[ad_1]
A recent study reveals the role of past cryovolcanism – and probably present – on the giant asteroid Ceres.
Ahuna Mons is an imposing mound on Ceres, probably created when a mixture of water and other compounds has burst from below. In this simulated perspective view, assembled from improved color images taken by NASA's Dawn spacecraft, the elevations are exaggerated by a factor or two.
NASA / JPL / UCLA / MPS / DLR / IDA / PSI
NASA's mission in Dawn has opened the world of 1 Ceres, the largest asteroid in our solar system. Now, a recent study, published today in The astronomy of nature, revealed something fascinating history of cryovolcanic activity on this dwarf planet, leading to the surface we see today.
cryovolcanisme involves rashes of water sludge mixed with some kind of antifreeze – maybe liquid ammonia (NH3) or methane (CH4) – at temperatures well below the usual freezing point of water. The resulting flows are the super-caloric equivalents of the familiar rock magma involved in volcanic activity on Earth.
As Ceres is believed to harbor a deep, saline mixture of liquids and solids under a rigid crust, geologists expect to find evidence of cryovolcanism on its surface. The large isolated mound called Ahuna Mons is a good example. The big question is why Ceres does not have much more of these ice pies.
The key seems to be in the subtleties of rheology, the physics of how the material (in this case, ice) flows in time in response to gravity. The study used a combination of flow modeling coupled with topographic analysis to reconstruct the cryovolcanic history of Ceres.
Domes, Mons and Tholi
Led by Michael Sori (University of Arizona), the researchers carefully analyzed the appearance of 22 domes on Ceres, their aspect ratio (their width relative to the height of the surrounding plain) and their distribution by latitude. For the purposes of the study, Tholi (the IAU-approved nickname for small eruptive domes) and mounted (larger mountains) were generically grouped together as "domes".
Compare the relatively large form of Ahuna Mons (left) to that of the old unnamed dome in the center. Yamor Mons (right) is perhaps 2 billion years old, but retains much of its original height due to its location near the ever – cold North Pole, where the ice 's ice can be seen. slows much more slowly over time.
Michael Mori et al / Astronomy of Nature
The team used the unique feature called Ahuna Mons as a reference standard. It dominates 4 km above the surrounding landscape and (given the scarcity of nearby impact craters), it does not appear to be more than 240 million years old – and it could be much younger. Ahuna Mons has a relatively high aspect ratio of almost 0.2 (its base is five times wider than its height).
The researchers discovered that the longer a large dome is on Ceres, or the closer it is to the equator of the asteroid, the lower its aspect ratio.
Michael Mori et al / Astronomy of Nature
Sori and her colleagues then estimated the age of 21 domes (all at least 10 km in diameter) assuming they consisted of large piles of cryovolcanic materials, such as Ahuna Mons, which subsided at a predictable rhythm over time. . (The technical term for this is viscous relaxation.) The age estimates obtained range from a few hundred million to 2 billion years.
The study also showed that the higher the latitude of a dome on Ceres, the higher its aspect ratio is high. For example, the domes near the Equator of Ceres have a low aspect ratio of about 0.06, while a large dome called Yamor Mons (at 85½ ° north) corresponds almost at 0.2 aspect ratio of Ahuna. Since Ceres poles are much colder, their slip speed should be slow.
How much and how often?
Mori and his team used the global distribution of domes and their estimated ages to infer the frequency of cryogenic activity on Ceres. They deduced that an eruption could occur once every 50,000 years on average. Over time, these eruptions could release approximately 10,000 cubic meters of slush per year. It's the equivalent of the volume of a large movie theater or four Olympic-size pools – but it's a tiny fraction of the rate on Earth, whose volcanoes spew out more. 39 billion cubic meters of molten rock each year.
"We are delighted to have been able to establish a cryovolcanic rate comparable to the rates of" regular "volcanism on terrestrial planets, which had never been done from observations," said Sori. Sky and telescope.
What makes cryovolcanism work on Ceres? Should not such a small body have cooled internally now? The most likely internal source feeding the cryovolcanic processes on Ceres is the radioactive decay of certain isotopes in the rocky parts of the small world, as happens on other planets. Another, more controversial hypothesis is that the significant impacts on the surface of Ceres start the process locally.
One of the first fascinating features that Dawn noticed during his initial approach to Ceres was fresh ice patches, known as faculae. (This is the same term used to describe the bright spots on the photosphere of the Sun.) These spots, found on the flanks of Ahuna Mons and on the floor of the Occcator crater, were the first clue for the researchers that cryovolcanism is still active on Ceres. .
An artist's design on how Ceres can be structured internally, from Dawn's data. The outer crust (light blue) consists largely of water ice, hydrated salts and minerals. Geochemists believe that under the crust is a thick shell, perhaps up to 100 km deep, consisting of a part of a brackish liquid.
NASA / JPL / UCLA / MPS / DLR / IDA
Cryovolcanism could also be an essential mechanism for resurfacing on other rocky worlds of the outer solar system. In addition to Ceres, we see the action of ice geysers and pack ice on the moon surfaces of Saturn Enceladus, the Pluto dwarf planet and the moon of Jupiter Europa.
"We have shown that Ceres may have been cryovolcanically active throughout its history, and it is not necessary to try to invoke a strange thermal / volcanic history that would have resulted in the formation of a single cryovolcano (Ahuna Mons) very recently, without volcanism. before that, "says Sori. "Cryovolcanism is important on Ceres, but not as dominant or important as rock volcanism was on Earth or Mars."
Unfortunately, the end is near for the Dawn mission. The engineers believe that the engine fuel of the machine should be exhausted between late September and mid-October, after which Dawn will remain permanently in orbit around Ceres. Launched on September 27, 2007, at the top of a Delta II rocket, Dawn flew over Mars in 2009, visited the asteroid 4 Vesta from 2011 to 2012 and explored Ceres from 2015.
Enjoy these final views of a cryovolcanic world, the only one we have sent a spaceship into orbit until now. Dawn should give planetary scientists a lot of data for years to come.
Source link
