Latest Research Reveals a Bizarre and Vibrant Rosetta’s Comet

We’ve subsisted for months on morsels of information coming from ESA’s mission to Comet 67P/Churyumov-Gerasimenko. Now, a series of scientific papers in journal Science offers a much more complete, if preliminary, look at Rosetta’s comet. And what a wonderful and complex world it is.

Scientists have defined 19 regions on Comet 67P/Churyumov-Gerasimenko’s nucleus according to terrain and named for Egyptian deities like Imhotep, Aten and Hathor. Credits: ESA/Rosetta/MPS/OSIRIS Team/UPD/LAM/IAA/SSO /INTA/UPM/DASP/IDA

Each of the papers describes a different aspect of the comet from the size and density of dust particles jetting from the nucleus, organic materials found on its surface and the diverse geology of its bizarre landscapes. Surprises include finding no firm evidence yet of ice on the comet’s nucleus. There’s no question water and other ices compose much of 67P’s 10 billion ton mass, but much of it’s buried under a thick layer of dust.

Despite its solid appearance, 67P is highly porous with a density similar to wood or cork and orbited by a cloud of approximately 100,000 “grains” of material larger than 2 inches (5 cm) across stranded there after the comet’s previous perihelion passage. Thousands of tiny comet-lets!

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 January 23,  2015 issue of the journal Science. Credit: ESA

Researchers have identified 19 distinct geological regions on the comet and five basic types of terrain: dust-covered, brittle material, large-scale depressions, smooth terrains and consolidated surfaces.

Features in the Hapi region show evidence of local gas-driven transport of dust producing dune-like ripples (left) and boulders with ‘wind-tails’ (right) – where boulders have acted as natural obstacles to the direction of the gas flow, creating streaks of material ‘downwind’ of it. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The Ma’at and Ash regions (in blue above) are buried in 3 to 15 feet of dust that once filled the comet’s coma and then slowly settled back to the surface. High resolution images show dune-like structures, ripples and rocks with wind tails. Winds on a comet? It’s thought that sublimating (vaporizing) CO and CO2 ice beneath the surface vents to the surface with a force strong enough to create fleeting gusty winds in the comet’s low gravity, lifting and moving dust across the landscape. Electrostatic levitation of dust charged by sunlight may also play a role.

The boundary between the Ash and Seth regions features a great example of fracturing and collapse of the dust-covered surface at A, B and C (left) caused by erosion through vaporization of ice. At right we see the border of a large circular depression in Seth. Look closely to the upper left and lower right of G and you’ll spot a chain of depressions likely caused by ice sublimating from beneath the surface. Click to enlarge. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The Seth and Imhotep regions are rich with brittle material liable to fracture and collapse when ices beneath them vaporize in sunlight to create voids. Given the comet’s extremely low gravity field, 67P must be composed of very fragile material for it to fall apart with such ease. Air candy anyone?

Impact craters appear to be rare on the comet. Explosive release of sublimating CO or CO2 may have excavated Hetmehit, the large circular depression on 67P’s smaller lobe. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

No impact craters have been positively identified yet, but the comet’s head and body each have a large, circular depression that may or may not be related to impact. Hatmehit, about 1 km in diameter, is located on the head of the nucleus with Aten on the body.

The irregular depression called Aten (center) may have been blasted out due to extreme temperature variations experienced by the comet during its orbit around the Sun. It’s thought that sublimating CO and CO2 ices did the job, not water. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Aten has a remarkably fresh floor free of dust. Nicolas Thomas, an OSIRIS camera co-investigator, speculates the depression may have formed as extreme changes in temperature experienced by the comet during its 6.5 year orbit around the Sun cracked and weakened the surface. Sublimating gas beneath the surface could then have explosively launched huge chunks of crust into space. Almost like watching an impact in reverse.

Every orbit, 67P it loses an estimated 1.1 million U.S. tons (3-5 billion kg) of material to space. To create the vast Aten chasm would have required some 10 to 20 orbits.

A pit in Seth at left clearly shows visible jetting in the enhanced contrast image (right). Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The neck of the comet, named Hapi, along with Imhotep and Anubis, are dominated by extremely smooth material (dust) with no obvious depressions and hardly a boulder. Along one side of Imhotep dozens of circular structures or pits several hundred meters across many of which appear to be filled with several meters of dust. Scientists suspect they were once active sites of vaporizing ices and are waiting to see if they might come alive again as the comet warms as it approaches its August perihelion.

