Raman imagery: a new approach to assess the geochemical maturity and biogenicity of permineralized precambrian fossils

Laser-Raman imagery is a non-intrusive, non-destructive analytical technique, recently introduced to Precambrian paleobiology, that can be used to demonstrate a one-to-one spatial correlation between the optically discernible morphology and kerogenous composition of permineralized fossil microorganisms. Made possible by the submicron-scale resolution of the technique and its high sensitivity to the Raman signal of carbonaceous matter, such analyses can be used to determine the chemical-structural characteristics of organic-walled microfossils and associated sapropelic carbonaceous matter in acid-resistant residues and petrographic thin sections. Here we use this technique to analyze kerogenous microscopic fossils and associated carbonaceous sapropel permineralized in 22 unmetamorphosed or little-metamorphosed fine-grained chert units ranging from approximately 400 to approximately 2,100 Ma old. The lineshapes of the Raman spectra acquired vary systematically with five indices of organic geochemical maturation: (1) the mineral-based metamorphic grade of the fossil-bearing units; (2) the fidelity of preservation of the fossils studied; (3) the color of the organic matter analyzed; and both the (4) H/C and (5) N/C ratios measured in particulate kerogens isolated from bulk samples of the fossil-bearing cherts. Deconvolution of relevant spectra shows that those of relatively well-preserved permineralized kerogens analyzed in situ exhibit a distinctive set of Raman bands that are identifiable also in hydrated organic-walled microfossils and particulate carbonaceous matter freed from the cherts by acid maceration. These distinctive Raman bands, however, become indeterminate upon dehydration of such specimens. To compare quantitatively the variations observed among the spectra measured, we introduce the Raman Index of Preservation, an approximate measure of the geochemical maturity of the kerogens studied that is consistent both with the five indices of organic geochemical alteration and with spectra acquired from fossils experimentally heated under controlled laboratory conditions. The results reported provide new insight into the chemical-structural characteristics of ancient carbonaceous matter, the physicochemical changes that accompany organic geochemical maturation, and a new criterion to be added to the suite of evidence by which to evaluate the origin of minute fossil-like objects of possible but uncertain biogenicity.     

Mineralogical biosignatures and the search for life on Mars

If life ever existed, or still exists, on Mars, its record is likely to be found in minerals formed by, or in association with, microorganisms. An important concept regarding interpretation of the mineralogical record for evidence of life is that, broadly defined, life perturbs disequilibria that arise due to kinetic barriers and can impart unexpected structure to an abiotic system. Many features of minerals and mineral assemblages may serve as biosignatures even if life does not have a familiar terrestrial chemical basis. Biological impacts on minerals and mineral assemblages may be direct or indirect. Crystalline or amorphous biominerals, an important category of mineralogical biosignatures, precipitate under direct cellular control as part of the life cycle of the organism (shells, tests, phytoliths) or indirectly when cell surface layers provide sites for heterogeneous nucleation. Biominerals also form indirectly as by-products of metabolism due to changing mineral solubility. Mineralogical biosignatures include distinctive mineral surface structures or chemistry that arise when dissolution and/or crystal growth kinetics are influenced by metabolic by-products. Mineral assemblages themselves may be diagnostic of the prior activity of organisms where barriers to precipitation or dissolution of specific phases have been overcome. Critical to resolving the question of whether life exists, or existed, on Mars is knowing how to distinguish biologically induced structure and organization patterns from inorganic phenomena and inorganic self-organization. This task assumes special significance when it is acknowledged that the majority of, and perhaps the only, material to be returned from Mars will be mineralogical.     

