Comet Notes 1 (private)

Jets / plumes of dust

Sublimation Notes

1. Summer fireworks on comet 67P

https://academic.oup.com/mnras/article/462/Suppl_1/S184/2633381

3 main dust plume morphologies : 

Type A: they produce a much collimated jet which expands beyond our field of view (typically 10 km). They extend further away from the nucleus than the other types.

Type B: broad plumes, or wide dust fans. They expand much more laterally than radially when compared to type A plumes.

Type C: complex events, often combining both a narrow and a broad feature. To the best of our knowledge both features arise from the same source, within the error ellipse of our detection.

It is important to stress that this classification is purely morphological. It is not clear whether the three types correspond to different mechanisms or if they are different stages of a same process.

We can constrain the dust velocity (velocity was not measured directly) in these plumes by using the cadence of our images. For instance, the plume associated with event #25 extends by at least 8,150 m and was not detected in the previous image acquired 10 min earlier. This implies that dust was ejected with a minimum velocity of 13 m/s. This is at least one order of magnitude larger than the typical velocity of dust grains in 67P's jets at 1 m/s, indicative of more energetic events.

If there is indeed an evolution, it seems that the most reasonable sequence would be type A > type C > type B.

The event starts with some dust and gas being ejected at high velocity in a narrow plume (type A). This is indicative of a small source area, possibly confined.

As the outbursts unravels, the local surface is modified (collapse or ‘eruption’) and exposes a larger fraction of fresh material leading to the formation of a broader plume (type C).

Finally, the morphology of the source area has changed enough to not be able to collimate the initial narrow flow anymore, and only the broad plume survives (type B).

 

Outbursts occur either in the early morning or shortly after the local noon, indicating 3 potential processes : 

- morning outbursts may be triggered by thermal stresses linked to the rapid change of temperature

- afternoon events are most likely related to the diurnal or seasonal heat wave reaching volatiles buried under the first surface layer. 

- another possibility is that most of the dust is released upon the collapse of a cliff.

Long-lasting repetitive jets - One of the striking discoveries of Rosetta has been the clockwork repeatability of jets from one rotation to the next, that too at the same exact features.

Transient events - These events are characterized by the sudden and short release of a dust, sometimes collimated but not necessarily.

While the typical jets are relatively faint (about 10 % of the nucleus surface brightness), the plumes ejected by these outbursts are usually as bright as the nucleus, and they can be detected in our images without enhancing the contrast. 

Contrary to the jets that last for several hours, most transient events are observed only once, indicating a shorter lifetime (jets that last for 5 min to 30 min).

An outburst is identified by a sudden brightness increase in the coma, associated with the release of gas and dust over a duration very short with respect to the rotation period of the nucleus. The dust plume is typically one order of magnitude brighter than the usual jets. 

Outbursts usually last less than 5 min. Maximum mass of dust ejected by such events is around 20–80 tons.

This mass loss per outburst is comparable to what has been observed on, at least, another comet. We have estimated the mass ejected in the outburst by 9P/Tempel 1 on 2005 Jul 2. 

This outburst was 2/3rd the brightness of the ambient coma at all radii and the total ejected mass was 300 tons over 10 min, therefore an outburst flux of 500 kg/s, of the same order as the values we have found for 67P.

We did observe a large outburst in 2014 April, at a distance of 4 au (snow line or frost line is at 3au according to ESA but others claim it goes up to 5au, a strangely huge margin of error & very convenient for their sublimation theories).

The next event occurred in 2015 Feb at a distance of 2.5 au. Although much smaller in scale than most of the other events, it is particularly interesting because it arose from an area that had been in the night for 5 hours, when the outbursts occurred.

Comparing the timeline of detected transient events with the cadence of our images gives us a hint of the completeness (or incompleteness) of our catalogue. For instance, one can see that we may have missed several events in the first half of 2015 July due to poor time coverage. The same is true for the last week of 2015 September during which we did not observe for 24 consecutive hours. 

However, the rest of the timeline is densely covered with observations, and the gaps in outburst detection cannot be explained by lack of imaging. This is particularly true for the first half of 2015 August or the week around 2015 September 10 during which we did not detect any event in spite of continuous high-cadence monitoring. In addition to that, it happened several times that we observed the comet in consecutive rotations for 1 or 2 d during which only one event was detected. For instance, the full set of observations acquired around perihelion from August 9 to 13 yielded only two events (August 9 and 12).

