497 research outputs found
The geochemical carbon cycle and the uptake of fossil fuel CO2
Atmospheric carbon dioxide levels are controlled over long time scales by the transfer of carbon between the atmosphere, oceans, and sedimentary rocksâ a process referred to as the CO2 geochemical cycle. Carbon dioxide is injected into the atmosphereâocean system by volcanism; it is removed by the weathering of silicate rocks on the continents followed by the deposition of carbonate minerals on the sea floor. Humans are currently perturbing the natural carbon cycle by burning fossil fuels and deforesting the tropics, both of which add CO2 to the atmosphere. The effects of human activities on future atmospheric CO2 levels can be estimated by including anthropogenic emissions in a model of the longâterm carbon cycle. The model predicts that CO2 concentrtions could increase by a factor of six or more during the next few centuries if we consume all of the available fossil fuels. Preserving existing forests and/or reforesting parts of the planet could mitigate the CO2 increase to some extent, but cannot be depended on to make a significant difference. Because the removal processes for atomspheric CO2 are slow, the maximum CO2 level reached is relatively insensitive to the fossil fuel burning rate unless the burning rate is many times smaller than its present value. The model also predicts that hundreds of thousand of years could pass before atmospheric CO2 returns to its original preindustral level. Implications of these results for future energy and land use policies are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87506/2/175_1.pd
Increased insolation threshold for runaway greenhouse processes on Earth like planets
Because the solar luminosity increases over geological timescales, Earth
climate is expected to warm, increasing water evaporation which, in turn,
enhances the atmospheric greenhouse effect. Above a certain critical
insolation, this destabilizing greenhouse feedback can "runaway" until all the
oceans are evaporated. Through increases in stratospheric humidity, warming may
also cause oceans to escape to space before the runaway greenhouse occurs. The
critical insolation thresholds for these processes, however, remain uncertain
because they have so far been evaluated with unidimensional models that cannot
account for the dynamical and cloud feedback effects that are key stabilizing
features of Earth's climate. Here we use a 3D global climate model to show that
the threshold for the runaway greenhouse is about 375 W/m, significantly
higher than previously thought. Our model is specifically developed to quantify
the climate response of Earth-like planets to increased insolation in hot and
extremely moist atmospheres. In contrast with previous studies, we find that
clouds have a destabilizing feedback on the long term warming. However,
subsident, unsaturated regions created by the Hadley circulation have a
stabilizing effect that is strong enough to defer the runaway greenhouse limit
to higher insolation than inferred from 1D models. Furthermore, because of
wavelength-dependent radiative effects, the stratosphere remains cold and dry
enough to hamper atmospheric water escape, even at large fluxes. This has
strong implications for Venus early water history and extends the size of the
habitable zone around other stars.Comment: Published in Nature. Online publication date: December 12, 2013.
Accepted version before journal editing and with Supplementary Informatio
Photochemistry of methane in the Earth's early atmosphere
A detailed model is presented of methane photochemistry in the primitive terrestrial atmosphere along with speculation about its interpretation. Steady-state CH4 mixing ratios of 10-6-10-4 could have been maintained by a methane source of about 1011 cm-2 s-1, which is comparable to the modern biogenic methane production rate. In the absence of a source, methane would have disappeared in 4 years, being either oxidized, or polymerized into more complex hydrocarbons. The source strength needed to maintain a steady CH4 mixing ratio and the degree to which methane could have polymerized to form higher hydrocarbons depend upon the amount of CO2 present in the early atmosphere. The dependence on H2 is much weaker. Infrared absorption by methane, and especially by other hydrocarbon species, may have supplemented the greenhouse warming due to carbon dioxide. A radiative model is needed to establish this effect quantitatively. The destruction of the methane greenhouse early in the Proterozoic may have triggered the Huronian glaciation.These calculations also suggest that atmospheres rich in both CO2 and CH4 may be photochemically unstable with respect to conversion to CO.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25194/1/0000633.pd
Water permeation through stratum corneum lipid bilayers from atomistic simulations
Stratum corneum, the outermost layer of skin, consists of keratin filled
rigid non-viable corneocyte cells surrounded by multilayers of lipids. The
lipid layer is responsible for the barrier properties of the skin. We calculate
the excess chemical potential and diffusivity of water as a function of depth
in lipid bilayers with compositions representative of the stratum corneum using
atomistic molecular dynamics simulations. The maximum in the excess free energy
of water inside the lipid bilayers is found to be twice that of water in
phospholipid bilayers at the same temperature. Permeability, which decreases
