126 research outputs found
Mesospheric vertical thermal structure and winds on Venus from HHSMT CO spectral-line observations
We report vertical thermal structure and wind velocities in the Venusian
mesosphere retrieved from carbon monoxide (12CO J=2-1 and 13CO J=2-1) spectral
line observations obtained with the Heinrich Hertz Submillimeter Telescope
(HHSMT). We observed the mesosphere of Venus from two days after the second
Messenger flyby of Venus (on June 5 2007 at 23:10 UTC) during five days.
Day-to-day and day-to-night temperature variations and short-term fluctuations
of the mesospheric zonal flow were evident in our data. The extensive layer of
warm air detected recently by SPICAV at 90 - to 100 km altitude is also
detected in the temperature profiles reported here. These data were part of a
coordinated ground-based Venus observational campaign in support of the ESA
Venus Express mission. Furthermore, this study attempts to cross-calibrate
space- and ground-based observations, to constrain radiative transfer and
retrieval algorithms for planetary atmospheres, and to contribute to a more
thorough understanding of the global patterns of circulation of the Venusian
atmosphere.Comment: 35 pages, 18 figures. Shortcut URL to this page:
http://www.sciencedirect.com/science/journal/0032063
The Nature and Frequency of the Gas Outbursts in Comet 67P/Churyumov-Gerasimenko observed by the Alice Far-ultraviolet Spectrograph on Rosetta
Alice is a far-ultraviolet imaging spectrograph onboard Rosetta that, amongst
multiple objectives, is designed to observe emissions from various atomic and
molecular species from within the coma of comet 67P/Churyumov-Gerasimenko. The
initial observations, made following orbit insertion in August 2014, showed
emissions of atomic hydrogen and oxygen spatially localized close to the
nucleus and attributed to photoelectron impact dissociation of H2O vapor.
Weaker emissions from atomic carbon were subsequently detected and also
attributed to electron impact dissociation, of CO2, the relative H I and C I
line intensities reflecting the variation of CO2 to H2O column abundance along
the line-of-sight through the coma. Beginning in mid-April 2015, Alice
sporadically observed a number of outbursts above the sunward limb
characterized by sudden increases in the atomic emissions, particularly the
semi-forbidden O I 1356 multiplet, over a period of 10-30 minutes, without a
corresponding enhancement in long wavelength solar reflected light
characteristic of dust production. A large increase in the brightness ratio O I
1356/O I 1304 suggests O2 as the principal source of the additional gas. These
outbursts do not correlate with any of the visible images of outbursts taken
with either OSIRIS or the navigation camera. Beginning in June 2015 the nature
of the Alice spectrum changed considerably with CO Fourth Positive band
emission observed continuously, varying with pointing but otherwise fairly
constant in time. However, CO does not appear to be a major driver of any of
the observed outbursts.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
Far-ultraviolet Spectroscopy of Venus and Mars at 4 A Resolution with the Hopkins Ultraviolet Telescope on Astro-2
Far-ultraviolet spectra of Venus and Mars in the range 820-1840 A at 4 A
resolution were obtained on 13 and 12 March 1995, respectively, by the Hopkins
Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the
Space Shuttle Endeavour. Longward of 1250 A, the spectra of both planets are
dominated by emission of the CO Fourth Positive band system and strong OI and
CI multiplets. In addition, CO Hopfield-Birge bands, B - X (0,0) at 1151 A and
C - X (0,0) at 1088 A, are detected for the first time, and there is a weak
indication of the E - X (0,0) band at 1076 A in the spectrum of Venus. The B -
X band is blended with emission from OI 1152. Modeling the relative intensities
of these bands suggests that resonance fluorescence of CO is the dominant
source of the emission, as it is for the Fourth Positive system. Shortward of
Lyman-alpha, other emission features detected include OII 834, OI lambda 989,
HI Lyman-beta, and NI 1134 and 1200. For Venus, the derived disk brightnesses
of the OI, OII, and HI features are about one-half of those reported by Hord et
al. (1991) from Galileo EUV measurements made in February 1990. This result is
consistent with the expected variation from solar maximum to solar minimum. The
ArI 1048, 1066 doublet is detected only in the spectrum of Mars and the derived
mixing ratio of Ar is of the order of 2%, consistent with previous
determinations.Comment: 8 pages, 5 figures, accepted for publication in ApJ, July 20, 200
Rosetta-Alice Observations of Exospheric Hydrogen and Oxygen on Mars
The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter
with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on
25 February 2007, closest approach being at 01:54UT. The Alice instrument on
board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for
in situ cometary spectroscopy in the 750-2000 A spectral band, was used to
study the daytime Mars upper atmosphere including emissions from exospheric
hydrogen and oxygen. Offset pointing, obtained five hours before closest
approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta
emissions from exospheric hydrogen out beyond 30,000 km from the planet's
center. These data are fit with a Chamberlain exospheric model from which we
derive the hydrogen density at the 200 km exobase and the H escape flux. The
results are comparable to those found from the the Ultraviolet Spectrometer
experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen
emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb
