Rosetta shows how comet interacts with the solar wind

This blog post is based on the papers “Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko: Observations between 3.6 and 2.0 AU ” by H. Nilsson et al.; “Rosetta observations of solar wind interaction with the comet 67P/Churyumov-Gerasimenko” by T.W. Broiles et al.; and “Solar Wind Sputtering of Dust on the Surface of 67P/Churyumov-Gerasimenko ” by Peter Wurz et al., which have all been accepted for publication in Astronomy and Astrophysics, andDynamical features and spatial structures of the plasma interaction region of 67P/Churyumov–Gerasimenko and the solar wind” by C. Koenders et al, which is published in Planetary and Space Science.

Rosetta is making good progress in one of its key investigations, which concerns the interaction between the comet and the solar wind.

The solar wind is the constant stream of electrically charged particles that flows from the Sun, carrying its magnetic field out into the Solar System. Like all comets, 67P/Churyumov–Gerasimenko must navigate this flow in its orbit around the Sun.

MeetAcomet_cropIt is the constant battle fought between the comet and the solar wind that helps to sculpt the comet’s ion tail. Rosetta’s instruments are monitoring the fine detail of this process.

Using the Rosetta Plasma Consortium Ion Composition Analyzer, Hans Nilsson from the Swedish Institute of Space Physics and his colleagues have been studying the gradual evolution of the comet’s ion environment. They have seen that the number of water ions – molecules of water that have been stripped of one electron – accelerated away from the comet increased hugely as 67P/C-G moved between 3.6AU (about 538 million km) and 2.0AU (about 300 million km) from the Sun. Although the day-to-day acceleration is highly variable, the average 24-hour rate has increased by a factor of 10,000 during the study, which covered the period August 2014 to March 2015.

The water ions themselves originate in the coma, the atmosphere of the comet. They are placed there originally by heat from the Sun liberating the molecules from the surface ice. Once in gaseous form, the collision of extreme ultraviolet light displaces electrons from the molecules, turning them into ions. Colliding particles from the solar wind can do this as well. Once stripped of some of their electrons, the water ions can then be accelerated by the electrical properties of the solar wind.

Not all of the ions are accelerated outwards, some will happen to strike the comet’s surface. Solar wind particles will also find their way through the coma to hit home. When this happens, they cause a process called sputtering, in which they displace atoms from material on the surface – these are then ‘liberated’ into space.

Peter Wurz from the University of Bern, Switzerland, and colleagues have studied these sputtered atoms with Rosetta’s Double Focussing Mass Spectrometer (DFMS), which is part of the ROSINA experiment.

They have so far discovered sodium, potassium, silicon and calcium, which are all present in a rare form of meteorites called carbonaceous chondrites. There are differences in the amounts of these atoms at the comet and in these meteorites, however. While the abundance of sodium appears the same, 67P/C-G shows an excess of potassium and a depletion of calcium.

Most of the sputtered atoms come from the winter side of the comet. Although this is the hemisphere that is mostly facing away from the Sun at present, solar wind particles can end up striking the surface because they are deflected during interactions with ions in the comet’s coma. This can be a significant process so long as the density of the coma ions is not too large. But at some point the comet’s atmosphere becomes dense enough to be a major defence, protecting the icy surface.

Screenshot from a simulation of plasma interactions between Comet 67P/C-G and the solar wind around perihelion.  Credit: Modelling and simulation: Technische Universität Braunschweig and Deutsches Zentrum für Luft- und Raumfahrt; Visualisation: Zuse-Institut Berlin

Screenshot from a simulation of plasma interactions between Comet 67P/C-G and the solar wind around perihelion. Click for full animation and detailed caption. Credit: Modelling and simulation: Technische Universität Braunschweig and Deutsches Zentrum für Luft- und Raumfahrt; Visualisation: Zuse-Institut Berlin

As the comet gets closer to the Sun, the sputtering will eventually stop because the comet will release more gas and the coma will become impenetrable. When this happens, the solar wind ions will always collide with atoms in this atmosphere or be deflected away before striking the surface.

The first evidence that this deflection is taking place at 67P/C-G has been measured with the Rosetta Plasma Consortium Ion and Electron Sensor, by Thomas Broiles of the Southwest Research Institute (SwRI) in San Antonio, Texas, and colleagues.

Their observations began on 6 August 2014 when Rosetta arrived at the comet, and have been almost continuous since. The instrument has been measuring the flow of the solar wind as Rosetta orbits 67P/C-G, showing that the solar wind can be deflected by up to 45° away from the anti-solar direction.

