Defining life

Any definition is intricately connected to a theory that gives it meaning. Accordingly, this article discusses various definitions of life held in the astrobiology community by considering their connected "theories of life." These include certain "list" definitions and a popular definition that holds that life is a "self-sustaining chemical system capable of Darwinian evolution." We then act as "anthropologists," studying what scientists do to determine which definition-theories of life they constructively hold as they design missions to seek non-terran life. We also look at how constructive beliefs about biosignatures change as observational data accumulate. And we consider how a definition centered on Darwinian evolution might itself be forced to change as supra-Darwinian species emerge, including in our descendents, and consider the chances of our encountering supra-Darwinian species in our exploration of the Cosmos. Last, we ask what chemical structures might support Darwinian evolution universally; these structures might be universal biosignatures.     

复材机身制造的自动化流水线工艺过程

AeroSuite给出了一个飞行器制造过程有效管理整个产品制造流程的解决方案。它包括FiberSIM复材工艺软件、SyncroFIT机身设计制造装配软件、对于首件鉴定过程的流程化的质量规划模块(QPE)。     

Cassini Imaging Science: Instrument Characteristics And Anticipated Scientific Investigations At Saturn

The Cassini Imaging Science Subsystem (ISS) is the highest-resolution two-dimensional imaging device on the Cassini Orbiter and has been designed for investigations of the bodies and phenomena found within the Saturnian planetary system. It consists of two framing cameras: a narrow angle, reflecting telescope with a 2-m focal length and a square field of view (FOV) 0.35^∘ across, and a wide-angle refractor with a 0.2-m focal length and a FOV 3.5^∘ across. At the heart of each camera is a charged coupled device (CCD) detector consisting of a 1024 square array of pixels, each 12 μ on a side. The data system allows many options for data collection, including choices for on-chip summing, rapid imaging and data compression. Each camera is outfitted with a large number of spectral filters which, taken together, span the electromagnetic spectrum from 200 to 1100 nm. These were chosen to address a multitude of Saturn-system scientific objectives: sounding the three-dimensional cloud structure and meteorology of the Saturn and Titan atmospheres, capturing lightning on both bodies, imaging the surfaces of Saturn’s many icy satellites, determining the structure of its enormous ring system, searching for previously undiscovered Saturnian moons (within and exterior to the rings), peering through the hazy Titan atmosphere to its yet-unexplored surface, and in general searching for temporal variability throughout the system on a variety of time scales. The ISS is also the optical navigation instrument for the Cassini mission. We describe here the capabilities and characteristics of the Cassini ISS, determined from both ground calibration data and in-flight data taken during cruise, and the Saturn-system investigations that will be conducted with it. At the time of writing, Cassini is approaching Saturn and the images returned to Earth thus far are both breathtaking and promising.This revised version was published online in July 2005 with a corrected cover date.     

The Search for Extraterrestrial Intelligence and the Nasa High Resolution Microwave Survey (HRMS): Historical perspectives

The modern era of the Search for Extraterrestrial Intelligence (SETI), which began with the paper of Cocconi and Morrison in 1959, reached a landmark with the inauguration of NASA's High Resolution Microwave Survey (HRMS) in 1992. SETI programs stand in the tradition of humanity's quest for its place in the Universe, and seek to test whether cosmic evolution results in widespread intelligence in our Galaxy. This paper reviews the history of the modern SETI era, with emphasis on the development of the NASA SETI program.     

Electron-Ion Temperature Equilibration in Collisionless Shocks: The Supernova Remnant-Solar Wind Connection

Collisionless shocks are loosely defined as shocks where the transition between pre-and post-shock states happens on a length scale much shorter than the collisional mean free path. In the absence of collision to enforce thermal equilibrium post-shock, electrons and ions need not have the same temperatures. While the acceleration of electrons for injection into shock acceleration processes to produce cosmic rays has received considerable attention, the related problem of the shock heating of quasi-thermal electrons has been relatively neglected. In this paper we review the state of our knowledge of electron heating in astrophysical shocks, mainly associated with supernova remnants (SNRs), shocks in the solar wind associated with the terrestrial and Saturnian bowshocks, and galaxy cluster shocks. The solar wind and SNR samples indicate that the ratio of electron temperature, (T _e ) to ion temperature (T _p ) declining with increasing shock speed or Alfvén Mach number. We discuss the extent to which such behavior can be understood on the basis of waves generated by cosmic rays in a shock precursor, which then subsequently damp by heating electrons, and speculate that a similar explanation may work for both solar wind and SNR shocks.     

Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments

Ongoing research on martian meteorites and a new set of observations of carbonate minerals provided by an unprecedented series of robotic missions to Mars in the past 15 years help define new constraints on the history of martian climate with important crosscutting themes including: the CO₂ budget of Mars, the role of Mg-, Fe-rich fluids on Mars, and the interplay between carbonate formation and acidity. Carbonate minerals have now been identified in a wide range of localities on Mars as well as in several martian meteorites. The martian meteorites contain carbonates in low abundances (<1 vol.%) and with a wide range of chemistries. Carbonates have also been identified by remote sensing instruments on orbiting spacecraft in several surface locations as well as in low concentrations (2–5 wt.%) in the martian dust. The Spirit rover also identified an outcrop with 16 to 34 wt.% carbonate material in the Columbia Hills of Gusev Crater that strongly resembled the composition of carbonate found in martian meteorite ALH 84001. Finally, the Phoenix lander identified concentrations of 3–6 wt.% carbonate in the soils of the northern plains. The carbonates discovered to date do not clearly indicate the past presence of a dense Noachian atmosphere, but instead suggest localized hydrothermal aqueous environments with limited water availability that existed primarily in the early to mid-Noachian followed by low levels of carbonate formation from thin films of transient water from the late Noachian to the present. The prevalence of carbonate along with evidence for active carbonate precipitation suggests that a global acidic chemistry is unlikely and a more complex relationship between acidity and carbonate formation is present.     

Response: Against manned space flight programs

Richard Rovetto has done a good job of assembling various arguments that are commonly advanced for a program of manned space flight. I will take them up one by one, and explain why I find them all unconvincing.     

Geochemical Consequences of Widespread Clay Mineral Formation in Mars’ Ancient Crust

Clays form on Earth by near-surface weathering, precipitation in water bodies within basins, hydrothermal alteration (volcanic- or impact-induced), diagenesis, metamorphism, and magmatic precipitation. Diverse clay minerals have been detected from orbital investigation of terrains on Mars and are globally distributed, indicating geographically widespread aqueous alteration. Clay assemblages within deep stratigraphic units in the Martian crust include Fe/Mg smectites, chlorites and higher temperature hydrated silicates. Sedimentary clay mineral assemblages include Fe/Mg smectites, kaolinite, and sulfate, carbonate, and chloride salts. Stratigraphic sequences with multiple clay-bearing units have an upper unit with Al-clays and a lower unit with Fe/Mg-clays. The typical restriction of clay minerals to the oldest, Noachian terrains indicates a distinctive set of processes involving water-rock interaction that was prevalent early in Mars history and may have profoundly influenced the evolution of Martian geochemical systems. Current analyses of orbital data have led to the proposition of multiple clay-formation mechanisms, varying in space and time in their relative importance. These include near-surface weathering, formation in ice-dominated near-surface groundwaters, and formation by subsurface hydrothermal fluids. Near-surface, open system formation of clays would lead to fractionation of Mars’ crustal reservoir into an altered crustal reservoir and a sedimentary reservoir, potentially involving changes in the composition of Mars’ atmosphere. In contrast, formation of clays in the subsurface by either aqueous alteration or magmatic cooling would result in comparatively little geochemical fractionation or interaction of Mars’ atmospheric, crustal, and magmatic reservoirs, with the exception of long-term sequestration of water. Formation of clays within ice would have geochemical consequences intermediate between these endmembers. We outline the future analyses of orbital data, in situ measurements acquired within clay-bearing terrains, and analyses of Mars samples that are needed to more fully elucidate the mechanisms of martian clay formation and to determine the consequences for the geochemical evolution of the planet.