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Headshot of Emily Lakdawalla

At last! A slew of OSIRIS images shows fascinating landscapes on Rosetta's comet

Posted By Emily Lakdawalla

26-01-2015 11:50 CST

Topics: Rosetta and Philae, pretty pictures, comets, comet Churyumov-Gerasimenko, explaining science, conference report

Last week a suite of eight peer-reviewed papers were published in Science magazine, detailing the first results of the Rosetta mission. The papers are open-access, so you can go read them all if you want. And don't miss the supplemental materials! The publication of these papers means that the OSIRIS camera team has finally released a large quantity of closeup images of comet Churyumov-Gerasimenko, taken in August and September of last year. Since OSIRIS images and derived data products are such a rare treat I figured I'd post them all here and discuss them based on the two papers and also my notes from the Rosetta presentations at the American Geophysical Union (AGU) meeting, which I never wrote up because I lacked the images to illustrate them!

Two papers concerned these images: "On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko", by Holger Sierks and coauthors, which I'll refer to as the nucleus paper, and "The morphological diversity of comet 67P/Churyumov-Gerasimenko", by Nicolas Thomas et al, which I'll call the morphology paper. The nucleus paper primarily described the images and the basic properties of the comet, while the morphology one ventured a little deeper into the fascinating landscapes on the comet's surface.

First, some geographical terminology. There are now agreed-upon names for 19 geomorphic regions on the comet, using names from Egyptian mythology. Regions on the head have names of female deities, while regions on the body have names of male deities. In the supplementary material to the morphology paper they say that the boundaries between regions were drawn "at sharp changes in surface morphology (e.g., rough vs smooth surface) or clear structural/ topographical boundaries (e.g., ridge or edge of a depression)." One of the speakers at AGU -- I no longer remember who -- got a laugh out of the audience by pointing out that there is a location on the comet that is now named the "Hapi-Babi region."

Map of comet Churyumov-Gerasimenko regions

ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Map of comet Churyumov-Gerasimenko regions
The 19 regions identified on Comet 67P/Churyumov–Gerasimenko are separated by distinct geomorphological boundaries. Following the ancient Egyptian theme of the Rosetta mission, they are named for Egyptian deities. They are grouped according to the type of terrain dominant within each region. Five basic categories of terrain type have been determined: dust-covered (Ma’at, Ash and Babi); brittle materials with pits and circular structures (Seth); large-scale depressions (Hatmehit, Nut and Aten); smooth terrains (Hapi, Imhotep and Anubis), and exposed, more consolidated (‘rock-like’) surfaces (Maftet, Bastet, Serqet, Hathor, Anuket, Khepry, Aker, Atum and Apis).

Having geographical names makes it easier and more precise to begin to discuss comet morphology and activity. For instance, the nucleus paper could state that most of the observed comet activity originated from the Hapi region. That's quite curious, because modeling indicates that Hapi receives less total energy input (because of shadowing) than the highest-latitude parts of the two comet lobes. "This suggests a large compositional or structural difference in Hapi relative to other regions," the paper states.

So far, Rosetta has imaged only 70% of the comet's surface, because of the season -- the south pole is in permanent winter shadow. The comet reaches equinox in May, and the south pole will be well illuminated when the comet is at perihelion in august. The lack of imaging in the south makes it a bit of a challenge to estimate comet nucleus properties like volume and therefore density, but the OSIRIS team was able to use data from the MIRO microwave radiometer to coarsely map the topography of the unlit south pole, completing their shape model. The team speculates that once we do see the south pole, it might look dramatically different from other areas of the comet, because it receives much more intense heating for much shorter time than the northern hemisphere does.

Here's a summary of the physical properties of the comet as presented in all the Science papers:

Comet 67P/Churyumov-Gerasimenko infographic

ESA

Comet 67P/Churyumov-Gerasimenko infographic
Summary of properties of Comet 67P/Churyumov–Gerasimenko, as determined by Rosetta’s instruments during the first few months of its comet encounter. The full range of values are presented and discussed in a series of papers published in the 23 January 2015 issue of the journal Science.

