Comet mission reveals 'missing link' in our understanding of planet formation

October 25, 2017, Royal Astronomical Society
Schematic representation of the porous surface structure of comet 67P/Churyumov-Gerasimenko. Based on the results of the Rosetta mission, Blum and colleagues conclude that comet 67P is composed of millimetre-sized dust pebbles. It is assumed that the pebbles inside the comet consist of a mixture of dust and ice (light blue spheres in the image) and only the uppermost layers, which are exposed to direct sunlight, do not contain ice (dark grey spheres). Credit: Maya Krause, TU Braunschweig.

The missing link in our understanding of planet formation has been revealed by the first ever spacecraft to orbit and land on a comet, say German scientists. The study is published in a recent edition of the journal Monthly Notices of the Royal Astronomical Society.

A research team led by Jürgen Blum (Technische Universität Braunschweig, Germany) have analysed data from the historic Rosetta mission to uncover how 67P/Churyumov-Gerasimenko, or "Chury" for short, came into existence more than four and a half billion years ago.

Understanding the evolution of our solar system and its planets was one of the main objectives of the Rosetta mission to comet 67P/Churyumov-Gerasimenko. For Jürgen Blum and his international team it was worth it, because results from the various Rosetta and Philae instruments have revealed that only one out of many proposed models can explain their observations. Comet 67P consists of 'dust pebbles' ranging between millimetres and centimetres in size.

Professor Blum explains the implications of the team's observations "Our results show that only a single model for the formation of larger solid bodies in the young solar system may be considered for Chury. According to this formation model, 'dust pebbles' are concentrated so strongly by an instability in the that their joint gravitational force ultimately leads to a collapse."

This process forms the missing link between the well-established formation of 'dust pebbles' ('planetary building blocks' formed in the solar nebula by sticking collisions between dust and ice particles) and the gravitational accretion of planetesimals into planets, which scientists have pondered over for years.

"Although it sounds very dramatic" Blum continues, "it's actually a gentle process in which the dust agglomerates are not destroyed, but are combined into a larger body with an even greater gravitational attraction - the accumulation of the dust agglomerates into a coherent body is virtually the birth of the comet." Due to the relatively small mass of comet 67P, the pebbles survived intact until today, allowing scientists to confirm the hypothesis for the first time.

In fact, the pebble-collapse formation model can explain many observed properties of comet 67P, for instance its high porosity and how much gas is escaping from inside. "Now all phases in the planet-formation model have been established", concludes Blum.

Explore further: Image: Rosetta's ever-changing view of a comet

More information: Jürgen Blum et al, Evidence for the formation of comet 67P/Churyumov-Gerasimenko through gravitational collapse of a bound clump of pebbles, Monthly Notices of the Royal Astronomical Society (2017). DOI: 10.1093/mnras/stx2741

Related Stories

Image: Rosetta selfie 16 km from comet

October 15, 2014

Using the CIVA camera on Rosetta's Philae lander, the spacecraft have snapped a 'selfie' at comet 67P/Churyumov–Gerasimenko from a distance of about 16 km from the surface of the comet. The image was taken on 7 October ...

Final descent image from Rosetta spacecraft

September 30, 2016

A new image of comet 67P/Churyumov-Gerasimenko was taken by the European Space Agency's (ESA) Rosetta spacecraft shortly before its controlled impact into the comet's surface on Sept. 30, 2016. Confirmation of the end of ...

Recommended for you

Unconfirmed near-Earth objects

June 22, 2018

Near-Earth objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth, potentially threatening a collision. NEOs are tracers of the composition, dynamics and environmental conditions ...

HESS J1943+213 is an extreme blazar, study finds

June 21, 2018

An international group of astronomers have carried out multi-wavelength observations of HESS J1943+213 and found evidence supporting the hypothesis that this gamma-ray source is an extreme blazar. The finding is reported ...

'Red nuggets' are galactic gold for astronomers

June 21, 2018

About a decade ago, astronomers discovered a population of small, but massive galaxies called "red nuggets." A new study using NASA's Chandra X-ray Observatory indicates that black holes have squelched star formation in these ...

The Rosetta stone of active galactic nuclei deciphered

June 21, 2018

A galaxy with at least one active supermassive black hole – named OJ 287 – has caused many irritations and questions in the past. The emitted radiation of this object spans a wide range – from the radio up to the highest ...


Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (5) Oct 27, 2017
That's a swell schematic representation of the theorized porous surface. Funny thing though, when Philae tried to drill the surface it found a rock hard surface. So hard in fact that it didn't get too far at all. Certainly not as if the surface were porous as suggested.
5 / 5 (3) Oct 27, 2017

Its easy to make shit up, and just as easy to prove you wrong.
not rated yet Oct 27, 2017
Theres no conclusive results from the philae drill experiment
5 / 5 (3) Oct 27, 2017
That's a swell schematic representation of the theorized porous surface. Funny thing though, when Philae tried to drill the surface it found a rock hard surface. So hard in fact that it didn't get too far at all. Certainly not as if the surface were porous as suggested.

A hard surface does not imply lack of porosity. What they also did was send radio waves through the lobe of the comet to investigate its properties. It is consistent with their findings (nothing like rock, by the way). MIRO probed the surface and near-subsurface thermal inertia. Again, consistent with their findings. An impact at Tempel 1 found it not to be rock. Calculation of the speed of the sound waves, when Philae was hammering the surface, show it to be of a strength consistent with sintered ice. Not rock, though.
All this information is available to those that really want to understand the science, instead of clinging to the failed pseudoscience of the idiot Thornhill.
5 / 5 (3) Oct 27, 2017
Theres no conclusive results from the philae drill experiment

Not strictly true, sir. They can rule certain things out. Such as the electric loonies claim that the comet is rock. http://adsabs.har...42A..04K

With the bulk density of 533 ± 6 kg/m3 as derived from tracking Rosetta (Pätzold et al., Nature, vol. 530, 2016), this velocity translates into a shear modulus of the comet material of at least 3.2 MPa. Shear modulus scales with velocity squared, so when taking into account the formal uncertainties arising from the arrival time inversion, the shear modulus may easily be as large as 10 MPa. This is still low compared to solid rock or monocrystalline ice, but is compatible with highly porous materials........We conclude that the results of CASSE listening to MUPUS support the hypothesis of surficial sintering at least for the Abydos site on 67P/Churyumov-Gerasimenko.

So, rock is ruled out :)
5 / 5 (3) Oct 27, 2017
Another way of looking at the subsurface is to wait for a cliff to collapse, and see what is revealed:

Ice would be the answer to that.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.