Today’s CometWatch entry was taken by Rosetta’s Navigation camera on 12 April from a distance of 146.8 km from the centre of Comet 67P/Churyumov-Gerasimenko. The image scale is 13m/pixel and has been processed to bring out the details of the comet’s activity. It is cropped to 10.3 km (the original frame, provided at the end of the post, measures 13 km across).
Comet 67P/C-G on 14 April 2015. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
In this orientation the comet’s small lobe is to the left and the large lobe is to the right. The large lobe includes an oblique view across Imhotep. The raised section on the top left side of the large lobe is at the boundary of Aten and Babi.
Discussion: 26 comments
Another great picture of the jets. I count 20-30 distinct smaller jets making up the larger jets, though much of it is fuzzy. This angle of course only shows the jets moving more or less perpendicular to Rosetta’s position. Obviously there are many more jets that are coming off the surfaces that are not so much on the viewing horizon moving toward or away from the camera that aren’t visible. So, there must be hundreds to thousands of active jets at this point. With what has been discovered and surmised about P67 so far, could Gerald or Harvey or someone else give an updated explanation, step by step and in detail, how these jets are formed via sublimation? Some questions that come to mind for this model are:
Are the volatiles still thought to be mainly ice, or are they thought to now be mainly other gases not as easily recognizable visually as ice is?
Are the sublimation chambers that are creating the explosive jets deeper than originally thought?
How are the jets producing both the hard and fluffy dust detected so far?
How long would one sublimation chamber be able to produce a jet? When does it “run out?”
Still wondering what the maximum diameter of a chamber opening (nozzle) could be to still produce a jet and maintain the necessary and apparently very steady internal pressure for the jet?
At what depth is the slight heat from the sun at this distance totally cancelled out by comet surface depth – 1cm, 2cm…1 meter?
How is the rate of flow through the chamber nozzles affected by the growing size of the chamber as it sublimates?
What is the life span of a chamber, or range of time?
What in the sublimation model accounts for the closer up pictures of jets that are quite wide at their base, where they originate from the comet?
P67 has taken many passes around the sun, no doubt producing many many sublimation jet chambers each time that would I imagine go inactive as it retreats from the sun. Are the jets we see now being produced from old chambers that were already there from previous trips, or from new chambers, or both?
Does the opening size of all the nozzles for the chambers have to be pretty much the same diameter for this model to work, and if so, how did nature create this perfectly uniformed, comet-wide phenomena? And wouldn’t just one or two other cracks or openings or surface porousity cancel the necessary pressure for the jet flow? And as surface material is removed, how is it that these nozzles would keep appearing – do they already exist at greater depths, or does the sublimation process create them?
Wouldn’t these nozzles be effected by the explosive pressure of the chambers (be quickly enlargened), to the point of decompressing the chamber and canceling the jet?
If chambers form under the surfaces that don’t have surface openings (nozzles), shouldn’t we be seeing surface explosions? Unless it is being proposed that all the heat driving sublimation is coming just through the nozzle, but then, seems the escaping gas would quickly cool and counteract the heat coming downward through the nozzle. So surely not all the surface heat is just affecting potential chambers with nozzles. In fact, it seems we should either being seeing slowly released gas/dust through surface cracks and porousity, OR explosive release of buried sublimation, and that jets would be either extremely rare and relatively brief, or non-existent. Perhaps if you could provide another example of something in nature that’s under explosive pressure similar to these chambers that maintains a steady, even, prolonged released, it would help.
Given that these emissions seem to be coming from pretty much the entire sunlit surface, and given that the comet has been found to be non-magnetic, therefore excluding any sort of “electric” discharge, perhaps you have an alternative idea?
ianw16, the nucleus has been found not to be composed of a significant amount of permanently magnetic material, from one cluster of measurments relating to the bouncing lander on a particularly inactive region of the surface. This does not exclude anything about a possible electrical explanation for the comet mechanism. The magnetic field that was detected had in that region the same intensity at the surface and at 17 km. That field was interpreted as being caused by the solar wind current flow and not influenced by the comet nucleus. An opinion. Many more results are required.
Have you an alternative suggestion or do you think it is best to give up now and stick with ice sublimation fantasy.
