How comets are born

How comets are born
Evidence that Comet 67P/Churyumov-Gerasimenko is composed of ancient material preserved from the formation of the early Solar System and that came together under low speed. The evidence collected by Rosetta lies in the comet's structural properties, the gases detected leaving the nucleus, and observations of surface features. Credit: Centre: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0; Insets: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Fornasier et al. (2015); ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S; Langevin et al. (2016)

Detailed analysis of data collected by Rosetta show that comets are the ancient leftovers of early Solar System formation, and not younger fragments resulting from subsequent collisions between other, larger bodies.

Understanding how and when objects like Comet 67P/Churyumov-Gerasimenko took shape is of utmost importance in determining how exactly they can be used to interpret the formation and early evolution of our Solar System.

A new study addressing this question led by Björn Davidsson of the Jet Propulsion Laboratory, California Institute of Technology in Pasadena (USA), has been published in Astronomy & Astrophysics.

If comets are primordial, then they could help reveal the properties of the solar nebula from which the Sun, planets and small bodies condensed 4.6 billion years ago, and the processes that transformed our planetary system into the architecture we see today.

The alternative hypothesis is that they are younger fragments resulting from collisions between older 'parent' bodies such as icy trans-Neptunian objects (TNOs). They would then provide insight into the interior of such larger bodies, the collisions that disrupted them, and the process of building new bodies from the remains of older ones.

"Either way, comets have been witness to important Solar System evolution events, and this is why we have made these detailed measurements with Rosetta – along with observations of other comets – to find out which scenario is more likely," says Matt Taylor, ESA's Rosetta project scientist.

During its two-year sojourn at Comet 67P/Churyumov-Gerasimenko, Rosetta has revealed a picture of the comet as a low-density, high-porosity, double-lobed body with extensive layering, suggesting that the lobes accumulated material over time before they merged.

The unusually high porosity of the interior of the nucleus provides the first indication that this growth cannot have been via violent collisions, as these would have compacted the fragile material. Structures and features on different size scales observed by Rosetta's cameras provide further information on how this growth may have taken place.

How comets are born
Rosetta navigation camera (NavCam) image taken on 22 March 2015 at 77.8 km from the centre of comet 67P/Churyumov-Gerasimenko. The image has been cropped and measures 6.0 km across; the resolution is about 6.6 m/pixel. Credit: European Space Agency

Earlier work showed that the head and body were originally separate objects, but the collision that merged them must have been at low speed in order not to destroy both of them. The fact that both parts have similar layering also tells us that they must have undergone similar evolutionary histories and that survival rates against catastrophic collision must have been high for a significant period of time.

Merging events may also have happened on smaller scales. For example, three spherical 'caps' have been identified in the Bastet region on the small comet lobe, and suggestions are that they are remnants of smaller cometesimals that are still partially preserved today.

At even smaller scales of just a few metres across, there are the so-called 'goosebumps' and 'clod' features, rough textures observed in numerous pits and exposed cliff walls in various locations on the comet.

While it is possible that this morphology might arise from fracturing alone, it is actually thought to represent an intrinsic 'lumpiness' of the comet's constituents. That is, these 'goosebumps' could be showing the typical size of the smallest cometesimals that accumulated and merged to build up the comet, made visible again today through erosion due to sunlight.

According to theory, the speeds at which cometesimals collide and merge change during the growth process, with a peak when the lumps have sizes of a few metres. For this reason, metre-sized structures are expected to be the most compact and resilient, and it is particularly interesting that the comet material appears lumpy on that particular size scale.

Further lines of evidence include spectral analysis of the comet's composition showing that the surface has experienced little or no in situ alteration by liquid water, and analysis of the gases ejected from sublimating ices buried deeper within the surface, which finds the comet to be rich in supervolatiles such as carbon monoxide, oxygen, nitrogen and argon.

These observations imply that comets formed in extremely cold conditions and did not experience significant thermal processing during most of their lifetimes. Instead, to explain the low temperatures, survival of certain ices and retention of supervolatiles, they must have accumulated slowly over a significant time period.

"While larger TNOs in the outer reaches of the Solar System appear to have been heated by short-lived radioactive substances, comets don't seem to show similar signs of thermal processing. We had to resolve this paradox by taking a detailed look at the time line of our current Solar System models, and consider new ideas," says Björn.

