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PubMed2000

Cleomedes (c. 1st century AD) on the celestial illusion, atmospheric enlargement, and size-distance invariance.

Cleomedes (Kleomedes) is a little-known Greek author (c. 1st century AD) who produced what is probably the earliest extant statement of size-distance invariance. He supported the Stoic philosophy and was concerned to discredit the Epicurean position that we perceive objects as having their true size. He explained the celestial illusion (the apparent enlargement of the sun near the horizon) in two ways: partly as a refractive effect of the atmosphere similar to angular enlargement when looking into water; and partly as a linear enlargement due to increased apparent distance in a misty atmosphere. He is the earliest extant author to offer apparent distance as a clear explanation of the celestial illusion. He attributed these views to Posidonius (c. 135-50 BC). His explanations remained at the geometrical level, and he did not speculate on sensory mechanisms.

Distance Perception↗
PubMed2003

Mach bands change asymmetrically during solar eclipses.

Observations made during two partial eclipses of the Sun show that the Mach bands on shadows cast by the Sun disappear and reappear asymmetrically as an eclipse progresses. These changes can be explained as due to changes in the shape of the penumbras of shadows as the visible portion of the Sun forms crescents of different orientation.

Humans↗
PubMed2006

The galactic cosmic ray ionization rate.

The chemistry that occurs in the interstellar medium in response to cosmic ray ionization is summarized, and a review of the ionization rates that have been derived from measurements of molecular abundances is presented. The successful detection of large abundances of H(3)(+) in diffuse clouds and the recognition that dissociative recombination of H(3)(+) is fast has led to an upward revision of the derived ionization rates. In dense clouds the molecular abundances are sensitive to the depletion of carbon monoxide, atomic oxygen, nitrogen, water, and metals and the presence of large molecules and grains. Measurements of the relative abundances of deuterated species provide information about the ion removal mechanisms, but uncertainties remain. The models, both of dense and diffuse clouds, that are used to interpret the observations may be seriously inadequate. Nevertheless, it appears that the ionization rates differ in dense and diffuse clouds and in the intercloud medium.

Astronomical Phenomena↗
PubMed1999

Case study of the effects of atmospheric aerosols and regional haze on agriculture: an opportunity to enhance crop yields in China through emission controls?

The effect of atmospheric aerosols and regional haze from air pollution on the yields of rice and winter wheat grown in China is assessed. The assessment is based on estimates of aerosol optical depths over China, the effect of these optical depths on the solar irradiance reaching the earth's surface, and the response of rice and winter wheat grown in Nanjing to the change in solar irradiance. Two sets of aerosol optical depths are presented: one based on a coupled, regional climate/air quality model simulation and the other inferred from solar radiation measurements made over a 12-year period at meteorological stations in China. The model-estimated optical depths are significantly smaller than those derived from observations, perhaps because of errors in one or both sets of optical depths or because the data from the meteorological stations has been affected by local pollution. Radiative transfer calculations using the smaller, model-estimated aerosol optical depths indicate that the so-called "direct effect" of regional haze results in an approximately 5-30% reduction in the solar irradiance reaching some of China's most productive agricultural regions. Crop-response model simulations suggest an approximately 1:1 relationship between a percentage increase (decrease) in total surface solar irradiance and a percentage increase (decrease) in the yields of rice and wheat. Collectively, these calculations suggest that regional haze in China is currently depressing optimal yields of approximately 70% of the crops grown in China by at least 5-30%. Reducing the severity of regional haze in China through air pollution control could potentially result in a significant increase in crop yields and help the nation meet its growing food demands in the coming decades.

Agriculture↗
PubMed1999

Ozone depletion and UVB radiation: impact on plant DNA damage in southern South America.

The primary motivation behind the considerable effort in studying stratospheric ozone depletion is the potential for biological consequences of increased solar UVB (280-315 nm) radiation. Yet, direct links between ozone depletion and biological impacts have been established only for organisms of Antarctic waters under the influence of the ozone "hole;" no direct evidence exists that ozone-related variations in UVB affect ecosystems of temperate latitudes. Indeed, calculations based on laboratory studies with plants suggest that the biological impact of ozone depletion (measured by the formation of cyclobutane pyrimidine dimers in DNA) is likely to be less marked than previously thought, because UVA quanta (315-400 nm) may also cause significant damage, and UVA is unaffected by ozone depletion. Herein, we show that the temperate ecosystems of southern South America have been subjected to increasingly high levels of ozone depletion during the last decade. We found that in the spring of 1997, despite frequent cloud cover, the passages of the ozone hole over Tierra del Fuego (55 degrees S) caused concomitant increases in solar UV and that the enhanced ground-level UV led to significant increases in DNA damage in the native plant Gunnera magellanica. The fluctuations in solar UV explained a large proportion of the variation in DNA damage (up to 68%), particularly when the solar UV was weighted for biological effectiveness according to action spectra that assume a sharp decline in quantum efficiency with increasing wavelength from the UVB into the UVA regions of the spectrum.