Electric currents produce magnetic fields.
The field both at Rosetta and at Philae had the same vector at the same time, over Philae’s bounce track.
The magnitude of the field was ~2nT; TINY.
As both Gerald’s and my quantitative calculations show this absolutely rules out any significant current flow within distances of order a km at the time of Philae’s landing, barring extremely unlikely (at two locations) cancellations.
You can assert all you like; the observation is directly fatal to any significant ‘discharge’ current flow at that time.
Hi SS,
I think Gerald and Harvey will respond to this, but here is my take. The gases detected indicate that since water vapour is the main constituent, water Ice would be the main source. Nozzles and chambers theories and ideas have too many unknowns and thus the problems you highlight may well be relevant, but we expect further observations may elucidate.
Not too much is ruled in or out otherwise, other than the main idea of solar thermal energy being the source of the energy, and water and other volatiles as being the source of the outgassing via sublimation. Ie. The specifics is up for grabs, but the overarching principles are going to be steadfast to the scientists for the foreseeable future.
The Circumstancial difficulties you site with the outgassing is not as relevant as other Circumstancial issues:
1) Ongoing lack of erosion, collapse or any other surface remodelling
2) Plausibility and evidence of liquid water internally.
3) outgassing is only vaguely corellated to insolation and surface temperature
Circumstancial evidence points to a deeper, more mysterious source of the outgassing. Mysterious because it is deeper and we have no real opportunity to see that far under the surface to guess what is driving the outgassing in that respect.
Hi Marco,
1) There is plenty of evidence of erosion, collapses and surface remodeling on 67P, e.g. dunes and boulders.
2) There is no liquid water or other liquid’s on comets – sublimation is solids turning to gases.
3) What is your evidence that outgassing is “only vaguely corellated to insolation and surface temperature”?
What I see is outgassing primarily from lighted areas of 67P – that is strong correlation to surface temperature.
We have seen your “nozzles and chambers” – they are the pits on the surface. AFAIK there are no actual nozzles and chambers. The jets are not jets from a jet engine! All you need is a pit to produce a jet. There is at least one image with what looks like jets emitted from within a pit..
ETA: We also saw surface remodeling in action on Tempel 1 where a mechanism proposed for some observed narrow jets was actual chambers and nozzles.
@ Ian
“What is your evidence that outgassing is “only vaguely corellated to insolation and surface temperature”?”
It’s not even the case of being “only vaguely correlated”, there is clear evidence of an INVERSE correlation (see, for example, my comment of some weeks back: https://blogs.esa.int/rosetta/2015/02/09/seasonal-forecasts-for-67pc-g/#comment-399635, with links to VIRTIS temperature data). The observed fact that the warmest part of the comet, all along the bottom of the “Hapi Valley” neck region where most of the jets have been concentrated over the past 8 months, also happens to be the one which is necessarily receiving the LEAST energy from sunlight because of near constant shading from the cliff walls on either side, still needs to be accounted for.
This one shows the correlation between solar illumination and jets very clearly:
https://www.esa.int/spaceinimages/Images/2015/04/Comet_on_15_April_2015_c_NavCam
Hi Ian,
“We have seen your “nozzles and chambers” – they are the pits on the surface. AFAIK there are no actual nozzles and chambers. The jets are not jets from a jet engine! All you need is a pit to produce a jet. There is at least one image with what looks like jets emitted from within a pit..”
There is considerable evidence that these jets provide thrust that can slow or speed up rotation. In this sense they are like a jet engine in that it is directional thrust, otherwise outgassing would be diffuse and thrust would be omnidirectional and random factors would thus cancel net thrust. With several observed comets, non-gravitational accelerations such as thrust are consistent over cometary orbits, which contradicts their postulated ephemeral nature.
Hi Ian
1) what I mean to say, is that in 8 months of close observations, there is no rock, cliff, or solid valley floor that has measurably eroded at all. The only possible visible change has been shifting sands in Hapi. Dunes are caused by exhaled dust relanding back on the comet, and boulders, etc. are caused by fracture with unknown and unobserved cause as yet, even with increasing activity. Erosion, collapses and surface remodelling are hypothesised, but not yet observed.