How comets are born
Two main theories exist for how comets are born. In both cases, 'pebbles' start assembling from debris in the solar nebula, reaching sizes of about 1 cm. Then, according to the collisional rubble pile theory (left column), large objects such as the trans-Neptunian objects (TNOs) formed rapidly, within the first one million year of the solar nebula, aided by turbulent gas streams and gravity that rapidly accelerated their growth to sizes of up to 400 km. These objects also underwent internal heating caused by the decay of radioactive substances, which resulted in their dense, low-porosity structure, and kept growing over the following 400 million years, some of them even reaching sizes of Pluto or Triton-sized objects. In this scenario, comets form from fragments created in collisions between TNOs in the outer Solar System, and therefore are relatively young. According to the primordial rubble pile theory (right), instead, comets took a different path. After the rapid initial growth phase of the TNOs, leftover grains and 'pebbles' of icy material in the cold, outer parts of the solar nebula started to come together at low speed, undergoing a gradual growth with no thermal processing to their interior and yielding comets roughly 5 km in size by the time gas has disappeared from the solar nebula. The larger TNOs played a further role in the evolution of comets: by 'stirring' the cometary orbits, additional material was accreted at somewhat higher speed over the next 25 million years, forming the outer layers of the comets. The stirring also made it possible for the few kilometre-sized objects in size to bump gently into each other, leading to the bi-lobed nature of some observed comets. In the second hypothesis, comets are ancient objects made out of debris left over from the main planet-building phase and which contain preserved remnants of the early solar nebula materials. Evidence collected by Rosetta strongly favours the primordial rubble pile hypothesis, namely that comets were built up slowly through low-speed accumulation of material into the shapes observed today. Credit: European Space Agency

Björn and colleagues propose that the larger members of the TNO population formed rapidly within the first one million years of the solar nebula, aided by turbulent gas streams that rapidly accelerated their growth to sizes of up to 400 km.

Around three million years into the Solar System's history, gas had disappeared from the solar nebula, only leaving solid material behind. Then, over a much longer period of around 400 million years, the already massive TNOs slowly accreted further material and underwent compaction into layers, their ices melting and refreezing, for example. Some TNOs even grew into Pluto or Triton-sized objects.

Comets took a different path. After the rapid initial growth phase of the TNOs, leftover grains and 'pebbles' of icy material in the cold, outer parts of the solar nebula started to come together at low velocity, yielding comets roughly 5 km in size by the time gas has disappeared from the solar nebula. The low speeds at which the material accumulated led to objects with fragile nuclei with high porosity and low density.

This slow growth also allowed comets to preserve some of the oldest, volatile-rich material from the , since they were able to release the energy generated by radioactive decay inside them without heating up too much.

The larger TNOs played a further role in the evolution of comets. By 'stirring' the cometary orbits, additional material was accreted at somewhat higher speed over the next 25 million years, forming the outer layers of comets. The stirring also made it possible for the few kilometre-sized objects in size to bump gently into each other, leading to the bi-lobed nature of some observed comets.

"Comets do not appear to display the characteristics expected for collisional rubble piles, which result from the smash-up of large objects like TNOs. Rather, we think they grew gently in the shadow of the TNOs, surviving essentially undamaged for 4.6 billion years," concludes Björn.

"Our new model explains what we see in Rosetta's detailed observations of its comet, and what had been hinted at by previous comet flyby missions."

"Comets really are the treasure-troves of the Solar System," adds Matt.

"They give us unparalleled insight into the processes that were important in the planetary construction yard at these early times and how they relate to the Solar System architecture that we see today."


Explore further

Team identifies clathrate ices in comet 67P

More information: B. J. R. Davidsson et al. The primordial nucleus of comet 67P/Churyumov-Gerasimenko, Astronomy & Astrophysics (2016). DOI: 10.1051/0004-6361/201526968
Journal information: Astronomy & Astrophysics

Citation: How comets are born (2016, July 28) retrieved 20 September 2019 from https://phys.org/news/2016-07-comets-born.html
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Jul 28, 2016
"If comets are primordial .. The alternative hypothesis is that they are younger fragments resulting from collisions between older 'parent' bodies .."

The substrate is not the collapse of a cloud of gas because we have a myriad of the core (Sun, planets, satellites, etc.), if are the collisions, how explain, within collapse, the formation of a number of bodies of the same age.
Therefore, the hypothesis without evidence and logic will remain only hypotheses.

Jul 28, 2016
"If comets are primordial .. The alternative hypothesis is that they are younger fragments resulting from collisions between older 'parent' bodies .."

