VIRTIS maps comet ‘hot spots’

The VIRTIS team have produced maps of the surface temperature of comet 67P/C-G showing how the temperature varies across regions and with local time. Some examples of these were shown today during the Rosetta special session at the European Planetary Science Congress (EPSC).

Over the past two months, the VIRTIS imaging spectrometer has recorded about three million spectra of the surface of 67P/C-G as Rosetta closed in from about 14000 km to less than 100 km from the comet’s nucleus. One thing that can be extracted from these spectra is the surface temperature, which is derived from measurements of the radiation emitted in the spectral range 4.5 to 5.1 microns.

Back in mid-July, the first temperature measurements – made when the comet occupied just a few pixels in the VIRTIS field of view – yielded an average surface temperature of 205 K, already at that time ruling out a surface covered exclusively in ice.

By early August, as the comet filled more of the VIRTIS field of view, variations in the temperature could be seen, modulated by the comet’s rotation period.

Following Rosetta’s arrival at the comet on 6 August, the VIRTIS team have been able to map variations in the surface temperature across the nucleus. These data have been an essential input to the on-going landing site selection process since one of the criteria for choosing a site is that it be neither too hot nor too cold for the Philae lander.

Temperature measurements of the surface of comet 67P/Churyumov-Gerasimenko generated from data recorded by the VIRTIS instrument. The maps, on an orthographic projection of the comet’s surface centered on the 0° meridian (left) and the 180° meridian (right), show the temperature for local time between 12h and 14h. The data were obtained in July and August 2014 when the comet was between 3.6AU and 3.45AU from the Sun, and the spacecraft was closing in from 14000km to less than 100 km from the comet nucleus. The locations of the five candidate landing sites for the Philae lander are indicated.

Temperature measurements of the surface of comet 67P/Churyumov-Gerasimenko generated from data recorded by the VIRTIS instrument. The maps, on an orthographic projection of the comet’s surface centred on the 0° meridian (left) and the 180° meridian (right), show the temperature for local time between 12h and 14h. The data were obtained in July and August 2014 when the comet was between 3.6AU and 3.45AU from the Sun, and the spacecraft was closing in from 14000km to less than 100 km from the comet nucleus. The locations of the five candidate landing sites for the Philae lander are indicated. Credit: ESA/Rosetta/VIRTIS/INAF-IAPS/OBS DE PARIS-LESIA/DLR

At the Rosetta special session today at the EPSC, the VIRTIS team described how they have made temperature maps of the entire sunlit surface of the comet, noting that temperatures as high as 230 K have been recorded. They have also been able to map how the surface temperature varies with local time – as can be seen in the figure above.

Measurements of the surface temperature can provide clues to the composition and physical properties of a comet. These new VIRTIS measurements have allowed the team to rule out some models of the comet surface and to favour a comet surface composed of a porous and  highly thermally insulating dusty crust that is depleted of water ice. As they reported today, this is also consistent with the VIRTIS global measurements of thermal inertia – a measure of a body’s resistance to changes in temperature – that is compatible with the value for high porosity dusty materials.

The relatively high spatial resolution of the VIRTIS measurements have also allowed the team to investigate thermal shocks that happen when a region enters or exits from shadow.  This is of interest because thermal shocks can give rise to stresses in the surface, which can lead to micro-cracks and eventually result in fractures in the surface.

The team have also been poring over the spectra to search for hints about the chemical makeup of the surface of comet 67P/C-G. Among the preliminary results reported today was no evidence of water ice on a global scale, confirming that the outer surface is generally dehydrated.

On the other hand, they see some strong hints of carbon-bearing compounds, with some spectral features that are compatible with the complex macromolecular carbonaceous materials found in the most primitive carbonaceous meteorites. These materials are often referred to as “organics”, even if their origin is unrelated to life.

The picture of comet 67P/C-G that is beginning to emerge from these early VIRTIS measurements is of a dark, dry, and dusty comet surface with a rich and complex chemistry.

Comments

32 Comments

  • Tractor says:

    My list of possible contents of comet and coma,

    hydrogen, helium, neon, argon, krypton, xenon, oxygen, nitrogen, water ice, hydrocarbons like methane, ethane, diacetylene, methylacetylene, acetylene and propane, cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, polycyclic aromatic hydrocarbons, tholins, minerals in the composition of what is found in meteorites due to influence from the asteroid belt as it passes this regularly. Amino acids, RNA, DNA are present in meteorites so most likely the comet piked up that as well. And sot due to the high energy radiation splitting molecules into atoms.

