Atmospheric effects on ejecta emplacement and crater formation on Venus from Magellan

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Abstract
The Venus cratering record provides a unique environment for assessing the effects of both gravity and an atmosphere on impact crater formation. This contribution uses surface signatures of energy partitioning as a framework for testing extrapolations from laboratory experiments and other planetary settings. Seven general conclusions can be drawn. First, the dense lower atmosphere of Venus takes on the role of a low‐density target for bodies smaller than about 4 km in diameter. Air blasts created by cratering in the atmosphere create distinctive surface signatures that allow the derivation of an independent assessment of impactor energy at the limit of break up. Second, dynamic pressures during entry of larger bodies will exceed their strength limit but may not prevent penetration of the atmosphere due to aerodynamic reshaping that minimizes the drag coefficient. Such a process may account for the formation of unusually small craters (1–3 km). Third, the dense atmosphere of Venus preserves signatures of early time cratering processes on the surface that are typically lost on atmosphere free surfaces. Such signatures not only provide another estimate of impactor energy but also include a distinctive record of the impactor (i.e., comet versus asteroid) in distinctive run‐out flows created before the crater has finished formation. Strong winds and turbulence associated with the atmospheric disturbance at later times create wind streaks behind topographic barriers. Fourth, ballistic ejection of excavated debris occurs from craters on Venus just as it does on planets without an atmosphere, thereby underscoring the fundamental mechanical transfer of energy from impactor to target. Fifth, ejecta emplacement is nonballistic due to the large dynamic forces acting on the advancing curtain and its constituent ejecta. The outward moving ejecta curtain induces strong response winds that entrain ejecta and drive a ground‐hugging debris flow outward without returning to the cavity. Flow separation creates an overriding run out ejecta flow further sustained by atmospheric turbulence and identified as radar dark lobate lobes. Sixth, radar‐dark parabolas are proposed to be late time fallout deposits created as the downrange fireball evolves aloft, perhaps analogous to terrestrial tektite strewn fields. And seventh, the response of crater formation to the atmosphere is conversely expressed by a reduction in cratering efficiency as revealed by the unusually large central peak complexes and the unexpected diameter‐depth relations of craters. Hence surface ages may be significantly underestimated.