The fact that I found most striking in here was the 70-80% porosity. I had a hard time wrapping my mind about what kind of interior structure could create such a huge amount of void space, but I looked up a paper on snow physics and found that this this is actually quite a bit less porous than fresh snow, more in the neighborhood of wind-packed snow or melt-freeze crusts atop snowbanks. Of course, the comet isn't just made of water ice; it has other ices of varying densities, and also organic and silicate materials, too. But I'm somewhat relieved by the fact that, the pososity of an icy world with nearly zero gravity is in family with snow.

One very curious aspect of Churyumov-Gerasimenko is how lacking in color it is. Sierks remarked in his AGU talk: "There is no color variation. Except a very few points, if you zoom in, you see some bright material falling off cliffs that are slightly less red in spectral data." The comet is everywhere the same shade of extremely dark brownish black. To my mind, that suggests that everything is mantled with some kind of dust, the way Mars looks the same shade of red everywhere because of its ubiquitous windblown dust. There are only a couple of spots where the OSIRIS team members saw any color or albedo variation. Here are two examples:

Slight color varations

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Slight color varations
Left: Color composite of an alcove at the Hathor-Anuket boundary. The surface of the consolidated material in the alcove contains distinct spots of brighter material. On the floor beneath, material 20% brighter and bluer than the surroundings can also be seen (e.g. A and B), and may be evidence of recently exposed ice. The picture is composed of three images captured through infrared and blue filters (central wavelengths of 882, 700 and 481 nanometers) of the OSIRIS narrow-angle camera and the image sequence was acquired on 21 August 2014. Right: Bright, highly reflective meter‐size boulders (C) at the Ash–Khepry boundary. An example of brittle fracturing is indicated at D. The image was acquired on 3 September 2014 with the OSIRIS narrow-angle camera.

Is it comet dust? It's hard to say. Some areas of the comet's surface are clearly covered with granular airfall material, which makes for smooth plains and softened-looking surfaces. This material probably comes from comet jets: "any ice chunks or particles larger than a few millimeters that are emitted from the source will not sublimate before falling back and could therefore form...ice deposits." Take this round depression, one of the few examples of possible impact craters on the comet. The 35-meter crater appears to be covered by airfall material with a depth of 1 to 5 meters (based on standard depth-to-diameter ratios for craters), according to the morphology paper.

Impact crater candidate

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Impact crater candidate
The surface of Comet 67P/Churyumove–Gerasimenko is almost devoid of recognisable impact craters, but this candidate may be an example formed by debris that was first ejected from the surface but subsequently fell back down. It measures about 35 m across and has been partially covered by dust. The image was acquired by the OSIRIS narrow-angle camera on 10 September 2014.

Some locations even contain dune-like features. In the morphology paper (and the captions posted with the images on the ESA website), they make it pretty clear that they believe that the dune-like appearance of these features is no coincidence; these really are wind-caused features. How can you have wind on an airless, nearly gravity-less body? Well, there are vents, and gas does come out of vents, and those may cause the kinds of shear stresses that could lift surface particles. They estimate that gas velocities of 300 meters per second are large enough to lift 100-micron particles. However, my notes from AGU indicate that there isn't consensus on the idea of these dunes being aeolian. Olivier Groussin -- who is, by the way, the 28th of 59 authors on the morphology paper -- remarked something to the effect of "I am not convinced they are actually dunes. You need good wind in good direction at good speed. Can you make them by compression or sliding? 'Dunes' is the word we've been using, but probably we will not be using it in next weeks or months."

Ripples and wind-tails

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Ripples and wind-tails
Features in the Hapi region show evidence of local gas-driven transport producing dune-like ripples (left) and boulders with ‘wind-tails’ (right) – where the boulder has acted as a natural obstacle to the direction of the gas flow, creating a streak of material ‘downwind’ of it. The images were taken with the OSIRIS narrow-angle camera on 18 September 2014.
Dune-like structures

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Dune-like structures
Evidence of dune-like structures in the surface dust of Comet 67P/Churyumov–Gerasimenko. Features like this may be the result of localised gas-driven transport of surface dust. The image was taken by the OSIRIS narrow-angle camera on 23 August 2014.