2) stardust has detected chemicals that only form in liquid water. As far as observations on 67P go, we have detected the gaseous volatiles, but not the quantity of solid volatiles as yet. To prove sublimation we need to see both the solid and the gas. Until we see the solid or liquid volatiles, we really cannot determine the mechanism for sure.
3) In this and other observed comets, jets have been seen coming from “pre-dawn” areas of the comet, which are the coldest. Also, areas of the neck which are shaded appear to be hotter in some cases. Not random, but not proportional in any way to measurable temperature, and temperature is not proportional spatially to insolation. Ie. Vaguely correlated until you look at particular areas and specific temperatures and outgassing.
Marco
Broadly I’d agree, except perhaps for the last sentence.
Solar input is now ~~~1kW/m^2; the comet absorbs well; that’s clearly the first place to look. We need to be very sure there is no way that obvious power source is responsible before invoking the ‘mysterious’ 🙂
Someone (forget who) suggested tidal heating; that bears proper re evaluation, it would provide a ‘deep heat source’. Intuitively, as its a gravity *gradient* effect & 67P is so small, you would think it would be very small, but its worth checking, not trivial. Easily accessible stuff tends to apply to low eccentricity, short orbital period moons; the comet is very different. I don’t think its *likely* its a major factor, but it should be checked, intuition could be unsafe in these circumstances.
I would however utterly rule out ‘discharges’ etc, which don’t bear a moments inspection of the data or consideration of how they could work, nor do the pictures even look like real discharges (as opposed to what people think discharges look like.)
The tidal acceleration at 1 AU should be about 1.6e-10 m/s².
” the solar tidal acceleration at the Earth’s surface along the Sun-Earth axis is about 0.52e−7 g” (with g = 9.80665 m/s²), and the tidal acceleration proportional to the radius of the orbiting object; the radius of the comet is about 1/3200 that of the Earth.
https://en.wikipedia.org/wiki/Tidal_force
The mass of the nucleus is about 1e13 kg.
An upper bound for the acting force should hence be F = m a = 1.6e-10 m/s² x 1e13 kg = 1600 N.
If he comet would be completey flattened by the tidal force, the energy would be W = F s = 1600 N x 2000 m = 3.2e6 J, or distributed over 6 hours P = W / t = 3.2e6 J / 21600 s = 148 W.
That’s less than the solar power of 1 m² fully illuminated surface. Since tidal deformations are much lower, the released power by tidal forces is also much lower than 148 Watts for the whole nucleus, likely below 1 W.
Hi Gerald, there is not much point calculating the tidal force and relating it to energy. The energy is coming from the rotational kinetic energy. The diurnal modulation of the gravity field is postulated to enable the rotational kinetic energy to be converted to heat. The tidal force is not causing the heat, so your calculation avoids the postulated energy source altogether.
On Earth, it is the rotation of the earth that is the source of the tidal friction. As Earths rotation slows down, it’s kinetic energy is converted to heat in the oceans. Some of it gets transferred to the moons orbital velocity to conserve AM.
Hi Marco, I’ve applied one of the techniques to estimate an upper bound of tidal heating. With more accurate calculations you would get an even much lower value for power produced by tidal heating.
You may like to read e.g. these little lecture notes for a better understanding:
https://www.astro.uvic.ca/~venn/A201/maths.5.tidal_heating.pdf
The energy stored in the rotation doesn’t help, since it needs to be transformed to tides for any heating effect.
Maybe you trust this paper on tidal heating of Enceladus:
“For a comparison for the Earth K=..1 Kelvin during 10^9 year was obtained”
https://hal.archives-ouvertes.fr/hal-00594415/document, page 5
Tidal heating effects for small bodies distant from the Sun or large planets are negligible.
Hi Gerald,
These papers do not bother to mention rotational kinetic energy because in cases of Io, Enceladus and many other objects where tidal effects are “obvious” being closer to the Roche zone, rotational effects are irrelevant in comparison. I understand what they are calculating there, and it is not what is causing the internal heat in 67P. It also not the energy source for the friction of the Oceans on the Earths surface due to the movements of water due to the diurnal tidal cycle. if the Earth was mutually tidally locked to the Moon, there would be no diurnal cycle in the waters heights, and no rotational momentum exchange.
