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At least 199 records · Page 11Linked to original sources
PubMed1987

Solid hydrocarbon aerosols produced in simulated Uranian and Neptunian stratospheres.

Optical constants n and k are measured for thin hydrocarbon films produced from charged particles (RF plasma) irradiation of (1) 100% CH4; (2) 7% CH4, 93% H2; (3) 0.5% CH4, 99.5% H2; (4) 0.0002% CH4, 99.3% H2 (with impurities); and (5) 3 to 25% CH4, 25% He, remainder H2--all at submillibar pressures. In all experiments, yellow to deep brown-red solid products are synthesized which are hypothesized to be, at least in part, the unidentified visible and near-UV chromophores in the stratospheres of Uranus and Neptune. Results for experiments 2, 3, and 4 are in good mutual accord, but are significantly different from experiments 1 and 5. He in the precursor gases affects the product composition. Typical solid products for experiments 5 show, at 0.55 micrometer wavelength, n = 1.60 +/- 0.05, 3 x 10(-2) > or = k > or = 3 x 10(-3), and [C/H] approximately equal to 0.7. These results are, for n and k respectively, consistent with and in excellent agreement with those derived from high phase angle Voyager 2 photometry of Uranus (Pollack et al., this issue). Aerosols produced directly from the atmosphere by precipitating magnetospheric charged particles may be competitive with those produced by UV and charged particle irradiation of simple hydrocarbon condensates. The optical and chemical properties of aerosols in the Uranian and Neptunian atmospheres may evolve toward higher values of n and k and higher carbon content as the particles sediment through changing radiation and thermal environments.

Aerosols↗
PubMed1994

Low density of neutral hydrogen and helium in the local interstellar medium: Extreme Ultraviolet Explorer photometry of the Lyman continuum of the hot white dwarfs MCT 0501-2858, MCT 0455-2812, HZ 43, and GD 153.

The first comprehensive sky survey of the extreme ultraviolet (EUV) spectral range performed by the Extreme Ultraviolet Explorer (EUVE) has uncovered a handful of very bright sources at wavelengths longer than the He I 504 angstroms photoionization edge. Among these objects are four white dwarfs with exceptionally low interstellar medium (ISM) column densities along the line of sight. Analysis of EUV photometry of the He-rich DO white dwarf MCT 0501-2858 and the H-rich DA white dwarf MCT 0455-2812 along one line of sight and of the DA white dwarfs HZ 43 and GD 153 near the north Galactic pole indicates that the overall minimum column density of the neutral material centered on the Sun is N(H I)= 0.5-1.0 x 10(18) cm-2. In the case of MCT 0501-2858, EUV photometric measurements provide a clear constraint to the effective temperature (60,000-70,000 K). Given these neutral hydrogen columns, the actual contribution to the density of neutral species from the immediate solar environment (the "local fluff") would only cover a distance of approximately 2-3 pc (assuming an average density n(H I) = 0.1 cm-3) leaving these lines of sight almost entirely within the hot phase of the ISM. A preliminary examination of the complete EUVE long-wavelength survey indicates that these lines of sight are exceptional and set a minimum column density in the solar environment.

Astronomy↗
PubMed1998

Detection of water ice on Nereid.

We report the detection of the 1.5 and 2.0 micrometers absorption bands of water ice in the near-infrared reflection spectrum of Neptune's distant irregular satellite Nereid. The spectrum and albedo of Nereid appear intermediate between those of the Uranian satellites Umbriel and Oberon, suggesting a surface composed of a combination of water ice frost and a dark and spectrally neutral material. In contrast, the surface of Nereid appears dissimilar to those of the outer solar system minor planets Chiron, Pholus, and 1997 CU26. The spectrum thus provides support for the hypothesis that Nereid is a regular satellite formed in a circumplanetary environment rather than a captured object.

Astronomical Phenomena↗
PubMed2001

Cave biosignature suites: microbes, minerals, and Mars.

