Comet’s firework display ahead of perihelion

This article is mirrored from the main ESA Web Portal.

In the approach to perihelion over the past few weeks, Rosetta has been witnessing growing activity from Comet 67P/Churyumov–Gerasimenko, with one dramatic outburst event proving so powerful that it even pushed away the incoming solar wind.

The comet reaches perihelion on Thursday, the moment in its 6.5-year orbit when it is closest to the Sun. In recent months, the increasing solar energy has been warming the comet’s frozen ices, turning them to gas, which pours out into space, dragging dust along with it.

The period around perihelion is scientifically very important, as the intensity of the sunlight increases and parts of the comet previously cast in years of darkness are flooded with sunlight.

Although the comet’s general activity is expected to peak in the weeks following perihelion, much as the hottest days of summer usually come after the longest days, sudden and unpredictable outbursts can occur at any time – as already seen earlier in the mission.

A short-lived outburst from Comet 67P/Churyumov–Gerasimenko was captured by Rosetta’s OSIRIS narrow-angle camera on 29 July 2015. The image at left was taken at 13:06 GMT and does not show any visible signs of the jet. It is very strong in the middle image captured at 13:24 GMT. Residual traces of activity are only very faintly visible in the final image taken at 13:42 GMT. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

A short-lived outburst from Comet 67P/Churyumov–Gerasimenko was captured by Rosetta’s OSIRIS narrow-angle camera on 29 July 2015. The image at left was taken at 13:06 GMT and does not show any visible signs of the jet. It is very strong in the middle image captured at 13:24 GMT. Residual traces of activity are only very faintly visible in the final image taken at 13:42 GMT. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

On 29 July, Rosetta observed the most dramatic outburst yet, registered by several of its instruments from their vantage point 186 km from the comet. They imaged the outburst erupting from the nucleus, witnessed a change in the structure and composition of the gaseous coma environment surrounding Rosetta, and detected increased levels of dust impacts.

Perhaps most surprisingly, Rosetta found that the outburst had pushed away the solar wind magnetic field from around the nucleus.

A sequence of images taken by Rosetta’s scientific camera OSIRIS show the sudden onset of a well-defined jet-like feature emerging from the side of the comet’s neck, in the Anuket region. It was first seen in an image taken at 13:24 GMT, but not in an image taken 18 minutes earlier, and has faded significantly in an image captured 18 minutes later. The camera team estimates the material in the jet to be travelling at 10 m/s at least, and perhaps much faster.

“This is the brightest jet we’ve seen so far,” comments Carsten Güttler, OSIRIS team member at the Max Planck Institute for Solar System Research in Göttingen, Germany.

“Usually, the jets are quite faint compared to the nucleus and we need to stretch the contrast of the images to make them visible – but this one is brighter than the nucleus.”

 

During an outburst of gas and dust from Comet 67P/Churyumov–Gerasimenko on 29 July 2015, Rosetta’s ROSINA instrument detected a change in the composition of gases compared with previous days. The graph shows the relative abundances of various gases after the outburst, compared with the composition measured two days earlier (water vapour is indicated by the black line).   Credits: ESA/Rosetta/ROSINA/UBern/ BIRA/LATMOS/LMM/IRAP/MPS/SwRI/TUB/UMich

During an outburst of gas and dust from Comet 67P/Churyumov–Gerasimenko on 29 July 2015, Rosetta’s ROSINA instrument detected a change in the composition of gases compared with previous days.
The graph shows the relative abundances of various gases after the outburst, compared with measurements two days earlier (water vapour is indicated by the black line).
Credits: ESA/Rosetta/ROSINA/UBern/ BIRA/LATMOS/LMM/IRAP/MPS/SwRI/TUB/UMich

Soon afterwards, the comet pressure sensor of ROSINA detected clear indications of changes in the structure of the coma, while its mass spectrometer recorded changes in the composition of outpouring gases.

For example, compared to measurements made two days earlier, the amount of carbon dioxide increased by a factor of two, methane by four, and hydrogen sulphide by seven, while the amount of water stayed almost constant.

“This first ‘quick look’ at our measurements after the outburst is fascinating,” says Kathrin Altwegg, ROSINA principal investigator at the University of Bern. “We also see hints of heavy organic material after the outburst that might be related to the ejected dust.

“But while it is tempting to think that we are detecting material that may have been freed from beneath the comet's surface, it is too early to say for certain that this is the case.”

Meanwhile, about 14 hours after the outburst, GIADA was detecting dust hits at rates of 30 per day, compared with just 1–3 per day earlier in July. A peak of 70 hits was recorded in one 4-hour period on 1 August, indicating that the outburst continued to have a significant effect on the dust environment for the following few days.

“It was not only the abundance of the particles, but also their speeds measured by GIADA that told us something ‘different’ was happening: the average particle speed increased from 8 m/s to about 20 m/s, with peaks at 30 m/s – it was quite a dust party!” says Alessandra Rotundi, principal investigator at the ‘Parthenope’ University of Naples, Italy.

The decrease in magnetic field strength measured by Rosetta’s RPC-MAG instrument during the outburst event on 29 July 2015. This is the first time a ‘diamagnetic cavity’ has been detected at Comet 67P/Churyumov–Gerasimenko and is thought to be caused by an outburst of gas temporarily increasing the gas flux in the comet’s coma, and pushing the pressure-balance boundary between it and incoming solar wind farther from the nucleus than expected under ‘normal’ levels of activity. Credits: ESA/Rosetta/RPC/IGEP/IC

The decrease in magnetic field strength measured by Rosetta’s RPC-MAG instrument during the outburst event on 29 July 2015. This is the first time a ‘diamagnetic cavity’ has been detected at Comet 67P/Churyumov–Gerasimenko and is thought to be caused by an outburst of gas temporarily increasing the gas flux in the comet’s coma, and pushing the pressure-balance boundary between it and incoming solar wind farther from the nucleus than expected under ‘normal’ levels of activity.
Credits: ESA/Rosetta/RPC/IGEP/IC

Perhaps the most striking result is that the outburst was so intense that it actually managed to push the solar wind away from the nucleus for a few minutes – a unique observation made by the Rosetta Plasma Consortium’s magnetometer.

