Exposed water ice detected on comet's surface

June 25, 2015 by Markus Bauer






, European Space Agency
Ice on Comet 67P/Churyumov-Gerasimenko. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Using the high-resolution science camera on board ESA's Rosetta spacecraft, scientists have identified more than a hundred patches of water ice a few metres in size on the surface of Comet 67P/Churyumov-Gerasimenko.

Rosetta arrived at the comet in August 2014 at a distance of about 100 km and eventually orbited the comet at 10 km or less, allowing of the surface to be acquired.

A new study just published in the journal Astronomy & Astrophysics focuses on an analysis of bright patches of exposed ice on the comet's surface.

Based on observations of the gas emerging from comets, they are known to be rich in ices. As they move closer to the Sun along their orbits, their surfaces are warmed and the ices sublimate into gas, which streams away from the nucleus, dragging along dust particles embedded in the ice to form the coma and tail.

But some of the comet's dust also remains on the surface as the ice below sublimates, or falls back on to the nucleus elsewhere, coating it with a thin layer of dusty material and leaving very little ice directly exposed on the surface. These processes help to explain why Comet 67P/Churyumov-Gerasimenko and other comets seen in previous flyby missions are so dark.

Despite this, Rosetta's suite of instruments has already detected a variety of gases, including water vapour, carbon dioxide and carbon monoxide, thought to originate from frozen reservoirs below the surface.

Now, using images taken with Rosetta's OSIRIS narrow-angle camera last September, scientists have identified 120 regions on the surface of Comet 67P/Churyumov-Gerasimenko that are up to ten times brighter than the average surface brightness.

Some of these bright features are found in clusters, while others appear isolated, and when observed at high resolution, many of them appear to be boulders displaying bright patches on their surfaces.

Icy clusters and individual boulders. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The clusters of bright features, comprising a few tens of metre-sized boulders spread over several tens of metres, are typically found in debris fields at the base of cliffs. They are most likely the result of recent erosion or collapse of the cliff wall revealing fresher material from below the dust-covered surface.

By contrast, some of the isolated bright objects are found in regions without any apparent relation to the surrounding terrain. These are thought to be objects lifted up from elsewhere on the comet during a period of cometary activity, but with insufficient velocity to escape the gravitational pull of the comet completely.

In all cases, however, the bright patches were found in areas that receive relatively little solar energy, such as in the shadow of a cliff, and no significant changes were observed between images taken over a period of about a month. Furthermore, they were found to be bluer in colour at visible wavelengths compared with the redder background, consistent with an icy component.

"Water ice is the most plausible explanation for the occurrence and properties of these features," says Antoine Pommerol of the University of Bern and lead author of the study.

"At the time of our observations, the comet was far enough from the Sun such that the rate at which water ice would sublimate would have been less than 1 mm per hour of incident solar energy. By contrast, if carbon dioxide or carbon monoxide ice had been exposed, it would have rapidly sublimated when illuminated by the same amount of sunlight. Thus we would not expect to see that type of ice stable on the surface at this time."

The team also turned to laboratory experiments that tested the behaviour of water ice mixed with different minerals under simulated solar illumination in order to gain more insights into the process. They found that after a few hours of sublimation, a dark dust mantle a few millimetres thick was formed. In some places this acted to completely conceal any visible traces of the ice below, but occasionally larger dust grains or chunks would lift from the surface and move elsewhere, exposing bright patches of water ice.

Colour composites of icy bright patches on comet. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

"A 1 mm thick layer of dark dust is sufficient to hide the layers below from optical instruments," confirms Holger Sierks, OSIRIS principal investigator at the Max Planck Institute for Solar System Research in Göttingen.

"The relatively homogeneous dark of the nucleus of Comet 67P/Churyumov-Gerasimenko, only punctuated by some metre-scale bright dots, can be explained by the presence of a thin dust mantle composed of refractory mineral and organic matter, with the bright spots corresponding to areas from which the dust mantle was removed, revealing a water-ice-rich subsurface below."

The team also speculates about the timing of the formation of the icy patches. One hypothesis is that they were formed at the time of the last closest approach of the comet to the Sun, 6.5 years ago, with icy blocks ejected into permanently shadowed regions, preserving them for several years below the peak temperature needed for sublimation.