    What of this the rosetta and philae are able to detect or what else might be found is beyond my knowledge, but i think my list covers the major part of it

    The dark surface color is due to the sot and the tholins (tar). Hopefully this layer is not too thick for Philaes drill probe.

    • Dan Delany says:

      Worth noting that, while *nucleobases*, which are the building blocks of DNA/RNA, have been found in meteorites, DNA/RNA have not, to my knowledge. That would be much bigger news.

    • THOMAS says:

      It's maybe a little premature to assert, as if it were proven fact, that "The dark surface color is due to the sot [sic] and the tholins (tar)." For their part, the people at Universetoday.com only go as far as saying that:

      "ASTRONOMERS HYPOTHESIZE (my bold) that a comet grows a dark ‘skin’ both from accumulated dust and irradiation of its pristine ices by cosmic rays. Cosmic rays loosen oxygen atoms from water ice, freeing them to combine with simple carbon molecules present on comets to form larger, more complex and darker compounds resembling tars and crude oil."

      It's worth checking out their webpage to get a clear idea of how uniformly charcoal-black and ice-free the comet actually seems to be:

      http://www.universetoday.com/114034/what-comets-parking-lots-and-charcoal-have-in-common/

      If European taxpayers have just spent billions of euros and over ten years sending Rosetta to get close-up images and information about this comet, I reckon it’s precisely because for the moment we only have unproven hypotheses whereas we actually need hard facts if we want to improve our understanding. The images and information collected (both present and upcoming) will likely falsify the “dirty snowball" model of comets and the hypotheses concerning “sot” (whatever that is) and “tholins” which now apparently go with it.

  • Tractor says:

    I forgot to say that practically all of it is in vapor and solid form, no liquids will be found and no hard rock will be found as the solids are dust created due to sublimation in vacuum. The structural integrity is about a burned and frozen toast and its density a third of water.

    I suggest that the thermal decay is measured when the surface of interest passes the shadow zone and gets cooled down, this indicates its thermal capacity, i have done this myself years ago in the 8 to 12 micrometer range on a field to detect buried obstacles . It worked very well.
    Around sunset and sunrise the thermal capacity profile shows the best contrast. A correlation of these images then results in a better then 98% found to lost ratio detection of buried obstacles in sand. In my case the remaining 2 % where deadly mines not found. On the comet this method could be used to indicate ice-patches or hollow spots depending if the thermal capacity is a peak or a valley.

    • THOMAS says:

      The assertion that “no hard rock will be found”, together with the rest of this paragraph, also seems rather premature given the spectacularly rocky images being provided by OSIRIS, and in particular:

      http://www.esa.int/spaceinimages/Images/2014/08/Rosetta_s_comet_in_3D
      and
      http://www.esa.int/spaceinimages/Images/2014/09/Comet_on_5_September_2014

      Personally, I prefer to trust my sense and my senses to interpret this sort of hard, “smoking-gun” type evidence, rather than denying it in favour of a simple belief in the postulates of the increasingly untenable “dirty snowball” theory which seems to be melting away under our very eyes….

      • Alter schwede says:

        So far any speculation is as good as any other one. Hard rock in earth term is heavy and solid in a way that pyramids are built out of it in a 1 G gravity field. This type of rock has a density of 3,5 kg / liter plus minus a bit.

        The comet has an average density of 0.3 kg/liter
        Thats a factor ten less and is not able to get "hard" due to its obvious porosity.

        The possibility that the comet has a dens core is out of reach to detect and if so then the surface is even less dense.

        The only "hard" stuff to be found is some of the micrometer scale dust grains for example silicon carbide.

        These will never form rocks as the mixture with other stuff and the very low G field are preventing this.

      • Marco says:

        Before you start saying that it "looks" like solid rock, remember that there most rock of any description is not pitch black. Also, note that Comet Wilde showed surface variation between visits that are not easily explained by rock. Soot is a weasel word for unknown carbon rich particles. Put any oil, tar, graphite particle into a detector and zap it it will behave like soot.