In 67P’s case the rotational kinetic energy is much greater than the tidal energy, and thus it is your tidal energy calculation which is irrelevant.
Perhaps tidal heating is the”wrong” term in this case, but it is about the diurnal cycle of the tidal effect being strongest at Equinox in May, and it being enough to unlock the rotational kinetic energy and converting it to heat internally. I guess the “thermal tides” would also be at a peak in May, so I am not discounting the possibility of that being the force that is unlocking the rotational kinetic energy, or a combination of the two.
Ok, assume some unknown mechanism converting all rotational energy of the comet to heat.
Assume all mass of the comet concentrated to 2 km away from the center of rotation to maximize the model assumptions about rotational energy.
The rotation period is 12.4 hours. Hence the velocity of the mass of the comet relative to the center of mass is 2000m * 2 * pi / 12.4 hours = 0.28 m/s.
Now slow down a mass from 0.28 m/s to zero. That’s an energy release of W/m = v²/2 = 0.0784 J / kg.
The specific heat capacity of liquid water
(https://en.wikipedia.org/wiki/Heat_capacity)
is 74.539 J/(mol⋅K) at 25°C, as an example.
One mol of water is 18g, hence 1 kg water is 1 kg / 0.018 kg = 55.6 mol. Hence you need 55.6 x 74.539 J = 4144 J to heat liquid water from 24.5° to 25.5° C.
The total available rotational kinetic energy of the comet, if made of water at 25°C, would heat up the comet by 0.0784 K / 4144 = 1.89e-5 K = 0.0000189 K.
Since only the comet spins down only a fraction of this, heat production by this presumed mechanism is even less.
To evaporate liquid water (https://en.wikipedia.org/wiki/Enthalpy_of_vaporization) you need 55.6 x 40650 J = 2260140 J.
The rotational energy of the comet could evaporate
(0.0784 J / kg) / 2260140 J = 3.47e-8 of its mass, if made of liquid water at 100 °C at normal pressure.
With a total mass of 1e13 kg, this would corrsepond to
3.47e-8 x 1e13 kg = 346 metric tons.
An evaporation rate of 1 kg/s could be sustained for 346e3 s = 4 days.
The sublimation energy is a little higher, so we can take the above model number as an upper bound.
… The heat of evaporation of water at 100°C normal pressure is 2260140 J / kg , of course. (I ‘ve noticed the missing “/ kg” after committing the above post)
Take 100 W/m² as a lower bound by solar heating (https://en.wikipedia.org/wiki/Solar_constant) as a comparison.
Take 10 km² = 10e6 m² as the area of the comet, which is effectively heated by this value.
We get solar energy of 100 W/m² * 10e6 m² = 1e9 W for the comet.
More than triple the 2260140 J / kg evaporation heat of water to 1e7 J/kg as an upper bound estimate for sublimation heat.
You still get sufficient energy to sublimate 1e9 W / 1e7 J/kg = 100 kg /s water ice at any time.
The solar constant at 1 AU is 1361.W/m².
Hence the 100 W/m² assumption is valid for sqrt(1361.W/m² / (100 W/m²)) = 3.69 AU.
In the meanwhile we are closer to the Sun.
A lengthy Q&A session:
Q: “Are the volatiles still thought to be mainly ice, or are they thought to now be mainly other gases not as easily recognizable visually as ice is?”
A: It’s thought to be mainly water ice at most locations, mainly CO2 ice at some locations.
https://www.hou.usra.edu/meetings/lpsc2015/pdf/2494.pdf
“VIRTIS-M data are in good agreement
with VIRTIS-H data obtained previously [3] and
indicate that water vapour is mainly present in the coma
above the active areas of the neck region where
carbon dioxide emission are not detected. Conversely,
the carbon dioxide emission seems to happen preferably above the head region far from the active areas.”
Q: “Are the sublimation chambers that are creating the explosive jets deeper than originally thought?”
A: As far as I understand, the jets originate at locations where ice (mostly water ice) is either exposed to the surface or only protected by a thin layer of dust.
