The age of the Medusae Fossae Formation: Evidence of Hesperian emplacement from crater morphology, stratigraphy, and ancient lava contacts

The Medusae Fossae Formation (MFF), covering about 2.1 × 10⁶ km² (with an estimated volume of 1.4 × 10⁶ km³) and straddling the equatorial region of Mars east of Tharsis, has historically been mapped and dated as Amazonian in age. Analysis of the MFF using a range of new observations from recent mission data at multiple resolutions reveals evidence that the formation is older than previously hypothesized, with parts of the MFF having formed in the Hesperian and parts having been reworked and reformed throughout the Amazonian, up to the present. Ancient outcroppings of the MFF, edged with jagged yardangs, became a “mold” for embaying Hesperian-aged lavas. The erosion of the MFF left solidified lava “casts” in the embaying lava unit. This lava edge morphology permits the identification of ancient contacts between the MFF and Hesperian-aged lava terrain. Additionally, the flanking fan of the Hesperian-aged Apollinaris Patera volcano embays the formation at its foot, indicating that parts of the MFF were formed in the Hesperian. Erosion has erased and inverted many of the superposed craters in the region, showing that very young Amazonian ages derived from impact crater size-frequency distributions are resurfacing ages, and not emplacement ages. We find abundant evidence that the formation is extremely mobile and continuously reworked. We conclude that a significant part of the MFF may have originally been emplaced in the Hesperian. These observations place new constraints on the mode of origin of the MFF.