Atmosphere↗
PubMed2004

Radiation hazard during a manned mission to Mars.

The radiation hazard of interplanetary flights is currently one of the major obstacles to manned missions to Mars. Highly energetic, heavy-charged particles from galactic cosmic radiation can not be sufficiently shielded in space vehicles. The long-term radiation effects to humans of these particles are largely unknown. In addition, unpredictable storms of solar particles may expose the crew to doses that lead to acute radiation effects. A manned flight to Mars currently seems to be a high-risk adventure. This article provides an overview on the radiation sources and risks for a crew on a manned flight to Mars, as currently estimated by scientists of the US National Administration for Space and Aeronautics (NASA) and the Space Studies Board (SSB) of the US National Research Council.

Earth, Planet↗
PubMed1989

Radiometry of near-earth asteroids.

We report 10 micrometers infrared photometry for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-Earth asteroids.

Astronomical Phenomena↗
PubMed1989

A survey of cyclopropenylidene (C3H2) in galactic sources.

We report the results of an initial survey in a variety of Galactic sources for cyclopropenylidene (C3H2), the first interstellar hydrocarbon ring molecule. C3H2 is found to be very widespread throughout the Galaxy. This, together with its large dipole moment and many observable transitions, makes cyclopropenylidene a promising probe for physical conditions in the interstellar medium. The ortho 1(10)-1(01) transition at 18 GHz is detected in a variety of environments, including giant molecular clouds, diffuse clouds, cold dark clouds, the spiral arm clouds in the direction of distant continuum sources, and the envelope of the carbon star IRC + 10216. The 2(20)-2(11) para line at 21.6 GHz was surveyed in many sources having strong 1(10)-1(01) emission, and, when detected, it was always seen in absorption. A more limited survey of the ortho 2(12)-1(01) transition at 85.3 GHz has been conducted. In addition, the 2(11)-2(02) line of the para species at 46.8 GHz was detected in the dark clouds TMC-1 and L134N. Maps have been made of the clouds TMC-1, L134N, W51, and Orion, confirming that the C3H2 emission is extended in these objects. The data obtained thus far suggest that C3H2 is one of the more abundant organic constituents of the dense interstellar medium.

Astronomical Phenomena↗
PubMed1992

The dust coma of Comet P/Giacobini-Zinner in the infrared.

We present 1-20 micrometers photometry of P/Giacobini-Zinner obtained at the NASA Infrared Telescope Facility, during 1985 June-September (r = 1.57-1.03 AU). A broad, weak 10 micrometers silicate emission feature was detected on August 26.6; a similar weak emission feature could have been hidden in the broadband photometry on other dates. The total scattering and emitting cross section of dust in the inner coma was similar to that in other short-period comets, but a factor of 10 (r = 1.56 AU) to 100 (r = 1.03 AU) lower than the amount of dust in Comet Halley. The thermal emission continuum can be fit with models weighted toward either small or large absorbing grains. The dust production rate near perihelion was approximately 10(5) g/s (small-grain model) to approximately 10(6) g/s (large-grain model). The corresponding dust/gas mass ratio on August 26 was approximately 0.1-1. A silicate-rich heterogeneous grain model with an excess of large particles is compatible with the observed spectrum of Giacobini-Zinner on August 26. Thus, weak or absent silicate emission does not necessarily imply an absence of silicates in the dust, although the abundance of silicate particles < or = 1 micrometer radius must have been lower than in Comet Halley.

Astronomy↗
PubMed1990

Ionized carbon in side-illuminated molecular clouds.