Another idea is that even at relatively large distances from the Sun, carbon dioxide and carbon monoxide driven-activity could eject the icy blocks. In this scenario, it is assumed that the temperature was not yet high enough for water sublimation, such that the -rich components outlive any exposed or ice.

"As the comet continues to approach perihelion, the increase in solar illumination onto the bright that were once in shadow should cause changes in their appearance, and we may expect to see new and even larger regions of exposed ," says Matt Taylor, ESA's Rosetta project scientist.

"Combining OSIRIS observations made pre- and post-perihelion with other instruments will provide valuable insight into what drives the formation and evolution of such regions."

Explore further: Rosetta's comet remains active after nightfall and emits dust jets into space

More information: "OSIRIS observations of meter-size exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments." DOI: dx.doi.org/10.1051/0004-6361/201525977

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15 comments

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jonesdave
3.9 / 5 (7) Jun 25, 2015
Ice on a comet. Who'd have thought? :)
rossim22
1 / 5 (4) Jun 25, 2015
Ice on a comet. Who'd have thought? :)


The presence of minuscule amounts of surface ice is not argued by anyone. However, they observe bright spots and have hypothesized that these are patches of exposed ice. It'll be interesting to see if these spots sublimate quickly, as the entire surface has apparently done much farther from the Sun.
richard_f_cronin
1 / 5 (3) Jun 25, 2015
Miniscule water on all the comets and comet trails. Per other Rosetta findings, water observed is more enriched with deuterium than terrestrial water. Deuterium water would be logical from solar or stellar origins, where protium has a longer exposure to a neutron rich fusion environment. The far better explanation, the water on all planets are from internal sources, such as the naturally occurring fission GeoReactor described by Dr. J. Marvin Herndon. The GeoReactor also gives a far better explanation for Expansion Tectonics (terrestrial Berylium 10 in magmas) vs. Plate Tectonics, the geomagnetic field, planetary rotation, stellar ignition (fission igniting fusion), global warming and cooling going back to "Snowball Earth", all simple life-giving molecules...including CO2. http://www.prnews...091.html
bluehigh
4.3 / 5 (3) Jun 25, 2015
For a moment I thought this looked like images of Ceres.

jonesdave
3.8 / 5 (10) Jun 25, 2015
Ice on a comet. Who'd have thought? :)


The presence of minuscule amounts of surface ice is not argued by anyone. However, they observe bright spots and have hypothesized that these are patches of exposed ice. It'll be interesting to see if these spots sublimate quickly, as the entire surface has apparently done much farther from the Sun.


As they mention in the paper, they'll sublimate at about 1mm per hour at 3.5 AU if water ice, much more quickly if C02 or CO. They should stick around for a while, even allowing for closer heliocentric distance. A 3m boulder of ice, at 1mm per hour, requires 3000 hours of sunlight at 3.5 AU, less at closer distances, but still a pretty decent lifetime.
jonesdave
3.8 / 5 (10) Jun 25, 2015
And don't forget, they spectrally identified surface ice at Tempel 1 (http://www.scienc...6/1453), and directly observed the subsurface ice excavated by the impact (http://www.leif.o...ts.pdf).
So I don't think they're going out on too much of a limb here!
rossim22
1 / 5 (7) Jun 25, 2015
The only people who provide an ample amount of skepticism regarding these theories are the scientists themselves. Nobody said they are going on a limb calling it water ice, nobody disputes water ice has been found on other comets (in tiny amounts) or that it may exist on the surface of 67P. I'm only acknowledging what the science writers do not. These scientists have not detected ice on the surface of 67P, that is a false confirmation which their paper does not express. The scientists found bright spots, which are hypothesized to be water ice. I'm not saying they're wrong, I just disagree with how the article represents the observations.
Vietvet
4.6 / 5 (9) Jun 25, 2015
@rossim

"OSIRIS observations of meter-size exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments"
http://www.aanda....01525977

That"s from the paper, not the article..
rossim22
1 / 5 (4) Jun 25, 2015

Yeah, like I said for the 25375825 time, I am not doubting their interpretation of the observed bright spots. But that's all they've detected, bright spots. The ice is a hypothesis yet to be tested with further observation, that's what the article should be addressing. Maybe your bias had made the assumption for you prematurely.

