“Evidence for Young Volcanism on Mercury from the Third MESSENGER Flyby”
Louise M. Prockter, Carolyn M. Ernst, Brett W. Denevi, Clark R. Chapman, James W. Head III, Caleb I. Fassett, William J. Merline, Sean C. Solomon, Thomas R. Watters, Robert G. Strom, Gabriele Cremonese, Simone Marchi, Matteo Massironi
Main Result:
The authors provide evidence that the newly identified, relatively young Rachmaninoff basin contains volcanic plains within the peak-ring and pyroclastic material on nearby regions of the basin floor. These deposits represent the youngest observed volcanic deposits on Mercury with implications for the thermal history of the planet.
Methods:
The authors used MDIS NAC and WAC color image data from the third MESSENGER flyby to analyze and map the newly identified Rachmaninoff basin.
Observations:
Rachmaninoff is a peak-ring basin with diameter of ~290 km. Similar in morphology to the previously identified Raditladi basin, Rachmaninoff is striking both for its relative youth (inferred from preservation state) and for the multiple distinct plains units on the floor of the basin. Figure 1 is taken from the paper, and shows and MDIS NAC image, geologic map, and MDIS WAC color-enhanced image of Rachmaninoff basin which highlights the multiple floor plains units. Between the peak ring and the rim there are three relatively lower- reflectance plains units—smooth plains, hummocky plains, and low-reflectance plains. The interior of the peak-ring is dominated by higher reflectance plains. The lower reflectance plains units are consistent with plains units observed in central peaks and crater ejecta deposits across Mercury, leading to the interpretation that these annular lower-reflectance plains are composed of impact melt. In contrast, the higher reflectance plains in the peak-ring interior embay the foot of the peak-ring and have obscured parts of the peak-ring, supporting the hypothesis that they represent a fluid unit which flowed over the area. These high reflectance plains are interpreted to be volcanic in origin.
Additional evidence for volcanism on the basin floor comes from a bright, high-reflectance patch in the SW portion of the crater between the peak-ring and the rim. This bright material is diffuse and similar in characteristics to other bright deposits observed on Mercury and interpreted to represent pyroclastic deposits (e.g. bright deposits and vents along the edge of the Caloris basin). However, despite the similarity in characteristics, there is no identified scalloped vent feature associated with the bright deposits in Rachmaninoff.The interior of the basin also has graben that indicate extensional strain within the crater. These graben are similar to features observed in Raditladi basin and the larger Caloris and Rembrandt basins, with the extension likely being a result of uplift within the basin floor. The authors propose that this uplift may be related to the volcanic plains formation.
Crater statistics comparisons of Rachmaninoff and Raditladi basin indicate that Rachmaninoff basin is older than Raditladi basin. The crater studies also indicate that the interior, higher-reflectance plains of Rachmaninoff are significantly younger than the annular lower-reflectance plains. Although age-dating of surfaces on Mercury is imprecise, it is clear that the volcanism within Rachmaninoff extended into the Mansurian and may be as young as 1Ga. This conclusion greatly extends the period of active volcanic plains formation on Mercury, and has implications for the thermal history of the planet.