1,579 research outputs found
Origin and thermal evolution of Mars
The thermal evolution of Mars is governed by subsolidus mantle convection beneath a thick lithosphere. Models of the interior evolution are developed by parameterizing mantle convective heat transport in terms of mantle viscosity, the superadiabatic temperature rise across the mantle, and mantle heat production. Geological, geophysical, and geochemical observations of the compositon and structure of the interior and of the timing of major events in Martian evolution are used to constrain the model computations. Such evolutionary events include global differentiation, atmospheric outgassing, and the formation of the hemispherical dichotomy and Tharsis. Numerical calculations of fully three-dimensional, spherical convection in a shell the size of the Martian mantle are performed to explore plausible patterns of Martian mantel convection and to relate convective features, such as plumes, to surface features, such as Tharsis. The results from the model calculations are presented
Doppler Effects from Bending of Light Rays in Curved Space-Times
We study Doppler effects in curved space-time, i.e. the frequency shifts
induced on electromagnetic signals propagating in the gravitational field. In
particular, we focus on the frequency shift due to the bending of light rays in
weak gravitational fields. We consider, using the PPN formalism, the
gravitational field of an axially symmetric distribution of mass. The zeroth
order, i.e. the sphere, is studied then passing to the contribution of the
quadrupole moment, and finally to the case of a rotating source. We give
numerical estimates for situations of physical interest, and by a very
preliminary analysis, we argue that analyzing the Doppler effect could lead, in
principle, in the foreseeable future, to the measurement of the quadrupole
moment of the giant planets of the Solar System.Comment: 16 pages, 2 EPS figures; to appear in the International Journal of
Modern Physics
The effects of childbirth on the pelvic-floor
Basically, vaginal delivery is associated with the risk of pelvic floor damage. The pelvic floor sequelae of childbirth includes anal incontinence, urinary incontinence and pelvic organ prolapse. Pathophysiology, incidence and risk factors for the development of the respective problems are reviewed. Where possible, recommendations for reducing the risk of pelvic floor damage are given
Abiotic formation of O2 and O3 in high-CO2 terrestrial atmospheres
Previous research has indicated that high amounts of ozone (O3) and oxygen
(O2) may be produced abiotically in atmospheres with high concentrations of
CO2. The abiotic production of these two gases, which are also characteristic
of photosynthetic life processes, could pose a potential "false-positive" for
remote-sensing detection of life on planets around other stars.We show here
that such false positives are unlikely on any planet that possesses abundant
liquid water, as rainout of oxidized species onto a reduced planetary surface
should ensure that atmospheric H2 concentrations remain relatively high, and
that O2 and O3 remain low. Our aim is to determine the amount of O3 and O2
formed in a high CO2 atmosphere for a habitable planet without life. We use a
photochemical model that considers hydrogen (H2) escape and a detailed hydrogen
balance to calculate the O2 and O3 formed on planets with 0.2 of CO2 around the
Sun, and 0.02, 0.2 and 2 bars of CO2 around a young Sun-like star with higher
UV radiation. The concentrations obtained by the photochemical model were used
as input in a radiative transfer model that calculated the spectra of the
modeled planets. The O3 and O2 concentrations in the simulated planets are
extremely small, and unlikely to produce a detectable signature in the spectra
of those planets. We conclude that with a balanced hydrogen budget, and for
planets with an active hydrological cycle, abiotic formation of O2 and O3 is
unlikely to create a possible false positive for life detection in either the
visible/near-infrared or mid-infrared wavelength regimes.Comment: 27 pages, 15 figures, Astronomy & Astrophysics accepte
Habitability of the Goldilocks Planet Gliese 581g: Results from Geodynamic Models
Aims: In 2010, detailed observations have been published that seem to
indicate another super-Earth planet in the system of Gliese 581 located in the
midst of the stellar climatological habitable zone. The mass of the planet,
known as Gl 581g, has been estimated to be between 3.1 and 4.3 Earth masses. In
this study, we investigate the habitability of Gl 581g based on a previously
used concept that explores its long-term possibility of photosynthetic biomass
production, which has already been used to gauge the principal possibility of
life regarding the super-Earths Gl 581c and Gl 581d. Methods: A thermal
evolution model for super-Earths is used to calculate the sources and sinks of
atmospheric carbon dioxide. The habitable zone is determined by the limits of
photosynthetic biological productivity on the planetary surface. Models with
different ratios of land / ocean coverage are pursued. Results: The maximum
time span for habitable conditions is attained for water worlds at a position
of about 0.14+/-0.015 AU, which deviates by just a few percent (depending on
the adopted stellar luminosity) from the actual position of Gl 581g, an
estimate that does however not reflect systematic uncertainties inherent in our
model. Therefore, in the framework of our model an almost perfect Goldilock
position is realized. The existence of habitability is found to critically
depend on the relative planetary continental area, lending a considerable
advantage to the possibility of life if Gl 581g's ocean coverage is relatively
high. Conclusions: Our results are a further step toward identifying the
possibility of life beyond the Solar System, especially concerning super-Earth
planets, which appear to be more abundant than previously surmised.Comment: 5 pages, 3 figures, 1 table; in pres
Channelized melt flow in downwelling mantle: Implications for 226Ra-210Pb disequilibria in arc magmas
We present the results of an analytical model of porous flow of viscous melt into a steadily dilating ‘‘channel’’ (defined as a cluster of smaller veins) in downwelling subarc mantle. The model predicts the pressure drop in the mantle wedge matrix surrounding the channel needed to drive melt flow as a function of position and time. Melt is sucked toward the dilatant region at a near-constant velocity (105 s1) until veins comprising the channel stop opening (t = t). Fluid elements that complete their journey within the time span t < t arrive at a channel. Our results make it possible to calculate the region of influence sampled by melt that surrounds the channel. This region is large compared to the model size of the channelized region driving flow. For a baseline dilation time of 1 year and channel half width of 2 m, melt can be sampled over an 80-m radius and has the opportunity to sample matrix material with potentially contrasting chemistry on geologically short timescales. Our mechanical results are consistent with a downgoing arc mantle wedge source region where melting and melt extraction by porous flow to a channel network are sufficiently rapid to preserve source-derived 238U-230Th-226Ra, and potentially also 226 Ra-210Pb, disequilibria, prior to magma ascent to the surface. Since this is the rate-determining step in the overall process, it allows the possibility that such short-lived disequilibria measured in arc rocks at the surface are derived from deep in the mantle wedge. Stresses due to partial melting do not appear capable of producing the desired sucking effect, while the order of magnitude rate of shear required to drive dilation of 107 s1 is much larger than values resulting from steady state subduction. We conclude that local deformation rates in excess of background plate tectonic rates are needed to ‘‘switch on’’ the dilatant channel network and to initiate the sucking effect
Can Life develop in the expanded habitable zones around Red Giant Stars?