The solar wind is the constant stream of electrically charged particles that flows out from the Sun, carrying its magnetic field out into the Solar System. Earlier measurements made by Rosetta and Philae had already shown that the comet is not magnetised,  so the only source for the magnetic field measured around it is the solar wind.

But it doesn’t flow past unimpeded. Because the comet is spewing out gas, the incoming solar wind is slowed to a standstill where it encounters that gas and a pressure balance is reached.

“The solar wind magnetic field starts to pile up, like a traffic jam, and eventually stops moving towards the comet nucleus, creating a magnetic field-free region on the Sun-facing side of the comet called a ‘diamagnetic cavity’,” explains Charlotte Götz, magnetometer team member at the Institute for Geophysics and extraterrestrial Physics in Braunschweig, Germany.

This image, taken on 12 April 2015 by the OSIRIS narrow-angle camera, identifies the source region of the outburst from Comet 67P/Churyumov–Gerasimenko observed by Rosetta’s instruments on 29 July. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

This image, taken on 12 April 2015 by the OSIRIS narrow-angle camera, identifies the source region of the outburst from Comet 67P/Churyumov–Gerasimenko observed by Rosetta’s instruments on 29 July.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Diamagnetic cavities provide fundamental information on how a comet interacts with the solar wind, but the only previous detection of one associated with a comet was made at about 4000 km from Comet Halley as ESA’s Giotto flew past in 1986.

Rosetta’s comet is much less active than Halley, so scientists expected to find a much smaller cavity around it, up to a few tens of kilometres at most, and prior to 29 July, had not observed any sign of one.

But, following the outburst on that day, the magnetometer detected a diamagnetic cavity extending out at least 186 km from the nucleu. This was likely created by the outburst of gas, which increased the neutral gas flux in the comet’s coma, forcing the solar wind to ‘stop’ further away from the comet and thus pushing the cavity boundary outwards beyond where Rosetta was flying at the time.

“Finding a magnetic field-free region anywhere in the Solar System is really hard, but here we’ve had it served to us on a silver platter – this is a really exciting result for us,” adds Charlotte.

“We’ve been moving Rosetta out to distances of up to 300 km in recent weeks to avoid problems with navigation caused by dust, and we had considered that the diamagnetic cavity was out of our grasp for the time being. But it seems that the comet has helped us by bringing the cavity to Rosetta,” says Matt Taylor, Rosetta Project Scientist.

“This is a fantastic multi-instrument event which will take time to analyse, but highlights the exciting times we’re experiencing at the comet in this ‘hot’ perihelion phase.”

 

 

Comments

102 Comments

  • Lucas says:

    For example, compared to measurements made two days earlier, the amount of carbon dioxide increased by a factor of two, methane by four, and hydrogen sulphide by seven, while the amount of water stayed almost constant.

    How is that possible in the "snowball" theory?

    • ianw16 says:

      Because, as already seen elsewhere, the more energetic jets tend to be driven by CO/ CO2. People shouldn't get confused by the word "ice'. It isn't just water ice, which is what we tend to think of when we hear the word, or see it on Earth.
      And these composition measurements are not of actual abundances, only relative abundances. Water production may well have increased in this outburst. They are simply plotting the relative abundance of other species in relation to water.

      • THOMAS says:

        @ ianw16
        "And these composition measurements are not of actual abundances, only relative abundances. Water production may well have increased in this outburst. They are simply plotting the relative abundance of other species in relation to water"

        Not so, Ian. Read the text again: "...*compared to measurements made two days earlier*, the amount of carbon dioxide increased by a factor of two, methane by four, and hydrogen sulphide by seven, while *the amount of water stayed almost constant.*

        Despite this extraordinarily explosive outburst, supposedly caused by gently sublimating, invisible, ices, the amount of water stayed constant in ABSOLUTE, not relative, terms ("compared to measurements made two days earlier"), while CO2 merely doubled. This spells big trouble for standard theory and your misreading of the text is simply wishful thinking since the implications of the actual data are too far-ranging to bear thought.

        • Gerald says:

          Constant means constant in absolute and in relative terms.
          The "Electric Universe / Electric Comet" nonsensical stuff is in big trouble, more and more with each new data. According to "Electric Comet" presumptions - if they are able to express anything at all - there should be an activity minimum right now; the contrary is the fact. And the high levels of methane and other reduced chemicals heavily contradict combustion-driven gas emanation. Water abundance stayed constant; it would have formed by consuming methane. Most of the solar wind - you need for your excessively high proton flux presumption - is shielded by the coma.
          What you call "standard theory" - actually a model description of physical reality - is exciting, and very valid. As usual, you're excessively confusing things, THOMAS.

          "Electric Universe" ideas are ruled out over and over again. Why are you unwilling to notice reality?

          • THOMAS says:

            Instead of attempting to create a diversion by resorting to the usual defensive tactic of blindly attacking the opposition, would you please just answer this simple, extremely precise, question: why did the abundance of detected H2O fail to increase at all IN REAL TERMS as a result of this exceptionally violent explosion of supposedly sublimating ice? Are you seriously claiming that this observation does not constitute yet one more major point of perplexity for the standard theory? How does violently sublimating ice manage to not produce vastly increase amounts of H2O? Please answer this simple question.

