¹Vicki Darlington,^1,2Tom Blenkinsop,¹Paul Dirks,³Jess Salisbury,³Andrew Tomkins
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12734]
¹College of Science and Engineering, James Cook University, Townsville, Queensland, Australia
²School of Earth and Ocean Science, Cardiff University, Cardiff, UK
³School of Geosciences, Monash University, Melbourne, Victoria, Australia
Published by arrangement with John Wiley & Sons

The Lawn Hill Impact Structure (LHIS) is located 250 km N of Mt Isa in NW Queensland, Australia, and is marked by a highly deformed dolomite annulus with an outer diameter of ~18 km, overlying low metamorphic grade siltstone, sandstone, and shale, along the NE margin of the Georgina Basin. This study provides detailed field observations from sections of the Lawn Hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the Lawn Hill impact. 40Ar-39Ar dating of impact-related melt particles provides a time of impact in the Ordovician (472 ± 8 Ma) when the Georgina Basin was an active depocenter. The timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed Thorntonia Limestone at the time of impact. The exposed annulus is remarkably well preserved, with preservation attributed to postimpact sedimentation. The LHIS has an atypical crater morphology with no central uplift. The heterogeneous target materials at Lawn Hill were probably low-strength, porous, and water-saturated, with all three properties affecting the crater morphology. The water-saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus, and restricted melt production. The impact timing raises the possibility that the Lawn Hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid-Ordovician (470 ± 6 Ma).

¹E. Chassefière,^2,3J. Lasue,⁴B. Langlais,⁵Y. Quesnel
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12784]
¹GEOPS, Univ. Paris-Sud, CNRS, Universite Paris-Saclay, Rue du Belvedere, Bat. 504-509, 91405 Orsay, France
²Universite de Toulouse, UPS-OMP, IRAP, Toulouse, France
³CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France
⁴LPG-CNRS & Universite de Nantes, Nantes, France
⁵Aix-Marseille Universite, CNRS, IRD, CEREGE UM34, Aix-en-Provence, France
Published by arrangement with John Wiley & Sons

CH4 has been observed on Mars both by remote sensing and in situ during the past 15 yr. It could have been produced by early Mars serpentinization processes that could also explain the observed Martian remanent magnetic field. Assuming a cold early Mars, a cryosphere could trap such CH4 as clathrates in stable form at depth. The maximum storage capacity of such a clathrate cryosphere has been recently estimated to be 2 × 1019 to 2 × 1020 moles of methane. We estimate how large amounts of serpentinization-derived CH4 stored in the cryosphere have been released into the atmosphere during the Noachian and the early Hesperian. Due to rapid clathrate dissociation and photochemical conversion of CH4 to H2, these episodes of massive CH4 release may have resulted in transient H2-rich atmospheres, at typical levels of 10–20% in a background 1–2 bar CO2 atmosphere. The collision-induced heating effect of H2 present in such an atmosphere has been shown to raise the surface temperature above the water freezing point. We show how local and rapid destabilization of the cryosphere can be induced by large events (such as the Hellas Basin or Tharsis bulge formation) and lead to such releases. Our results show that the early Mars cryosphere had a sufficient CH4 storage capacity to have maintained H2-rich transient atmospheres during a total time period up to several million years or tens of million years, having potentially contributed to the formation of valley networks during the Noachian/early Hesperian.