We present some new ideas about the possibility of life developing around
sub-giant and red giant stars. Our study concerns the temporal evolution of the
habitable zone. The distance between the star and the habitable zone, as well
as its width, increases with time as a consequence of stellar evolution. The
habitable zone moves outward after the star leaves the main sequence, sweeping
a wider range of distances from the star until the star reaches the tip of the
asymptotic giant branch. If life could form and evolve over time intervals from
to years, then there could be habitable planets with
life around red giant stars. For a 1 M star at the first stages of
its post main-sequence evolution, the temporal transit of the habitable zone is
estimated to be of several 10 years at 2 AU and around 10 years at 9
AU. Under these circumstances life could develop at distances in the range 2-9
AU in the environment of sub-giant or giant stars and in the far distant future
in the environment of our own Solar System. After a star completes its first
ascent along the Red Giant Branch and the He flash takes place, there is an
additional stable period of quiescent He core burning during which there is
another opportunity for life to develop. For a 1 M star there is an
additional years with a stable habitable zone in the region from 7 to 22
AU. Space astronomy missions, such as proposed for the Terrestrial Planet
Finder (TPF) and Darwin should also consider the environments of sub-giants and
red giant stars as potentially interesting sites for understanding the
development of life
The habitability of super-Earths in Gliese 581
Aims: The planetary system around the M star Gliese 581 consists of a hot
Neptune (Gl 581b) and two super-Earths (Gl 581c and Gl 581d). The habitability
of this system with respect to the super-Earths is investigated following a
concept that studies the long-term possibility of photosynthetic biomass
production on a dynamically active planet. Methods: A thermal evolution model
for a super-Earth is used to calculate the sources and sinks of atmospheric
carbon dioxide. The habitable zone is determined by the limits of biological
productivity on the planetary surface. Models with different ratios of land /
ocean coverage are investigated. Results: The super-Earth Gl 581c is clearly
outside the habitable zone, since it is too close to the star. In contrast, Gl
581d is a tidally locked habitable super-Earth near the outer edge of the
habitable zone. Despite the adverse conditions on this planet, at least some
primitive forms of life may be able to exist on its surface.Therefore, Gl 581d
is an interesting target for the planned TPF/Darwin missions to search for
biomarkers in planetary atmospheres.Comment: 6 pages, 4 figures, 2 table
Late veneer and late accretion to the terrestrial planets
It is generally accepted that silicate-metal (`rocky') planet formation
relies on coagulation from a mixture of sub-Mars sized planetary embryos and
(smaller) planetesimals that dynamically emerge from the evolving circum-solar
disc in the first few million years of our Solar System. Once the planets have,
for the most part, assembled after a giant impact phase, they continue to be
bombarded by a multitude of planetesimals left over from accretion. Here we
place limits on the mass and evolution of these planetesimals based on
constraints from the highly siderophile element (HSE) budget of the Moon.
Outcomes from a combination of N-body and Monte Carlo simulations of planet
formation lead us to four key conclusions about the nature of this early epoch.
First, matching the terrestrial to lunar HSE ratio requires either that the
late veneer on Earth consisted of a single lunar-size impactor striking the
Earth before 4.45 Ga, or that it originated from the impact that created the
Moon. An added complication is that analysis of lunar samples indicates the
Moon does not preserve convincing evidence for a late veneer like Earth.
Second, the expected chondritic veneer component on Mars is 0.06 weight
percent. Third, the flux of terrestrial impactors must have been low (
<=10^(-6) M_earth/Myr) to avoid wholesale melting of Earth's crust after
4.4~Ga, and to simultaneously match the number of observed lunar basins. This
conclusion leads to an Hadean eon which is more clement than assumed
previously. Last, after the terrestrial planets had fully formed, the mass in
remnant planetesimals was ~10^(-3) M_earth, lower by at least an order of
magnitude than most previous models suggest. Our dynamically and geochemically
self-consistent scenario requires that future N-body simulations of rocky
planet formation either directly incorporate collisional grinding or rely on
pebble accretion.Comment: Accepted for publication in Earth and Planetary Science Letter
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