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PubMed2004

Looking for planetary moons in the spectra of distant Jupiters.

More than 100 nearby stars are known to have at least one Jupiter-sized planet. Whether any of these giant gaseous planets has moons is unknown, but here we suggest a possible way of detecting Earth-sized moons with future technology. The planned Terrestrial Planet Finder observatory, for example, will be able to detect objects comparable in size to Earth. Such Earth-sized objects might orbit their stars either as isolated planets or as moons to giant planets. Moons of Jovian-sized planets near the habitable zones of main-sequence stars should be noticeably brighter than their host planets in the near-infrared (1-4 microm) if their atmospheres contain methane, water, and water vapor, because of efficient absorption of starlight by these atmospheric components. By taking advantage of this spectral contrast, future space observatories will be able to discern which extrasolar giant planets have Earth-like moons capable of supporting life.

Astronomical Phenomena↗
PubMed2005

Vegetation's red edge: a possible spectroscopic biosignature of extraterrestrial plants.

Earth's deciduous plants have a sharp order-of-magnitude increase in leaf reflectance between approximately 700 and 750 nm wavelength. This strong reflectance of Earth's vegetation suggests that surface biosignatures with sharp spectral features might be detectable in the spectrum of scattered light from a spatially unresolved extrasolar terrestrial planet. We assess the potential of Earth's step-function-like spectroscopic feature, referred to as the "red edge," as a tool for astrobiology. We review the basic characteristics and physical origin of the red edge and summarize its use in astronomy: early spectroscopic efforts to search for vegetation on Mars and recent reports of detection of the red edge in the spectrum of Earthshine (i.e., the spatially integrated scattered light spectrum of Earth). We present Earthshine observations from Apache Point Observatory (New Mexico) to emphasize that time variability is key to detecting weak surface biosignatures such as the vegetation red edge. We briefly discuss the evolutionary advantages of vegetation's red edge reflectance, and speculate that while extraterrestrial "light-harvesting organisms" have no compelling reason to display the exact same red edge feature as terrestrial vegetation, they might have similar spectroscopic features at different wavelengths than terrestrial vegetation. This implies that future terrestrial-planet-characterizing space missions should obtain data that allow time-varying, sharp spectral features at unknown wavelengths to be identified. We caution that some mineral reflectance edges are similar in slope and strength to vegetation's red edge (albeit at different wavelengths); if an extrasolar planet reflectance edge is detected care must be taken with its interpretation.

Chlorophyll↗
PubMed2005

Why O2 is required by complex life on habitable planets and the concept of planetary "oxygenation time".

Life is constructed from a limited toolkit: the Periodic Table. The reduction of oxygen provides the largest free energy release per electron transfer, except for the reduction of fluorine and chlorine. However, the bonding of O2 ensures that it is sufficiently stable to accumulate in a planetary atmosphere, whereas the more weakly bonded halogen gases are far too reactive ever to achieve significant abundance. Consequently, an atmosphere rich in O2 provides the largest feasible energy source. This universal uniqueness suggests that abundant O2 is necessary for the high-energy demands of complex life anywhere, i.e., for actively mobile organisms of approximately 10(-1)-10(0) m size scale with specialized, differentiated anatomy comparable to advanced metazoans. On Earth, aerobic metabolism provides about an order of magnitude more energy for a given intake of food than anaerobic metabolism. As a result, anaerobes do not grow beyond the complexity of uniseriate filaments of cells because of prohibitively low growth efficiencies in a food chain. The biomass cumulative number density, n, at a particular mass, m, scales as n (> m) proportional to m(-1) for aquatic aerobes, and we show that for anaerobes the predicted scaling is n proportional to m (-1.5), close to a growth-limited threshold. Even with aerobic metabolism, the partial pressure of atmospheric O2 (P(O2)) must exceed approximately 10(3) Pa to allow organisms that rely on O2 diffusion to evolve to a size approximately 10(3) m x P(O2) in the range approximately 10(3)-10(4) Pa is needed to exceed the threshold of approximately 10(2) m size for complex life with circulatory physiology. In terrestrial life, O(2) also facilitates hundreds of metabolic pathways, including those that make specialized structural molecules found only in animals. The time scale to reach P(O(2)) approximately 10(4) Pa, or "oxygenation time," was long on the Earth (approximately 3.9 billion years), within almost a factor of 2 of the Sun's main sequence lifetime. Consequently, we argue that the oxygenation time is likely to be a key rate-limiting step in the evolution of complex life on other habitable planets. The oxygenation time could preclude complex life on Earth-like planets orbiting short-lived stars that end their main sequence lives before planetary oxygenation takes place. Conversely, Earth-like planets orbiting long-lived stars are potentially favorable habitats for complex life.

Atmosphere↗
PubMed2006

Astrobiologically interesting stars within 10 parsecs of the sun.