exponentially with the free energy barrier, is reduced by several orders of
magnitude as compared to with phospholipid bilayers. The average time it takes
for a water molecule to cross the bilayer is calculated by solving the
Smoluchowski equation in presence of the free energy barrier. For a bilayer
composed of a 2:2:1 molar ratio of ceramide NS 24:0, cholesterol and free fatty
acid 24:0 at 300K, we estimate the permeability P=3.7e-9 cm/s and the average
crossing time \tau_{av}=0.69 ms. The permeability is about 30 times smaller
than existing experimental results on mammalian skin sections.Comment: latex, 8 pages, 6 figure
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Separation of trivalent lanthanides and actinides by solvent extraction without aqueous complexing agents
A method of separating the trivalent actinides, mainly Am and Cm, from trivalent lanthanides is presented. This method embodies the sequential use of two different solvent extractants; the first extractant would remove the heavy lanthanides from the lighter lanthanides and Am--Cm, while the second would extract Am--Cm in preference to the lighter lanthanides. In this scheme, no additional complexing agents are required. Thus, waste disposal and corrosion problems are minimized. Overall separation factors for Am--Cm from lanthanide fission products in reactor wastes may be as high as several thousand. (auth
Kepler-47: A Transiting Circumbinary Multi-Planet System
We report the detection of Kepler-47, a system consisting of two planets
orbiting around an eclipsing pair of stars. The inner and outer planets have
radii 3.0 and 4.6 times that of the Earth, respectively. The binary star
consists of a Sun-like star and a companion roughly one-third its size,
orbiting each other every 7.45 days. With an orbital period of 49.5 days,
eighteen transits of the inner planet have been observed, allowing a detailed
characterization of its orbit and those of the stars. The outer planet's
orbital period is 303.2 days, and although the planet is not Earth-like, it
resides within the classical "habitable zone", where liquid water could exist
on an Earth-like planet. With its two known planets, Kepler-47 establishes that
close binary stars can host complete planetary systems.Comment: To appear on Science Express August 28, 11 pages, 3 figures, one
table (main text), 56 pages, 28 figures, 10 table
P/2010A2 LINEAR - I: An impact in the Asteroid Main Belt
Comet P/2010A2 LINEAR is a good candidate for membership with the Main Belt
Comet family. It was observed with several telescopes (ESO NTT, La Silla;
Gemini North, Mauna Kea; UH 2.2m, Mauna Kea) from 14 Jan. until 19 Feb. 2010 in
order to characterize and monitor it and its very unusual dust tail, which
appears almost fully detached from the nucleus; the head of the tail includes
two narrow arcs forming a cross. The immediate surroundings of the nucleus were
found dust-free, which allowed an estimate of the nucleus radius of 80-90m. A
model of the thermal evolution indicates that such a small nucleus could not
maintain any ice content for more than a few million years on its current
orbit, ruling out ice sublimation dust ejection mechanism. Rotational spin-up
and electrostatic dust levitations were also rejected, leaving an impact with a
smaller body as the favoured hypothesis, and ruling out the cometary nature of
the object.
The impact is further supported by the analysis of the tail structure.
Finston-Probstein dynamical dust modelling indicates the tail was produced by a
single burst of dust emission. More advanced models, independently indicate
that this burst populated a hollow cone with a half-opening angle alpha~40degr
and with an ejection velocity v_max ~ 0.2m/s, where the small dust grains fill
the observed tail, while the arcs are foreshortened sections of the burst cone.
The dust grains in the tail are measured to have radii between a=1-20mm, with a
differential size distribution proportional to a^(-3.44 +/- 0.08). The dust
contained in the tail is estimated to at least 8x10^8kg, which would form a
sphere of 40m radius. Analysing these results in the framework of crater
physics, we conclude that a gravity-controlled crater would have grown up to
~100m radius, i.e. comparable to the size of the body. The non-disruption of
the body suggest this was an oblique impact.Comment: 15 pages, 11 figures, in pres
An extrasolar planetary system with three Neptune-mass planets
Over the past two years, the search for low-mass extrasolar planets has led
to the detection of seven so-called 'hot Neptunes' or 'super-Earths' around
Sun-like stars. These planets have masses 5-20 times larger than the Earth and
are mainly found on close-in orbits with periods of 2-15 days. Here we report a
system of three Neptune-mass planets with periods of 8.67, 31.6 and 197 days,
orbiting the nearby star HD 69830. This star was already known to show an
infrared excess possibly caused by an asteroid belt within 1 AU (the Sun-Earth
distance). Simulations show that the system is in a dynamically stable
configuration. Theoretical calculations favour a mainly rocky composition for
both inner planets, while the outer planet probably has a significant gaseous
envelope surrounding its rocky/icy core; the outer planet orbits within the
habitable zone of this star.Comment: 17 pages, 3 figures, preprint of the paper published in Nature on May
18, 200
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