(So even at the same 2.5 au distance, the comet's activity is not consistent. outburst on Aug 9, nothing on 10 & 11, outburst again on 12, nothing on 13)

3.3 Source locations and local morphology

All outbursts sources are located close to morphological boundaries, i.e. areas where we observe discontinuities in the local terrain, either textural or topographic. This seems to indicate a link between morphology and outbursts, although it is not clear which one influences the other.

We found that 45 per cent of the outbursts occurred after about 3 h of illumination, i.e. shortly after the local noon. The other 55 per cent of events appear to arise from surfaces that saw their last morning more than 10 h before. If the comet would be a sphere, this would mean night side outbursts.

However, due to the very complex shape of 67P, one has to look at these events case by case, and indeed they all seem to arise from a very early local morning, as the Sun just starts to shine on the source area.

There is no correlation between the type of outburst and local time. The dichotomy is more likely related to a different mechanism : 

(1) noon outbursts may be linked to buried pockets of volatiles, which need time to get heated enough to trigger an outburst. Shortly after noon is when the local surface reaches its maximum temperature. 

(2) Early morning outbursts, however, occur almost immediately as the Sun rises. Although the temperature might not yet be too high, the very low thermal inertia ensures that these local times display the steepest temperature gradient. The surface heats up in a few minutes.

More than half of the outbursts observed by Rosetta on comet 67P were dawn outbursts.

Hence, for a region of homogeneous surface properties, most outbursts should occur around the same local time/local solar incidence. As explained in Section 3.4, we do not see this on 67P; the same area can outburst shortly after noon or early in the morning.

On the basis of our observations of the dust plumes and of the local nucleus morphology, we can infer a possible mechanism generating the outbursts. Vincent et al. (2016) have proposed receding fractured cliffs as the major process responsible for the usual dust jets seen around 67P and other comets. In this scheme, small fractures lead to enhanced inward heat flux and acceleration of the gas in a nozzle-like structure, which form of small jets that can merge into a larger feature as they expand away from the surface. Hoefner (2016) has shown that fractures are an efficient heat trap, but require specific illumination conditions to achieve full potential, namely the Sun shining directly into the fracture so that the maximum input flux reaches the bottom. This means that local activity can only be sustained by having many small fractures subsequently activated as the nucleus rotates. This is indeed the case in many areas, as discussed in Vincent et al. (2016). But what if the solar insolation reaches its maximum over a large fracture? For instance, the crack in the Hapi region that seems to separate the two lobes of the comet (Thomas et al. 2015), or a 500 m long fracture in the Anuket region (El-Maarry et al. 2015a), both a few metres wide? One would expect the same process as for the jets to take place, although enhanced by at least an order of magnitude due to the larger size of those fractures. This high solar input concentrated on a very localized area would lead to an outburst. Unfortunately, although the right illumination conditions are achieved regularly, we have never detected an outburst from a large fracture. This is perhaps not surprising as the heat trap effect would prevent them from retaining much volatile material anyway. The only possibility would be a sudden opening of the fracture either laterally or in depth, which would expose fresh material again.

Interestingly, we did not observe outbursts from any of the large fracture systems (e.g. Wosret's fractured terrains), and only once from a smooth terrain (though notably one of the strongest events, see #16 in Table 1 and Fig. 3). All outburst-related structural features appear in pre-perihelion images  and have not been created by the outbursts themselves, but perhaps modified. 

Here we suggest an alternative (cliff collapse), new process, which could explain the observed outbursts without the need for local ice reservoirs, or specific illumination conditions. Jet activity from fractured cliffs leads to a weakening of the wall structure until it collapses, a phenomenon observed on 67P and other comets, and described in detail in Vincent et al. (2016). As most outbursts are located near cliffs presenting evidence of mass wasting, it is tempting to link the two processes. That is to say that with the proposed mechanism, most of the dust is being generated during the collapse, rather than ejected from the surface by an explosion. Of course, we certainly need a gas flow to accelerate the dust away from the surface but the collapse itself may not be triggered by activity. This process requires less energy input than other mechanisms because cometary cliffs are extremely weak (tensile strength <100 Pa) and any small perturbation can lead to their fall. Additionally, once the dust is released it is easier to accelerate it away because the gas flow does not need to overcome cohesion forces that were keeping the grains on the nucleus surface.