The existence of life based on carbon chemistry and water oceans relies upon planetary properties, chiefly climate stability, and stellar properties, such as mass, age, metallicity, and galactic orbits. The latter can be well constrained with present knowledge. We present a detailed, up-to-date compilation of the atmospheric parameters, chemical composition, multiplicity, and degree of chromospheric activity for the astrobiologically interesting solar-type stars within 10 parsecs of the Sun. We determined their state of evolution, masses, ages, and space velocities, and produced an optimized list of candidates that merit serious scientific consideration by the future space-based interferometry probes aimed at directly detecting Earthsized extrasolar planets and seeking spectroscopic infrared biomarkers as evidence of photosynthetic life. The initially selected stars number 33 solar-type within the total population (excluding some incompleteness for late M-dwarfs) of 182 stars closer than 10 parsecs. A comprehensive and detailed data compilation for these objects is still lacking; a considerable amount of recent data has so far gone unexplored in this context. We present 13 objects as the nearest "biostars," after eliminating multiple stars, young, chromospherically active, hard x-ray- emitting stars, and low metallicity objects. Three of these "biostars"-- Zeta Tucanae, Beta Canum Venaticorum, and 61 Virginis -- closely reproduce most of the solar properties and are considered as premier targets. We show that approximately 7% of the nearby stars are optimally interesting targets for exobiology.

Evolution, Planetary↗
PubMed2006

Microorganisms metabolizing on clay grains in 3-km-deep Greenland basal ice.

We have discovered > 10(8) microbial cells/cm3 attached to clay grains in the bottom 13 m of the GISP2 (Greenland Ice Sheet Project) ice core. Their concentration correlates with huge excesses of CO2 and CH4. We show that Fe-reducing bacteria produce most of the excess CO2 and methanogenic archaea produce the excess CH4. The number of attached cells per clay grain is proportional to grain perimeter rather than to area, which implies that nutrients are accessed at grain edges. We conclude that Fe-reducing microbes immobilized on clay surfaces metabolize via "shuttle" molecules that transport electrons to grain edges, where they reduce Fe(III) ions at edges to Fe(II) while organic acid ions are oxidized to CO2. Driven by the concentration gradient, electrons on Fe(II) ions at grain edges "hop" to Fe(III) ions inward in the same edges and oxidize them. The original Fe(III) ions can then attach new electrons from shuttle molecules at the edges. Our mechanism explains how Fe-reducers can reduce essentially all Fe(III) in clay minerals. We estimate that the Fe(III) in clay grains in the GISP2 silty ice can sustain Fe-reducing bacteria at the ambient temperature of -9 degrees C for approximately 10(6) years. F420 autofluorescence imaging shows that > 2.4% of the cells are methanogens, which account for the excess methane.

Aluminum Silicates↗
PubMed2001

Can skin wrinkling in a site that has received limited sun exposure be used as a marker of health status and biological age?

OBJECTIVES: to determine if skin wrinkling in a site that had received limited sun exposure may be a marker of health status and biological age. DESIGN: population-based, cross-sectional study. PARTICIPANTS: we evaluated the health status of representative samples of elderly Greek-born people living in Melbourne, Greeks living in rural Greece, Anglo-Celtic Australians living in Melbourne and Swedes living in Sweden. We carried out microtopographic assessment of their skin and measured plasma dehydroepiandrosterone concentrations. METHODS: we derived activities of daily living, well-being, memory and general health status scores from a cross-cultural questionnaire. We measured skin wrinkling using cutaneous microtopographic methods and plasma dehydroepiandrosterone by enzyme immuno-assay. RESULTS: skin wrinkling was positively correlated with age (r(s)=0.27, P<0.0001) and negatively with body mass index (r(s)=-0.19, P<0.0001). Therefore, all analyses were controlled for these variables. Plasma dehydroepiandrosterone was higher in smokers than non-smokers (2.86 vs 2.08; P<0.001) and men had significantly higher plasma dehydroepiandrosterone than women (2.74 vs 1.69; P<0.0001). In the pooled data, skin wrinkling was negatively associated with general health score (r(s)=-0.13, P<0.01) and activities of daily living score (r(s)=-0.14, P<0.05) after controlling for age, body mass index and smoking. These associations were more pronounced in women. Finally, those with the least skin wrinkling had the highest dehydroepiandrosterone level (r(s)=-0.12, P=0.06) after adjusting for age, smoking and sex. CONCLUSION: skin wrinkling in a site with limited sun exposure might be used as a marker of health status and, to some extent, biological age--particularly for women.

Aged↗
PubMed2004

TP53 mutations in squamous-cell carcinomas of the conjunctiva: evidence for UV-induced mutagenesis.