        • ianw16 says:

          You might want to get on Google Scholar, and have a look at the data for EPOXI at Hartley 2. Lots of very energetic, long lived jets (it was a 'hyperactive' comet). Spectrally resolved as being composed of mainly neutral CO2. Temperature measurements also made at that comet (as well as Tempel 1 and 67P).
          Zero indication of electric woo.
          Is it very cold electric woo, that is somehow producing neutral species?
          And who said the 'jets' were due to 'gentle' sublimation?
          You seriously need to catch up on the last 3 decades or so of published findings regarding comets

    • Gerald says:

      By release of supervolatiles accumulated in a subsurface chamber, where temperatures are below the sublimation point of water ice.
      A similar thermal gradient is established during fractional distillation, to separate liquids of different boiling points:
      https://en.wikipedia.org/wiki/Fractional_distillation#/media/File:Crude_Oil_Distillation.png
      Without openings, the chamber bursts at some point and releases the supervolatiles suddenly.

      • logan says:

        Another model could be a 'burping' from a formerly sealed organics layer. Here we have a 'dry' out-gassing 😉

  • Gerald says:

    Interesting, the period of low magnetic field strength is followed by a peak near 13:30 UT, like a (slightly underdamped) harmonic oscillator:
    https://upload.wikimedia.org/wikipedia/commons/4/4f/Step_response_for_two-pole_feedback_amplifier.PNG

    • Kamal Lodaya says:

      Here is a 15-year old prediction:

      P.L. Israelevich, A.I. Ershkovich: Neutral Points at the Cometary Diamagnetic Cavity Boundary as Possible Source of Cometary Rays, Earth, Moon and Planets, 05-1998, Volume 81, Issue 2, pp 123-133

    • logan says:

      Could that reboot be useful for a raw estimate of actual height of pile-up?

    • Harvey says:

      It looks a bit like that, but I don't thing it would be a safe deduction in the noisy data. There does seem to be a clear periodic element, it would be interesting to see a Fourier transform of the data.

  • Gerald says:

    ... and a little less scientific: High H2S and CH4? The comet must have eaten onions or something a few days before the outburst...

  • Gerald says:

    Back to exploration: This eruption should have left over significant changes at the surface of the comet.
    This one probably won't be the last outburst of the phase near perihelion.
    Investigating the surface changes since the mapping last year will be going to become thrilling in a few months.

  • WayneC says:

    Great!
    Hopefully, here come the fireworks!

  • Dave says:

    Anybody have any explanation as to why the amount of many gases increased dramatically but detection of water stayed the same through out the episode?
    Also not sure how the solar wind was pushed away by the dust and how the magnet field evaporated to nothing. Harvey do you have any ideas from your experience ? Was it the physical movement of the dust or was it the existence of another magnetic field that canceled out the exististing field for a while?
    Nice to have some interesting info so quickly after the event, thanks esa
    Regards

    • ianw16 says:

      "Anybody have any explanation as to why the amount of many gases increased dramatically but detection of water stayed the same through out the episode?"

      As known from other comets, the more energetic jets tend to be powered by CO and/or CO2. As explained before, this is a measure of relative abundance. H2O may have increased in this episode, but they are merely measuring abundances relative to H2O.

      The dust doesn't push away the solar wind. The increase in neutrals from the comet does this. This was observed at Halley in 1986. A bow shock region forms, where the solar wind is essentially blocked by the outflowing neutrals, and flows around the comet.
      There was plenty of literature on the Halley results, some of which is freely accessible. The following link is a decent guide to what happens, including the diamagnetic cavity: http://www2.ku.edu/~kuspace/aeronomy/comet-tutorial.pdf

      • THOMAS says:

        @ianw16
        "H2O may have increased in this episode"

        It didn't, it stayed constant compared to the data recorded two days earlier, as is explicitly stated in the release:
        "...*compared to measurements made two days earlier*, the amount of carbon dioxide increased by a factor of two, methane by four, and hydrogen sulphide by seven, while *the amount of water stayed almost constant.*"

        Your point is invalid.

      • Shannon says:

        No, they are not measuring abundances relative to H20. The post clearly states:

        "For example, compared to measurements made two days earlier, the amount of carbon dioxide increased by a factor of two, methane by four, and hydrogen sulphide by seven, while the amount of water stayed almost constant."

        • ianw16 says:

          So what? The ice that is being released is richer in those molecules. Your point is?

        • Jacob nielsen says:

          Yes they are (view graph) AND H2O is near constant.

        • originalJohn says:

          Methane and hydrogen sulphide are unreacted reactants Shannon. The findings are explained by a change in the ration of available oxygen to reactants. This ratio anyway would not be expected to be uniform over the nucleus surface.
          So the balance of products would be changing all the time.

    • Gerald says:

      Dave, my first ad-hoc explanation is, that CH4 and H2S are more volatile than H2O, particularly CH4. Ices of these (super-) volatiles start to sublimate at lower temperatures than water ice. Their origin is hence from a deeper (and colder) subsurface layer than water vapor, where water ice stays frozen. The high amount of gasses isn't explained by continuous sublimation at the surface (by solar heating), but by accumulation of the gasses in a subsurface chamber due to the seasonal longer term warming, and sudden release as the covering layers open a fracture, or collapse.

      Water ice sublimates over large surface areas and has a high absolute abundance. Local fluctuations of the composition of the gasses are hence less distinct for the relative water vapor abundance.