We have observed the 2P3/2-2P1/2 fine-structure line of C II at 1900 GHz in five sources with ionization fronts nearly perpendicular to the plane of the sky. The LSR velocity of the C II emission is generally in good agreement with that observed for molecular species such as CO. However, the observed line widths of 3-14 km s-1 are typically wider than those of molecular lines and often show rapid spatial variations in the regions observed. In some sources this may indicate that part of the C II emanates from an ionized gas component, while for others it suggests an association between C II emission and an outflow. The C II brightness temperatures are typically equal to or slighty higher than the dust temperature at all locations observed. In the optically thin approximation, C II excitation temperatures are > or = 100 K and column densities are < or = 10(18) cm-2 for all sources except M17, which has a more intense and complicated line profile with a larger spatial extent than any other source observed. The quoted column density estimates derived in the optically thin limit appear to be somewhat lower than those predicted by models of photodissociation regions for sources with a side-illuminated geometry, but uncertainties in the UV flux and geometry of the ionization front preclude a definitive comparison. The estimated column densities would be higher if the C II emission were somewhat optically thick, in which case the ionized carbon would be more in equilibrium with the dust at temperatures lower than predicted by current models.

Astronomical Phenomena↗
PubMed1990

Heating and melting of small icy satellites by the decay of 26Al.

We study the effect of radiogenic heating due to 26Al on the thermal evolution of small icy satellites. Our object is to find the extent of internal melting as a function of the satellite radius and of the initial 26Al abundance. The implicit assumption, based on observations of young stars, is that planet and satellite accretion occurred on a time scale of approximately 10(6) yr (comparable with the lifetime of 26Al). The icy satellites are modeled as spheres of initially amorphous ice, with chondritic abundances of 40K, 232Th, 235U, 238U, corresponding to an ice/dust mass ratio of 1. Evolutionary calculations are carried out, spanning 4.5 x 10(9) yr, for different combinations of the two free parameters. Heat transfer by subsolidus convection is neglected for these small satellites. Our main conclusion is that the initial 26Al abundance capable of melting icy bodies of satellite size to a significant extent is more than 10 times lower than that prevailing in the interstellar medium (or that inferred from the Ca-Al rich inclusions of the Allende meteorite, approximately 7 x 10(-7) by mass). We find, for example, that an initial 26Al mass fraction of approximately 4 x 10(-8) is sufficient for melting almost completely icy spheres with radii of 800 km, typical of the larger icy planetary satellites. We also find that for any given 26Al abundance, there is a narrow range of radii below which only marginal melting occurs and above which most of the ice melts (and refreezes later). Since extensive melting may have important consequences, such as differentiation, gas release, and volcanic activity, the effect of 26Al should be included in future studies of satellite interiors.

Aluminum↗
PubMed1990

Gas release in comet nuclei.

The evolution of a comet nucleus is investigated, taking into account the crystallization process by which the gas trapped in the ice is released to flow through the porous ice matrix. The equations of conservation of the energy and of the masses of ice and gas are solved throughout the nucleus, to obtain the evolution of the temperature, gas pressure and density profiles. A spherical nucleus composed of cold, porous amorphous ice, with 10% of CO trapped in it, serves as initial model. Several values of density (porosity) and pore size are considered. For each combination of parameters the model is evolved for 20-30 revolutions in comet P/Halley's orbit. Two aspects of the release of gas upon crystallization are analyzed and discussed: (a) the resulting continuous outward flux with high peaks at the time of crystallization, which is a cyclic process in the low-density models and sporadic in the high-density ones; (b) the internal pressures obtained down to depths of a few tens to approximately 200 m (depending on parameters), that are found to exceed the compressional strength of cometary ice. As a result, both cracking and explosions of the overlying ice layer and ejection of gas and ice/dust grains are expected to follow crystallization. They should appear as outbursts or sudden brightening of the comet. The model of 0.2 g cm-3 density is found to reproduce quite well many of the light-curve and activity characteristics of comet P/Halley.

Astronomical Phenomena↗
PubMed1991

Abundances of hydrogen sulfide in star-forming regions.

Interstellar hydrogen sulfide (H2S) and its isotopic variant H2 34S have been observed toward several star-forming regions via their 1(10)-1(01) transitions at 2 mm, using the FCRAO telescope. In sources where both isotopic species H2S and H2 34S were observed, column densities of approximately 10(16) cm-2 were measured. Column density lower limits of approximately 10(14) cm-2 for H2S were found for other sources, where only the main isotopic line was observed. The fractional abundances of H2S relative to molecular hydrogen appear to be enhanced by at least an order of magnitude relative to quiescent cloud values (approximately 10(-9)) for many of the observed sources. Such enhancement toward star-forming clouds suggests that some process involving elevated temperature aids in producing this species; this could be gas-phase reactions, grain-related processes, or both.

Astronomical Phenomena↗
PubMed1991

The interstellar C-H stretching band near 3.4 microns: constraints on the composition of organic material in the diffuse interstellar medium.