during limb scans shortly after closest approach. However, the derived oxygen
scale height is not consistent with recent models of oxygen escape based on the
production of suprathermal oxygen atoms by the dissociative recombination of
O2+.Comment: 17 pages, 8 figures, accepted for publication in Icaru
An Upper Limit on Gas Production from 3200 Phaethon
Asteroid 3200 Phaethon resembles a comet in some ways, including a
highly-eccentric orbit (e=0.89) and a strong associated meteor shower (the
Geminids). Yet this object has never been observed to exhibit any cometary
activity, i.e., gas production. We observed 3200 Phaethon with the Caltech
Submillimeter Observatory on two occasions, once while it was near its closest
approach to Earth as it neared perihelion, and another while it was further
from Earth post-perihelion. Observations of the J=2-1 and J=3-2 rotational
transitions of 12CO, typically strong lines in comets and indicative of gas
production, yielded no detection. Upper limits on the 12CO production of 1.8e28
molecules/s and 7.6e28 molecules/s for Phaethon were determined on these two
occasions
Ultraviolet Spectroscopy of Comet 9P/Tempel 1 with Alice/Rosetta during the Deep Impact Encounter
We report on spectroscopic observations of periodic comet 9P/Tempel 1 by the
Alice ultraviolet spectrograph on the Rosetta spacecraft in conjunction with
NASA's Deep Impact mission. Our objectives were to measure an increase in
atomic and molecular emissions produced by the excavation of volatile
sub-surface material. We unambiguously detected atomic oxygen emission from the
quiescent coma but no enhancement at the 10% (1-sigma) level following the
impact. We derive a quiescent water production rate of 9 x 10^27 molecules per
second with an estimated uncertainty of 30%. Our upper limits to the volatiles
produced by the impact are consistent with other estimates.Comment: 11 pages, 4 postscript figures. Accepted for publication in Icarus
special issue on Deep Impac
A multi-site campaign to measure solar-like oscillations in Procyon. II. Mode frequencies
We have analyzed data from a multi-site campaign to observe oscillations in
the F5 star Procyon. The data consist of high-precision velocities that we
obtained over more than three weeks with eleven telescopes. A new method for
adjusting the data weights allows us to suppress the sidelobes in the power
spectrum. Stacking the power spectrum in a so-called echelle diagram reveals
two clear ridges that we identify with even and odd values of the angular
degree (l=0 and 2, and l=1 and 3, respectively). We interpret a strong, narrow
peak at 446 muHz that lies close to the l=1 ridge as a mode with mixed
character. We show that the frequencies of the ridge centroids and their
separations are useful diagnostics for asteroseismology. In particular,
variations in the large separation appear to indicate a glitch in the
sound-speed profile at an acoustic depth of about 1000 s. We list frequencies
for 55 modes extracted from the data spanning 20 radial orders, a range
comparable to the best solar data, which will provide valuable constraints for
theoretical models. A preliminary comparison with published models shows that
the offset between observed and calculated frequencies for the radial modes is
very different for Procyon than for the Sun and other cool stars. We find the
mean lifetime of the modes in Procyon to be 1.29 +0.55/-0.49 days, which is
significantly shorter than the 2-4 days seen in the Sun.Comment: accepted for publication in Ap
Martian Atmospheric Hydrogen and Deuterium: Seasonal Changes and Paradigm for Escape to Space
Mars\u27 water history is fundamental to understanding Earth-like planet evolution. Water escapes to space as atoms, and hydrogen atoms escape faster than deuterium giving an increase in the residual D/H ratio. The present ratio reflects the total water Mars has lost. Observations with the Mars Atmosphere and Volatile Evolution (MAVEN) and Hubble Space Telescope (HST) spacecraft provide atomic densities and escape rates for H and D. Large increases near perihelion observed each martian year are consistent with a strong upwelling of water vapor. Short-term changes require processes in addition to thermal escape, likely from atmospheric dynamics and superthermal atoms. Including escape from hot atoms, both H and D escape rapidly, and the escape fluxes are limited by resupply from the lower atmosphere. In this paradigm for the escape of water, the D/H ratio of the escaping atoms and the enhancement in water are determined by upwelling water vapor and atmospheric dynamics rather than by the specific details of atomic escape
The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets
This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics
Epigenetic and Transcriptional Variability Shape Phenotypic Plasticity.
Epigenetic and transcriptional variability contribute to the vast diversity of cellular and organismal phenotypes and are key in human health and disease. In this review, we describe different types, sources, and determinants of epigenetic and transcriptional variability, enabling cells and organisms to adapt and evolve to a changing environment. We highlight the latest research and hypotheses on how chromatin structure and the epigenome influence gene expression variability. Further, we provide an overview of challenges in the analysis of biological variability. An improved understanding of the molecular mechanisms underlying epigenetic and transcriptional variability, at both the intra- and inter-individual level, provides great opportunity for disease prevention, better therapeutic approaches, and personalized medicine
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