2. Comet’s firework display ahead of perihelion

https://blogs.esa.int/rosetta/2015/08/11/comets-firework-display-ahead-of-perihelion/

On 29 July, 2015, 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 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.

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.

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 2 days earlier, the amount of carbon dioxide increased by a factor of 2, methane by 4, and hydrogen sulphide by 7, while the amount of water stayed almost constant.

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 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.

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 nucleus. 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.

3. Sunset / Nightime Jets on comet 67P

https://blogs.esa.int/rosetta/2015/06/08/sunset-jets/

Some regions on Comet 67P remain active even after nightfall, like this image showing jets emanating from the comet's small lobe.

The OSIRIS team think that the comet can store the incoming heat for some time beneath its surface, resulting in sustained activity from these regions even after nightfall. (how can it store heat when its thermal inertia is poor ? how can any ice remain till nightfall and not sublimate away during the day ? if sub-surface heat is the cause for night jets, then how do dawn jets form in the absence of sub-surface heat ? why does it need the morning sun ?)

“While the dust covering the comet’s surface cools rapidly after sunset, deeper layers remain warm for a longer period of time,” says OSIRIS scientist Xian Shi from the MPS, who is studying the sunset jets. (how can deeper layers remain warm ? the deeper layer is supposed to be ice)

The scientists suspect that the comet’s supply of ices that fuel the comet’s activity exists in these deeper layers (defies logic)

Previous comet missions, such as Stardust’s flyby of Comet 81P/Wild 2 and Deep Impact’s mission to Comet 9P/Tempel 1, also found evidence of jets sustained on the night side.

4. Rosetta unravels formation of sunrise jets

https://www.mps.mpg.de/Rosetta-unravels-formation-of-sunrise-jets

Daily recurring outbursts at sunrise can be observed. In these, evaporating gas and entrained dust are concentrated to form jet-like structures. A new study now identifies the rugged, duck-shaped structure of the comet as the main cause of these jets. Not only do concave regions collimate gas and dust emissions similar to an optical lens, the complex topography also provide some areas of the surface with more sunlight than others.

Paper vomits sublimation nonsense. 

During Giotto mission, which flew by comet 1P/Halley in 1986, distinct jets of gas and dust were visible within the coma. They reach up to several kilometers into space. 

2 types of jets on comet 67P - eruptive, sudden outbursts of gas and dust, as well as jets that are stable for a longer time. 

"When the Sun rises over a part of the comet, the surface along the terminator almost instantaneously becomes active," first author Dr. Xian Shi from MPS describes. "The jets of gas and dust, which we then observe within the coma, are very reliable: they are found each morning in the same places and in a similar form," she adds. Responsible for this early morning activity is the frost, which forms at night on the cold comet surface. As soon as the Sun's rays touch it, it begins to evaporate.

"Outbursts can often be traced back to a small area on the surface where suddenly frozen water is exposed, for example due to a landslide," explains Dr. Holger Sierks from the MPS, OSIRIS Principal Investigator. (if ice is exposed, why should it be dark ? logic says it should be bright. The justification for a dark comet was that the ice is covered by dust. So when the ice gets exposed, we should see bright patches, which we don't)

"In the case of cometary activity at sunrise, this is different. The frost is distributed fairly evenly over the entire surface." But then why do the gas and dust emissions form jets? Why do they not create a completely homogeneous cloud?

The new study shows for the first time that mainly the unusual shape and jagged topography of the comet are responsible for this phenomenon.

On a spherical or even potato-shaped comet, these structures within the coma might not be as prominent. Gas and dust would be much more evenly distributed. (Enceladus moon blatantly proves these claims to be bogus, check my tweet on it)

(nighttime jets are attributed to latent sub-surface heat. how a thin coating of frost can the survive the whole day defies logic. anyway, in the case of dawn jets, there is no latent heat at all. these are all bogus nonsensical illogical explanations)

17. ---------------------------- from comet 67P page

Rosetta’s big day in the sun

https://blogs.esa.int/rosetta/2015/08/13/rosettas-big-day-in-the-sun/

ESA, Aug 2015

Extract - Perihelion distance of comet 67P - 186 million km from the Sun (1.24 AU).