The jets thus far are not too virulent. Gas expands with about the speed of sound, maybe supersonic, after exposure to the near-vacuum, but it’s dilute. It drags dust with lower speed, observations thus far up to 10 m/s dust velocity, theoretical models allow for higher dust velocities.
The super-volatiles can sublimate deeper in the subsurface and are more likely to form pockets, with the potential to erupt explosively.
Q: “How are the jets producing both the hard and fluffy dust detected so far?”
A: The gas drags dust from near the cometary surface. The dust appears to consist of a fluffy interstellar (presolar) dust component and a much more abundant
(by mass) component of higher bulk density (not immediately of presolar origin).
Q: “How long would one sublimation chamber be able to produce a jet? When does it ‘run out?'”
A: Sublimation (mostly of water ice) seems to occur near-surface. That’s evidenced by increased activity on the day side. Sublimation from deeper layers or chambers would continue almost uniformly on the night side of the rotating comet.
Q: “Still wondering what the maximum diameter of a chamber opening (nozzle) could be to still produce a jet and maintain the necessary and apparently very steady internal pressure for the jet?”
A: Assuming a chamber for the jets is probably only marginally correct. A better approximation seems to be a dust/ice mix covered by a thin dust layer.
Q: “At what depth is the slight heat from the sun at this distance totally cancelled out by comet surface depth 1cm, 2cm, 1 meter?”
A: The heat from the sun is more than “slight”. We are talking about 100K or more between day and night at the very surface. The temperature gradient in the fluffy dust cover is estimated up to about 50 K / cm at peak solar illumination for the top 1cm dust layer.
https://www.hou.usra.edu/meetings/lpsc2015/pdf/2595.pdf
Assuming an exponential law, we are talking about a few 10s of centimeters at most for dust-covered surfaces, until we are below significant diurnal effects.
Q: “How is the rate of flow through the chamber nozzles affected by the growing size of the chamber as it sublimates?
What is the life span of a chamber, or range of time?”
A: There may from pockets, but that’s only marginally related to continuous jets.
Q: “What in the sublimation model accounts for the closer-up pictures of jets that are quite wide at their base, where they originate from the comet?”
A: The area of subsurface ices accessible for solar heating is increasing closer to the sun. The crust / dust layer varies in thickness, composition and thermal conductivity.
Q: “P67 has taken many passes around the sun, no doubt producing many many sublimation jet chambers each time that would I imagine go inactive as it retreats from the sun. Are the jets we see now being produced from old chambers that were already there from previous trips, or from new chambers, or both?”
A: Eight of the currently close (1.3 AU) passes since 1959, when it came close to Jupiter. Pockets may have formed near perihelion; they may have erupted explosively or formed sinkholes leading to the cratered-looking surface.
If ices are exposed at those “craters” they undergo sublimation, when heated by the sun.
New pockets may form due to the sublimation of supervolatiles as protecting layers sublimate away. The pocket structure as such isn’t the relevant factor for jets; it’s the exposure of ice by whatever reason (sublimation, sinkholes, explosion, …).
Q: “Does the opening size of all the nozzles for the chambers have to be pretty much the same diameter for this model to work, and if so, how did nature create this perfectly uniformed, comet-wide phenomena?”
A: It works a different way.
Q: “And wouldn’t just one or two other cracks or openings or surface porousity cancel the necessary pressure for the jet flow?”
A: This may apply to explosive eruptions; continuous jets work different.
Q: “And as surface material is removed, how is it that these nozzles would keep appearing, do they already exist at greater depths, or does the sublimation process create them?”
A: As surface material is removed, new surface material is exposed; sublimation continues, the jets are sustained, independent of the formation of nozzles or pockets.
Q: “Wouldn’t these nozzles be effected by the explosive pressure of the chambers (be quickly enlargened), to the point of decompressing the chamber and canceling the jet?”
A: This applies to explosive eruptions, not to continuous jets.
Q: “If chambers form under the surfaces that don’t have surface openings (nozzles), shouldn’t we be seeing surface explosions?”