Squamous cell carcinoma of the conjunctiva is associated with sun exposure and often occurs in HIV-positive individuals. We have analysed TP53 mutations in 21 cases of squamous cell carcinoma and 22 controls with benign conjunctival lesions from a region (Uganda, Africa) with a high prevalence of heavy sun exposure and HIV infection. TP53 mutations were detected in 11 cases (52%) and 3 controls (14%). Seven of the mutations (6 in cases and 1 in controls) were CC-->TT transitions, a molecular signature of mutagenesis by solar UV rays. A similar prevalence (56%) of TP53 mutations was found in 18 squamous cell carcinoma cases positive for epidermodysplasia verruciformis human papillomavirus types. The prevalence of CC-->TT transitions reported here is the highest observed in any cancer type and matches that of skin cancers in subjects with xeroderma pigmentosum, an inherited disease with hypersensitivity to UV damage. These results confirm at the molecular level the causal role of solar UV rays in the aetiology of squamous cell carcinoma of the conjunctiva and suggest that infection with epidermodysplasia verruciformis types of human papillomavirus may act as a cofactor to increase the sensitivity of conjunctiva cells to UV-induced mutagenesis.

Adult↗
PubMed1996

Skin cancer in a subtropical Australian population: incidence and lack of association with occupation. The Nambour Study Group.

Because it is not possible to monitor skin cancer accurately using routine methods, special surveys have been undertaken in Nambour, a typical subtropical community in Queensland, Australia. Estimates of incidence reported here are based on skin cancers medically treated between 1985 and 1992 and new cases diagnosed by dermatologists in two examination clinics in 1986 and 1992. Among men and women aged 18-69 years in 1986, age-adjusted incidence rates of basal cell carcinoma were 2,074 and 1,579 per 100,000 per year, respectively-the highest incidence rates of a specific cancer ever reported. Squamous cell carcinoma occurred at half the rate of basal cell carcinoma among men and at about one third the rate among women. Although as expected, fair skin, a history of repeated sunburns, and nonmalignant solar skin damage diagnosed by dermatologists were strongly associated with both types of skin cancer, outdoor occupation was not. Significant self-selection was observed among outdoor workers, whereby people with fair or medium complexions and a tendency to sunburn were systematically underrepresented among those in long-term outdoor occupations although they accounted for more than 80 percent of the community study sample. The mitigating effect of this selection bias may partly explain the paradox of the lack of quantitative evidence of a causal link between sun exposure and skin cancer in humans.

Adult↗
PubMed2002

Mutations in the TP53 gene in human malignant melanomas derived from sun-exposed skin and unexposed mucosal membranes.

Mutations in the p53 tumour suppressor gene ( ) have been linked to several types of cancer. We therefore investigated whether such mutations occur in malignant melanomas and, if so, whether they are linked to ultraviolet (sun) light exposure. For the first time, mutations in mucosal membranes and adjacent tissues shielded from sunlight were compared with those in cutaneous melanomas from sun-exposed skin. Archival tissues were obtained from 35 patients with a primary melanoma taken from unexposed mucosal areas and from 34 patients with a primary melanoma located in chronically sun-exposed head and neck skin. was characterized by means of polymerase chain reaction amplification and single-strand conformation polymorphism assay followed by nucleotide sequencing. The results showed that 17.6% of the primary cutaneous and 28.6% of the primary mucosal melanomas had point mutations in. Among the cutaneous melanomas, one showed three mutations in exon 7, and one had two mutations in exon 5; the mutation was in the same allele in both cases. One mucosal melanoma had two mutations in exon 7, both in the same allele, and another had two mutations, one in exon 7 and one in intron 6, both in the same allele. C<--T mutations at dipyrimidine sites, considered fingerprints for ultraviolet light-induced mutations, were about equally distributed among patients with melanomas from chronically sun-exposed areas (six out of nine; 67%) and those with melanomas from unexposed mucosal areas and adjacent skin (eight out of 14; 57%). Our data, demonstrating the presence of such mutations even in melanomas from mucosal membranes, clearly suggest that factors other than, or additional to, ultraviolet radiation are operational in the induction of mutations in melanomas.

Adult↗
PubMed2005

Imaging sunspots using helioseismic methods.

The origin of sunspots is one of the most fundamental and yet poorly understood areas in solar physics. Imaging local anomalous features in the solar interior offers a direct way to unravel the underlying physical processes of sunspots and the mechanisms behind their formation. The advent of local helioseismology in the last few years has, for the first time, made it possible to image local internal solar structures. High-resolution satellite and ground telescope data which reveal the details of the vibrations of the visible solar surface are essential in the development of local helioseismology. We are now in a position to transfer the seismic methods that have traditionally been used to study the Earth's interior to solar investigations. This interdisciplinary approach to developing seismic imaging techniques is opening up new ways of understanding the flow and other structural characteristics beneath sunspots. In this article, I review recent progress in the imaging of sunspots and the surrounding solar active regions. By highlighting the strengths of seismic methods and the challenges facing local helioseismology, I discuss some of the new research directions and possibilities that have arisen from this novel type of seismic imaging.

