Ceres: Riddles about missing craters

Dwarf Planet has much less large craters and impact basins than it would need

Dwarf planet Ceres has only 16 craters with more than 100 kilometers in diameter - far too little. © Southwest Research Institute / Simone Marchi.
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Astonishing shortage: Ceres again provides guesswork, because the dwarf planet has only 16 large impact craters - but would have more than 200 of them, as model simulations reveal. Finally, Ceres circles in the asteroid belt and thus in the middle of the most turbulent part of the solar system. Some possible explanations are now presented by researchers in the journal "Nature Communications" - but they do not have a clear answer yet.

The dwarf planet Ceres is by far the largest object in the asteroid belt between Mars and Jupiter - and possibly one of the oldest. "It is believed that Ceres' accretion took place a few million years after the condensation of the first solids - and that the dwarf planet has therefore largely witnessed the evolution of the solar system, " said Simone Marchi of the Spouthwest Research Institute (SwRI) in Boulder and his colleagues.

Enigmatic Kratermangel

But the appearance of the dwarf planet does not fit in with its long history: if he has been residing in the turbulent environment of the asteroid belt for such a long time, Ceres would often have collided with other planetesimals and large asteroids in his early days. For example, Vesta, which was about half the size of the asteroid, experienced two massive impacts that caused enormous dents in the asteroids.

Unlike Ceres: Enigmatic, the small planet is indeed littered with smaller craters, large impact sinks of more than 280 kilometers in diameter but completely missing. Marchi and his colleagues have now evaluated the Krat distribution on Ceres, as determined by the space probe Dawn, and compared it with what would be expected from current models.

"Completely incompatible with the models"

Their result: Actually, there should be 80 to 180 craters on Ceres with a diameter of more than 100 kilometers, 40 to 70 craters of more than 150 kilometers and still nine to 14 impact basins of more than 400 kilometers in size. But today, only 16 craters larger than 100 kilometers can be found on the dwarf planet - and thus only a fraction of what it actually should have. display

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"These results are completely incompatible with current collision models - across a wide range of assumptions, " notes Marchi and his colleagues. "Assuming a Poisson distribution, the probability for Ceres to have no impact basins of more than 400 kilometers at just 0.3 percent." Even for the only 16 gro craters are by chance unexplainable.

Is a late arrival to blame?

But where are the missing impact craters? A possible explanation could be that Ceres originated further out in the solar system. He would then only at the beginning of the so-called Late Heavy Bombardment about 4.1 billion years ago been flung to its current location.

But as the researchers now prove, the dwarf planet would have to have at least 24 to 43 100-kilometer craters and three to four impact basins at least 400 kilometers in diameter. But that is not the case. "We conclude that Ceres Kratermuster can not revert to a delayed arrival in the asteroid belt, " said Marchi and his colleagues. The likelihood of this is only two percent.

Ice volcanoes or viscous surface?

But what is the reason for the ridiculous absence of large craters? Some researchers speculated that once a thick layer of ice surrounded the Ceres and therefore kept him from lasting impact marks. However, according to Marchi and her colleagues, this is also unlikely because the dwarf planet would have lost this layer of ice when he arrived at the asteroid belt.

The 800-kilometer Vendemia Planitia dip could be a primeval impact basin on Ceres. Southwest Research Institute / Simone Marchi

Instead, the surface of the dwarf planet itself must have ensured that the traces of large impacts have disappeared over time, as the researchers explain. Possible mechanisms would include an elastic-to-surface or a strong cryovolcanism, through which material from the interior of the celestial body is repeatedly brought to the surface and thus covered the craters.

Mechanism unclear

Weak remains of such activity can still be seen in the outpouring of water vapor and the white salt spots that appear in some craters. However, according to the researchers, so far no traces of former extensive exits have been found on Ceres.

"Regardless of the specific mechanisms, our findings indicate that there must have been an active crater removal even after the Late Heavy bombardment, " concludes Marchi and her colleagues. "The crater distribution of Ceres is inseparably bound up with its peculiar composition and inner evolution."

There is a big depression

After all: The weak traces of at least one large impact basin could have Marchi and her colleagues on Ceres aufgesp rt: On further analysis of the topography they discovered an oval, about 800 kilometers large sink the surface of the dwarf planet. The center of this basin called Vendemia Planitia is sunken into the area around three to four kilometers, the margin slightly increased.

"These are all clear indications of an impact structure, " the researchers say. "Therefore, this depression is the best candidate for a large impact crater on Ceres so far." However, if it was actually caused by a collision, it must have taken place in the early early days of the dwarf planet. (Nature Communications, 2016; doi: 10.1038 / ncomms12257)

(Nature, 27.07.2016 - NPO)