Monday, 22 December 2014

The Surface of a Comet

Philae landing
This sequence of images captures the landing location of Rosetta’s Philae lander. The first image in the sequence shows the pre-landing surface, at a resolution of 1.3 m/pixel, acquired 3.5 minutes before first touchdown. The second image in the sequence shows the landing site about 1.5 minutes after first touchdown. The large circle on the left highlights the plume of dust Philae raised when it bounced off the surface on its first touchdown. The smaller circles on the right point out where the Philae lander finally settled after bouncing twice on the surface of the comet. More details about this image can be found at:
Image Credit:  ESA/Rosetta/NavCam
It has been a very exciting summer and fall for planetary exploration, but I have been far too busy with a new job and home renovations to write about it. Finally, I have managed to squeeze out a bit of time, just before the end of the year, to summarize the European Space Agency’s awesome landing on the surface of Comet 67P/Churyumov-Gerasimenko.

Comet Churyumov–Gerasimenko is shaped kind of like a barbell, with one smaller lobe and another larger lobe connected by a narrow “neck”. Detailed images from the OSIRIS (Optical, Spectrocopic and Infrared Remote Imaging System) camera on board the Rosetta spacecraft provided the high resolution images that were needed to understand the comet’s form and to select a target site for the Philae lander. An area on the outer edge of the comet’s smaller lobe, just outside a large, circular depression, was selected as the landing site. The selection criteria considered scientific interest, safety, and operations. This particular site was chosen because it was thought to 1) have a relatively smooth and flat surface (at least locally), providing a safe place to land, 2) receive sufficient sunlight to re-charge Philae’s solar batteries, making the location operationally viable, and 3) be close to active processes and provide access to pristine materials, making it scientifically interesting.

Shape model of comet
Comet 67P/Churyumov–Gerasimenko has a very irregular shape. Images taken by the OSIRIS camera on the Rosetta spacecraft have allowed this 3D shape model to be calculated. Philae landed on the outer edge of the comet’s smaller lobe, just outside of the large, circular depression located there.
More details about this image can be found at:

Active processes on a comet can be considerably different from those on other solid bodies in the solar system, like asteroids, moons, and planets. For one, comets have a high content of volatile material that regularly sublimates every time the comet comes close to the sun (at perihelion). Thus, sublimation is the dominant process operating on comet surfaces, rather than impacts, which tend to dominate the other solar system bodies.

Cometary surface processes have been studied in the past, most notably on comet 19/P Borrelly, which was observed in 2001 when the Deep Space 1 mission flew by.  Comet Borrelly is a Jupiter-family comet, like Cheryumov-Gerasimenko, therefore both comets are expected to have similar orbits, periods, and active processes. The most notable observation on comet Borrelly was the complete absence of impact craters, down to the resolution limit of 200 m.  Small depressions of 200-300 meters were observed, but their morphology and distribution argued against an impact origin. Instead, Dr. Dan Britt and his team of researchers, who studies the comet’s surface, proposed that these pits were caused by sublimation. 
Philea landing location – 50 km
This image from Rosetta’s OSIRIS narrow-angle camera, taken from 50 km above the comet’s surface, shows the location of the Philae landing site, just outside a large circular depression on the outer edge of the comet’s smaller lobe.
More details about this image can be found at:
Philea landing location – 30 km
In this image from Rosetta’s OSIRIS narrow-angle camera, taken from 30 km above the comet’s surface, more details of Philae’s landing site start to become visible. The lander appears to have touched down on a smooth elevated surface within a rugged terrain.
More details about this image can be found at:

During its perihelion pass around the sun, volatile materials are sublimated from the surface of the comet. Non-volatile materials, in contrast, are left behind, building up an insulating “lag” layer that protects the surface from further sublimation and erosion. If this layer is not too thick, thermal instabilities can cause pits to form, exposing the volatile materials below. Impacts can also do the same for thicker lag deposits. In both cases, the volatiles exposed sublimate away, undermining the protective layer and causing the depressions to grow in size. This kind of growth, over may frequent perihelion passes, is thought produce a very rugged topography, with many flat surfaces and steep slopes.

The surface features that Dr. Britt and his team observed at comet Borrelly support this hypothesis. They saw smooth areas, which were interpreted to represent accumulations of non-volatile materials. They saw pits, which they think are indicative of relatively thin lag deposits. And they saw rugged topography, with many steep slopes interspersed with relatively flat regions, suggesting prolonged erosion of a lag deposit through undermining at steep slopes.

Philea landing location - 40 m
This image, taken by Philae's down-looking descent ROLIS imager from 40 m above the comet’s surface, shows that the landing surface is quite rough at smaller scales, in comparison to its smooth appearance in earlier images. The area is covered by debris, ranging in size from mm to meters, with the big bolder at the top right corner being 5 m in diameter. The black bar in the same corner is a section of Philae’s landing gear.
More details about this image can be found at:
Image Credit:  ESA/Rosetta/Philae/ROLIS/DLR
The topography of comet Cheryumov-Gerasimenko appears to have many similar elements. Philae’s landing area in particular seems to have an abundance of steep slopes with flat surfaces at their tops and bottoms. Upon closer approach, these “flat” areas are shown to be covered by rugged debris, containing many different particle sizes, from fine dust to meter-sized boulders. Although the resolution of the Deep Space 1 images was not sufficient to observe roughness of this size on comet Borelly, photometric analysis of that data did suggest that smooth units were rougher at smaller scales. Thus, even in this respect, comet Borelly and comet Cheryumov-Gerasimenko appear to be similar.

First Views from the lander
This is the first image ever taken from the surface of a comet! Acquired by Philae's CIVA camera, this two-image mosaic shows what appears to be a very rough and rugged cliff, illuminated by the sun. One of the lander’s three feet can be seen in the centre left.
More details about this image can be found at:
Image Credit:  ESA/Rosetta/Philae/CIVA
All of this implies that the surface of Cheryumov-Gerasimenko was probably formed by similar processes as comet Borelly. Thus, the surface of Cheryumov-Gerasimenko most likely represents erosive sublimation processes, where a relatively thin lag deposit was breached, presumably by thermal instabilities from below, creating pits that grew and coalesced to form the rugged terrain we see.
However, it is still not clear how the large circular feature near the Philae landing site was formed. It is possible that this feature represents a pit that grew to a particularly large size. It is also possible that this feature is an old impact crater that was too big to be completely eroded away by sublimation. More work is clearly required to answer this question.

Unfortunately, we may never get any more data from Philae on the surface of the comet. Because of its two bounces, Philae did not stay where it was originally targeted to land, but instead came to rest in the shadow of a cliff.  As a result, the solar panels could not keep the lander operational and at half past midnight (GMT) on Nov 15, 2014 Philae stopped transmitting to the Rosetta spacecraft. There is a slight possibility that when the comet makes its closest approach to the sun on August 13, 2015, Philae’s solar panels may receive enough energy to wake up the lander and re-establish communications. Until then, we will have to be satisfied with orbiter data from the Rosetta spacecraft, which continues to collect data, currently from 20 km above the comet’s surface, but with future flybys planned to approach closer than 8 km. Exciting times, indeed!

Britt, D.T. et al. The morphology and surface processes of Comet 19/P Borrelly. Icarus  167, 2004, DOI: 10.1016/j.icarus.2003.09.004.

“J” Marks the Spot for Rosetta’s Lander, ESA’s Rosetta Blog, Nov. 15, 2014.

Pioneering Philae Completes Main Mission before Hibernation, ESA’s Rosetta Blog, Sept. 15, 2014.