One image taken by Rosetta’s navigation camera was acquired just an hour before the moment of perihelion, from a distance of around 327 km from the comet (still the comet was not significantly more active, despite being just 1 hr away perihelion)

Rosetta’s measurements suggest the comet is spewing up to 300 kg of water vapour – roughly the equivalent of two bathtubs – every second. This is a 1,000 times more than was observed this time last year when Rosetta first approached the comet. Then, it recorded an outflow rate of just 300 g per second, equivalent to two small glasses of water.

Along with gas, the nucleus is also estimated to be shedding up to 1,000 kg of dust per second

1,000/300 means that dust is 3.3 times more than gas (water vapour). or the gas to dust ratio is 1:3.3

Not long after arriving, surface temperatures of around –70ºC were recorded. By April–May 2015, this had risen to only a few degrees below zero celsius, and now highs of a few tens of degrees above zero are forecast for the next month.

Rosetta is far too close to the comet to see its growing tail, but images collected over the past few months with telescopes across the world show that it already extends more than 120 000 km.

A lop-sided coma, with a notable high-density region away from the main tail, was revealed in various images, including some taken last week from the Gemini-North telescope on Mauna Kea, Hawaii.

Ground-based images of the comet reveals some asymmetries in the comet’s tail.  

18. ---------------------------- from comet 67P page

Icy surprises at Rosetta’s comet

https://blogs.esa.int/rosetta/2016/11/17/icy-surprises-at-rosettas-comet/

ESA, Nov 2016

Rosetta’s comet approached its most active period last year, the spacecraft spotted carbon dioxide ice – never before seen on a comet – followed by the emergence of two unusually large patches of water ice.

The carbon dioxide ice layer covered an area comparable to the size of a football pitch, while the two water ice patches were each larger than an Olympic swimming pool and much larger than any signs of water ice previously spotted at the comet.

“We know comets contain carbon dioxide, which is one of the most abundant species in cometary atmospheres after water, but it’s extremely difficult to observe it in solid form on the surface,” explains Gianrico Filacchione from Italy’s INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, who led the study.

In the comet environment, carbon dioxide freezes at -193ºC, much below the temperature where water turns into ice. Above this temperature, it changes directly from a solid to a gas, hampering its detection in ice form on the surface.

The patch, consisting of a few percent of carbon dioxide ice combined with a darker blend of dust and organic material, was observed on two consecutive days in March. This was a lucky catch: when the team looked at that region again around three weeks later, it was gone.

Assuming that all of the ice had turned into gas, the scientists estimated that the 80 x 60 m patch contained about 57 kg of carbon dioxide, corresponding to a 9 cm-thick layer. Its presence on the surface is likely an isolated rare case, with the majority of carbon dioxide ice being confined to deeper layers of the nucleus.

Gianrico and his collaborators believe the icy patch dates back a few years, when the comet was still in the cold reaches of the outer Solar System and the southern hemisphere was experiencing its long winter. At that time, some of the carbon dioxide still outgassing from the interior of the nucleus condensed on the surface, where it remained frozen for a very long while, and vaporised only as the local temperature finally rose again in April 2015.

This reveals a seasonal cycle of carbon dioxide ice, which unfolds over the comet’s 6.5 year orbit, as opposed to the daily cycle of water ice, also spotted by VIRTIS shortly after Rosetta’s arrival.

Interestingly, shortly after the carbon dioxide ice had disappeared, Rosetta’s OSIRIS narrow-angle camera detected two unusually large patches of water ice in the same area, between the southern regions of Anhur and Bes.

“We had already seen many metre-sized patches of exposed water ice in various regions of the comet, but the new detections are much larger, spanning some 30 x 40 m each, and they persisted for about 10 days before they completely disappeared,” says Sonia Fornasier from LESIA–Observatoire de Paris and Université Paris Diderot, France, lead scientist of the study focusing on seasonal and daily surface colour variations.

These ice-rich areas appear as very bright portions of the comet surface reflecting light that is bluer in colour compared with the redder surroundings. 

