Ceres is the largest object of the asteroid belt, the band of rubble orbiting the Sun between Mars and Jupiter. About 1,000 kilometers wide, it is technically a protoplanet, becoming a full-fledged planet before running out of equipment to feed and grow. It was more than 4 billion years ago. So you could expect the world to be dead and frozen, with nothing new geologically, with the exception perhaps of a strange asteroidal impact or two.
But the only thing that the Universe does well is to pervert our expectations. When the Dawn space shuttle arrived at Ceres in 2015, she actually witnessed a world marked by impacts. But he also saw two very, very strange things. One of them consisted of a series of bright reflective plates scattered on the surface, usually in craters, and easy to spot, unlike the usually very dark gray surface.
The other was Ahuna Mons.
It is an oblong mountain that protrudes beyond the protoplanet and is 17 km wide at its base and extends for 4 km in the airless sky. It's flat, like a mesa, surrounded by long, bent flanks that surround it like a skirt. There is nothing else in Ceres, and with the exception of one adjacent crater, the terrain around it is relatively flat and devoid of any structure. Ahuna Mons is literally alone.
And it's weird. Ceres has no tectonic activity and therefore can not grow mountains this way. In any case, it usually forms mountain ranges (such as continental plate collisions on Earth that created the Rocky Mountains and the Himalayas).
This leaves only another explanation: upwelling. Literally, something under the surface does its best to go out.
This is not a new idea, and in fact, it was proposed just after the first appearance of Ahuna Mons. It was thought that it was a kind of cryovolcano – literally, a volcano powered by water (yes, water) instead of melted rock – but the details were not clear. New research, however, has refined the idea a bit and have even been able to focus on the lift material: not just water, but also a lot of suspended rock particles.
How is it possible?
Ceres is not like other asteroids. It was big enough when it started to differentiate in part, which meant that when it was still hot, heavier materials such as rocks and some metals fell to the center, while lighter materials such as water, aluminum rich materials and some low density rocks outside. Eventually, it cooled, forming a crust of about 40 km in average thickness on an ice coat of about 400 km deep. Dawn's measurements indicate that the mantle is about twice as dense as the earth's crust (1.3 grams per cubic centimeter versus 2.4, where the water has a density of 1). In addition, the decay of the small amount of radioactive material inside Ceres can warm it up enough so that the water below the surface can still be liquid, even after all this time.
Much of the water in the mantle is liquid and the elements at the bottom of the mantle will be hotter than those at the top. This means that the water will convect, with warmer objects rising and colder falling objects. If the pressure is low enough, the water will raise the surface deep through the cracks.
Many rocks and others are dissolved in the water, making it briny. When it hits the surface, it freezes and sunlight finally turns it into gas. It evaporates (technically, it sublimates), leaving behind the brine: that is what makes the bright dots that we see everywhere in Ceres.
Sometimes, however, the water rises in a plume, like the one that makes the Canary Islands and Hawaii. It fits the data better, and the authors think that it may be what happens beneath the surface to create Ahuna Mons.
In itself, it may not penetrate the surface. However, if it finds a finer spot in the crust and there is a duct in the crust not exceeding 10 meters in diameter, this could explain what we see. The water that comes to the surface freezes, but more water comes out of the plume, pushing the ice up and up. It ends up forming a mountain, essentially a slow volcano. It does not have much structural strength, so it collapses on the sides to form the skirt.
This new work uses a clever method to determine the material of Ahuna Mons. Dawn's orbit depends on the total mass of Ceres (this is how gravity works), but if Dawn passes over a local region of higher density, it would accelerate. By carefully measuring the Doppler shift of radio signals back to Earth, scientists can accurately measure the orbit of the spacecraft, and thus the density of the material below at a given moment.
Measuring the severity of Ceres, they found an anomaly at the position of Ahuna Mons. Using certain physical characteristics to adapt computer models to gravity, they found that the best choice was a drop of material about 80 x 40 km and a depth of 30 km. The density of the material was about 2.4 grams / cc, which corresponds to the density of the mantle. This suggests that this is a spot of the mantle springing from the surface (probably where the curve was less thick, because it is an average of 40 km deep).
By examining the morphology of Ahuna Mons, the physics of the movement of materials inside Ceres and what happens when it cools, they find that the material contained 55 to 70% of the material. water in volume, the remaining 30 to 45% being "insoluble", solid particles, essentially a mud ".
In other words, mud.
This could explain why Ahuna Mons is the only mountain of this type as well; plumes will not be common, especially near areas where the crust is thinner. And if others formed, they would collapse and probably collapse in the course of tens or hundreds of millions of years. Which also means that Ahuna Mons is young, in the geological sense of the word. It is quite possible that the plume is still active now. It may take some time to find out, perhaps checking for landslides now as the material grows.
Unfortunately, Dawn no longer has fuel by the end of 2018 and scientific observations have ceased. Hope we send another probe into this strange little world to see what happens …. and what has changed since Dawn. I love the way we continue to be surprised by our solar system, which challenges expectations at every turn. But as the saying goes, if we knew what we would discover there, it would not be exploration.