Between 2011 and 2018, NASA’s Dawn mission conducted extended observations of Ceres and Vesta, the largest bodies in the Main Asteroid Belt. The mission’s purpose was to address questions about the formation of the Solar System since asteroids are leftover material from the process, which began roughly 4.5 billion years ago. Ceres and Vesta were chosen because Ceres is largely composed of ice, while Vesta is largely composed of rock. During the years it orbited these bodies, Dawn revealed several interesting features on their surfaces.
This included mysterious flow features similar to those observed on other airless bodies like Jupiter’s moon Europa. In a recent study, Michael J. Poston, a researcher from the Southwest Research Institute (SWRI), recently collaborated with a team at NASA’s Jet Propulsion Laboratory to attempt to explain the presence of these features. In the paper detailing their findings, they outlined how post-impact conditions could temporarily produce liquid brines that flow along the surface, creating curved gullies and depositing debris fans along the impact craters’ walls.
Michael J. Poston, the lead author of the study, is the Group Leader of Laboratory Studies (Space Science) at the SwRI. He was joined by a team of researchers from NASA JPL at the California Institute of Technology (Caltech) and the Airborne Snow Observatories, including Jennifer Scully – a NASA JPL planetary geologist and an Associate on the Dawn science mission team. The paper that describes their findings, “Experimental Examination of Brine and Water Lifetimes after Impact on Airless Worlds,” was published on October 21st in The Planetary Science Journal.
Airless bodies are frequently struck by asteroids, meteorites, and other debris that form impact craters and cause temporary atmospheres to form above them. On icy bodies or those with sufficient amounts of volatile elements (possibly beneath the surface), this will trigger temporary outflows of liquid water. However, water and other volatiles (like ammonia, carbon dioxide, methane, etc.) will lose stability in strong vacuum conditions. For their study, the team sought to examine how long liquid could potentially flow on the surfaces of airless bodies (such as Ceres and Vesta) before refreezing.
To this end, they simulated the pressures that ice on Vesta experiences after a meteoroid impact and how long it would take the liquid released from the subsurface to refreeze. “We wanted to investigate our previously proposed idea that ice underneath the surface of an airless world could be excavated and melted by an impact and then flow along the walls of the impact crater to form distinct surface features,” said Scully in a recent SwRI press release.
To this end, the team placed liquid-filled sample containers in a modified test chamber at NASA JPL to simulate the rapid pressure decreases that occur after an impact on airless bodies. In so doing, they were able to simulate how liquid behaves as the temporary atmosphere created by an impact dissipates. According to their results, the pressure drop was so fast that test liquids immediately and dramatically expanded, ejecting material from the sample containers. As Poston explained:
“Through our simulated impacts, we found that the pure water froze too quickly in a vacuum to effect meaningful change, but salt and water mixtures, or brines, stayed liquid and flowing for a minimum of one hour. This is sufficient for the brine to destabilize slopes on crater walls on rocky bodies, cause erosion and landslides, and potentially form other unique geological features found on icy moons.”
These findings could help explain the origins of similar features on other airless bodies, like Europa’s smooth plains and the spider-like feature in its Manannán impact crater (which is due to “dirty ice” existing alongside “pure” water ice). They could also shed light on post-impact processes on bodies with very thin atmospheres, like Mars. This includes its gullies, which have dark features that flow downhill, and fan-shaped debris deposits that form in the presence of flowing water. Last, the study could support the existence of subsurface water in other inhospitable environments throughout the Solar System.
“If the findings are consistent across these dry and airless or thin-atmosphere bodies, it demonstrates that water existed on these worlds in the recent past, indicating water might still be expelled from impacts,” said Poston. “There may still be water out there to be found.” This could have profound implications for future missions to these bodies, including NASA’s Europa Clipper mission. This mission launched on October 14th, 2024, and will establish orbit around Europa by April 2030.
Further Reading: SwRI, The Planetary Science Journal