THE ORIGIN OF MERCURY’S NORTHERN VOLCANIC PLAINS

Smooth plains represent the youngest (~3.7–3.9 Ga) major terrain type on Mercury. The northern volcanic plains (NVP) and the plains interior and exterior to the Caloris impact basin make up 70% of this terrain type by area. The Caloris interior plains are interpreted to be volcanic; some combination of volcanism and impact ejecta likely comprises the exterior plains. Of the remaining smooth plains units, 65% are found either partially or entirely within earlier formed impact structures at volumes far in excess of those expected from impact melt alone. These observations, together with the recognition that global contraction of Mercury induced by interior cooling would yield a lithospheric stress state inhibitive of expansive dike-fed volcanism, raise the question: was the emplacement of Mercury’s smooth plains, and the NVP in particular, causally linked to impacts? Large impacts would remove overburden, deposit subsurface heat, and release pre-existing stress at sites of potential magma generation. The NVP occupy a low-lying region high in Mercury’s northern hemisphere, but although Mercury has a large-scale oblate spheroid shape there is no southern counterpart to the NVP. Moreover, north polar flattening is slightly greater (~500 m) than that at the south pole, supporting a contribution from impact-related excavation of the surface. A best-fit circle applied to the central portion of the NVP is consistent with a basin ~1,700 km in diameter, centered on the free-air gravity anomaly associated with the relative topographic high termed the “northern rise.” There is no resolvable corresponding Bouguer anomaly here as there is for Caloris, but circular Bouguer anomalies occur elsewhere within the NVP, bound by arcuate or near-circular portions of the perimeter that in places are surrounded by high terrain that may be ejecta. The voluminous NVP lavas may therefore have ponded within a set of large, coalesced impact craters. Yet the causal link between impacts and volcanism remains uncertain: the size–frequency distribution of 140 flooded craters within the NVP gives an age of ~4.1 Ga for the underlying surface, 02–0.4 Ga older than the plains themselves. Testing whether a similar age relation exists for other impact feature-hosted smooth plains may help determine if—and how—impacts and volcanism are related on Mercury.