To better constrain and quantify the composition of material in the diffuse interstellar medium (ISM), absorption spectra between 3600 and 2700 cm-1 (2.8 and 3.7 microns) have been taken of objects which have widely varying amounts of visual extinction along different lines of sight. The spectra of these objects contain a broad feature centered at approximately 3300 cm-1 (approximately 3.0 microns), attributed to O-H stretching vibrations, and/or a feature near 2950 cm-1 (3.4 microns) attributed to C-H stretching vibrations. The lack of correlation between the strengths of these two bands indicates that they do not arise from the same molecular carrier. The features in the 3100-2700 cm-1 (3.2-3.7 microns) region fall into one of two classes. We attribute the first class of features to material in the diffuse ISM on the basis of the similarity between the band profiles along the very different lines of sight to Galactic center source IRS 7 and VI Cygni #12. Similar features are also reported for Galactic center source IRS 3, Ve 2-45, and AFGL 2179. Higher resolution spectra of the objects OH 01-477 and T629-5, which are known to be M stars, are dominated by a series of narrow bands in this region. These bands are largely due to OH in the stars' photospheres. While the spectra of OH 01-477 and T629-5 are likely to contain C-H absorption from diffuse ISM dust, the strength of the overlapping photospheric OH features presently prevents us from quantifying the depths of the interstellar C-H feature towards these objects. The interstellar feature for Galactic center source IRS 7 has subpeaks near 2955, 2925, and 2870 cm-1 (+/- 5 cm-1), which we attribute to C-H stretching vibrations in the -CH2- and -CH3 groups of aliphatic hydrocarbons. These band positions fall within 5 cm-1 of the values normal for saturated aliphatics. The absence of a distinct band near 2855 cm-1 suggests that the material contains small amounts of electronegative groups like -O-H or -C triple bond N. The relative strengths and profiles of the 2955 and 2925 cm-1 features towards five objects suggests an average diffuse ISM line-of-sight -CH2-/-CH3 ratio of about 2.5, indicating the presence of relatively complex organic materials. The strengths of the subpeaks at 2925 and 2955 cm-1, due to -CH2- and -CH3 groups, respectively, correlate with visual extinction, strongly suggesting that the C-H stretching band is a general feature of the material along different lines of sight in the diffuse ISM. We find average ratios of A nu/tau(2925 cm-1) = 240 +/- 40 and A nu/tau(2955 cm-1) = 310 +/- 90 for the objects we have observed. We deduce that 2.6%-35% of the cosmic carbon in the ISM is tied up in the carrier of this band with the most likely value falling near 10%. The interstellar C-H band is remarkably similar to the feature in lab residues produced by irradiating analogs of dense molecular cloud ices. This is consistent with a model in which the hydrocarbon component in the diffuse interstellar medium consists of complex hydrocarbons containing aliphatic side chains and bridges which are produced in dense molecular clouds and subsequently modified in the diffuse medium.

Astronomical Phenomena↗
PubMed1991

Spectral observations of the extreme ultraviolet background.

A grazing incidence spectrometer was designed to measure the diffuse extreme ultraviolet background. It was flown on a sounding rocket, and data were obtained on the diffuse background between 80 and 650 angstroms. These are the first spectral measurements of this background below 520 angstroms. Several emission features were detected, including interplanetary He I 584 angstroms emission and geocoronal He II 304 angstroms emission. Other features observed may originate in a hot ionized interstellar gas, but if this interpretation is correct, gas at several different temperatures is present. The strongest of these features is consistent with O V emission at 630 angstroms. This emission, when combined with upper limits for other lines, restricts the temperature of this component to 5.5 < log T < 5.7, in agreement with temperatures derived from O VI absorption studies. A power-law distribution of temperatures is consistent with this feature only if the power-law coefficient is negative, as is predicted for saturated evaporation of clouds in a hot medium. In this case, the O VI absorption data confine the filling factor of the emission of f < or = 4% and the pressure to more than 3.7 x 10(4) cm-3 K, substantially above ambient interstellar pressure. Such a pressure enhancement has been predicted for clouds undergoing saturated evaporation. Alternatively, if the O V emission covers a considerable fraction of the sky, it would be a major source of ionization. A feature centered at about 99 angstroms is well fitted by a cluster of Fe XVIII and Fe XIX lines from gas at log T = 6.6-6.8. These results are consistent with previous soft X-ray observations with low-resolution detectors. A feature found near 178 angstroms is consistent with Fe X and Fe XI emission from gas at log T = 6; this result is consistent with results from experiments employing broad-band soft X-ray detectors.

Astronomy↗