The deflection is largest for the lighter ions, such as protons, and not so much for the heavier ions derived from helium atoms. For all ions the deflection is set to increase as the comet gets closer to the Sun and the coma becomes ever denser.

As all this happens, Rosetta will be there to continue monitoring and measuring the changes. This was the raison d’être for the rendezvous with this comet. Previous missions have taken snapshots during all too brief fly-bys but Rosetta is showing us truly how a comet behaves as it approaches the Sun.

Read “Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko: Observations between 3.6 and 2.0 AU” by H. Nilsson et al. here

Read “Rosetta observations of solar wind interaction with the comet 67P/Churyumov-Gerasimenko” by T.W. Broiles et al. here

Read “Solar Wind Sputtering of Dust on the Surface of 67P/Churyumov-Gerasimenko” by Peter Wurz et al. here.

Read Dynamical features and spatial structures of the plasma interaction region of 67P/Churyumov–Gerasimenko and the solar wind” by C. Koenders et al. here.

 

Comments

30 Comments

  • logan says:

    Evidence accumulating about 67P being a single object 🙂

  • ianw16 says:

    Thanks for these. Interesting stuff. Good to see that the two I have were available as free access. Hoping the other two are the same.
    Very interesting regarding the various Na/K ratios for solar system objects.

  • sol88 says:

    Well Done!

  • Harvey says:

    The (very interesting indeed) simulation assume 5*10^27 molecules/sec gas production. It been a *very* long time since we've had an update on the current best estimate for that critical parameter.

    It would be really interesting to have a current or near current figure to compare to 5*10^27, have an idea of mass loss, etc etc.

    Any chance one could be provided?

    • Harvey says:

      To answer my own question.
      Recently ESA have quoted 300kg/sec of water, which I make 10^28 molecules/sec.
      So the simulation is very much in the right region - which I'm sure is no coincidence!

      I need to go dig in the papers. Not only water will contribute, other species could be photoionised too, & 67P produces a lot of them. I wonder if they do contribute significantly?

  • originalJohn says:

    "The water ions themselves originate in the coma, the atmosphere of the comet. They are placed there originally by heat from the Sun liberating the molecules from the surface ice. "

    Once again hypothesis presented as fact and used as the basis for the interpretation. So the interpretation is hypothetical too.

    The rest of it uses a speculative computer model of the interaction of the solar wind with the comet coma with scattered bits of data inserted speculatively in it. The objective appears to be confirmation of beliefs. The bow shock
    idea is a belief with no published data supporting it.

    The ion measuring instruments would be far more beneficially deployed near the surface of the nucleus where the significant reactions are occurring. It is hypothesised that some protons reach and sputter the nucleus surface but
    that most are deflected by the comet coma. What is this process whereby the "gas" of the coma deflects protons. And what is the measured proton current density at the nucleus surface compared to the solar wind proton density impinging on the coma boundary.

    It is observed that results from the DFMS instrument show that the products of sputtering from the nucleus surface contain a collection of elements found too in carbonaceous chondrite meteorites, but in different proportions.
    No comment though on the significance of this in the context of the hypothesised ice origin and composition of comets.

    "But at some point the comet’s atmosphere becomes dense enough to be a major defence, protecting the icy surface."

    What icy surface ? And what is the density of the coma at this point. Is there any data for change of coma density with time. Is there any data for proton flux density within the coma.

    • Harvey says:

      Spacecraft have flown right through the shock and seen its unmistakeable signature.
      http://onlinelibrary.wiley.com/doi/10.1029/96JA04002/abstract
      'No published data'; really? This is direct, unequivocal observations! Figure 1 of the paper, staring you in the face.

      In fact it would be amazing if you *didn't* see a shock, if the density is adequate, for exactly the same reason a supersonic plane greats a boom.

      Nobody suggests that the *neutral gas* deflects the protons; it's the resulting magnetised plasma that does that.

      The instruments can't be deployed near the surface without unacceptable risk to Rosetta.

      But on one thing we agree,mid low some near surface densities and gas production rates, none given in quite a while.

      • originalJohn says:

        You see a bow shock in figure 1 Harvey, I see an increase in density of all species as a boundary is crossed. The concept of a bow shock implies a collision between incoming particles and a barrier which changes their direction and perhaps increases their density at the barrier, not increases particle density as the boundary is crossed and maintains that increase or increases it further as the motion progresses.

