13,838 research outputs found
The Origin of Life
The origin of life is in a sense a genetic problem, for, as H. J. Muller pointed out many years ago, the essential attribute that identifies living matter is its capacity to replicate itself and its variants (1). Because this uniquely biological property has its physical basis in proteins and nucleic acids, the goal of modern work on the origin of life is to discover the manner of origin of these polymers and of the interactions between them that constitute the genetic mechanism. In attempting to review this subject in a limited space, we cannot undertake an exhaustive treatment. Rather, we summarize work published principally since 1970 in the following areas, with emphasis on those aspects that are of greatest current interest: 1. precambrian paleontology, 2. chemical evolution of genetically important monomers, 3. prebiotic dehydration-condensation reactions, 4. organic compounds in meteorites and interstellar space, and 5. biological exploration of the planets.
A large number of review articles (2-5), critical and theoretical discussions (6-8), books (9-16), and conference proceedings (17-21) dealing with the origin of have appeared in recent years. In addition, a new serial, the Journal of Molecular Evolution, publishing papers on this and related subjects, appeared in 1971; the journal Space Life Sciences has been renamed "Origins of Life," and a society, the International Society for the Study of the Origin of Life, was recently founded
Models of Saturn's Interior Constructed with Accelerated Concentric Maclaurin Spheroid Method
The Cassini spacecraft's Grand Finale orbits provided a unique opportunity to
probe Saturn's gravity field and interior structure. Doppler measurements
yielded unexpectedly large values for the gravity harmonics J_6, J_8, and J_10
that cannot be matched with planetary interior models that assume uniform
rotation. Instead we present a suite of models that assume the planet's
interior rotates on cylinders, which allows us to match all the observed even
gravity harmonics. For every interior model, the gravity field is calculated
self-consistently with high precision using the Concentric Maclaurin Spheroid
(CMS) method. We present an acceleration technique for this method, which
drastically reduces the computational cost, allows us to efficiently optimize
model parameters, map out allowed parameter regions with Monte Carlo sampling,
and increases the precision of the calculated J_2n gravity harmonics to match
the error bars of the observations, which would be difficult without
acceleration. Based on our models, Saturn is predicted to have a dense central
core of 15-18 Earth masses and an additional 1.5-5 Earth masses of heavy
elements in the envelope. Finally, we vary the rotation period in the planet's
deep interior and determine the resulting oblateness, which we compare with the
value from radio occultation measurements by the Voyager spacecraft. We predict
a rotation period of 10:33:34 h +- 55s, which is in agreement with recent
estimates derived from ring seismology.Comment: 12 color figures, 5 tables, Astrophysical Journal, in press (2019
Assessing the Effectiveness of Saving Incentives
In this paper, we argue that there is more to be learned from recent research on the effectiveness of targeted saving incentives than is suggested by the wide variation in empirical estimates. First, we conclude that characterizations of saving appear to stimulate moderate amounts of new saving. Second, we suggest a cost-benefit approach to ask: What is the incremental gain in capital accumulation per dollar of foregone revenue? We find that for quite conservative measures of the saving impacts of IRAs or 401(k)s, the incremental gains in capital accumulation per dollar of lost revenue are large
Effects of Helium Phase Separation on the Evolution of Extrasolar Giant Planets
We build on recent new evolutionary models of Jupiter and Saturn and here
extend our calculations to investigate the evolution of extrasolar giant
planets of mass 0.15 to 3.0 M_J. Our inhomogeneous thermal history models show
that the possible phase separation of helium from liquid metallic hydrogen in
the deep interiors of these planets can lead to luminosities ~2 times greater
than have been predicted by homogeneous models. For our chosen phase diagram
this phase separation will begin to affect the planets' evolution at ~700 Myr
for a 0.15 M_J object and ~10 Gyr for a 3.0 M_J object. We show how phase
separation affects the luminosity, effective temperature, radii, and
atmospheric helium mass fraction as a function of age for planets of various
masses, with and without heavy element cores, and with and without the effect
of modest stellar irradiation. This phase separation process will likely not
affect giant planets within a few AU of their parent star, as these planets
will cool to their equilibrium temperatures, determined by stellar heating,
before the onset of phase separation. We discuss the detectability of these
objects and the likelihood that the energy provided by helium phase separation
can change the timescales for formation and settling of ammonia clouds by
several Gyr. We discuss how correctly incorporating stellar irradiation into
giant planet atmosphere and albedo modeling may lead to a consistent
evolutionary history for Jupiter and Saturn.Comment: 22 pages, including 14 figures. Accepted to the Astrophysical Journa
Sequence composition and environment effects on residue fluctuations in protein structures
The spectrum and scale of fluctuations in protein structures affect the range
of cell phenomena, including stability of protein structures or their
fragments, allosteric transitions and energy transfer. The study presents a
statistical-thermodynamic analysis of relationship between the sequence
composition and the distribution of residue fluctuations in protein-protein
complexes. A one-node-per residue elastic network model accounting for the
nonhomogeneous protein mass distribution and the inter-atomic interactions
through the renormalized inter-residue potential is developed. Two factors, a
protein mass distribution and a residue environment, were found to determine
the scale of residue fluctuations. Surface residues undergo larger fluctuations
than core residues, showing agreement with experimental observations. Ranking
residues over the normalized scale of fluctuations yields a distinct
classification of amino acids into three groups. The structural instability in
proteins possibly relates to the high content of the highly fluctuating
residues and a deficiency of the weakly fluctuating residues in irregular
secondary structure elements (loops), chameleon sequences and disordered
proteins. Strong correlation between residue fluctuations and the sequence
composition of protein loops supports this hypothesis. Comparing fluctuations
of binding site residues (interface residues) with other surface residues shows
that, on average, the interface is more rigid than the rest of the protein
surface and Gly, Ala, Ser, Cys, Leu and Trp have a propensity to form more
stable docking patches on the interface. The findings have broad implications
for understanding mechanisms of protein association and stability of protein
structures.Comment: 8 pages, 4 figure
Recreational Food Service: A Growing Employment Option
The recreational food service industry represents a wealth of potential job opportunities for graduates of hospitality management degree programs. Most hospitality management curricula are saturated with core courses and basic hospitality education course work, leaving little room for additional components. Recreational food service, however, could be easily integrated into an existing pro- gram and made available for students interested in this growing employment option. The author presents one option as a model curriculum approach
Photocatalytic production of organic compounds from CO and H2O in a simulated Martian atmosphere
[14C]CO2 and [14C]organic compounds are formed when a mixture of [14C]CO and water vapor diluted in [12C]CO2 or N2 is irradiated with ultraviolet light in the presence of soil or pulverized vycor substratum. The [14C]CO2 is recoverable from the gas phase, the [14C]organic products from the substratum. Three organic products have been tentatively identified as formaldehyde, acetaldehyde, and glycolic acid. The relative yields of [14C]CO2 and [14C]organics are wavelength- and surface-dependent. Conversion of CO to CO2 occurs primarily at wavelengths shorter than 2000 angstrom, apparently involves the photolysis of water, and is inhibited by increasing amounts of vycor substratum. Organic formation occurs over a broad spectral range below 3000 angstrom and increases with increasing amounts of substratum. It is suggested that organic synthesis results from adsorption of CO and H2O on surfaces, with excitation of one or both molecules occurring at wavelengths longer than those absorbed by the free gases. This process may occur on Mars and may have been important on the primitive earth
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