      We see the dust in images, but relevant for the magnetic field is the (optically invisible) accompaying gas. It pushes away the solar wind. The magnetic field is entangled in the solar wind plasma, and hence is pushed away, too.

      • Gerald says:

        ... To be more complete: Some of the magnetic field is located in the comet's magnetosphere, where the gas is less dense and partially ionized. It has been pushed away by neutral gas, as well.

      • originalJohn says:

        All assumption Gerald. No evidence that any of the list of thirteen compounds detected by ROSINA are sublimation products from frozen solids. Very likely indeed that they are combustion products. They are all typical. No evidence for that either though. However, if you have a temperature measurement for the jet ( you would have done that wouldn't you) that will tell you categorically.

        Oh and if you have an analysis for ion species within the "firework" jet that will tell you whether it consisted of neutral gas or a plasma current with its own magnetic field.

        • ianw16 says:

          No magnetic fields within the diamagnetic cavity. That, for your information, is why it is called a "diamagnetic cavity". Google the word "Diamagnetic".
          What do you think they they saw in the coma after this eruption? Look at the graphic. Wasn't anything that EU predicted.
          Sorry, mate, you don't have a clue.
          Give it up before you get even more embarrassed.
          Like I have previously said; you believe in fairy tales, with no evidence to back them up.
          No harm in that. Just don't try to pass it off as science.

        • Gerald says:

          Originaljohn, you are calling H2S, CH4 (methane, aka ), methanole, etc. "combustion products"?
          Which are the ingredients for a combustion resulting in these products? I'm much more used to using most of them as fuels.
          https://en.wikipedia.org/wiki/Natural_gas

          • originalJohn says:

            That's right Gerald, they are fuels at the reaction site but when they appear in the products of the reaction they are combustion products. This is normal in uncontrolled combustion reactions where there are periods of oxygen deficiency.

    • logan says:

      Hi Dave.

      The solar wind was pushed UP by the emitted gases, more than anything else [H20 wasn't among them].

      The magnetic field was not evaporated, just pushed up also. Above ROSETTA's sky, Minutes later felt back [and rebooted a little].

      Magnetic perturbation amounted to 30 min. An left parasitic turbulences that reduced the field strength in 4/5 of former value.

      Hoping for Harvey comments, too 🙂

  • Sovereign Slave says:

    Hi Emily, thanks for sharing, great picture. If you have time, perhaps you (or anyone else out there) could answer this. After seeing this picture, was wondering if it's been determined if pretty much ALL of the dust coming from the comet is columnized in jets. We seem to see nothing but jets of dust, but no dust clouds or hazes. Shouldn't a lot, or even most, of the gas and dust coming from the comet be pretty dispersed and not traveling a high speed within jets? And if some is dispersed, have they determined what percentage is jets vs dispersed?

    • Gerald says:

      The jet didn't point to Rosetta. That's by visual evidence, and since the dust particles would have arrived at Rosetta within 4 hours at most (300 km / 20 m/s = 15,000s = 4.1667h), not after 14 hours or even after three days on 1 August. Nevertheless, the recorded dust grains peaked, according to the blog post:
      "Meanwhile, about 14 hours after the outburst, GIADA was detecting dust hits at rates of 30 per day, compared with just 1–3 per day earlier in July. A peak of 70 hits was recorded in one 4-hour period on 1 August, indicating that the outburst continued to have a significant effect on the dust environment for the following few days."

      Hence a cloud of dispersed dust seems to have formed in addition to the columnized jet.

      But I can't determine the ratio of dispersed vs. "columnized" dust, not easily at least.

    • Sovereign Slave says:

      Emily, was also wondering what the scale of this picture is. Seems it's up close and detailed enough to be able to get a fairly accurate measurement of the width of the jet at it's base. Knowing the approximate width seems like useful information to me at least, especially when going back and looking at close up OSIRIS pictures of features in that specific area from directly above that could produce a jet of that width. Would it be 1 meter wide, 10 meters, 100 meters?

    • Sovereign Slave says:

      Sorry, on a roll. But, one other thing occurs to me that is a very well defined argument with only two possibilities, for which evidence can be lined up to support one or the other. There are only two options for the dust movement in the jets - either the dust is being pushed, or it is being pulled. Pushed would support sublimation, pulled would support EU, and they are mutually exclusive. So, what are the characteristics of something like dust in a vacuum being pushed as opposed to pulled, and is there any evidence so far that would support one over the other? And no, proving pushing or pulling would not definitely prove the one, but it would disprove the other.

      • THOMAS says:

        As Originaljohn has pointed out several times over the past year, the simplest tests of the nature of the jets are 1) their observed temperature relative to that of the comet surface they are issuing from and 2) the detection of any plasma activity within them. This totally basic data has necessarily been collected now for large numbers of observed jets, but has still not been disclosed. One increasingly wonders why, since it would settle the discussion one way or the other at a stroke...

        • ianw16 says:

          You obviously missed the data from Hartley 2. The jets were spectrally resolved as CO2. Neutral. Temperature measurements at that comet also rule out any electric woo.
          The question was settled a long time ago.

    • Harvey says:

      Dave.
      The dust and neutral gas do not contribute directly to the formation of the diamagnetic cavity. The gas becomes ionised by solar UV, and it's the resulting expanding plasma which causes the cavity. (The dust has no role in this.)
      It's not easy to explain in words, and many of the papers on it are either extremely hard to read or not free access.
      I dug around a bit, this one is accessible and the introduction not too horrendous to read!
      http://deepblue.lib.umich.edu/bitstream/handle/2027.42/95513/grl13492.pdf?sequence=1&isAllowed=y

      • Harvey says:

        This is good on the comet's magnetic environment:
        http://www2.ku.edu/~kuspace/aeronomy/comet-tutorial.pdf

      • THOMAS says:

        @Harvey
        "The dust and neutral gas do not contribute directly to the formation of the diamagnetic cavity. The gas becomes ionised by solar UV, and it's the resulting expanding plasma which causes the cavity."

