NASA's "Dawn" spacecraft has revealed mysterious channels on the surface of Vesta. New research suggests that short-lived liquid brines created by meteoroid impacts may be the key to understanding the phenomenon.
Evidence for cosmic collisions
The crater-scarred surfaces of many celestial bodies in our solar system are clear evidence of a 4.6 billion-year history of impact by meteoroids and space-traveling debris. But there are worlds, such as the giant asteroid Vesta studied by NASA's "Dawn" mission, where the surface also has channels, whose structure is still a mystery.
Hypotheses about channels and flows of brines
According to one of the leading theories these channels were formed by dry debris flows caused by geophysical processes, such as meteorite impacts or temperature changes due to exposure to the sun. But a new study offers new evidence pointing to a different process: short-lived water flows caused by impacts. These flows may have carved the channels and left sediment deposits behind. The researchers used laboratory equipment to recreate the conditions in Vesta and detailed for the first time the possible composition of this liquid and how long it could have remained liquid before freezing.
Although the existence of frozen brine deposits on Vesta has not been verified, scientists have previously speculated that meteoroid impacts could have exposed and melted subsurface ice on worlds like Vesta. In this scenario, flows from this process could create channels and other landforms that resemble those on Earth.
The challenges of liquids in airless worlds
But on airless worlds - celestial bodies without an atmosphere and exposed to the strong vacuum of space - how can liquids exist on the surface long enough to flow? Such a process would be contrary to the understanding that liquids rapidly lose stability in a vacuum, and change to a gas as the pressure decreases.
"The impacts cause fluids to flow across the surface, and the fluids are active long enough to create specific landforms," said project leader Jennifer Scully of JPL, where the experiments were conducted. "But how long? Most liquids lose stability quickly in these airless bodies, where the vacuum of space is tough."
The role of salt in extending the stability of liquids
It turns out that the critical component is sodium chloride - table salt. In experiments it was found that under conditions like Vesta, pure water froze almost immediately, while liquid salts remained liquid for at least an hour. "This is enough to create the land features associated with the flows detected in Vesta, which are estimated to have taken up to half an hour," says lead author Michael J. Poston.
The "Dawn" spacecraft launched in 2007 flew to the main asteroid belt between Mars and Jupiter and then orbited Vesta for 14 months and Ceres for nearly four years. Before it ended in 2018, the mission discovered evidence that Becker had an underground reservoir of brine and that there may still be a transfer of brine from the interior to the surface. The latest study provides insights into processes in the hook but focuses on Vesta, where ice and salts may form liquid brines when an impact heats them, the scientists said.
Simulation of Vesta's unique environment
To recreate Vesta-like conditions that would occur after a meteorite impact, the scientists relied on an experimental chamber at JPL called DUSTIE. By rapidly reducing the air pressure around samples of liquids, they simulated the environment around a liquid reaching the surface. When exposed to a vacuum, pure water immediately froze. But salty liquids lasted longer, continuing to flow before freezing.
The depth of the brines in the experiments was close to three centimeters. The scientists concluded that flows in the Vesta that are meters to tens of meters deep will take longer to freeze again.
The researchers were also able to recover the "caps" of frozen material that probably formed on the brines. The lids, essentially a frozen top layer, stabilize the liquid underneath, and protect it from exposure to the vacuum of space - or in this case, of the DUSTIE - and help the liquid flow longer before refreezing.
This phenomenon is similar to how lava on Earth flows farther in lava tubes than when exposed to the cold temperatures of the surface. It also lends itself to modeling research on potential Martian mud domes and volcanoes that may have ejected icy material on Jupiter's moon Europa.
More findings from the asteroid Vesta
Haim Mazar
On the slopes of the craters of the asteroid Vesta, formations of channels were found, which on Earth are formed by the flow of water. These configurations are called gullies. Vesta is too small a body to hold an atmosphere and therefore cannot have water bodies such as seas and rivers on its surface. Research in recent years has brought a surprising conclusion. Indeed these formations were created by flowing water. The question asked is what is the source of the water?
For the purpose of this research they used a facility called DUSTIE. This name is an acronym for Dirty Under-vacuum Simulation Testbed for Icy Environments. Exposing samples of salt-free water to vacuum showed that they froze immediately. Salt water continues to flow for an additional period of time until it freezes. In this experiment the salty water was tested at a depth of several centimeters. Additional flow time was required until they froze. In Vesta, the salty water found at a depth of several meters will require more time before it freezes. In another experiment, they allowed a frozen substance that "covers" liquids underneath and stabilizes them, to flow for extended periods of time despite the vacuum conditions. This process simulates lava flows on Earth where isolated lava tubes allow for longer movement than open air lava.
If this hypothesis is correct, another conclusion is called for. The "life expectancy" of water flowing on Vesta is very short. A rough estimate is tens of minutes at most and then they evaporate. Such a short flow of water cannot create these channels. This takes a long time in geological time terms. Which means that those areas had prolonged meteorite bombardments. Is prolonged bombardment of meteorites sufficient to create these gullies(1). Perhaps another factor was at work. Were there in the past, and perhaps even today, geyser bursts similar to those found on Saturn's moon Enceladus_ the author's hypothesis - Haim Mazar). It is possible that in the future it will be necessary to put a spacecraft into orbit around an asteroid for an extended stay of years).
Similar to the Earth, Vesta's interior is divided into 3 parts: a metallic core, a silicate shell and a thin basaltic crust. Its first 30 million years at least were volcanic in character. That was 4.565 million years ago. Assuming that all the materials supplying the radioactive elements such as aluminum 26 completely decayed in the initial period, it is possible that pockets of magma survived and were related to the slow and partial cooling of a lava ocean below the surface of the asteroid. It is possible that during this time frame collisions of large asteroids created craters that reached a depth of 10 km and more under the volcanically active crust.
The deepest places where the impact rocks were close enough to the mantle that they were affected by the asteroid's heat show and thus metamorphosed.
The information transmitted from the Dawn spacecraft confirmed the hypothesis that Vesta's first lava flows were buried deep within the crust by later lava flows. It is possible that in the future by testing soil samples from the asteroid it will be possible to make stratigraphic sections. At this stage the lava was heated by the earlier mantle and changed the rocks(2).
Sources
1. Clarence Oxford – Lab experiments explore origins of gullies on Asteroid vesta. 23.12.2024
2. Turbulent times revealed on Asteroid 4 Vesta. Pert Australia (SPX) 27.2.2020
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