        • Harvey says:

          What that means I've no idea.
          It has ALL the characteristics of a bow shock, it's a bow shock. You just don't see what you don't want to see.
          It would be amazing if there *wasn't* a bow shock, that would be really hard to explain!
          Essentially it would be Concorde, well supersonic, with no 'bang'; now that would be interesting.

    • ianw16 says:

      @OJ

      "What icy surface ?

      The icy surface and near subsurface that is producing all that H2O.

      "And what is the density of the coma at this point. Is there any data for change of coma density with time. Is there any data for proton flux density within the coma."

      There is a lot of data from the various missions to Halley, Giacobini-Zinner, Grigg-Skjellerup and Borrelly. Far too much to sum up here. A thorough understanding would actually involve making a study of that easily available and well known data. However, to sum up the findings that are most relevant to your question: "The plasma in the inner coma of comet Halley (within r = 10^5km) was observed to be almost stationary (less than several km/s) and entirely of cometary origin." From "Comets in the Post-Halley Era, Volume 2." pp. 1231. Viewable online at https://books.google.co.uk/books?id=QmydbA0TswgC&pg=PA1211&lpg=PA1211&dq=plasma;+comets&source=bl&ots=Q72DNKSXhO&sig=FIYyC0GAZPajnITpi_upcFXXODo&hl=ensa=X&ved=0CCcQ6AEwATgKahUKEwiX7--o_pvHAhUiK9sKHbKcA6I#v=onepage&q=plasma%3B%20comets&f=false

      Therefore, as the comet becomes more active the less solar wind protons reach the surface, until a point where, to all intents and purposes, none are. This is already seen in the data for the sputtering results, where there is an almost total anti-correlation of the Si and H2O detection locations. As the authors say, the H2O emissions are essentially preventing the solar wind from reaching the surface there, so the Si detections are only seen from low to zero outgassing regions. This was all some months ago, of course. By now I expect there to be little if any solar wind interaction with the surface, as already seen at the aforementioned comets.

      • originalJohn says:

        Why rely on old flyby results of dubious interpretation w16 when we have a loitering mission in front of us now.
        The capability is there to map the ion density of the coma. Let us see it.
        Too much inferred from random isolated results up to now.
        I say the jets are the result of the impact of solar wind protons with the nucleus surface. Prove me wrong with results from this mission.

        • ianw16 says:

          "The plasma in the inner coma of comet Halley (within r = 10^5km) was observed to be almost stationary (less than several km/s) and entirely of cometary origin."

          Nothing dubious about that. And the solar wind carries a magnetic field. And electric currents create magnetic fields. So where are they within the diamagnetic cavity at Halley and, now, 67P?
          You're still clinging to a crackpot idea that has zero evidence to back it up, and plenty against it. Kind of resembles a religion.

        • Harvey says:

          The magnetic field data alone proves that.

      • logan says:

        "The plasma in the inner coma of comet Halley (within r = 10^5km) was observed to be almost stationary (less than several km/s) and entirely of cometary origin."

        r=100,000km is a lot of inner coma, for any comet.

    • Gerald says:

      Don't you think, it's time to say "good-bye" to the unphysical "electric comet" idea, originaljohn?

  • ianw16 says:

    @OJ
    "The bow shock idea is a belief with no published data supporting it."

    Really? I found loads. Such as http://www-personal.umich.edu/~tamas/TIGpapers/1986/1986_Gringauz_nature.pdf which describes both the Vega spacraft encoutering and flying through the bow wave/shock at Halley a mere 29 years ago. And also from Giotto: http://onlinelibrary.wiley.com/doi/10.1029/JA095iA12p20701/full. Not to mention other results from Grigg-Skjellerup & Borrelly. It is also mentioned in the paper "Rosetta observations of solar wind interaction with the comet 67P/Churyumov-Gerasimenko", where it says "Observations of the interaction between the solar wind and cometary ionospheres were studied previously at the comets Giaccobini-Zinner, Halley, Grigg-Skjellerup, Borrelly. With exception to Giaccobini-Zinner, each spacecraft passed through a cometary bow shock...." and gives relevant references.

    • originalJohn says:

      The results you are talking about w16 are interpreted as bow shock because that was the expectation. The solar wind is a plasma, a stream of charged particles. The comet coma is a plasma. The boundary between plasmas of different characteristics is recognised as a particular phenomenon, nothing to do with collision between neutral gas particles. Any interpretation which fails to address the plasma aspect is misguided.

