[meteorite-list] Comet Surface Changes Before Rosetta's Eyes

[Ron Baalke]

http://blogs.esa.int/rosetta/2015/09/18/comet-surface-changes-before-rosettas-eyes/

Comet surface changes before Rosetta's eyes
ESA Blog
September 18, 2015
 			
In the months leading to the perihelion of Comet 67P/Churyumov-Gerasimenko, 
Rosetta scientists have been witnessing dramatic and rapid surface changes 
on the Imhotep region, as reported in a paper to be published in Astronomy 
& Astrophysics

Since arriving at Comet 67P/C-G in August 2014, Rosetta has been witnessing 
an increase in the activity of the comet, warmed by the ever-closer Sun. 
A general increase in the outflow of gas and dust has been punctuated 
by the emergence of jets and dramatic rapid outbursts in the weeks around 
perihelion, the closest point to the Sun on the comet's orbit, which occurred 
on 13 August 2015.

But in June 2015, just two months before perihelion, Rosetta scientists 
started noticing important changes on the surface of the nucleus itself. 
These very significant alterations have been seen in Imhotep, a region 
containing smooth terrains covered by fine-grained material as well as 
large boulders, located on 67P/C-G's large lobe.

[Images]
Sequence of ten images showing changes in the Imhotep region on Comet 
67P/C-G. The images were taken with the OSIRIS narrow-angle camera on 
Rosetta between 24 May and 11 July 2015. The individual images are also 
available separately. Credits: ESA/Rosetta/MPS for OSIRIS Team 
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

"We had been closely monitoring the Imhotep region since August 2014, 
and as late as May 2015, we had detected no changes down to scales of 
a tenth of a metre," comments Olivier Groussin, an astronomer at the Laboratoire 
d'Astrophysique de Marseille, France, OSIRIS Co-Investigator and lead 
author of the study.

"Then one morning we noticed that something new had happened: the surface 
of Imhotep had started to change dramatically. The changes kept going 
on for quite a while."

First evidence for a new, roughly round feature in Imhotep was seen in 
an image taken with Rosetta's OSIRIS narrow-angle camera on 3 June. Subsequent 
images later in June showed this feature growing in size, and being joined 
by a second round feature. By 2 July, they had reached diameters of roughly 
220 m and 140 m, respectively, and another new feature began to appear.

By the time of the last image used in this study, taken on 11 July, these 
three features had merged into one larger region and yet another two features 
had appeared.

[Images]
Same sequence as above, with indication of dates and location of the morphological 
changes. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

"These spectacular changes are proceeding extremely rapidly, with the 
rims of the features expanding by a few tens of centimetres per hour. 
This highlights the complexity of the physical processes involved," adds 
Olivier.

The sublimation of volatile species is clearly an important factor, as 
colour images of this region reveal the signature of exposed ice on some 
of the rims of the newly-formed surface features. The rapid rate of expansion 
is unexpected, however: models of sunlight-driven sublimation would predict 
erosion rates of just a few centimetres per hour, and thus the scientists 
believe that additional mechanisms are required to explain the observations.

A simple possibility is that the surface material is very weak, allowing 
for more rapid erosion, but it is also possible that the crystallisation 
of amorphous ice or the destabilisation of so-called "clathrates" (a lattice 
of one kind of molecule containing other molecules) could liberate energy 
and thus drive the expansion of the features at faster speeds.

[Images]
Colour images of the Imhotep region on Comet 67P/C-G, taken with the OSIRIS 
narrow-angle camera on Rosetta on 18 June (upper row), 2 July (middle 
row) and 11 July 2015 (lower row). The first column shows images taken 
in the orange filter (649 nanometres); the second column shows the ratio 
between images taken with the blue filter (481 nanometres) and the orange 
filter for the 18 June and 2 July images, and the ratio between images 
taken with the blue and the red (701 nanometres) filters for the 11 July 
image; the third column shows a composite obtained by combining the images 
in the previous two columns. The yellow arrows indicate some of the new 
features that were detected on Imhotep. These colour images show that 
some patches on the surface of the comet reflect orange/red light less 
effectively and blue light more effectively than their surroundings. They 
appear as white in the central column, where the colour ratio is shown. 
This indicates the presence of frozen water ice at or just below the surface 
of these patches. Credits: ESA/Rosetta/MPS for OSIRIS Team 
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The erosion could be accompanied by increased rates of gas outflow, including 
H2O, CO2, or CO. The scientists also searched in OSIRIS images for evidence 
of increased dust rising from Imhotep as the surface morphology evolved, 
but did not find any.

While it is unlikely that many small (micron-sized) dust particles were 
released as the features formed and expanded, it is possible that the 
same amount of mass was released in a smaller number of larger (millimetre-sized) 
particles, which would produce less reflected light and thus be harder 
to detect with OSIRIS.

In addition, a significant fraction of the dust released may have immediately 
fallen back to the surface, accumulating at the base of the expanding 
rims.

[Image]
Activity seen above the Imhotep region with the OSIRIS narrow-angle camera 
on Rosetta on 23 May 2015 (left), before significant morphological changes 
were seen in this region, and on 23 June 2015 (right), after the changes 
had begun to appear. (Times are in UT.) The positions of the first two 
new features that were seen in Imhotep are marked with A and B. The white 
arrows indicate the direction along which an increase of activity would 
have been seen in the case of jets lifting from the newly arisen features. 
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Although the scientists were initially surprised to see such significant 
changes taking place on smooth terrains such as those seen in Imhotep, 
the location of this region close to the comet's equator guarantees that 
it receives large amounts of sunlight.

"We are looking forward to combining our OSIRIS observations with data 
from other instruments on Rosetta, to piece together the origin of these 
curious features,' concludes Olivier.