Relaxation States of Large Impact Basins on Mercury Based on MESSENGER Data

The crustal structure of Mercury's large impact basins provides valuable insights into the planet's geological history. For a warm crust, a post‐impact basin structure will viscously relax with inward flow of crustal materials toward the basin center. This effect drastically diminishes the crustal thickness contrasts and associated Bouguer gravity contrasts between the basin center and its surroundings. Here, we analyze Bouguer contrasts of 36 basins (diameter > ${ >} $300 km) located in the northern hemisphere as a proxy for viscoelastic relaxation. Thermal evolution models, assuming the present 3:2 spin‐orbit configuration, are used to predict crustal temperatures. Our analysis reveals that the expected correlation between zones of warm crust and low Bouguer contrast from relaxation is not observed in the available data. This suggests that crustal temperatures have changed in the past, potentially due to a change in Mercury's orbit or to a major volcanic event associated with smooth plain formation. Plain Language Summary Mercury's impact basins and their crustal structure provide important clues about the planet's geological past and present‐day state. We study Mercury's crust at large impact basins by using information from gravity measurements and temperature estimates from thermal evolution models that consider the surface temperature pattern caused by Mercury's present‐day orbit. In regions with warm interior, the crustal structure of large basins is expected to relax more readily compared to colder regions. Here, we examine 36 large basins (diameter > ${ >} $300 km) in the northern hemisphere, where the gravity data is well resolved, to investigate this process. Our results show no correlation between gravity data at basins' location, modeled local temperature, and expected impact basin relaxation. This suggests that crustal temperatures have changed in the past, potentially due to a change in Mercury's orbit or to a major volcanic event associated with smooth plain formation. Key Points Observed Bouguer anomaly contrasts and predicted crustal temperatures are used to investigate viscoelastic relaxation of Mercury's basins Our findings do not indicate the anticipated pattern where basins exhibiting greater relaxation are situated in areas with hotter crust Crustal temperatures varied over time, likely due to rotation state changes or major volcanic events linked to smooth plain formation