The substrate is not the collapse of a cloud of gas because we have a myriad of the core (Sun, planets, satellites, etc.), if are the collisions, how explain, within collapse, the formation of a number of bodies of the same age.
Therefore, the hypothesis without evidence and logic will remain only hypotheses.


Did you read all 30 pages of the paper to be able to come out with that nonsense? Or just made it up on the spur of the moment?

Jul 28, 2016
The substrate is not the collapse of a cloud of gas because we have a myriad of the core (Sun, planets, satellites, etc.),......


I'm not even sure what that means, so difficult to argue with it. If it was from a collapsing cloud of dust and gas (as seen around a number of stars), then the dust and gas is not going to hang around for very long, once it starts to be accreted by proto-planets. Therefore, by the time planetary formation is over, everything is going to be roughly the same age.

Jul 29, 2016
Please explain: how the collapse of gas has no one already a million cores (they say age is the same for all)?

Jul 29, 2016
Stupendous, another ad hoc patch to explain an unexpected observation. Nevermind that the dual-lobed structure is a common occurrence of high energy electric discharge. Nope, let us postulate the cosmic fender bender that causes two snowballs to make cosmic snowmen. I think Mr. Barnum had a quote for how easily it is to dupe the ignorant such as jonesdumb.

Jul 29, 2016
another ad hoc patch to explain an unexpected observation
cdTROLL
no, it's the scientific method in action:
throw out the falsified garbage (eu) and continue to update the theory with valid information from observation, experimentation etc

considering the eu can't actually field a working hypothesis that isn't already falsified & they can't actually make predictions...
the dual-lobed structure is a common occurrence of high energy electric discharge
source material?
peer reviewed links/studies?
anything from astrophysics that can validate the claim?
pseudoscience don't count
I think blah blah dupe the ignorant such as...
i wonder if @24volts is gonna come along and downrate your posts and report you for your continued ad hominem posts because you can't actually provide evidence of your claims?

well 24?

is there anything scientific that can be validated in the comment?
NOPE

where is your righteous indignation for cd?
the eu's falsified posts?

Jul 29, 2016
Stupendous, another ad hoc patch to explain an unexpected observation.


One common theme I see in the comment sections is that people don't seem to understand science. Science is the act of "patch[ing] to explain an unexpected observation," as it means your current model, while correct on some things, is evidently incomplete. If your model/theory accurately predicts other aspects but not something new, then you augment and/or patch the model so that it accurately explains the new and the old... This is how science functions and how we arrived to where we are now. So, I find this entire comment after that statement confusing.
If you have a competing theory, that's great, that is what science is about... However, if that's the case, rigorously prove it via predicted and experimental results. If it accurately explains things just as well or better, it will become a competing theory.

Jul 29, 2016
The paywalled paper doesn't say how old their primordial rubble pathway would be, but I note that the 400 Myrs era likely is set by late bombardment models while the primary body formation seems to be over in 25 Myrs. Perhaps this is another failed test of the late bombardment hypothesis, if so it follows from another putative failed test of the new Moon Imbrium observations that puts the late bombardment dating from Apollo rocks in tension.

Re the anti-science trolls, I also note that the paper has some 40 expert authors and is based on several years of Rosetta observations. Against which the bloviating windbags offer nothing quantifiable touching the data at hand. A boring fail^2.

@Jarrod: And the "ad hoc" claim isn't true since the models is consistent with what has gone before. It is of course the trolls attempt to criticism which is coming out of the (lightning free) blue.

Jul 29, 2016
Stupendous, another ad hoc patch to explain an unexpected observation. Nevermind that the dual-lobed structure is a common occurrence of high energy electric discharge. Nope, let us postulate the cosmic fender bender that causes two snowballs to make cosmic snowmen. I think Mr. Barnum had a quote for how easily it is to dupe the ignorant such as jonesdumb.


Ignorant? That is rich coming from you!!!! Who couldn't tell the difference between a diamagnetic cavity and a non-existent double layer. Lol.
So tell us what the electrical processes are. How are they caused, what is the evidence? Where is the theory? Links, please.

Jul 30, 2016
@torbjorn,
The paywalled paper doesn't say............


The paper should be showing as free access. Click on PDF option on the right hand side.