        According to the graph, the duration of this huge diamagnetic cavity was less than ten minutes and it disappeared as sharply and suddenly as it appeared, presumably when the violent outburst suddenly ceased.

        Are you seriously suggesting that it was the action of solar UV ionizing the escaping 'neutral' gas over a period of less than ten minutes which violently repulsed the incoming ions of the solar wind to a distance of "least 186 km" from the nucleus? If so, could you explain to us what physical process suddenly managed to reverse the procedure by de-ionizing that vast amount of ionized gas inside the coma almost instantaneously, such that, ten minutes later, the diamagnetic cavity had reverted to its normal, insignificant distance from the nucleus?

        • Harvey says:

          Primarily quite simply the expansion of the 'puff' of photoionised plasma beyond that volume and resulting drop in density.
          There is no problem at all.

          Perhaps you could explain in turn how intense electrical discharges could conceivably result in such a drop - or indeed in the total absence of any magnetic fields other than those broadly predicted from the usual models.

      • Kamal Lodaya says:

        Is it something like friction generating static electricity when you sometimes comb your hair?

        • Harvey says:

          Not, not really analogous to that, which is triboelectricity.
          However that process might occur in the dust on 67P, it would not surprise me if small scale static electricity discharges were to occur on the surface.
          Unfortunately these magneto hydrodynamic effects really are not easy to explain in a few words here; they are quite complex. The links I gave do a better job than I can here.

    • Harvey says:

      That's an interesting point.
      Some pictures do seems to show a more general low velocity haze, and of course integrated up over much larger areas than the dramatic visible jets that could carry a lot of material.
      Purely thinking out loud; perhaps where gas is just 'percolating' through the surface the dust is simply not accelerated to escape velocity, just shuffles about a bit, and only where a suitable channel forms can it get accelerated, pack it's bags and leave? But I stress a highly speculative not thought through concept.

    • logan says:

      Sensitive question, Sovereign Slave.

      You can see a hint of what you are looking for at top horizon in:

      http://www.esa.int/spaceinimages/Images/2015/06/Comet_on_25_April_2015_Osiris

      Magnificent HR OSIRIS document.

    • logan says:

      For all of the places where there is not 'collimated' jets, dust behaves according to classical models.

      And that is good.

    • logan says:

      Dust just happen to be around there. Dust have kinetics of his own.

      Maybe contributing some reflectance, 'fluorescence'.

  • logan says:

    "... the amount of water stayed almost constant".

    Strongly suggesting pressure buildup not related to water vapor. The less energy budget needed, the most credible it results.

    Thanks Emily and All Teams.

  • Phil Stooke says:

    Fantastic observation, thanks for reporting it.

  • ianw16 says:

    Amazing. Massive jet and a huge DROP in magnetic field strength. Who'd have thought.

  • Dave says:

    Logan,
    Re - Water stayed constant

    Was also my sentiment to, this makes the theory of hints pits formation even more difficult to understand if it's based on sublimation of water ice under the crust. Anyway gonna be interesting
    Regards

  • logan says:

    Please Help, Gerald: Is it plausible that inside of big dust clouds be more diamagnetic than the outside?

    • Gerald says:

      The diamagnetism and the dust cloud may have a common cause: Release of neutral gas.
      The dust itself may be diamagnetic, too, but taken allone, this wouldn't disturb magnetic fields pretty much.

      • Harvey says:

        Without a doubt the direct diamagnetic contribution of the dust is negligible. Even if it were a solid lump of any of the candidate materials, the effect would be very small, and this is mainly vacuum.
        The effect comes essentially entirely from the moving conductor formed by photoionised degassed material.

  • Guili says:

    Wonderful science.
    Thanks

  • originalJohn says:

    Gas will not deflect a magnetic field. Charged particles will. An obvious explanation is that the short term jet was a stream of charged particles, a plasma jet. This does not apparently occur to the investigators and they explain it with a gas pressure model. Any measurement of ion species in the jet.

    It is also clear that the jet was narrow, as with other jets recently, unlike in the earlier phase of approach when the jets were typically several hundred metres across. No comment on this from the team. No significance then ?

    • Gerald says:

      The investigators obviously know what they are talking of.
      "Electric universe" stuff fails completely, as usual.

    • sjastro says:

      Then there wouldn't be a diamagnetic cavity given the "plasma jet" has its own magnetic field.

    • ianw16 says:

      @OJ
      "This does not apparently occur to the investigators and they explain it with a gas pressure model."

      They did all that nearly 30 years ago:
      "It has been demonstrated that, unlike Venus (for low solar wind dynamic pressure conditions; Luhmann and
      Cravens, 1991), outward ion-neutral friction is the dominant force responsible for the existence of the
      cavity (Ip and Axford, 1987; Cravens, 1986). That is, the inward J x B force associated with the barrier is
      balanced by the outward ion-neutral friction force of neutrals flowing at about 1 km/s past stagnated ions.
      A simple analytic application of this force balance reproduces the observed magnetic field profile very
      nicely (Ip and Axford, 1987; Cravens, 1986)."
      http://www2.ku.edu/~kuspace/aeronomy/comet-tutorial.pdf

      And, as the data revealed at Halley, the only thing seen within the cavity were cometary ions and cometary neutrals, moving at around 1- a few km s^-1.