Although the dune-like features are interpreted here to be made of fluffy, unconsolidated material, the morphology paper goes on to say that once material falls out of the sky onto the surface, it may not stay so fluffy. I was really struck by this sentence: "Given the likelihood of a high organic content for the airfall deposit (e.g., cometary tholin) and exposure to sunlight, the production of a form of organic, possibly polymerized, matrix from this deposit (after the loss of any volatiles) is also conceivable." You know, we have a lot of polymerized organic material on Earth. We call that "plastic." Is the surface of this comet made of powdery snow cross-linked by a matrix of plastic?

However, there are large areas of the comet that do not appear to be covered by soft, fluffy stuff. Some areas form very steep cliffs, with slopes in some spots reaching greater than 45 degrees, particularly the steep, striated cliffs of Hathor, bordering the neck. In fact, according to the morphology paper, the 900-meter-tall cliff of Hathor is "roughly parallel to the local gravitational acceleration," that is to say, it's nearly vertical. They see vertical linear features as well as "roughly perpendicular linear features aligned with small terraces, which might suggest inner layering." They think that this is an eroded surface and therefore that Hathor is "showing us the internal structure of the head." The nucleus paper speculaets that the cracks might result from thermal shock, due to the huge variety in temperature that the comet experiences along its orbit, up to 150K.

Hapi and Hathor

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Hapi and Hathor
The boulder-strewn, smooth Hapi region in Comet 67P/Churyumov-Gerasimenko’s neck, with the Hathor cliff face to the right.

The subsurface is competent enough that it can exhibit parallel sets of fractures, sometimes everal criscrossing sets like the ones in this photo.

Tectonic features in Aker

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Tectonic features in Aker
Tectonic feature in the Aker region on the comet body, imaged on 18 September 2014 with the OSIRIS narrow-angle camera.

There's another kind of fracture that appears markedly different. It crosses Anuket, and appears to split the head and body:

Crack in Anuket

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / Emily Lakdawalla

Crack in Anuket
This OSIRIS narrow-angle camera image shows part of a large fracture running across Comet 67P/Churyumov–Gerasimenko’s neck, in particular where it has left Hapi and is extending into Anuket. In this orientation, the Seth region is at the uppermost left and Hapi in the lower left. This image has been resized from the original release (which had been enlarged about 8 times).

A particularly cool feature of this fracture is what it does when it crosses into the softer surface of Hapi. It turns into a chain of pits, meaning that as the crack opens, loose material from Hapi is falling down into the void.

Crack in Hapi

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / Emily Lakdawalla

Crack in Hapi
This OSIRIS narrow-angle camera image shows part of a large fracture running through the Hapi region in Comet 67P/Churyumov–Gerasimenko’s neck. At the location of the right arrow, the fracture appears as a chain of pits. This image has been resized to 40% of the original image release (which appears to have been enlarged by 500%).

There are lots of other features on the comet that suggest ongoing mass wasting processes:

Mass wasting processes on Churyumov-Gerasimenko

ESA / Rosetta / Philae / ROLIS / DLR; ESA / Rosetta / MPS for OSIRIS-Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA; Emily Lakdawalla

Mass wasting processes on Churyumov-Gerasimenko
In this OSIRIS narrow-angle camera image the boundary between the Ash and Seth regions is highlighted. The left-hand image shows evidence of collapse along the boundary, with debris at the base of a cliff (labelled A and B). Fracturing to produce crevices (C) is also observed with ‘deflation’ of the surface (D and E) appearing as a possible precursor to fracture or collapse. Surface deflation is also indicated in the right-hand image (G), above a fractured cliff face (F). The image also highlights the contrast between the dust covered horizontal slope and the ‘clean’ vertical face. The left image was taken on 7 August 2014, the right image on 2 September 2014.