From the paper:

"Our preferred interpretation of the meter-sized bright spots identified in OSIRIS NAC images acquired between August and Novermber 2014 is the exposure of water ice at the surface."

"Monitoring the evolution of the BRIGHT SPOTS over time will thus be the best way to assess whether the HYPOTHESES formed in this article are correct. "

HannesAlfven
1.6 / 5 (7) Jun 25, 2015
Let nobody forget that Philae likely never stood a chance of connecting to 67P for the very reason that it was equipped with ice screws. You'd think that this would inspire some humility and a desire to keep alternative hypotheses in the center of the conversation from that point on. But, what we've seen is an eagerness to ignore the excess of electrons observed near the surface. And all of this leads to the distinct sense that we will see reports many years from now which support a far more electrical interpretation.
Torbjorn_Larsson_OM
5 / 5 (5) Jun 26, 2015
@rossim: And we repeat again, from the paper you should read, that these are constrained (tested) hypotheses: "All these observations are consistent with exposure of water ice at the surface of boulders produced by ...". And the immediate context to your sentence, the preceding sentence you chose not to show, is "As the comet approaches perihelion, the increase in insolation in the illuminated regions should induce a fast sublimation of the water-ice exposures ...". That is: a) it is claimed to be an observation of ice and b) they make auxiliary hypotheses.

So unless you have a competing theory that does as well, the hypotheses has survived a lot of testing already. That is what we commonly call an observation, a fact: "the water-ice exposures" is a factful claim.
Torbjorn_Larsson_OM
5 / 5 (6) Jun 26, 2015
@HA: So every time your crackpot theory get winged by actual and successful science despite being unquantifiable, or more precisely because it doesn't offer an alternative, you claim victory? Color us others unsurprised.

Meanwhile, science progresses despite your ineffectual nattering and boring insistence to not show an empirical (quantifiable) basis for your ideas.
rossim22
1 / 5 (4) Jun 26, 2015
@rossim: And we repeat again, from the paper you should read, that these are constrained (tested) hypotheses: "All these observations are consistent with exposure of water ice at the surface of boulders produced by ...". And the immediate context to your sentence, the preceding sentence you chose not to show, is "As the comet approaches perihelion, the increase in insolation in the illuminated regions should induce a fast sublimation of the water-ice exposures ...". That is: a) it is claimed to be an observation of ice and b) they make auxiliary hypotheses.

So unless you have a competing theory that does as well, the hypotheses has survived a lot of testing already. That is what we commonly call an observation, a fact: "the water-ice exposures" is a factful claim.


No you're absolutely correct. It's called a hypothesis just to be politically correct, but it's already a confirmed fact. NASA and ESA shouldn't spend another dime, we know everything already. Nothing new left.
rossim22
1.8 / 5 (5) Jun 26, 2015
You guys act as if the scientists cannot be wrong. That's not what the paper states, it's that simple. Do you really think that a skeptic must have an alternate theory in order to question the validity of a statement? That's ridiculous and unscientific. If you read this paper and finish it thinking the observation may show exposed ice, then great, the scientists provide a lot of support. But if you think it's confirmed that the bright spots must be and can only be ice, then you need to pick up a book on scientific philosophy.
jonesdave
4.2 / 5 (5) Jun 27, 2015
You guys act as if the scientists cannot be wrong. That's not what the paper states, it's that simple. Do you really think that a skeptic must have an alternate theory in order to question the validity of a statement? That's ridiculous and unscientific. If you read this paper and finish it thinking the observation may show exposed ice, then great, the scientists provide a lot of support. But if you think it's confirmed that the bright spots must be and can only be ice, then you need to pick up a book on scientific philosophy.


I think it is worth a 99% bet that it is ice. They explained why it is unlikely to be CO2. It is even less likely to be chalk or some other form of mineral. Given the spectral analysis of the bright spots on Tempel 1, and the indications on 67P, then I think both the authors, and anybody who reads that paper, is justified in saying it is almost certainly water ice. Very small chance of other types of ice.
Pretty much zero chance of anything else.

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