The two newly detected patches contain 20–30% of water ice mixed with darker material, forming a layer up to 30 cm thick of solid ice. One of them was likely lurking underneath the carbon dioxide ice sheet revealed by VIRTIS about a month before. 2 papers referenced in this aricle : 

Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P

https://science.sciencemag.org/content/early/2016/11/16/science.aag3161

Rosetta’s comet 67P sheds its dusty mantle to reveal its icy nature

https://science.sciencemag.org/content/early/2016/11/16/science.aag2671

19. ----------------------------

Rosetta’s last words : Science descending to a comet

https://blogs.esa.int/rosetta/2016/12/15/rosettas-last-words-science-descending-to-a-comet/

ESA, Dec 2016

ROSINA measured an increase in the surrounding gas pressure by more than a factor of 100 as the spacecraft neared the surface.

“We saw the gas velocity and ram pressure drop to zero before we reached the ground, suggesting there is an interesting acceleration of gas slightly away from the nucleus,” says Kathrin. “We also collected good data with the DFMS, and will be looking at which kind of atoms and molecules were present in the gas.”

During the descent, MIRO collected continuum emission data from the nucleus, providing temperature measurements 1 cm and 5 cm below the surface.

“Over the last few hours, we see temperatures varying between about 80 K and 160 K (-193 C to -113 C) as our beam moved across the nucleus. We think these differences are due to topography and shadowing.

RPC-LAP and RPC-MIP both reported very low plasma densities through the descent, though slowly and evenly increasing, similar to that seen by ROSINA-COPS for neutral gas. However, the plasma reached a broad peak of up to about 100–150 cm^-3 (preliminary measurement) at about 2 km from surface, before dropping off again. This is as expected for a plasma originating from the neutral gas released by the comet: its density must be low at the surface since the molecules found there have just left the nucleus and have not had any time to become ionised.

Fracture networks on 67P

https://www.planetary.org/space-images/fracture-networks-on-67p

Fractures on comet 67P/Churyumov-Gerasimenko observed by Rosetta/OSIRIS

https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1002/2015GL064500

El-Maarry et al,, 2015

Fractures vary greatly in length from a few meters to 250 m in length.

Surface Morphology of Comets and Associated Evolutionary Processes: A Review of Rosetta’s Observations of 67P

https://link.springer.com/article/10.1007/s11214-019-0602-1

Sub-meter-wide fractures form 2-dimensional surface patterns with different topologies. They vary in length from 100s of meters down to mm-scale, as observed by the Philae lander at its final landing site. The longest fractures (seen by Philae) reach up to 10s of cm in length.

Astronomers Document Remarkable Changes on Comet 67P

https://scitechdaily.com/astronomers-document-remarkable-changes-on-comet-67p/

The comet’s increasing spin rate in the lead-up to perihelion is thought to be responsible for a 500 meters fracture spotted in August 2014 that runs through the comet’s neck. The fracture, which originally extended a bit longer than the Empire State Building is high, was found to have increased in width by about 100 feet (30 meters) by December 2014. Furthermore, in images taken in June 2016, a new 500- to 1,000-foot-long (150 to 300 meters)  fracture was identified parallel to the original fracture.

Fractures on comet 67P 

https://www.researchgate.net/publication/281268620_Fractures_on_comet_67P_SupportingMaterial

Refractory to ice dust ratio

8% ice on comet 67P :

https://www.aanda.org/articles/aa/full_html/2020/04/aa37485-20/aa37485-20.html

We determine that the refractory-to-ice mass ratio ranges from 6 to 110 in the perihelion-eroded pristine nucleus, providing a pristine ice mass fraction of 8% in mass.

(dust to ice ratio ranges from 6:1 to 110:1, this indicates very poor in ice & rich in dust, that dust comes from rock)

OSIRIS detected both small dust particles in the size range from 3 × 10−3 up to 1 cm (Fulle et al. 2016) and larger particles, named chunks. These objects are defined as pieces of the nucleus of an average mass of 1 kg, in the 10 to 20 cm range (Fulle et al. 2019).