      NASA were expecting to find a bow shock at the boundary between the heliosphere and the interstellar medium. It did not materialize.

      Bow shock is a fluid interaction phenomenon, actually with a solid but could be two fluids. Plasmas are not fluids and do not behave like fluids. Plasma boundaries can, to simplify it, accelerate or repel charged particles, depending on the relative charge.

      • Harvey says:

        Comet bow shocks *are plasma phenomena* for heavens sake! Not shock waves in the neutral gas!
        The fact that there is no shock there has no relevance; modelling showed it might, or might not occur; they thought it probably would, but it doesn't, the parameters are such that no shock forms.
        Near a comet, if the degassing produces a sufficient density of photoionised plasma, it can form - or not, if out gassing is too weak.
        .

        • originalJohn says:

          That's it Harvey. It is plasma. So you agree that any change in the direction or velocity of the solar wind or the interstellar medium can only be caused by the magnetic field of any other plasma it encounters, a comet coma or the heliosphere in the cases in question.

          That is charged particles interacting with a magnetic field. So it is not a fluid effect, you would agree, and not therefore a bow shock.

          Irrespective of the magnetic effect the charged particle stream can be either accelerated straight through the boundary, or decelerated, or stopped and reversed in direction at the plasma boundary depending on the relative charge. No fluid model required.

  • ianw16 says:

    @OJ
    ""The water ions themselves originate in the coma, the atmosphere of the comet. They are placed there originally by heat from the Sun liberating the molecules from the surface ice. "

    "Once again hypothesis presented as fact and used as the basis for the interpretation. So the interpretation is hypothetical too."

    How so? Given that ice has been identified on the surface of this comet: http://www.hou.usra.edu/meetings/lpsc2015/pdf/2021.pdf, as well as Tempel 1, and that ice was excavated from the subsurface of Tempel 1, then I'd say that sublimation of ice is the only scientifically valid mechanism for getting the measured quantities of H2O neutral and ionic species into the coma of a comet.
    Given the density and porosity of the comet, it's hard to see what else it would be.
    Unless you have a scientifically sensible alternative mechanism, that is backed up by observation?

    • originalJohn says:

      Ice has not been identified w16. Results have been interpreted as ice when other explanations are equally plausible.
      The amount of ice ( frost patches) on Tempel ! was less than 1% of that necessary to account for the water production volume.

      If you can't think of another alternative w16 it does not mean that the one you have thought of is right. I can think of a highly likely alternative. The surface combustion reaction between the surface hydrocarbon layer and oxygen from the rock. The opportunity for observation is there now.

      • ianw16 says:

        The fact that the DI impact excavated a large amount of directly observed H2O from the subsurface shows where the H2O is coming from.

        You can dream up all sorts of fantasies for what you would like to be happening, but the fact is that the electric rocky comet is still, and will forever remain, an evidence free zone.

      • Gerald says:

        Originaljohn. "I can think of a highly likely alternative. "
        Which one?
        Silicate rock doesn't release relevant amounts of oxygen to "combust" hydrocarbons. Not even with solar protons. So, what "highly likely" alternative can you think of?

      • Harvey says:

        The fact that no such reactions exist is of course a minor impediment to that explanation. The 'solar wind mediated' version doesn't work either; only energy deposited near the interface can contribute, the particle density is umpteen orders of magnitude too low, and the reaction products most likely to remain bound. If the organic layer is even a micron thick, the solar wind proton won't even reach the interface. And of course the comet is largely shielded from the solar wind now anyway.
        Aside from that it's a great 'explanation' .
        (And 'rock' on 67P is in even shorter supply than ice!)

      • Harvey says:

        Hydrocarbons sit in the pores of sand stone reservoirs, in intimate contact with it, for millions of years, at temperatures higher than 67P; no reaction.
        In some of those reservoirs there is enough radioactivity to yield recoverable helium - multi MeV alpha particles, yet the oil is still there.
        Fire bricks, furnace bricks, in intimate contact with coal, carbon at over 1000C, no reaction.
        My gas BBQ, so called 'lava' rocks I'd estimate at about 900C, in contact with combusting propane, been using them for years, still fine.
        Standard texts say carbon doesn't reduce SI-O below c1700C
        If you want really inert glassware, 'quartz', fused silica, is the material of choice - and glass blowers manipulate it with graphite tools.

        Just a few problems there.

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