Another Ceres Mystery: The Dwarf Planet’s Large Impact Craters Have Vanished

Scientists have been confronted with another mystery regarding dwarf planet Ceres: It would appear that the planetoid is relatively bereft of large impact craters. That is correct — Ceres, which should be pitted with numerous large impact craters across its surface depicting collisions over its 4.5-billion-year history, has only a few large craters. So where are they? reported on July 26 that Ceres, the largest object in the Asteroid Belt, was found to be missing a few topographical features — namely, large impact craters — on its surface when the Dawn space probe reached its orbit. NASA expected to see the rocky planetoid covered with both large and medium-sized impact craters but, instead, was faced with only one large crater and just over a dozen medium-sized impact craters.

Computer models had predicted that a planetary body the size of Ceres should have 10 to 15 large impact craters measuring at least 250 miles in diameter. It also should have at least 40 impact craters larger than 60 miles in diameter. However, Ceres had only one large impact crater, the Kerwan Basin, which stretches 177 miles in diameter. Not particularly minute, but one crater three-fifths the size of the projected size of at least a dozen craters makes for a poor showing. And since there were only 16 craters on all of Ceres that were 60 miles in diameter or larger.

Somehow, the large impact craters on Ceres had been removed.

Enter the Dawn team set on figuring out what might have happened to the large craters. Led by Simone Marchi of the Southwest Research Institute in Boulder, Colorado, the team reported — in the online journal Nature Communications — on their findings regarding their computer simulations of Ceres’ history. And they discovered that the dwarf planet has been home to considerable geological evolution, so much so that it likely contributed to the vanishing of the large basins.

Arizona State University’s David Williams, the director of the Ronald Greeley Center for Planetary Studies in ASU’s School of Earth and Space Exploration and a member of the Dawn team, says that it was very noticeable that Ceres was without the expected crater-pocked surface of an ancient planetoid that undoubtedly saw myriad impacts over its geological lifetime. And yet, even Vesta, the asteroid that Dawn visited prior to moving on to explore Ceres, had two large impact craters — and it was half the size of Ceres.

“But at Ceres, all we saw was the Kerwan Basin, just 177 miles in diameter. That was a big red flag that something had happened to Ceres.”

So what happened to the craters? Did the largest body in the asteroid belt simply get lucky on a cosmic scale and then somehow miraculously avoid colliding with other objects for over four billion years? Hardly.

Marchi sums up the team’s findings.

“We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, which is likely the result of Ceres’ peculiar composition and internal evolution.”

The computer simulations, Williams explains, aided the Dawn team in ascertaining that the mystery of the vanishing impact craters was due to three possible and probably coexisting causative factors.

One: Ceres’ crust was already known to contain significant amounts of ice, so it is believed that said ice, if mixed with salts, could weaken the crust, allowing the impact basins to relax, thus becoming smoother, more uniform, and, eventually, disappearing.

Two: The dwarf planet likely generated its own internal heat from decaying radioactive elements after its formative years, a process that could have softened or even erased the large-scale topographic markings on Ceres’ surface.

Three: There is evidence that cryovolcanism — volcanic activity produced by icy water — exists on the dwarf planet’s surface. It presents as the now famous “bright spots.”

Williams notes that the Occator Crater contains a ready example where “the central bright spot is a domed feature which looks as if it has erupted or been pushed up from below.”

Ceres has been a small world of mysteries for NASA thus far. Even as the Dawn space probe approached the dwarf planet, there was worldwide speculation as to what the mysterious “bright spots” glittering on the surface were composed of and even if they were natural or artificial. Some went so far as to speculate that they were created by aliens or might actually be by-products of an alien civilization. But the more pragmatic scientists stuck with ice volcano effluvia, salt deposits, or some other form of ice deposits.

In June, NASA announced that the bright spots on Ceres were a result of intense hydrothermal activity and that they consisted of carbonates. The carbonic acid salts are the largest deposit of the minerals found outside of the Earth itself.

Williams says that geological maps of Ceres will be completed in the next year. Along with compositional and gravity data analyses, scientists will develop a better understanding of the evolution of the dwarf planet.

“Ceres is revealing only slowly the answers to her many mysteries,” he says.

The Dawn Mission, as noted at the NASA website, was launched on September 7, 2007, with the space probe’s targets set as the asteroid Vesta and the dwarf planet Ceres. Both Vesta and Ceres would be studied in order to hopefully gain an understanding into how the Solar System formed.

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