Astronomy↗
PubMed2005

An overview of coronal seismology.

The idea of exploiting observed oscillations as a diagnostic tool for determining the physical conditions of the coronal plasma was first suggested several decades ago (Roberts et al. 1984 Astrophys. J. 279, 857). Until recently, the application of this idea has been very limited by a lack of high-quality observations of coronal oscillations. However, during the last few years, this situation has changed dramatically, especially due to space-based observations by the Solar and Heliospheric Observatory and the Transition Region and Coronal Explorer and waves and oscillations have now been observed in a wide variety of solar structures, such as coronal loops, polar plumes and prominences. This paper will briefly summarize MHD wave theory, which forms the basis for coronal seismology, as well as present an overview of the variety of recently observed waves and oscillations in the solar corona. The present state of coronal seismology will also be discussed. Currently, the uncertainty associated with the obtained parameters is still considerable and, hence, the results require a cautious interpretation. However, these examples do show that coronal seismology is rapidly being transformed from a theoretical possibility to a viable technique.

Computer Simulation↗
PubMed1991

All the observed universe has contributed to life.

This paper presents evidence that virtually all electrons and nuclei of the atoms that are or have been part of living matter on Earth came from almost all stars in our and nearby galaxies and even from all other galaxies in the Universe that have produced observed high-energy gamma rays. However, a standard 70 kg human is always making about 7 3He, 600 40Ca, and 3000 14N nuclei every second by radioactive decay of 3H, 40K, and 14C, respectively.

Animals↗
PubMed2006

Palaeoclimates: the first two billion years.

Earth's climate during the Archaean remains highly uncertain, as the relevant geologic evidence is sparse and occasionally contradictory. Oxygen isotopes in cherts suggest that between 3.5 and 3.2 Gyr ago (Ga) the Archaean climate was hot (55-85 degrees C); however, the fact that these cherts have experienced only a modest amount of weathering suggests that the climate was temperate, as today. The presence of diamictites in the Pongola Supergroup and the Witwatersrand Basin of South Africa suggests that by 2.9 Ga the climate was glacial. The Late Archaean was relatively warm; then glaciation (possibly of global extent) reappeared in the Early Palaeoproterozoic, around 2.3-2.4 Ga. Fitting these climatic constraints with a model requires high concentrations of atmospheric CO2 or CH4, or both. Solar luminosity was 20-25% lower than today, so elevated greenhouse gas concentrations were needed just to keep the mean surface temperature above freezing. A rise in O2 at approximately 2.4 Ga, and a concomitant decrease in CH4, provides a natural explanation for the Palaeoproterozoic glaciations. The Mid-Archaean glaciations may have been caused by a drawdown in H2 and CH4 caused by the origin of bacterial sulphate reduction. More work is needed to test this latter hypothesis.

Carbon Dioxide↗
PubMed2000

Linking solar and long baseline terrestrial neutrino experiments.

We show that, in the framework of three light neutrino species with hierarchical masses and assuming no fine tuning between the entries of the neutrino mass matrix, one can use the solar neutrino data to obtain information on the element U(e3) of the lepton mixing matrix. Conversely, a measurement of U(e3) in atmospheric or long baseline accelerator or reactor neutrino experiments would help discriminate between possible oscillation solutions of the solar neutrino problem.

Earth, Planet↗
PubMed2000

Creation of magnetic energy in the solar atmosphere.

We show that a magnetic flux tube can grow in strength and size provided the temperature increases outward at the edge of the tube where the axial magnetic field declines to its external value. The radius of the tube increases at a rate determined by our theory. It is the coincidence of temperature and magnetic field gradients of opposite sign that generates the new field, and if this is lost the tube ceases to grow. The phenomenon is illustrated by adopting plausible distributions for the temperature and magnetic field strength, which yield an expression for the rate of growth of the magnetic field magnitude B. The mechanism provides a possible explanation of the fibrous nature of solar magnetic fields.

Astronomy↗
PubMed2001

Black hole thermodynamics from calculations in strongly coupled gauge theory.

We develop an approximation scheme for the quantum mechanics of N D0-branes at finite temperature in the 't Hooft large- N limit. The entropy of the quantum mechanics calculated using this approximation agrees well with the Bekenstein-Hawking entropy of a ten-dimensional nonextremal black hole with 0-brane charge. This result is in accordance with the duality conjectured by Itzhaki, Maldacena, Sonnenschein, and Yankielowicz [Phys. Rev. D 58, 046004 (1998)]. Our approximation scheme provides a model for the density matrix which describes a black hole in the strongly coupled quantum mechanics.

Astronomical Phenomena↗