Earth's subsurface offers one of the best possible sites to search for microbial life and the characteristic lithologies that life leaves behind. The subterrain may be equally valuable for astrobiology. Where surface conditions are particularly hostile, like on Mars, the subsurface may offer the only habitat for extant lifeforms and access to recognizable biosignatures. We have identified numerous unequivocally biogenic macroscopic, microscopic, and chemical/geochemical cave biosignatures. However, to be especially useful for astrobiology, we are looking for suites of characteristics. Ideally, "biosignature suites" should be both macroscopically and microscopically detectable, independently verifiable by nonmorphological means, and as independent as possible of specific details of life chemistries--demanding (and sometimes conflicting) criteria. Working in fragile, legally protected environments, we developed noninvasive and minimal impact techniques for life and biosignature detection/characterization analogous to Planetary Protection Protocols. Our difficult field conditions have shared limitations common to extraterrestrial robotic and human missions. Thus, the cave/subsurface astrobiology model addresses the most important goals from both scientific and operational points of view. We present details of cave biosignature suites involving manganese and iron oxides, calcite, and sulfur minerals. Suites include morphological fossils, mineral-coated filaments, living microbial mats and preserved biofabrics, 13C and 34S values consistent with microbial metabolism, genetic data, unusual elemental abundances and ratios, and crystallographic mineral forms.

Exobiology↗
PubMed2003

A perspective on the importance of reproductive mode in astrobiology.

Reproduction is a vital characteristic of life, and sex is the most common reproductive mode in the eukaryotic world. Sex and reproduction are not necessarily linked mechanisms: Sexuality without reproduction exists, while several forms of asexual reproduction are known. The occurrence of sexuality itself is paradoxical, as it is very costly in evolutionary terms. Most of the hypotheses (more than 20) attempting to explain the prevalence of sex fall into two categories: Sex either creates good gene combinations for adaptation to environments or eliminates bad gene combinations counteracting the accumulation of mutations. In spite of this apparent wealth of beneficial effects of sex, asexuality is not rare. Most eukaryotic, asexual lineages are short-lived and can only persist through the presence of sexual roots, but at least two animal groups, bdelloid rotifers and darwinulid ostracods, seem to claim the status of ancient asexuals. Research on (a)sexuality is relevant to astrobiology in a number of ways. First, strong relationships between the origin and persistence of life in extreme environments and reproductive mode are known. Second, the "habitability" of nonterrestrial environments to life greatly depends on reproductive mode. Whereas asexuals can do equally well or better in harsh environments, they fail to adapt fast enough to changing abiotic and biotic environments. Third, it has been shown that plants reproduce mainly asexually in space, and sperm production and motility in some vertebrates are hampered. Both findings indicate that extraterrestrial life under conditions different from Earth might be dominated by asexual reproduction. Finally, for exchange of biological material between planets, the choice of reproductive mode will be important.

Animals↗
PubMed1999

[Effect of space flight on ultrastructure in medicinal plant Datura innoxia Mill].

OBJECTIVE: To substantiate the effect of space environment on medicinal plants. METHOD: The seeds of Datura innoxia were set up in retrievable satellites. After returning to earth, the ultrastructural changes were observed by means of electron microscope. RESULTS: In cells of weightless group and hit group, some changes were found in chloroplast grana, starch grains and vesicles in chloroplasts. No obvious changes were observed in nuclei. CONCLUSION: Space environment has some effect on the ultrastructure in D. innoxia, and chloroplast is more sensitive to space environment than other organs.

Chloroplasts↗
PubMed2006

Astrochemistry: Complex organic matter in Titan's aerosols?

On 14 January 2005, the Huygens probe entered the atmosphere of Titan after a seven-year interplanetary flight as part of the Cassini mission to Saturn. Huygens carried, among other instruments, an aerosol collection and pyrolysis (ACP) device. Its designers, Israël et al., now claim to have detected complex organic matter in two aerosol samples collected at different altitudes (130-35 km and 25-20 km, respectively), on the basis of their detection of ammonia (NH3) and hydrogen cyanide (HCN) when the sample oven was heated to 600 degrees C. However, the authors' remarkable conclusions, which would have far-reaching consequences for our understanding of the chemical environment prevailing on Saturn's largest moon, are not supported by their limited data.

Aerosols↗
PubMed2005

Energetic neutral atom emissions from Titan interaction with Saturn's magnetosphere.

The Cassini Magnetospheric Imaging Instrument (MIMI) observed the interaction of Saturn's largest moon, Titan, with Saturn's magnetosphere during two close flybys of Titan on 26 October and 13 December 2004. The MIMI Ion and Neutral Camera (INCA) continuously imaged the energetic neutral atoms (ENAs) generated by charge exchange reactions between the energetic, singly ionized trapped magnetospheric ions and the outer atmosphere, or exosphere, of Titan. The images reveal a halo of variable ENA emission about Titan's nearly collisionless outer atmosphere that fades at larger distances as the exospheric density decays exponentially. The altitude of the emissions varies, and they are not symmetrical about the moon, reflecting the complexity of the interactions between Titan's upper atmosphere and Saturn's space environment.