Production of neutrons from interactions of GCR-like particles

In order to help assess the risk to astronauts due to the long-term exposure to the natural radiation environment in space, an understanding of how the primary radiation field is changed when passing through shielding and tissue materials must be obtained. One important aspect of the change in the primary radiation field after passing through shielding materials is the production of secondary particles from the breakup of the primary. Neutrons are an important component of the secondary particle field due to their relatively high biological weighting factors, and due to their relative abundance, especially behind thick shielding scenarios. Because of the complexity of the problem, the estimation of the risk from exposure to the secondary neutron field must be handled using calculational techniques. However, those calculations will need an extensive set of neutron cross section and thicktarget neutron yield data in order to make an accurate assessment of the risk. In this paper we briefly survey the existing neutron-production data sets that are applicable to the space radiation transport problem, and we point out how neutron production from protons is different than neutron production from heavy ions. We also make comparisons of one the heavy-ion data sets with Boltzmann-Uehling-Uhlenbeck (BUU) calculations.     

One hundred US EVAs: a perspective on spacewalks

In the 36 years between June 1965 and February 2001, the US human space flight program has conducted 100 spacewalks, or extravehicular activities (EVAs), as NASA officially calls them. EVA occurs when astronauts wearing spacesuits travel outside their protective spacecraft to perform tasks in the space vacuum environment. US EVA started with pioneering feasibility tests during the Gemini Program. The Apollo Program required sending astronauts to the moon and performing EVA to explore the lunar surface. EVA supported scientific mission objectives of the Skylab program, but may be best remembered for repairing launch damage to the vehicle and thus saving the program. EVA capability on Shuttle was initially planned to be a kit that could be flown at will, and was primarily intended for coping with vehicle return emergencies. The Skylab emergency and the pivotal role of EVA in salvaging that program quickly promoted Shuttle EVA to an essential element for achieving mission objectives, including retrieving satellites and developing techniques to assemble and maintain the International Space Station (ISS). Now, EVA is supporting assembly of ISS. This paper highlights development of US EVA capability within the context of the overarching mission objectives of the US human space flight program.     

U.S. prebreathe protocol

This paper identifies and describes the prebreathe protocol currently used by the U.S. Space Shuttle Program to provide astronauts the capability to safely perform extravehicular activity. A comparison of planned vs actual prebreathe experience through the STS-37 Mission is also provided.     

Identification and status of design improvements to the NASA Shuttle EMU for International Space Station application

To meet the significant increase in EVA demand to support assembly and operations of the International Space Station (ISS), NASA and industry have improved the current Shuttle Extravehicular Mobility Unit (EMU), or "space suit", configuration to meet the unique and specific requirements of an orbital-based system. The current Shuttle EMU was designed to be maintained and serviced on the ground between frequent Shuttle flights. ISS will require the EMUs to meet increased EVAs out of the Shuttle Orbiter and to remain on orbit for up to 180 days without need for regular return to Earth for scheduled maintenance or refurbishment. Ongoing Shuttle EMU improvements have increased reliability, operational life and performance while minimizing ground and on-orbit maintenance cost and expendable inventory. Modifications to both the anthropomorphic mobility elements of the Space Suit Assembly (SSA) as well as to the Primary Life Support System (PLSS) are identified and discussed. This paper also addresses the status of on-going Shuttle EMU improvements and summarizes the approach for increasing interoperability of the U.S. and Russian space suits to be utilized aboard the ISS.     

欧洲未来的数据中继卫星系统技术方案

本文介绍并比较了能满足欧洲九十年代需要的未来数据中继卫星系统的各种技术方案,列出了目前己知的各种工作要求及各种系统方案,还分析了各种可能的中继技术,其中有一项技术涉及激光线路。本文还介绍了预先研究和研制情况,最后简要介绍初步确定的参考系统方案和数据中继卫星(DRS)计划进度示意表。     

利用甚长基线干涉测量技术跟踪同步卫星

本文分析比较了用无线电干涉测量跟踪地球同步卫星的四种方法。以类星体为参考的差分甚长基线干涉测量技术(△VLBI)需要用非常灵敏的接收机,利用双基线系统可使位置精度达到米级。以卫星为参考的△VLBI使用简单、便宜的接收机,所得精度稍低一些。非差分VLBI利用GPS进行不太精确的传播修正及时钟校准,双基线系统的定位精度仍可达5～10米。对于倾角较大的轨道,这四种干涉测量方法都能用单基线给出上述定位精度的六个卫星状态分量。