According to a new paper in the journal Nature, two comets collided in the early Solar System to give rise to the extraordinary shape of Comet 67P/Churyumov-Gerasimenko.
This false-color four-image mosaic comprises images taken from a distance of 28.7 km from the center of Comet 67P/Churyumov-Gerasimenko on 3 February 2015. The mosaic measures 4.2 x 4.6 km. Image credit: ESA / Rosetta / NAVCAM / CC BY-SA IGO 3.0.
“It is most likely that two comets collided in the early Solar System, forming the double body that we see today,” said co-author Dr Ekkehard Kührt of the German Aerospace Center.
“In order to explain the measured low density and well-preserved layer structures of both the comet’s lobes, the collision must have been gentle and occurred at low speed.”
“This finding provides important information on the physical condition of the early Solar System 4.5 billion years ago,” he said.
The origin of the comet’s double-lobed form has been a key question since ESA’s Rosetta orbiter first revealed its surprising shape in 2014.
Two leading ideas emerged – scientists suspected either a collision of two bodies or particularly intense erosion at the site that evolved into the ‘neck.’
Analysis of high-resolution images of the comet acquired by Rosetta’s Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) between 6 August 2014 and 17 March 2015 has now provided the answer to the riddle.
OSIRIS images used to identify patterns in 67P/Churyumov-Gerasimenko’s extensive layering. Top left: main terraces (green) and exposed layers (red dashed lines) seen in the Seth region on the comet’s large lobe. The terraces become more inclined towards the comet neck region. The close-up shows terraces in two locations (thin white and yellow arrows) together with examples of parallel lineaments (large white arrows) that define a continuous stratification. Bottom left: outline of exposed layers (red dashed lines) primarily in the Imhotep and Ash region on the comet’s large lobe. The terraces in Ash change their dip direction from that in Seth to very slightly dip towards Imhotep. Some layers are also indicated on the comet’s small lobe in the background. The close-up shows the details of the parallel layers in a section along the Imhotep-Ash boundary. Top right: main layers (red dashed lines) and cross-cutting fractures (blue dashed lines) in the Hathor cliff face on the comet’s small lobe. No abrupt change in the orientation of the layers is seen between Hathor and Ma’at. The close-up shows stratification in an alcove at the Hathor-Anuket boundary, providing a view of the Anuket inner structure, which appears to extend under Ma’at. Terraces on Anuket (white arrows) are seen in different orientations to neighboring regions. Taken together, this reinforces the idea that Hathor represents the inner comet structure that has been exposed, with Anuket as the remnant. Bottom right: layers (white dashed lines) at the boundary of Anubis and Seth on the comet’s large lobe. This continuous scarp suggests the thickness of the Seth region is about 150 m. The three arrow heads point to a terrace margin in Anubis and the single white arrow points to a terrace in the adjacent Atum region. Image credit: ESA / Rosetta / MPS / OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / M. Massironi et al.
“It is clear from the images that both lobes have an outer envelope of material organized in distinct layers, and we think these extend for several hundred meters below the surface,” said lead author Dr Matteo Massironi of the University of Padova in Italy.
“You can imagine the layering a bit like an onion, except in this case we are considering two separate onions of differing size that have grown independently before fusing together.”
To reach their conclusion, Dr Matteo and co-authors first used OSIRIS images to identify over 100 terraces seen on the surface of the comet, and parallel layers of material clearly seen in exposed cliff walls and pits.
A 3D shape model was then used to determine the directions in which they were sloping and to visualize how they extend into the subsurface.
It soon became clear that the features were coherently oriented all around the comet’s lobes and in some places extended to a depth of about 650 m.
“This was the first clue that the two lobes are independent, reinforced by the observation that the layers are inclined in opposite directions close to the comet’s neck,” Dr Matteo said.
“To be sure, we also looked at the relationship between the local gravity and the orientations of the individual features all around the reconstructed comet surface.”
The methods used by the team to determine that 67P/Churyumov–Gerasimenko’s shape arises from two separate comets. Left: high-resolution OSIRIS images were used to visually identify over 100 terraces (green) or strata – parallel layers of material (red dashed lines) – in exposed cliff walls and pits all over the comet surface. Middle: a 3D shape model was used to determine the directions in which the terraces/strata are sloping and to visualize how they extend into the subsurface. The strata ‘planes’ are shown superimposed on the shape model (left panel) and alone (right panel) and show the planes coherently oriented all around the comet, in two separate bounding envelopes (scale bar indicates angular deviation between plane and local gravity vector). Right: local gravity vectors visualized on the comet shape model perpendicular to the terrace/strata planes further realize the independent nature of the two lobes. Image credit: ESA / Rosetta / MPS / OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / M. Massironi et al.
Layers of material should form at right angles to the gravity of an object. The scientists used models to compute the strength and direction of the gravity at the location of each layer.
In one case, they modeled 67P/Churyumov-Gerasimenko as a single body with a center of mass close to the neck. In the other, they worked with two separate comets, each with its own center of mass.
They found that orientation of a given layer and the direction of the local gravity are closer to perpendicular in the model with two separate objects, rather than in the one with a single combined nucleus.
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Matteo Massironi et al. Two independent and primitive envelopes of the bilobate nucleus of comet 67P. Nature, published online September 28, 2015; doi: 10.1038/nature15511