  • JCG says:

    It will be interesting to see if this prediction from 1980 holds true:
    Letters, Nature 283, 840 - 842 (28 February 1980); doi:10.1038/283840a0
    The composition of the Trojan asteroids
    Jonathan Gradie & Joseph Veverka
    Laboratory for Planetary Studies, Cornell University, Ithaca, New York 14853
    Physical studies have shown that most asteroids are covered with material of low visual albedo (p v~0.04) having flat, nearly featureless reflection spectra between 0.4 and 1.1 µm (ref. 1). This large class of objects, the C asteroids, accounts for a progressively larger fraction of minor planets as one goes from the inner to the outer regions of the asteroid belt, a trend which has been interpreted as evidence of a decrease in effective condensation temperature in the original solar nebula with increasing distance from the proto-Sun2. We suggest here that the very low albedos and red spectra of some Trojan asteroids can be explained by the presence of kerogen-like organic compounds. Materials containing these types of carbonaceous substances, rather than those found in the more familiar carbonaceous chondrite matrix, may have been the primary rocky condensate in the outer Solar System and may therefore be typical of the rocky component of comet nuclei.

  • Alter schwede says:

    All earth minerals i found on wiki with one exception are a lot heavier then water ranging from 1.7 to 12 kg / liter in general about 3 kg/liter.

    The comet has 0.3 kg/ liter density in average most likely uniform, when not then denser in the center, this equals some volcanic ash. Its porosity is obvious. Why searching for hollywood like vent holes as if the comet was a pressurized vessel waiting to split open. Most likely the temperature, a couple of tens of meters under the surface is stable and averaged trough its entire orbit, my guess to about 35K, Its in practically vacuum as well making it well insulated. It was never created out of liquids that solidified, Its just a heap of collected dust and vapor that sublimated in vacuum to some kind of snow of more then just water.

    • Marco says:

      @Alter Schwede. Vent holes are not theorised, they have been observed on other comets, particularly Hartley, from close range. They will be certainly found on C-G whether they are specifically searched for or not. 35 Kelvin or whatever insulated subsurface temperature is not enough to sublimate the volatiles in the quantities observed. Some of our current premises about comets are wrong - take your pick, but denying that we have observed vents because it contradicts our porosity/density calculations is wrong. Vents do imply at least some subsurface pressure to power them. Pressure implies the possibility of subsurface liquids, however transient. Some minerals found in STARDUST imply liquid water processing. We cannot deny the data that we are getting because it contradicts models that otherwise make sense.

      • Dave says:

        Marco

        vents have not been seen, emissions of material have been seen, not the holes they come from.
        If the debri is from sublimating subsurface ice, then at some time we should see the holes that it is coming from.

        If the emissions are a result of an electrical activity, then we would see no vent holes but could expect escarpments and craterwalls being erroded away. To see this we would expect white out areas on the comet when its happening and we wouuld be able to verify the change in the profile of what ever was eroding, comaparing before and after pictures.
        Its best not to make up your mind which is correct, a bit of both or none of the above until we see the evidence.

        Remember the last time something was shot towards a comet it hit solid ground, also there was a discharge flash just prior to impact and at impact. The energy released from this impact was orders of magnitude higher than expected, indicating some other force was at work, possibly electric.
        I am not worried which is the correct answer but want to ascertain for sure what comets are made of, where were they made and how old are they.
        I waiting for more detail

      • logan says:

        Agree with Marco. Liquid water necessary for crystallization evidence seen in other comets.

        • Erich says:

          Chrystal structures can very well form without liquids of any kind present.

          Low pressure and plasma can do this trick and even form diamond layers, not carbon layers i said diamond.

          Also a lot of other Chrystaline matrixes can be formed with this methods.

          • Marco says:

            @Erich Show me how cubanite can form without liquid processing. I believe physical evidence over models, when they contradict each other.

          • Dertutenaix says:

            Cubanite is fools cold containing iron coper and sulphur. It is formed at temperatures about +205deg C and it is found in some meteorites. I have no clue how it is done in those meteorites that practically contain no water al all.

          • logan says:

            Hi Erich. I badly selected my words. You are right 🙂

      • logan says:

        Structures seen on published 3D Osiris suggests a kind of 'muddy' forming. (So liquids suggested too).

      • Erich says:

        A lot can be said about the images of the Hartley flyby.
        For example great image processing and photo montage.
        This comet is tiny and in accordance to the presented images evaporates enormous amounts of vapor and dust. The areas of this visualized outbursts are spot-like but i see no VENT HOLES on Island i saw some geyser that was great and that was vent holes for real.

        A fun thing is that this comet has a shape of body neck and head and its orbiting time is about 6 years as well.

        So if i may extrapolate what will happen when 67p/C-G is closest to the sun it will be a spectacular firework of geysers and kilometer deep vent cavities.

        Or just a couple of great photomontage images where the out burst i 100 times more illuminated the the comet surface, check for your self and make your own conclusion.

