Impact cratering in H₂O‐bearing targets on Mars: Thermal field under craters as starting conditions for hydrothermal activity

Abstract

Abstract-We present a case modeling study of impact crater formation in H 2 O-bearing targets. The main goal of this work was to investigate the postimpact thermal state of the rock layers modified in the formation of hypervelocity impact craters. We present model results for a target consisting of a mixture of H 2 O-ice and rock, assuming an ice ⁄ water content variable with depth. Our model results, combined with results from previous work using dry targets, indicate that for craters larger than about 30 km in diameter, the onset of postimpact hydrothermal circulation is characterized by two stages: first, the formation of a mostly dry, hot central uplift followed by water beginning to flow in and circulate through the initially dry and hot uplifted crustal rocks. The postimpact thermal field in the periphery of the crater is dependent on crater size: in midsize craters, 30-50 km in diameter, crater walls are not strongly heated in the impact event, and even though ice present in the rock may initially be heated enough to melt, overall temperatures in the rock remain below melting, undermining the development of a crater-wide hydrothermal circulation. In large craters (with diameters more than 100 km or so), the region underneath the crater floor and walls is heated well above the melting point of ice, thus facilitating the onset of an extended hydrothermal circulation. These results provide preliminary constraints in characterizing the many water-related features, both morphologic and spectroscopic, that high-resolution images of Mars are now detecting within many Martian craters.

The theoretical plausibility of central pit crater formation via melt drainage

“…This ground ice would be melted and/or vaporized during an impact (Ivanov and Pierazzo, 2011). However, in large craters (D > 100 km) the central uplift will be made of mostly dry rocks initially located below the aquifer (Ivanov and Pierazzo, 2011). This may explain why central pits do not occur in craters with diameters larger than 114 km on Mars (Barlow, 2006).…”

“…However, Mars has ice beneath its surface (Byrne et al, 2009) and may have had more in the past (Baker et al, 1991). This ground ice would be melted and/or vaporized during an impact (Ivanov and Pierazzo, 2011). However, in large craters (D > 100 km) the central uplift will be made of mostly dry rocks initially located below the aquifer (Ivanov and Pierazzo, 2011).…”

Impact-induced phyllosilicate formation from olivine and water

“…Post-impact hydrothermal activity on Mars has been proposed previously (e.g., Schwenzer and Kring, 2009;Barnhart et al, 2010;Marzo et al, 2010;Ivanov and Pierazzo, 2011). Some nakhlites contain iddingsite (an assemblage of phyllosilicates, carbonates, sulfates, and iron hydroxides) along fractures in olivine (Treiman et al, 1993;Bridges and Grady, 2000;Noguchi et al, 2009;Changela and Bridges, 2010).…”

Environmental Effects of Impact Events

“…Theoretical studies suggest that large impacts can generate large amounts of heat (e.g. Ivanov and Pierazzo, 2011) which could drive substantial hydrothermal activity for hundreds of thousands of years, even under cold climatic conditions, supporting the idea that impact events may have played an important biological role on early Earth and on Mars (Abramov andKring, 2004, 2005).…”