Map of the northern polar region of Ceres. Colored areas are areas that are continuously in shadow during a Ceres year and are therefore very cold. – ERWAN MAZARICO/GSFC.
April 17 () –
The ice deposits observed in craters of Ceres, the largest asteroid in the Solar System, are extremely recent: less than 6,000 years in a body 4,000 million years old.
This is the conclusion of a new study published in The Planetary Science Journalwhich is based on data collected by the NASA Dawn missionwho in 2016 glimpsed these permanently dark craters and saw shiny ice deposits in some of them.
“The 2016 discovery posed a puzzle: many craters in the polar regions of Ceres remain in shadow year-round (which on Ceres lasts 4.6 Earth years) and therefore remain frigidly cold, but only a few few of them host ice deposits,” he said it's a statement Norbert Schorghofer, scientist at the Planetary Science Institute (PSI) and lead author of the study.
“Soon, another discovery provided a clue as to why: Ceres's spin axis oscillates back and forth every 24,000 years due to the tides of the Sun and Jupiter. When the tilt of the axis is high and the seasons strong, only a few Few craters remain in shadow all year round, and “These are the craters that contain bright ice deposits.”
To determine the size of thousands of shadows inside craters from years ago, scientists build digital elevation maps and then perform ray tracing calculations on them to theoretically reconstruct the shadows cast on the crater floors. The results are only as reliable as the digital shape models on which they are based. It must be taken into account that the floors of these craters are always in shadow, so it is not easy to measure their depth.
The Dawn spacecraft had a very sensitive camera, which could discern features at the bottom of the shadowed crater. Stereo images of sunlit regions are often used to construct digital elevation maps of sunlit regions, But making an elevation map of shaded terrain is a challenge that has rarely been taken on.. As part of the new study, PSI scientist Robert Gaskell developed a new technique to reconstruct heights even in the shadowed parts of a pair of stereo images. These improved elevation maps can then be used for ray tracing to predict the extent of cold, permanently shadowed regions.
These more precise maps yielded a surprising result: When Ceres reaches its maximum axis tilt, which last occurred about 14,000 years ago, no craters on Ceres remain perennially shadowed and the ice in them must have rapidly sublimated into space. “This leaves only one plausible explanation: the ice deposits must have formed more recently. The results suggest that all of these ice deposits must have accumulated in the last 6,000 years or less. Considering Ceres is more than 4 billion years old, that's a remarkably young age.“Schorghofer said.
“Ceres is an ice-rich object, but almost none of this ice is exposed on the surface. The so-called polar craters and some small areas outside the polar regions are the only ice exposures. However, ice is ubiquitous at close range. depth, as PSI scientist Tom Prettyman and his team discovered in 2017, so even a small dry impactor could vaporize some of that ice,” Schorghofer explained.
“A fragment of an asteroid may have collided with Ceres about 6,000 years ago, creating a temporary water atmosphere. Once a water atmosphere is generated, ice would condense in the cold polar craters, forming the bright deposits that we still see today. Alternatively, the ice deposits could have formed by avalanches of ice-rich material. This ice would then survive only in the cold, shadowed craters. “Either way, these events were very recent on an astronomical time scale.”
The study also examined the possibility that other types of ice, besides water ice, could become trapped in these unusual craters on Ceres. On our Moon, parts of the polar craters are so cold that even CO2 ice and some other chemical species could last in them for billions of years. Ceres is further away from the Sun, so you might expect its polar craters to be even colder than those of the Moon.
Schorghofer calculated the temperatures inside the polar craters of Ceres, something that had never been done before. The answer was surprising: Although these craters are cold enough to retain water ice, they are too hot to retain other common types of ice. Two circumstances contribute to this. First, the tilt of Ceres' axis, currently 4 degrees, is greater than the 1.5 degree tilt of the Moon, so more of the crater rims are illuminated by the sun and more light is scattered over the crater floor. Second, Ceres simply does not have permanently shadowed craters very close to the north pole, unlike the Moon, where one crater lies almost exactly at the south pole. For these reasons, temperatures are not as low on Ceres as on parts of the lunar surface.