Ice on comet 67P

Exposed water ice on the nucleus of comet 67P

https://www.nature.com/articles/nature16190

Abstract : "...limited evidence for exposed water-ice regions on the surface of the nucleus (of comet 67P) has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far"

The nucleus of 67P appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material (assumes sub-surface ice bcos it was not found on the surface. there is absolutely no evidence behind this faith of sub-surface ice. No scientist or institution on the planet can replicate cometary features with ice coated with dust. Why ?)

https://phys.org/news/2016-01-ice-surface-comet-67p.html

"Keep in mind that comets are very porous, like cotton candy," Gudipati said. "Seventy percent of this comet is a void, and because of that, the heat from the surface does not go that deep."

It should be noted that water ice made up just a small percentage of the material scanned by VIRTIS, suggesting that it is mixed in with the refractory materials in the upper layers of the comet.

https://www.esa.int/Science_Exploration/Space_Science/Rosetta/Rosetta_reveals_comet_s_water-ice_cycle

10–15% ice in a studied patch of 1 square km region on the comet’s neck. It means 85-90% of the material rock + dust

From these data, it is possible to estimate the relative abundance of water ice with respect to other material. (10-15% is very poor, not abundant. If you got 15 marks in a 100 marks test, would you describe your marks as abundant ?) Down to a few cm deep over the region of the portion of the comet nucleus that was surveyed, water ice accounts for 10–15% of the material and appears to be well-mixed with the other constituents.

Scientists using Rosetta’s Visible, InfraRed and Thermal Imaging Spectrometer, VIRTIS, have identified a region on the comet’s surface where water ice appears and disappears in sync with its rotation period. 

“We saw the tell-tale signature of water ice in the spectra of the study region but only when certain portions were cast in shadow,” says Maria Cristina.

The data suggest that water ice on and a few centimetres below the surface ‘sublimates’ when illuminated by sunlight, turning it into gas that then flows away from the comet. Then, as the comet rotates and the same region falls into darkness, the surface rapidly cools again.

(So this thin layer of frost ice explains the grand visual phenomenon of a comet ? where is the experimental verification ? what's the problem ? why failure ?)

However, the underlying layers remain warm owing to the sunlight they received in the previous hours, and, as a result, subsurface water ice keeps sublimating and finding its way to the surface through the comet’s porous interior.

But as soon as this ‘underground’ water vapour reaches the cold surface, it freezes again, blanketing that patch of comet surface with a thin layer of fresh ice.

(The coldest temp on 67P was -93C, too warm for ice to exist on surface, unless it was temporarily created in a endothermic reaction). Sub-surface ice has not been detected, nor experimentally success in recreating the observed visual  phenomenon of a comet)

Frost line (comets, wrong info)

http://astroweb.case.edu/ssm/astr101/S19c24.pdf

CWRU Astronomy is a community of active researchers and educators

https://en.wikipedia.org/wiki/2I/Borisov

Dave Jewitt and Jane Luu extrapolate that comet Borisov became active in June 2019 when it was between 4 and 5 au from the Sun. A search of image archives found precovery observations of 2I/Borisov as early as 13 December 2018, but not on 21 November 2018, indicating it became active between these dates.

Comet Borisov made its closest approach to the Sun at just over 2 AU on 8 December 2019.

https://www.esa.int/Science_Exploration/Space_Science/Rosetta/Rosetta_and_the_chameleon_comet

Fairytale icy fantasies, in total defiance of what they instruments found :

Colour changes on 67P reflect the amount of water ice that is exposed on the surface and in the surroundings of the comet. At the beginning of Rosetta’s mission, the spacecraft rendezvoused with the comet while it was still a long way from the Sun. At such distances, the surface was covered in layers of dust and little ice was visible. This meant the surface appeared red when analysed with the VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) instrument.

As the comet drew closer it crossed an important boundary, known as the frost line. Occurring at a distance around three times further from the Sun than the Earth (3au), anything within the frostline will be heated sufficiently by the Sun that the ice will turn into a gas, a process called sublimation.

As Rosetta followed 67P/C-G across the frostline, VIRTIS began to notice the colour of the comet change. As the comet approached the Sun, the heating increased and the hidden water ice began to sublime pushing away the dust grains too. This revealed layers of pristine ice, which made the nucleus turn bluer in colour as seen by VIRTIS. 

Ice on asteroid Gaspra & 24 Themis

*** 

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