PhD, Indian Institute of Science
Earth is the laboratory to understand the processes on other planets in Solar System
The evolution of Earth since its birth 4.5 billion years ago is unique compared to its neighbors in the Solar System. However, there are fundamental processes that have occurred on Earth as well as other planets like Mars. Impact cratering or collision between objects in the Solar System is one such process. Remote sensing images of planets and their satellites show that impact craters are ubiquitous in the Solar System. The study of impact craters is made even more significant because these craters provide a landing spot for rovers in planetary missions; for example, the Curiosity rover to Mars landed in Gale Crater. Hence, a proper understanding of the impact cratering process, such as the interaction between the impactor and the target rock, is required for interpretation of data that are obtained through these missions. Such understanding can be obtained from the terrestrial impact craters, but the lack of physical preservation of the impactor (meteorite) and alteration of the actual crater morphology due to weathering agents, complicate the research. The clues to the type of impactor and evidences of the impact cratering process is only preserved in the form of chemical signatures: excesses or depletions of certain elements and their isotopes, which have a contrasting composition in the meteorite and the target rock. A second process that is common to Earth and its neighbors like Moon or Mars is alteration by water. Flowing water is not present in any of the terrestrial planets in the present day, but signatures of aqueous alteration is preserved in the form of hydroxide bearing minerals. Research on the effect of aqueous alteration has gained importance in the last decade after the discovery of hydroxide bearing minerals by the Mars Exploration Rovers. These minerals, generally called clay minerals, are abundant on Earth since water is a common agent of weathering or alteration on Earth. However, the type of aqueous alteration products, found on Mars, are different from those on Earth; for example, the presence of higher Iron and Magnesium in clay minerals on Mars and the presence of sulfate, along with hydroxide, indicating water on Mars being acidic. Despite the general differences, there are certain locations on Earth, where the type of aqueous alteration products mimic those on Mars. A detailed understanding of the chemistry behind the formation of Mars-like clay minerals will guide us to the relevance of water in the evolution of Mars.
Abstract: The Lonar impact crater, India, is one of the few known terrestrial impact craters excavated in continental basaltic target rocks (Deccan Traps, ~65 Ma). The impactites reported from the crater to date mainly include centimeter- to decimeter-sized impact-melt bombs, and aerodynamically shaped millimeter- and submillimeter-sized impact spherules. They occur in situ within the ejecta around the crater rim and show schlieren structure. In contrast, non–in situ glassy objects, loosely strewn around the crater lake and in the ejecta around the crater rim do not show any schlieren structure. These non–in situ fragments appear to be similar to ancient bricks from the Daityasudan temple in the Lonar village. Synthesis of existing and new major and trace element data on the Lonar impact spherules show that (1) the target Lonar basalts incorporated into the spherules had undergone minimal preimpact alteration. Also, the paleosol layer as preserved between the top-most target basalt flow and the ejecta blanket, even after the impact, was not a source component for the Lonar impactites, (2) the Archean basement below the Deccan traps were unlikely to have contributed material to the impactite parental melts, and (3) the impactor asteroid components (Cr, Co, Ni) were concentrated only within the submillimeter-sized spherules. Two component mixing calculations using major oxides and Cr, Co, and Ni suggest that the Lonar impactor was a EH-type chondrite with the submillimeter-sized spherules containing ~6 wt% impactor components.
Pub.: 07 May '17, Pinned: 03 Dec '17