Atmosphere↗
PubMed1992

Comets and the formation of biochemical compounds on the primitive Earth--a review.

Thirty years ago it was suggested that comets impacting on the primitive Earth may have represented a significant source of terrestrial volatiles, including some important precursors for prebiotic synthesis (Oró, 1961, Nature 190: 389). This possibility is strongly supported not only by models of the collisional history of the early Earth, but also by astronomical evidence that suggests that frequent collisions of comet-like bodies from the circumstellar disk around the star beta Pictoris are taking place. Although a significant fraction of the complex organic compounds that appear to be present in cometary nuclei were probably destroyed during impact, it is argued that cometary collisions with the primitive Earth represented an important source of both free-energy and volatiles, and may have created transient, gaseous environments in which prebiotic synthesis may have taken place.

Atmosphere↗
PubMed1989

Microgravity particle research on the space station: the Gas-Grain Simulation Facility.

In the gravitational field on Earth, the large settling rate of micron-sized particles and the effects of gravity-induced convection prohibit many interesting studies of phenomena such as coagulation, collisions, and mutual interactions of droplets, dust grains and other particles. Examples of exobiology experiments involving these phenomena are the simulation of organic aerosol formation in Titan's atmosphere, studies of the role of comets in prebiotic chemical evolution, and simulations of carbon grain interactions in various astrophysical environments. The Gas-Grain Simulation Facility (GGSF) is a proposed Earth-orbital laboratory that will allow present ground-based experimental programs which study processes involving small particles and weak interactions to be extended to a new domain. Physics issues that scientists wishing to propose GGSF experiments must consider are reviewed in this paper. Specifically, coagulation, motion in gases and vacua, and wall deposition of particles in a microgravity environment are discussed.

Acceleration↗
PubMed1997

Titan under a red giant sun: a new kind of "habitable" moon.

We explore the response of Titan's surface and massive atmosphere to the change in solar spectrum and intensity as the sun evolves into a red giant. Titan's surface temperature is insensitive to insolation increases as the haze-laden atmosphere "puffs up" and blocks more sunlight. However, we find a window of several hundred Myr exists, roughly 6 Gyr from now, when liquid water-ammonia can form oceans on the surface and react with the abundant organic compounds there. The window opens due to a drop in haze production as the ultraviolet flux from the reddening sun plummets. The duration of such a window exceeds the time necessary for life to have begun on Earth. Similar environments, with approximately 200K water-ammonia oceans warmed by methane greenhouses under red stars, are an alternative to the approximately 30OK water-CO2 environments considered the classic "habitable" planet.

Ammonia↗
PubMed2005

A volcanic environment for bedrock diagenesis at Meridiani Planum on Mars.

Exposed bedrocks at Meridiani Planum on Mars display chemical and mineralogical evidence suggesting interaction with liquid water. On the basis of morphological observations as well as high abundances of haematite and sulphate minerals, the rocks have been interpreted as sediments that were deposited in a shallow body of briny water with subsequent evaporation leaving behind the sulphate minerals. The iron-sulphur mineralization at Meridiani has also been inferred to be analogous to that produced during oxidative weathering of metal sulphide minerals, such as occurs at acid mine drainage sites. Neither of these interpretations, however, is consistent with the chemical composition of the rocks. Here we propose an alternative model for diagenesis of Meridiani bedrock that involves deposition of volcanic ash followed by reaction with condensed sulphur dioxide- and water-bearing vapours emitted from fumaroles. This scenario does not require prolonged interaction with a standing body of surface water and may have occurred at high temperatures. Consequently, the model invokes an environment considerably less favourable for biological activity on Mars than previously proposed interpretations.

Exobiology↗
PubMed2002

Activities of the Japanese space weather forecast center at Communications Research Laboratory.

The International Space Environment Service (ISES) is an international organization for space weather forecasts and belongs to the International Union of Radio Science (URSI). There are eleven ISES forecast centers in the world, and Communications Research Laboratory (CRL) runs the Japanese one. We make forecasts on the space environment and deliver them over the phones and through the Internet. Our forecasts could be useful for human activities in space. Currently solar activity is near maximum phase of the solar cycle 23. We report the several large disturbances of space environment occurred in 2001, during which low-latitude auroras were observed several times in Japan.

Extraterrestrial Environment↗