      I'm sure there will be further analysis of this, and other data, in the months to come, but I would bet my house on the fact that it won't show anything that is particularly different from the data we knew about 30 years ago.

  • originalJohn says:

    Also there are numerous other fainter jets in the background. The investigators observe that this one is the brightest yet seen, brighter than the nucleus (by far) and required no picture enhancement to see it clearly. Why might this be? Did this jet unlike those around it reflect vastly more sunlight. Of course not. It is intrinsically brighter. It would not be at all surprising if it signifies a step increase in current ie an arc mode discharge. Any useful measurements ?

    The list of compounds detected by the ROSINA instrument is also informative. Typical products of the combustion of complex hydrocarbons in oxygen, hydrocarbons containing nitrogen and sulphur, high sulphur. Hard to imagine how all of these reaction products could have been released from the hydrocarbon layer by a stream of gas rushing by.

    • Gerald says:

      The almost zero magnetic field clearly rules out significant discharges.
      CH4 is a reduced gas and rules out combustion.
      The dust in the jet either just scatters sunlight (https://en.wikipedia.org/wiki/Light_scattering_by_particles), or it is additionally composed of more bright material, including ice. Remember: The surface of the comet is almost black.
      Everything well within standard physics. "Electric comet" daydreams ruled out once more.

      • Adam A says:

        The current model which is proposed to explain the jet(s) is a build up of subsurface temperature then pressure which leads to a 'blast' of the contained gas/plasma through some sort of confining structure which directs and (for a while) confines the eruption.

        If an underground overpressure occurred, then the eruption would throw the overburden away in all directions, and the overburden would have a wide range of particle sizes from dust up to rocks or clods of the frozen surface. The inertia of the widely graded aggregate would see a wide dispersion in departure velocities, leading to a plume of debris with smaller higher velocity particles at the leading edge, followed by heavier and slower fragments.

        I have looked at images of many jets and I do not see any debris field either in the leading edge of the jet emission plume, or travelling in the 'sideways' directions a sub-ground burst would give rise to.

        Thus the observations do not fit the proffered model for these events.

        Interesting.

        • Gerald says:

          They (OSIRIS) recently observed an ejected boulder (recent press conference). Although there seems to be no published information about the ejection event itself.
          A generally increased dust level has been observed after the outburst.
          "The" truth is probably more complex than anticipated, as so often.

        • logan says:

          "...I have looked at images of many jets and I do not see any debris field...".

          An out-burst is a liberation of a lot of potential energy. Even if [Robin's] mud-volcano scenario there should be one.

        • Harvey says:

          Yes, in an explosive event, a new cavity roof collapse *if* that is what is happening, I'd agree you'd expect things like that.
          Trouble is as far as I know this is really the only event of that type, and we don't have a 'video', just three shots at. 18 minute intervals. So maybe such things happened unfortunately u observed?
          But the other jets are more CW things, and it's not so clear one should expect those kind of effects.
          The mechanisms really remain very obscure.
          As I've said many times, 'sublimation driven' theories still have a heck of a lot to explain. But remain the only credible game in town.

      • originalJohn says:

        It requires detailed and systematic measurement Gerald before any conclusions about magnetic field (an extended
        phenomenon) can be drawn. Random point measurements tell you virtually nothing.

        As for your assumption that the presence of CH4 indicates a reduction process and that that assumption alone allows you
        to rule out all possibility of combustion. Wrong Gerald. CH4 is a probable reactant. A constituent of the hydrocarbon mix that coats the nucleus surface. It is normal in uncontrolled combustion reactions for some reactant to appear in the products, particularly with variable and sometimes inadequate oxygen supply. So the presence of methane
        signifies a typical natural combustion process. Any other of the long list products you would like to comment on, many of which are clearly oxidation products.

        Combustion very much alive. Any scientist would want to investigate it. A simple start would be some temperature measurements at the reaction sites.

        • Gerald says:

          As you say CH4 would be a reactant (educt) in a combustion process, not a product.
          The presence of fuel doesn't show the presence of a flame.
          Your combustion hypothesis is completely unsubstantiated, and beyond this energetically as reasonable as olive oil spontaneously reacting with the glass of its bottle at room temperature in the absence of oxygen.

    • ianw16 says:

      @OJ
      ".....if it signifies a step increase in current ie an arc mode discharge. Any useful measurements ?"
      Yes, no electric currents have been detected, no anomalously high magnetic data at the time of the jet (quite the opposite), no anomalous temperature readings on the surface, taken at times when the comet has been active, and, given the diamagnetic cavity, no explanation for how these currents would be formed or maintained.
      Apart from that.....................

      • THOMAS says:

        @ ianw16
        "no electric currents have been detected"

        Mission scientists disagree with you, Ian. In yesterday's post "What made the comet sing?" , the presentation of Karl-Heinz Glassmeier and the RPC team’s findings states that "In the plasma environment around the comet’s nucleus, the newborn ions move perpendicularly to the magnetic field, forming what is called a cross-field electric current. It turns out that this current is unstable, and ultimately, it is what makes the comet sing."

        Now that the existence of an electric current within the comet coma is being admitted by the Rosetta team, we eagerly await their explanations as to the nature of the anode and the cathode (i.e. the charge separation) driving the current, and the precise mechanism involved.

        • sjastro says:

          Gosh the cross field current is nothing more than an example of E X B ion and electron drift encountered in plasmas. Particles that undergo cyclotron motion around magnetic lines in the plasma come under the influence of neighbouring intrinsic electric fields and undergo drift.

          THIS DOES NOT REQUIRE AN EXTERNAL EMF.