Some of the most impressive features on the comet are circular, flat-floored, steep-walled depressions, like this one in the Seth region. They bear similarities to features comet missions have seen before, on Wild 2 and Tempel 1. It's clear from the OSIRIS images that at least some of these host comet jets, which become visible when you crank up the contrast in the photo:

Before & after: Active pit in Seth region of comet Churyumov-Gerasimenko
Before & after: Active pit in Seth region of comet Churyumov-Gerasimenko
 

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Before & after: Active pit in Seth region of comet Churyumov-Gerasimenko
Active pit detected in Seth region of Comet 67P/Churyumov–Gerasimenko. This is an OSIRIS narrow-angle camera image acquired on 28 August 2014 from a distance of 60 km. The image resolution is 1 m/pixel. Enhancing the contrast reveals fine structures in the shadow of the pit, interpreted as jet-like features rising from the pit.

How do these depressions form? My notes from Olivier Groussin's talk at AGU suggest he thinks they've always been there, but I'm guessing this is a topic of much internal team debate: "Are these primary voids, from their formation process, or evolutionary voids, from activity? I tend to favor the first scenario, because the second requires large mass loss. The depth-to-diameter ratio is much larger than craters; think they are collapse pits like sinkholes. It's not due to erosion, probably. Erosion is happening there, but it's not the dominant process. Also erosion would not create circular features." Later in his talk, he mentioned that "large primordial voids imply a gentle formation process," with comet-forming material accreting at low speeds, in the neighborhood of a meter per second.

That image, of the pit in Seth, was taken quite early in the orbital mission, from a great distance. When Rosetta got closer, it saw very strange details in the walls of those pits. At AGU, Holger Sierks called these "dinosaur eggs." They've since changed the metaphor to "goosebumps." The nucleus paper describes them thus: "The bumps themselves exhibit a characteristic scale of about 3 m. Although we do not yet have an interpretation of these features, the fact that a single characteristic scale is present may be an important clue to formation processes, as many of the possible processes have characteristic scales." Since they presumably represent the interior structure of the comet, I'd like a more three-dimensional metaphor: maybe gumballs, or the pebbly texture of expanded polystyrene foam. Sierks talked about these features at length at AGU. What are they? We don't know. I think they're the most intriguing and unusual thing OSIRIS has discovered so far. I wish these two images hadn't been so badly munged up before release.

Dinosaur eggs or goosebumps in a comet pit

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Dinosaur eggs or goosebumps in a comet pit
Close-up of a curious surface texture nicknamed ‘goosebumps’. The characteristic scale of all the bumps seen on Comet 67P/Churyumov–Gerasimenko by the OSIRIS narrow-angle camera is approximately 3 m, extending over regions greater than 100 m. They are seen on very steep slopes and on exposed cliff faces, but their formation mechanism is yet to be explained.
Dinosaur eggs or goosebumps in a comet cliff

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Dinosaur eggs or goosebumps in a comet cliff
Close-up of a curious surface texture nicknamed ‘goosebumps’. The characteristic scale of all the bumps seen on Comet 67P/Churyumov–Gerasimenko by the OSIRIS narrow-angle camera is approximately 3 m, extending over regions greater than 100 m. They are seen on very steep slopes and on exposed cliff faces, but their formation mechanism is yet to be explained.

All of the OSIRIS images shown in this post are image files that were prepared for print publication, and as far as I can tell, almost all have been cropped, resized, and processed in different ways before being posted to the Web by ESA. My guess is that they were placed in some kind of desktop publishing software before being exported; in many cases the gray levels in the original images were smashed down into a 16-level color table, with dithering, and were then enlarged at least twice before export (once by a nearest-neighbor process and once with interpolation). All that processing destroyed subtle shading and detail -- in particular, it made it impossible to see features in shadows, or bring out jets and coma material in the sky beyond the comet. So we're not really seeing anything like the full capability of OSIRIS in most of these images. Some of the most ridiculously enlarged photos I downsized before posting here in order to make them play nicer with your browsers.

