Karin Louzada

Geology of Lonar Crater, India

Lonar Crater, India, is one of the youngest and best preserved impact structures on Earth. The 1.88-km-diameter simple crater formed entirely within the Deccan traps, making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the Moon. In this study, we present a meter-scale–resolution digital elevation model, geological map of Lonar Crater and the surrounding area, and radiocarbon ages for histosols beneath the distal ejecta. Impact-related deformation of the target rock consists of upturned basalt fl ows in the upper crater walls and recumbent folding around rim concentric, subhorizontal, noncylindrical fold axes at the crater rim. The rim-fold hinge is preserved around 10%– 15% of the crater. Although tearing in the rim-fold is inferred from fi eld and paleomagnetic observations, no tear faults are identifi ed, indicating that large displacements in the crater walls are not characteristic of small craters in basalt. One signifi cant normal fault structure is observed in the crater wall that offsets slightly older layer-parallel slip faults. There is little fl uvial erosion of the continuous ejecta blanket. Portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules, in particular in the eastern and western rim, as well as in the depression north of the crater known as Little Lonar. The emplacement of the continuous ejecta blanket can be likened to a radial groundhugging debris fl ow, based on the preserved thickness distribution of the ejecta, the effi cient exchange of clasts between the ejecta fl ow and the underlying histosol, and the lack of sorting and stratifi cation in the bulk of the ejecta. The ejecta profi le is thickened at the distal edge and similar to fl ejecta structures observed on Mars.

Shock Demagnetization of Pyrrhotite (Fe1−xS, x⩽0.13) and Implications for the Martian Crust and Meteorites

After cessation of the dynamo on Mars, giant impact events should have demagnetized large regions of the crust. Models of the decay of shock pressure with distance indicate that the demagnetized zones are bound by peak shock pressures between 1 and 3 GPa. We performed the first planar shock recovery experiments at these pressures on natural pyrrhotite, a magnetic mineral found in Martian meteorites. Post‐shock magnetic measurements show that pyrrhotite demagnetizes significantly (∼85–90%) when subject to shock pressures between 1 and 4 GPa. Permanent changes to the magnetic properties of recovered samples include an increase in the saturation remanence and the mean destructive field, indicating that shocks harden the coercivity. We conclude that pyrrhotite is a candidate carrier for the magnetization in the Martian crust and that pyrrhotite in meteorites shocked to modest pressures may retain a pre‐shock remanence.