          Even if this was the case why should the current be unstable, as you have quoted, given you have assumed an anode and cathode already exist?

          Once again its the case of not comprehending the physics but simply reading you own ideas into the subject.

        • Harvey says:

          It was 'admitted' decades ago. The existence of a low density plasma with associated currents and magnetic fields is well known.
          The fantasy is that it has anything to do with gross material removal and transport.

          Regarding which it is always just *assumed* that a discharge would transport neutrals. But in fact discharges are extremely poor at transporting neutrals, because the ions and electrons are travelling in opposite directions - the ions towards the comet if it is supposed to be negative! In laboratory discharges the transport is known to be wall mediated, and not that big even for example in high current density laser discharges.
          So even if there were a discharge - not that it's possible off an insulating body and for a dozen other reasons - there is no reason to think it would accelerate a jet of neutral gas.

          • logan says:

            Fully, completely agree Harvey.

            Could you accept to displace that 'insulating' for 'highly resistive'? This is an heterogeneous object.

          • Harvey says:

            Logan.
            I'm not aware there is any strict definition of what should be called 'insulating' as opposed to 'highly resistive'. It can depend on context; currents, voltages, powers.
            The EU concept would require very large currents to be flowing (without of course any mechanism to drive them!) so even rather low resistivities, would b e a serious problem.
            So given the nature of everything we see in terms of materials detected (and indeed proposed) and the CONSORT results, I think 'insulating' is fair enough. Wouldn't really argue with 'highly resistive' - but what's the difference? Where do insulators 'start'?
            >10^6 ohm cm, >10^10 ohm cm?
            Even 10^3 would be a huge problem (and would not be considered insulating or 'highly resistive'.)

      • originalJohn says:

        .No temperature measurements of the jets at all w16, these recent arc mode jets or any other jets. Just what it requires to maintain the absurd sublimation hypothesis. No temperature measurement. No ion species measurement. No current density measurements.
        No looking equals no detection.

        • Gerald says:

          You call reality an "absurd hyothesis"?

          Of course the data provide evidence for real physical processes.

          "Electric comets" are not real, they just exist in the virtual "Electric Universe" faith.
          But the data are about real comets (including their electromagnetic properties). So don't expect real data supporting virtual "electric comets".

          • originalJohn says:

            Temperature and ion content of the jets would be real and valuable data Gerald, data that might allow you to construct a realistic and viable hypothesis for the comet mechanism.

            Until you measure the temperature of the jets you have no evidence that they are cold sublimed gas so sublimation is not reality it is a hypothesis.

            From what you say it appears that your recommendation would be to avoid collecting any data that might support the electric comet theory. Current density measurements within the coma, particularly close to the nucleus for example would do nothing for your sublimation hypothesis, but would be essential to verify the plasma discharge hypothesis.

            Interestingly measurement of temperature of the jets could confirm the combustion hypothesis but could also refute it and fully ratify your cold gas sublimation hypothesis. If you are confident you are observing sublimation why don't you get on with the temperature data collection and get the proof you anticipate.

          • Gerald says:

            The VIRTIS instrument is designed to measure IR spectra of the nucleus and the coma. From the IR data the temperature can be inferred. Why do you think, those measurements haven't been done?

            http://virtis-rosetta.lesia.obspm.fr/VIRTIS-the-instrument.html

        • Harvey says:

          I predicted months ago that eventually 67P would head off towards aphelion, Rosetta would sadly eventually 'die' - and the EU community would be saying 'you took the wrong measurements, you are hiding the data that proves us right, there is a conspiracy to support sublimation....'
          You just posted the first verse of that song.

        • ianw16 says:

          Hartley 2. Jets were NEUTRAL CO2. This has been known for some years. Did it pass you by?

    • Harvey says:

      Yes, very useful measurements; the magnetic field falling to near zero as expected, completely ruling this stuff out.

  • Jon says:

    Is that jet emanating from the clifff wall or neck?

    Thanks.

    • Claudia says:

      Hi Jon,
      As shown in the last image, the jet emanates from the region Anuket on the comet's neck.

  • Robin Sherman says:

    Great post Emily, lots of science to try and make sense of.

    Would I be right in thinking that the apparent lack of change in the amount of Water is due to the normal relative abundance of these gases. The normal Water output being so much larger, the extra from this outburst is so small in comparison it hardly makes a difference. Whereas for the other gases with far smaller normal abundances the extra output from this outburst becomes significant.

    What I find intriguing is the wide variety of molecular species that are present within the cometary material. It suggests all these different molecules have formed in significant amounts in open space or within the pre-solar nebula indicating just how much chemistry actually occurs in the perceived "cold vacuum" of space.

    Lots of Sulphur compounds in relatively large amounts too, which seems a bit unexpected for an object formed in the Kuiper Belt region. I suppose, if 67P formed by accretion as suggested, who knows where some of the smaller planetesimals could have migrated from in the Solar Nebula?

  • logan says:

    Water is being broken, strongly. If Lyman lines are dominant components on the UV range, Then what could impede other breaking lines from being dominant on the range of NAVCAM?

    http://www.esa.int/spaceinimages/Images/2015/06/Rosetta_uncovers_processes_at_work_in_comet_s_coma

    • Harvey says:

      Logan, I'm not sure I fully understand your query.
      The Lyman series are all deep in the ultraviolet, where the Navcam is very unlikely to have any sensitivity. Normal lens materials are opaque, the standard silicon detector extremely inefficient, if it works at all.
      ALICE, the imaging UV spectrograph, would be the relevant instrument.

