New craters on asteroids may host streams that flow for hours.

Impact events create extreme conditions. Under high temperatures and pressure, rocks break apart, melt, and even vaporize. As they cool, streams of water mixed with dust and stones crawl down the walls of the newly formed crater. More precisely, not water, but likely a strong "brine" from melted ice deposits within the rock. This is how scientists explain the ravines and depressions in the terrain of many celestial bodies in the Solar System. On Mars, these traces raise few questions, but how did they appear on the asteroid Vesta?

Vesta is the largest asteroid by size and mass in the main asteroid belt of the Solar System, located between the orbits of Mars and Jupiter. Before the Dawn mission flew, it was believed that there were no easily vaporizing compounds like water there. The Dawn spacecraft was launched to study the objects in the asteroid belt: Vesta and the dwarf planet Ceres.

In the images captured by Dawn, scientists observed traces of liquid flows in eight of Vesta's craters. Unfortunately, such distinct formations could not be discerned on Ceres, but this is likely a resolution issue. The average width of these "ravines" on Vesta is 30 meters. If they were slightly smaller, they would be indistinguishable in the images of Vesta. The resolution of images from Ceres is indeed lower. Overall, scientists hypothesize that Ceres also possesses such "ravines."

Since traces of water flows were found only in Vesta's craters, scientists logically assumed that their source is subsurface deposits of water ice melted by the impact event. Such deposits certainly exist on the dwarf planet Ceres, so it is likely they are present on the asteroid Vesta, given that traces of flows have already been discovered there.

During an impact event, most of this ice immediately vaporizes, but some deposits find themselves under suitable temperature and pressure conditions to transition into a liquid state. Of course, they freeze as well, but not immediately. Previous calculations have shown that for "ravines" to form, liquid must flow down slopes for several tens of minutes.

Scientists from the Southwest Research Institute (USA) along with colleagues from NASA's Jet Propulsion Laboratory (USA) experimentally identified the parameters for the formation of such flows. Impact event conditions were simulated in the DUSTIE chamber in the laboratory.

"We wanted to explore our previously proposed idea that an impact event could expose and melt subsurface ice in worlds without an atmosphere, which then flows down the walls of the crater and shapes the terrain," — explained the study leader Jennifer Scully from NASA's Jet Propulsion Laboratory.

The authors of the new study conducted experiments with pure water, a sodium chloride solution, as well as with additives in the form of glass beads and fragments of artificial regolith. Scientists analyzed how quickly water, "brine," and "mud" freeze under vacuum conditions, both in calm and when mixed. The results of the research work are described in the journal The Planetary Science Journal.

Pure water freezes quickly, while the sodium chloride solution is much slower. The surface of the sample in the chamber froze within seconds, while under the frozen "lid," the liquid state persisted for at least 54 minutes. The depth of the sample in the experiment was measured in centimeters. The width of the "ravines" on Vesta is 30 meters, so the liquid state should last for hours there.

Glass beads and artificial regolith accelerated the freezing process, but scientists suggested that the sample simply lacked depth to maintain the liquid state within. Mixing the sample disrupted the protective "crust" and only sped up the freezing. Moreover, the dynamics of the "stream" significantly differ from the mixing dynamics that the researchers tested in the chamber.

According to the authors' hypothesis, in real conditions, dust, stones, and rock debris help form a protective "crust" on the surface of the flow. On the other hand, rock impurities accelerate solidification within. Scientists hope that their work will serve as a foundation for further experiments and computer modeling.

Another important question that remains to be answered is: where could sodium chloride or other compounds suitable for forming a weakly freezing "brine" come from on Vesta? They could have been delivered by meteorites, but likely not in sufficient quantities to create 30-meter-wide "streams." Notably, Ceres, where "ravines" have yet to be discerned, has all the necessary ingredients. We must wait for new studies and observational data.