Controversy About How 67P Got Its Shape?

One of the surprises of the Rosetta mission to comet 67P/CG was the discovery of the unusual double lobed shape of the comet. Distant observations of a small body could never give enough detail to reveal this, only by visiting it could we see the shape.

This discovery posed a question for geologists: how did the comet attain this configuration. There are several candidate explanations, formation from two blobs, erosion of one body, or a collision of two bodies (in something like that order of “a priori” likelihood according to earlier understanding).

To decipher this question, the team examined the detailed imagery and data from reported by Rosetta. This data is very good, with a resolution up to .1m per pixel, and includes three dimensional shape maps and measurements of local gravity and other phenomena. From this data, it was possible to identify and map strata that reveal the geological history of the comet.

The team concluded that 67P/CG was once two separate (more or less spherical) bodies, each with “onion layers” of strata. At some point in time, the two bodies collided at low speed, mooshing together without smashing one or both.

This conclusion specifically rules out the idea that there was one larger comet that has eroded or distorted over time, which was probably the first guess of most observers.

This conclusion is supported by considerable evidence and some very cool (if complicate) computational modelling, as explained in a letter published in Nature (ref 1 below, full text available from most major libraries). Notably, one source of insight is the local gravity vectors, which are consistent with two different sets of strata. Interesting.

These results were described in an ESA blog, which generated considerable commentary disputing the conclusion and arguing for the hypothesis that 67P/CG was a single body, distorted and eroded to the current shape.

This alternative interpretation was mainly based on feature matching from the Rosetta imagery, which are seen by some to suggest continuous features that extend across the central fissure.

This controversy was addressed in another blog entry by Dr Matteo Massironi of the University of Padova, Italy who led the published study.  His remarks are mostly patient and polite, if not especially respectful. He stands by the expert analysis, and points out that the blog commentary relied on faulty data, incorrect interpretation of imagery, and questionable arguments.  Massironi restrained himself from openly saying, “please read the damn paper”, although he did advise that one should learn some damn science.

Massironi points out that the Rosetta team was well aware of the alternative explanations, and because of the controversial conclusions they marshaled multiple sources of evidence.

“Due to the controversial implications that the onion-like contact binary raises, we tried to find other lines of evidence that might undermine what was apparent from the former observations. This is why, from the best fitting planes, we passed to the geological sections and afterwards worked on the angular relationships between strata and the local gravity vectors. All these independent observations based on primary structures support the view in which the comet derives from a contact binary of two comets with an onion-like interior.

 I don’t know if this satisfies the blogosphere, but it gives me some confidence.

Again, we see here the difference between real science and Hollywood/Internet science. You can’t just look at some pictures and tell a story. You need to actually learn some science, and do some serious work with the data.

Title The comet’s two lobes Released 28/09/2015 3:00 pm Copyright ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; M. Massironi et al (2015)


  1. Mateo Massironi, Emanuele Simioni, Francesco Marzari, Gabriele Cremonese, Lorenza Giacomini, Maurizio Pajola, Laurent Jorda, Giampiero Naletto, Stephen Lowry, Mohamed Ramy El-Maarry, Frank Preusker, Frank Scholten, Holger Sierks, Cesare Barbieri, Philippe Lamy, Rafael Rodrigo, Detlef Koschny, Hans Rickman, Horst Uwe Keller, Michael F. A/’Hearn, Jessica Agarwal, Anne-Therese Auger, M. Antonella Barucci, Jean-Loup Bertaux, Ivano Bertini, Sebastien Besse, Dennis Bodewits, Claire Capanna, Vania Da Deppo, Bjorn Davidsson, Stefano Debei, Mariolino De Cecco, Francesca Ferri, Sonia Fornasier, Marco Fulle, Robert Gaskell, Olivier Groussin, Pedro J. Gutierrez, Carsten Guttler, Stubbe F. Hviid, Wing-Huen Ip, Jorg Knollenberg, Gabor Kovacs, Rainer Kramm, Ekkehard Kuhrt, Michael Kuppers, Fiorangela La Forgia, Luisa M. Lara, Monica Lazzarin, Zhong-Yi Lin, Jose J. Lopez Moreno, Sara Magrin, Harald Michalik, Stefano Mottola, Nilda Oklay, Antoine Pommerol, Nicolas Thomas, Cecilia Tubiana, and Jean-Baptiste Vincent, Two independent and primitive envelopes of the bilobate nucleus of comet 67P. Nature, advance online publication  09/28/online 2015.


Space Saturday

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