the best job possible at unravelling how comets work."

Notes for Editors

"Large heterogeneities in comet 67P as revealed by active pits from 
sinkhole collapse," by Jean-Baptiste Vincent et al is published in Nature.

About OSIRIS

The scientific imaging system OSIRIS was built by a consortium led by 
the Max Planck Institute for Solar System Research (DE) in collaboration 
with CISAS, University of Padova (IT), the Laboratoire d'Astrophysique 
de Marseille (FR), the Instituto de Astrofisica de Andalucia, CSIC (ES), 
ESA's Scientific Support Office (NL), the Instituto Nacional de T?cnica 
Aeroespacial (ES), the Universidad Polit?chnica de Madrid (ES), the Department 
of Physics and Astronomy of Uppsala University (SE), and the Institute 
of Computer and Network Engineering of the TU Braunschweig (DE). OSIRIS 
was financially supported by the national funding agencies of Germany 
(DLR), France (CNES), Italy (ASI), Spain (MEC) and Sweden (SNSB) and the 
ESA Technical Directorate.

About Rosetta

Rosetta is an ESA mission with contributions from its Member States and 
NASA. Rosetta's Philae lander is contributed by a consortium led by DLR, 
MPS, CNES and ASI.

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

Jean-Baptiste Vincent
Max Planck Institute for Solar System Research, Gottingen, Germany
Email: vincent at mps.mpg.de

Sebastien Besse
ESA-ESTEC
Email: sebastien.besse at esa.int

Matt Taylor
ESA Rosetta project scientist
Email: matt.taylor at esa.int