      • logan says:

        Lots of queries, amused here, Harvey 🙂

        If water is electron dissociating in the immediate gas column [500 m], Why not higher in the column? Why not the other sublimates? Could those other 'fresh', hypothetically complex dissociating sublimates emit spectra on the green, red, blue? What about those electron/ion products interaction with dust?

        • Harvey says:

          Logan. Water will indeed still be photo dissociating and photoionising 'higher' up. But collisional processes will become rapidly less important as the density decreases. The other molecules like CO2 etc can all undergo similar processes, but the rates depend on the cross sections for them and the intensity of the solar spectrum at the relevant wavelengths where they are strong.
          67P produces a lot of them relative to water, so they may be more important here than at other comets; I'd have to dig out the relevant cross section data to be sure.
          If indeed any sort of discharge was occurring, there would most certainly be intense line emission in the visible and the UV from many species. The absence of any observations of that (even from Earth -easy to measure emission) is telling.
          Once the dust is quite a short distance from the comet collisions with gas etc become very rare, and probably only photo effects are significant on the dust I'd think.

    • Gerald says:

      The Balmer series of hydrogen would be in the visible part of the spectrum. But relative to the sunlight scattered by the dust, that's probably too faint.

  • Adam A says:

    The occurrence of the jet being virtually coincident with a change in the magnetic field (and hence the electrostatic field) around the comet could suggest that the breakdown of the dielectric ‘around the comet, with the jet arising from an electrical connection between the comet surface and the kept-at-bay solar wind ('like an air' to ground lightning strike+its sprite). The prompt creation of a diamagnetic cavity’ would also have its electrostatic component.

    Recent work by other satellites have confirmed the presence of plasma / electrical 'ropes' connecting the Sun and Earth, so it would not b unreasonable to anticipate similar ropes of EMF connecting to the comet. Maintain a voltage difference between ground and sky long enough, something is going to break down.

    The trick is resolving which came first - the change in the electrical/magnetic environment, or the observed jet?

    So are the pits on the comet created by electrostatic milling, rather than by localised 'boiling' events within the comet's topsoil?

  • Dave says:

    Harvey,
    And a couple of others, thanks for some of the explanations.
    It's still not clear though, obviously I have some research to do.
    Could it be that the solar wind has an unstable flow around the comet and so looses the cavity through instability, this then drops out the field?
    Sort of chicken or egg, just a thought.

    • ianw16 says:

      Dave:
      The solar wind is getting nowhere near the nucleus. As shown at Halley. 29 years ago. This should/ does not come as a surprise to anyone that has studied comets. over the last 40 years.
      Why are you even questioning this?
      Unless you have evidence to the contrary.

      • THOMAS says:

        There are very good reasons for questioning it, Ian, despite your 40 years of studying comets.

        That was Halley 29 years ago, with extremely limited data acquired by Giotto during a simple, very brief, fly-by. This is 67P right here and now, with vast quantities of data acquired by Rosetta for over a year now and still going strong. We now know that there are considerable fluctuations in the interactions between the solar wind and the comet nucleus (cf. the 19/08 post ‘What made the Comet Sing?’), including the distance at which the solar wind is brought up short of the nucleus by what is happening in the coma (cf. the explosive July 29 'diamagnetic cavity' event). And this is just the beginning of Rosetta’s revelations, there is clearly a lot more to come. (Karl-Heinz Glassmeier’s findings of a “cross-field electric current” in the coma which “is what makes the comet sing” is based on data up until November 2014 only…).

        Your false impression that what was observed at Halley is cast in scientific stone forever is rendered worthless by the simple fact that Giotto couldn’t hang around long enough to observe any fluctuations at all. Metaphorically speaking, Giotto took a blurred black-and-white photo, whereas Rosetta is actually filming events in glorious Technicolor. What we are learning from Rosetta has quite logically relegated the scant, 29-year-old, Halley data and the preliminary conclusions which were drawn from it at the time to a state of scientific obsolescence.

        • Harvey says:

          In other words, let's ignore perfectly good data - which of course has its limitations - because it doesn't fit my pet theory.
          But in fact Rosetta has limitations too; it's very close to 67P, great for seeing the details of what is happening on he nucleus, but *too* close to observe some of the plasma structures,, it is inside them.
          You can't disgrace the old data; you need a model consistent with *both*, recognising the limitations of *both*, and the fact that these are different comets.

    • Harvey says:

      Dave. I tried to come up with something better, but in a few words with no diagrams, I think you are better off with the papers I cited.
      i always really hate saying 'it's too complicated to explain here.....'; but there are some cases where it's just true!
      These magnetised plasma effects are a bad case of that. Unfortunately without a knowledge of vector calculus, the equations are impenetrable. Even the figures can be hard to interpret.
      Sorry! Pleasure to see someone keen to understand it.

      • Gerald says:

        I wonder, whether there is some Faraday cage analog to the diamagnetic cavity.
        One thing, a Faraday cage analog fails, are the zero points.

        The hairy ball theorem of algebraic topology may be more intuitive as an explanation:
        https://en.wikipedia.org/wiki/Hairy_ball_theorem
        "you can't comb the hair on a coconut".

        ... just some informal brainstorming.

    • logan says:

      "...Could it be that..". Good seeing you on alternative scenarios.

      "...solar wind has an unstable flow around the comet..", Could it be responsible for the 10nT pulsating amplitude patterning before the 'event'?

      "...and so looses the cavity through instability, this then drops out the field...". Maybe wrong, but if field [gigantic exosphere structure] damaged then why it reappears almost fully formed minutes later?

      • Harvey says:

        Because the 'bubble' of higher density plasma has expanded and the density fallen back towards its pre-outburst value. If you do the sums the timescale is perfectly sensible.

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