Preservation of martian organic and environmental records: final report of the Mars biosignature working group

The Mars Science Laboratory (MSL) has an instrument package capable of making measurements of past and present environmental conditions. The data generated may tell us if Mars is, or ever was, able to support life. However, the knowledge of Mars' past history and the geological processes most likely to preserve a record of that history remain sparse and, in some instances, ambiguous. Physical, chemical, and geological processes relevant to biosignature preservation on Earth, especially under conditions early in its history when microbial life predominated, are also imperfectly known. Here, we present the report of a working group chartered by the Co-Chairs of NASA's MSL Project Science Group, John P. Grotzinger and Michael A. Meyer, to review and evaluate potential for biosignature formation and preservation on Mars. Orbital images confirm that layered rocks achieved kilometer-scale thicknesses in some regions of ancient Mars. Clearly, interplays of sedimentation and erosional processes govern present-day exposures, and our understanding of these processes is incomplete. MSL can document and evaluate patterns of stratigraphic development as well as the sources of layered materials and their subsequent diagenesis. It can also document other potential biosignature repositories such as hydrothermal environments. These capabilities offer an unprecedented opportunity to decipher key aspects of the environmental evolution of Mars' early surface and aspects of the diagenetic processes that have operated since that time. Considering the MSL instrument payload package, we identified the following classes of biosignatures as within the MSL detection window: organism morphologies (cells, body fossils, casts), biofabrics (including microbial mats), diagnostic organic molecules, isotopic signatures, evidence of biomineralization and bioalteration, spatial patterns in chemistry, and biogenic gases. Of these, biogenic organic molecules and biogenic atmospheric gases are considered the most definitive and most readily detectable by MSL.     

Fossil Lipids for Life-Detection: A Case Study from the Early Earth Record

The geological preservation of lipids from the cell membranes of organisms bestows a precious record of ancient life, especially for the Precambrian eon (>542 million years ago) when Earth life was largely microbial. All organisms produce lipids that, if the lipids survive oxidative degradation, become molecular fossils entrained with information on biological diversity, environmental conditions, and post-depositional alteration history. As with most biosignatures, the molecular fossil record that is indigenous (of the same place) and syngenetic (of the same age) to host rocks can be compromised by the introduction from and reaction with foreign or younger materials (e.g., petroleum or endolithic life). Deciphering the resulting complex pool of organic signals requires tests for the provenance of molecular fossils and the overall quality of the geobiological record itself. This paper reviews the basis for the very existence of a molecular fossil record from lipid biochemistry to mechanisms of organic-matter preservation and geochemical alteration. A systematic approach to resolving the provenance of molecular fossils and historical qualities of the record is presented in a case study of an early Earth record. This example demonstrates the value of geological context and the integration of independent geobiological parameters, which are critical to the detection and understanding of the ecological processes responsible for records of life.     

The Mars Science Laboratory Organic Check Material

Mars Science Laboratory’s Curiosity rover carries a set of five external verification standards in hermetically sealed containers that can be sampled as would be a Martian rock, by drilling and then portioning into the solid sample inlet of the Sample Analysis at Mars (SAM) suite. Each organic check material (OCM) canister contains a porous ceramic solid, which has been doped with a fluorinated hydrocarbon marker that can be detected by SAM. The purpose of the OCM is to serve as a verification tool for the organic cleanliness of those parts of the sample chain that cannot be cleaned other than by dilution, i.e., repeated sampling of Martian rock. SAM possesses internal calibrants for verification of both its performance and its internal cleanliness, and the OCM is not used for that purpose. Each OCM unit is designed for one use only, and the choice to do so will be made by the project science group (PSG).     

等离子喷涂Al_2O_3涂层的电击穿机理

采用大气等离子喷涂技术在铜基体上沉积了Al2O3涂层。采用XRD和SEM对涂层微观结构进行了表征。通过探讨孔隙率和吸潮行为对绝缘性能的影响,分析了等离子喷涂Al2O3涂层结构与电绝缘失效机理的关系。结果表明:等离子喷涂Al2O3涂层较致密,界面结合较好。随涂层厚度不同其孔隙率在5%~7%范围变化。等离子喷涂Al2O3涂层结构中的孔洞是电绝缘失效的主要部位且呈典型电晕击穿形貌。电晕击穿诱发的裂纹沿击穿方向扩展形成击穿隧道。击穿方向与电极极性无关而由击穿孔洞位置决定。涂层厚度与涂层击穿强度呈现倒数关系。吸潮会诱发导电通路形成降低Al2O3涂层抗击穿能力。