A: Pockets may collapse and produce explosions, yes. One such event is probably responsible for temporariliy increased cometary activity during the approach phase of Rosetta. It’s one of the hazards the mission needs to be able to deal with, at any time.
Q: “Unless it is being proposed that all the heat driving sublimation is coming just through the nozzle, but then, seems the escaping gas would quickly cool and counteract the heat coming downward through the nozzle.”
A: Solar heating and cooling by sublimation should keep the ices near their sublimation point during sunshine. Excess heat from the sun goes into sublimation, or if ices are under a thick thermally insulating dust cover, heat is thermally radiated away over night.
Q: “So surely not all the surface heat is just affecting potential chambers with nozzles.”
A: True.
Q: “In fact, it seems we should either being seeing slowly released gas/dust through surface cracks and porousity, OR explosive release of buried sublimation, …”
A: Currently we mostly see jets caused by slowly released gas/dust, most of it froms near surface by sublimation of ices. But explosive events become more likely near perihelion. As previously mentioned a likely virulent event happened during Rosetta’s approach phase.
Q: “…and that jets would be either extremely rare and relatively brief, or non-existent.”
A: Wrong, since the cause for the jets is different.
Q: ” Perhaps if you could provide another example of something in nature that’s under explosive pressure similar to these chambers that maintains a steady, even, prolonged released, it would help.”
A: The cometary environent and Earth are very different. Any examples from Earth therefore match only to some degree.
Atmospheric pressure on Earth make an example with water ice difficult. So if you’ve access to dry ice (CO2 ice) – be careful! or take it as a thought experimant, put it in the sunshine onto a thermally insulating layer. It will sublimate away; of course you usually don’t get the jet, due to interaction with the atmosphere. You’ll also find videos about CO2 sublimation in YouTube, e.g. this one:
https://www.youtube.com/watch?v=6JzQ08AGuhI
Remember, that the visible jets aren’t the sublimated ice, but dragged dust. (The video above takes a spoon instead of dust.)
I can’t recommend to make experiments with supersonic gas dragging dust, unless you have access to an appropriate vacuum chamber.
Geothermal steam vents are only remotely similar, since the heat source in this case comes from Earth’s interiour, whereas for comets the heat source is the exteriour solar illumination.
@ Gerald
“A lengthy Q&A session:”
It is lengthy indeed.
Your lengthy “answers” would have been more convincing, however, if they had been much shorter and, above all, actually addressed the precise questions asked. These, for the most part, required (even approximately) *quantified* responses based on the elementary principles of hydrodynamics.
Just one example:
Q: “How long would one sublimation chamber be able to produce a jet? When does it ‘run out?”
A: “Sublimation (mostly of water ice) seems to occur near-surface. That’s evidenced by increased activity on the day side. Sublimation from deeper layers or chambers would continue almost uniformly on the night side of the rotating comet.”
There are several other examples of this sort of simple avoidance of the crucial issues behind the questions asked.
The question was simple and direct (like all the others). Why couldn’t you simply answer it, shortly and to the point?
Simply because the question is based on wrong presumptions.
I could also have said, that all questions based on the chamber model are essentially baseless, not worth to be answered.
Thanks for this avowal, Gerald. Why did you even bother, in that case, with such a lengthy pseudo-Q&A session?
As regards your new answer, I thought the “chamber model” was the only one left in the standard theory arsenal to explain this sort of sudden, explosive occurrence, for those who still believe in sublimation.
So what alternative mechanism do you propose?
If it would be “sudden and explosive”, it would apply. Thus far by far the most jets, if not all, aren’t of that type.
High gas velocities are just an effect of exposure of gas to the vacuum, independent of topographical detail.
The gas is a result of sublimation/decomposition of ices/clathrates due to heating driven by solar illumination, including possible “self-heating” from illuminated surfaces.
This may or may not include “chambers”.
Pockets containing supervolatiles are a possible cause for virulent eruptions. What I’ve seen in the images thus far has been rather benign.
Explosive eruptions would eject clouds of larger fragments. Maybe we’ll see such events – or their effects – later, near perihelion.
Hi Emily. “…The raised section on the top left side of the large lobe is at the boundary of Aten and Babi.”
Is it near South Pole?