Suncups form when sunlight ablates or melts compacted snow. Credit: Derek Harper / Wikipedia

Close-up photos of one of the pits revealed that material had oozed out like a homemade vinegar and baking soda volcano hinting that pressures beneath the surface can rise high enough to convert the ice and dust into a fluid mix. Quiet for the moment, some pits may turn into smoking geysers when the time is right.

Smooth terrain in the Imhotep region showing layering (B) and circular structures or pits (circled). Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Pits may also have formed as “suncups” or depressions carved out by ice vaporizing in the Sun’s heat. If you’ve ever walked across a old snowfield, you’re probably familiar with their scooped appearance.

Close-up of a curious surface texture nicknamed ‘goosebumps’ taken with the OSIRIS narrow-angle camera. The bumps are approximately 9 feet across, extending over regions greater than 300 feet. They’re seen on very steep slopes such as in this pit and exposed cliff faces, but their formation mechanism is yet to be explained. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

In what could be the most fascinating finding yet, walls in some of the pits are studded with 9-foot-diameter (3-m) nodules nicknamed “goosebumps” or “dinosaur eggs” that some researchers believe may be the original chunks of primitive matter that glommed together 4.5 billion years ago to create the comet. Seeing them reminds me of how we might build a fireplace by stacking and cementing wave-rounded boulders.

Though dominated by dust, the neck or Hapi region was the primary source of water outgassing when Rosetta pulled up to the comet last August. Further study as 67P becomes more active on its approach to the Sun show many more sources. Some release primarily water vapor, others carbon monoxide or carbon dioxide gas that vary according to season and Sun exposure.

Several views of 67P’s giant cliff, Hathor. Erosion here in the form of linear grooves reveals the internal structure of the head. At lower right, Hathor meets the Hapi region. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The grandest of the consolidated surfaces – the fifth type of terrain – is Hathor, a towering 2,950-foot (900 meter) cliff that dominates the underside of the comet’s duck-like head. Its distinctive linear features, which run both up and down and across for much of its height, reveal brighter material that suggests we’re seeing the internal structure of the comet’s head. Tucked along an alcove on the cliff are additional bright white spots less than 30 feet (10-m) across that may be patches of sublimating ice.

Local bright spots, less than 30 feet across (labelled) and seen in an alcove in the Hathor region are compositionally distinct from the surrounding terrain. The image was taken by the OSIRIS narrow-angle camera on 7 August 2014. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Another region named Anuket borders Hathor and exhibits significant cracks that may have formed from stress induced by the comet’s rotation. For a complete synopsis of what these cracks could mean for 67P’s future, please check out Universe Today’s Tim Reyes’ article on the topic.

An exposed spur of material shows irregular fracturing (left). At right is a circular depression that appears to have been uplifted on the left side with vertical fractures (C). Collapse on the right generated a cascade of debris (D). Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Using a spectrometer to scan the comet’s surface researchers discovered complex organic (carbon-based) molecules that could include carboxylic acids – a component of amino acids. These organics only form in deep space where cosmic rays and ultraviolet light strike ice-coated dust to energize the creation of new compounds. Finding them in 67P could indicate that the comet formed farther from the Sun than expected and therefore may be more pristine than other comets. Finally, water vapor given off by 67P has been analyzed and discovered to have a higher amount of deuterium to hydrogen compared to water on Earth, indicating that 67P and its brethren were probably not responsible for seeding our planet’s oceans.

OSIRIS wide-angle camera image taken on November 22 from a distance of 18.6 miles (30 km) from 67P reveals a host of jets of sublimating ice and dust. The vertical line in the bottom right of the image, which seems to separate two regions of the coma with slightly different brightness, is the shadow of the nucleus cast onto the coma.

Churyumov-Gerasimenko is active right now and will only become more so as it approaches perihelion this summer. Expect the landscape to transform before our eyes. Cliffs that look solid now will give way and crumble, pits will break their silence and erupt as geysers as chunks of comet take flight. Here on Earth the tumult will reveal itself as an delicate coma followed by the streak of a tail.

Below you’ll find links that will take you to all the papers in Science and a wonderful website filled with hi-resolution images of Rosetta’s comet:

* Science: Catching a Comet

* Rosetta Gallery

Bob King

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. I also write a daily astronomy blog called Astro Bob. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, will publish in April. It's currently available for pre-order at Amazon and BN.

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