Investigating the Formation and Structure of Mercury's Caloris Impact Basin

Introduction: With a diameter of ~1500 km [1,2], Caloris is the largest impact basin on Mercury and one of the largest within the Solar System. Caloris, formed ~3.9 Ga [3], is also the best-preserved large mercurian basin. The basin was first imaged in its entirety by the Mercury Dual Imaging System (MDIS) onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, during its first flyby. Imaging showed the basin to be filled with a large expanse of smooth volcanic plains, like much of the northern latitudes of Mercury [1,4]. Post-impact modification has also resulted in some parts of Caloris' interior exceeding its basin rim by 1 km [5,6]. Caloris has two main interior units: low reflectance material (LRM) and high reflectance plains (HRP) [1,2,7]. The volcanic HRP unit covers the majority of the basin floor interior; LRM has been exposed on the basin surface via cratering events. The LRM is thought to be a minimum of 7.5-8.5 km thick [8,9] and may possibly represent basin floor material [9] and, therefore, be lower crustal and/or upper mantle-like in composition. Here, numerical modeling of the Caloris impact is undertaken to investigate basin formation and structure. Distribution of crustal and mantle material post-impact is analyzed and quantitative values of melt volume and thickness are calculated and used to interpret the origin of the LRM. Transient crater properties are used to explore any effects of the impact on Mercury's large core. Finally, modeling of Caloris will allow insight into basin formation on Mercury, which may be different than basin formation on the Moon [10]. Methods: The iSALE shock physics code [11-13] was used to model Caloris-sized impacts. iSALE has previously been used to study other large-scale impact basins within the Solar System including Chicxulub, Earth [14] and South Pole-Aitken, the Moon [15]. The impacts were modeled into a halfspace target divided into a 50 km thick crust [9,16], on top of a mantle 350 km thick, with an iron core beneath. Semi-analytical equations of state (ANEOS) for basalt [17], dunite [18] and iron [19] were used to represent the mercurian crust, mantle and core, respectively. Dunite was additionally used to represent the impactor which varied in size and velocity from 50-250 km and 15-50 km/s, respectively. Grid cell size was 5 km, comparable to other large-scale basin modeling [15]. Surface gravity was kept constant at 3.7 m/s 2 . Two target thermal profiles, suitable for the time of the Caloris impact [20-22], were investigated. The profiles had gradients of 8 K/km and 15 K/km. Following previous modeling of large basin-scale impacts [15], an effective viscosity of 10 10 Pa s for partially molten (super solidus) material was included. Results: Figure 1 illustrates two time steps in a Caloris-sized basin-forming impact. The top panel shows the transient crater (defined as forming once the expanding transient cavity reaches its maximum volume, in line with previous numerical modeling) which reaches this state 4.33 minutes after initial impact. The lower panel shows the basin after dynamic processes have ceased 120 minutes after impact.