Jul 30, 2016
The paper should be showing as free access
@Torbjorn &@jonesdave
Yup: free access: http://www.aanda....8-15.pdf

it does call out a reference for said rubble (Weissman 1986)
See also PG 2
We argue that the primordial disk was dynamically cold, and that the survival of small bodies against collisional disruption was high. We call these bodies primordial rubble piles, and we demonstrate in Sect. 5 that their properties match those of comet nuclei. In Sect. 6 we discuss in detail the reasons why our conclusions differ from those of Morbidelli & Rickman (2015) and Rickman et al. (2015),
it also says to see section 5 "The primordial rubble-pile hypothesis" PG-24, if you're interested


Jul 30, 2016
Jones and Captain, thank you for correcting my mistake!

Unless my hasty browsing is incorrect, the inherent time scale in their comet formation model is indeed ~ 25 Myrs (with the initial phases on the order of 1 Myrs).

While the 400 Myrs is set by TNO large object models - which seems to me, but the explicit description is lacking - are scaled to the Nice model, rather old, and arguable models at that as the references admit.

The point being - to me - that a Jupiter/Saturn resonance, which is the nice Nice solution to our system's architecture, could have happened before or shortly after Moon formation as regards comet formation. Comets had already formed mainly before but also (getting bi-lobed) a few tens of Myrs after the protoplanetary disk cleared.

Jul 30, 2016
One other piece of evidence that points to the comet's bi-lobed shape being due to the collision of two bodies, is that the larger lobe is ~5-15% denser than the small lobe. I think it is mentioned in this paper, although I first saw mention of it in another paper:

"The global shape, density and rotation of Comet 67P/Churyumov-Gerasimenko from preperihelion Rosetta/OSIRIS observations"
Jorda, L., et al.
http://www.scienc...16301385

Paywalled sadly, but where there is a will there is a way :)

Aug 01, 2016
One other piece of evidence that points to the comet's bi-lobed shape being due to the collision of two bodies,

That's not evidence, it's wild conjecture in order to support a failed guess.

Aug 01, 2016
One other piece of evidence that points to the comet's bi-lobed shape being due to the collision of two bodies,

That's not evidence, it's wild conjecture in order to support a failed guess.


No, it's from actual measurement and observation. Something you would not be familiar with as an EU cultist. As mentioned, the measurements come from a different paper, by different authors. Perhaps you could quote the relevant part of Jorda's paper that you disagree with? Deconstruct the maths regarding the spin/ precession parameters to show where they went wrong, maybe?

Unless, of course, they are all part of the worldwide anti-EU conspiracy (most of them have never heard of EU, by the way) that the Grand Poobah of Science (identity cannot be revealed, on pain of death) has instigated, due to the huge threat it poses to *real* science.

Aug 01, 2016
How comets are born

How are little cutesy comets born?
They are not. They sadly have no mommies.
Now back to kittens:
https://www.youtu..._E7efGWE


Awww. Kittens are cute, but ninja cats rock!
http://www.imageb...97870205

Aug 01, 2016
More ninja cats, whilst we await the latest science free offering from the Cult of the Invisible Lightning Bolts:

http://www.imageb...97873689

http://www.imageb...97873783


Aug 01, 2016
That's not evidence, it's wild conjecture in order to support a failed guess
@cd
is that what your con-men told you?

see, science uses hypothesis to constrain the data to determine facts, not guesses like you... then they insure said method is validated for use

Plus, and i know this will mess up your head, science has multiple ways of determining and constraining the data of measurements because they've actually found various ways to track, measure and determine things, like combining the data from Heggy et al (2012), Keller et al (2015), Groussin et al (2007), Feldman et al (2015), Gulkis et al (2015), Rotundi et al (2015) and more... that is only the top 6 papers i found

how many papers can you show supporting your false claim that it's conjecture?

what?
NO peer reviewed papers to help you?

and you wonder why the eu is considered pseudoscience?
(no one with sense wonders - keywords "with sense", eliminating rc, benji, bschitt, eu cult etc)

Aug 01, 2016
@cd cont'd

of course, ignoring all of the evidence above, if you had read the paper linked in the article you would also notice section 2.1 "Mass, bulk density, and porosity" and it's called out references

more to the point: had you actually read the study you would be able to refute any mentioned data with an equivalent study, if, and only if, you were at least marginally competent in the scientific method

given that you can't actually provide legitimate source material and peer reviewed references that refute any scientific claims (or even that support your claims against the science) then it demonstrates, with obvious evidence, that you're situation isn't tenable and that you must, by definition, be aligned with a religion or pseudoscience (same thing, really)

and given your known association with conspiracist ideation (links available upon request)...

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