ESA released one final image as part of the press release, which was used on the cover of Science -- since it is 2048 pixels square, I think this is the only one that actually represents the pixels as they came off the detector. It's also the most recent image of the bunch, acquired three months ago.

Comet landscape from 8 kilometers (OSIRIS)

ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Comet landscape from 8 kilometers (OSIRIS)
In October 2014, Rosetta ventured to within 8 kilometers of the surface of comet Churyumov-Gerasimenko -- lower than the cruising altitude of a passenger plane -- to capture this photo of smooth plains and steep cliffs with its OSIRIS science camera.

In this image, at least, fine details lurking in the shadows have not been destroyed by processing. Enhance the contrast and you can see beautiful intersecting sets of fractures in the cliff wall, lit by the wan light reflected off the smoother plains below the cliffs.

Before & after: Fractures in shadowed Churyumov-Gerasimenko cliffs
Before & after: Fractures in shadowed Churyumov-Gerasimenko cliffs
 

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / Emily Lakdawalla

Before & after: Fractures in shadowed Churyumov-Gerasimenko cliffs
Enhancing the brightness of OSIRIS images can reveal details in shadows only indirectly lit. Here, some cliffs on the head of the comet are revealed to have at least two criscrossing sets of fractures.

Untold treasures await among the vast archive of image data being gathered by Rosetta. Mark your calendars -- the first pile of data should be released to the public on May 19, 2015, including images gathered up to November 19, 2014.

 
See other posts from January 2015

 

Read more blog entries about: Rosetta and Philae, pretty pictures, comets, comet Churyumov-Gerasimenko, explaining science, conference report

Comments:

David Frankis: 01/26/2015 02:57 CST

For people who find the Science papers heavy going, several of them have been summarised on the Rosetta blog: http://blogs.esa.int/rosetta/

Sherbert: 01/26/2015 05:38 CST

Hi Emily. Nice to hear from someone who knows what was said by the OSIRIS team at the AGU meeting. Yesterday I tried posting about the OSIRIS images and their quality on the ESA Rosetta blog, the comments were almost the same as you used here. I added some detailed comments about the features and morphology seen in the ONE full sized and two cropped native resolution images. The post has not appeared. After ESA's DG's comments recently about images it seems to be a touchy subject.

lakeman: 01/27/2015 02:08 CST

Thanks Emily,A fantastic piece of work, must have taking hours of hard work, much appreciation. dave

lodaya: 01/27/2015 04:17 CST

There is a Nature report (http://www.nature.com/news/hunt-for-philae-hangs-in-the-balance-1.16775) on discussion within the ESA on whether Rosetta's mid-February near-flyby should be used for locating and picturing Philae, or to leave that to a later date, and concentrate more on obtaining zero-shadow images of the surface. It is not an easy decision, but I think the location of Philae is reasonably delineated inside the Ma'at region, so leaving actual location to a picture when it emerges more into daylight, or to a month later is fine, since it is expected to become active only about March. It might be a good idea to go where the action is happening now and try to grab it, in the Seth and Ash regions. Once it gets some proper sunlight Ma'at might well become an active region itself and flying by it might yield more interesting results.

lodaya: 01/27/2015 06:54 CST

I made a mistake in my Egyptian locations: Philae may not be in Ma'at but in Maftet, one needs a more detailed map of the region behind the Hatmehit crater. See Robin Sherman's map indicating the Consert area in www.flickr.com/photos/124013840@N06/16373930901/

Marco Parigi: 01/28/2015 01:16 CST

a lot of the "bizarre" morphologies, or those which defy explanation, are quite easily explained by stretch theory. Imhotep's morphology has just been explained in scute1133.wordpress.com, for instance.

Morgan: 01/29/2015 12:54 CST

Thank you so much for condensing all that work into an accessible article. As 67p nears the sun we should start to see the jets. Gives me 'goosebumps'.

Marco: 02/01/2015 04:22 CST

The cover shot for science is Hatmehit, isn't it? It appears quite distorted, although that could be the perspective and closer distance making the whole area look shorter and the cliffs very tall.

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