  • Alter schwede says:

    Anyone with a vacuum pump that reaches below 2 milli Bars can put some liquid water of any temperature this person likes in an vessel that can be evacuated with this pump and see for how long this water stays liquid.

    It starts to boil even at 0 deg C and finally forms ice.

    Then make the reverse and put steam of water into the evacuated vessel that is cooled to just below 0 deg C.
    It will only frost the walls.

    Now this is practically experiments for those who do not believe in physical facts. Water does not exist in liquid form in vacuum, at least not for long and what long means in a cosmic time scale is up to anyone to find out for them self.

    If someone is able to put 1E13 liter of liquid water in space it would not stay in liquid form very long after a while. Water vapor and water ice is what is left like it or not.

    • logan says:

      Unless comets has some kind of natural 'pressure vessels'. Our Earth has natural 'pressure vessels' containing water or/and hydrocarbons.

      • logan says:

        'Pressure vessels can develop fissures, sometimes tragic explosions.

      • logan says:

        You have strong electro fields like Sun or Jupiter to heat those 'pressure vessels'. Given enough thermal insulation even corrosion could add to the energy balance.

        • logan says:

          Nice part of models like this is that strong electro fields occurs in many situations beyond stars and big planets.

        • logan says:

          Remember Io? Maybe it's not only the gravitational tide.

    • bob johnson says:

      my iq isnt as high as you guys, but can the enviornment of 'space' and its gravity, temp, etc be re created like in a room with clear walls here on earth?

  • Dertutenix says:

    If you google for triple point of water and find this in wikipedia you are able to learn something.
    At 6 milli bar and a temperature of + 0,0098deg C water can exist as ice vapor and liquid all at the same time.
    Now 0.006 Bar is faaaar from vacuum. SO below 6 millibar there is NO liquid water and water does know what to do so does vapor and ice. There is no way to fool it.

    One 0 deg C body of water has a minimum size to produce a pressure at its center to produce 6 millibar.

    The comet has 1e13 Kg mass and at its center , if the comet is uniform, the pressure will be a bit over this required 6 millibars to let water become liquid if it had a temperature about 0 deg C. The core of the comet will never get that hot and the surface will never reach a pressure of 6 millibars. In sum no liquid water on this place no matter how hard you wish for it. Nature never makes a mistake only humans do that kind of things.

    • Marco says:

      @dertutenix. Wha t is being suggested is coming from direct evidence. .006 bar might be a long way from a vacuum, but it doesn't require much strength of material to hold in either. If there are voids below, but near the surface, and an oily, gooey material on the surface, gases will be held in by at least that amount of pressure.

      • Dertutenaix says:

        Well, far out the surface temperature is about 50K or less.
        Google for triple points of gasses that are supposed to be present. Right now the surface temperature is 200K same procedure again. Try to find out what kind of mixture that makes this place work like a Pizza doe ready to pop open with vents. If this is the case that vent cavities is the way this comet gets rid of its inner vapors then those would not vanish at every orbit and as it has done huge amount of those orbits already the comet would be smart enough to use the same vents all over and over again looking like a Swiss cheese by now. Tell me when you find the first clue of anything that almost not looks like a vent cavity at all. The risk that Philae lands on a place where its probability to get blown in the wind or even can observe this kind of spectacularity is less the molecular. To me this is not disappointing as i never do expect this kind of scenario to happen. There will for certain be some kind of outgassing from the surface also carrying som dust away, the risk is that this outgassing condenses on Philaes cold side an affects its instruments for a while. This evaporation might be uniform or spot-like and has a great deal with electrostatic activity to accompany it, vacuum is an excellent insulator and the vapor streams act like poor conductors, the solar wind is partly charged. Even without those longed for vent holes it might be spectacular. A comet tail has a two parts , one is the dust tail and another one is the plasma tail. Electric charges will be present, put don't start to think i terms of lightning.. The major risk is that some structural changes of the comet makes problems. The seismic microphones in the feet of the lander will then be the first record of the sound of a comet ever made.

  • Mattias Malmer says:

    I reprojected the VIRTIS image into a lat-long projection and draped it over my shapemodel.

    It is much easier to understand where the heat is if you see it on the actual comet:

    http://mattias.malmer.nu/2014/10/virtis-heatmap/

  • Andy Mudd says:

    I note that the landing site must be neither too hot nor too cold for the lander. Can anyone tell me what the temperature range can be, and what the limiting factor is? ie what bit of technology goes wrong first if the limits are exceeded? Thanks

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