The formation of infilled craters on Mars: Evidence for widespread impact induced decompression of the early martian mantle?

• Martian flat high inertia crater floors formed by volcanic infilling at ∼3.5 Ga.
• Impact events removed portions of the upper crust causing mantle decompression melting.
• This process was widespread on early Mars and has gone previously undocumented.

Flat-floored craters have long been recognized on Mars with early work hypothesizing a sedimentary origin. More recently, high-resolution thermal inertia measurements show that these craters contain some of the rockiest materials on the planet, inconsistent with poorly consolidated sedimentary materials. In this study, the distribution, physical properties (morphology and thermal inertia), and composition of these craters are thoroughly investigated over the entire planet. The majority of the ∼2800 rocky crater floors identified are concentrated in the low albedo (0.1–0.17), cratered southern highlands. These craters were infilled at ∼3.5 Ga and are associated with the highest thermal inertia values and some of the most mafic materials identified on the planet. Although several processes may have led to the formation of the crater floors, the most likely scenario is volcanic infilling through fractures created by the impact event. The primitive magma source directly results from decompression melting of the martian mantle by the removal of the crustal material excavated by the impactor. Volcanic infilling of craters by decompression melting appears to only have occurred in early martian history when the lithosphere was still relatively thin and the thermal gradient was high. This process was widespread and responsible for the eruption of significant volumes of primitive material, inside and likely outside of craters. Impact induced decompression melting of the martian mantle accounts for the unusual infilling of martian craters and is a widespread planetary process that has gone previously undocumented.