5,752 research outputs found
Water Delivery and Giant Impacts in the 'Grand Tack' Scenario
A new model for terrestrial planet formation (Hansen 2009, Walsh et al. 2011)
has explored accretion in a truncated protoplanetary disk, and found that such
a configuration is able to reproduce the distribution of mass among the planets
in the Solar System, especially the Earth/Mars mass ratio, which earlier
simulations have generally not been able to match. Walsh et al. tested a
possible mechanism to truncate the disk--a two-stage, inward-then-outward
migration of Jupiter and Saturn, as found in numerous hydrodynamical
simulations of giant planet formation. In addition to truncating the disk and
producing a more realistic Earth/Mars mass ratio, the migration of the giant
planets also populates the asteroid belt with two distinct populations of
bodies--the inner belt is filled by bodies originating inside of 3 AU, and the
outer belt is filled with bodies originating from between and beyond the giant
planets (which are hereafter referred to as `primitive' bodies).
We find here that the planets will accrete on order 1-2% of their total mass
from primitive planetesimals scattered onto planet-crossing orbits during the
formation of the planets. For an assumed value of 10% for the water mass
fraction of the primitive planetesimals, this model delivers a total amount of
water comparable to that estimated to be on the Earth today. While the radial
distribution of the planetary masses and the dynamical excitation of their
orbits are a good match to the observed system, we find that the last giant
impact is typically earlier than 20 Myr, and a substantial amount of mass is
accreted after that event. However, 5 of the 27 planets larger than half an
Earth mass formed in all simulations do experience large late impacts and
subsequent accretion consistent with the dating of the Moon-forming impact and
the estimated amount of mass accreted by Earth following that event
A low mass for Mars from Jupiter's early gas-driven migration
Jupiter and Saturn formed in a few million years (Haisch et al. 2001) from a
gas-dominated protoplanetary disk, and were susceptible to gas-driven migration
of their orbits on timescales of only ~100,000 years (Armitage 2007).
Hydrodynamic simulations show that these giant planets can undergo a two-stage,
inward-then-outward, migration (Masset & Snellgrove 2001, Morbidelli & Crida
2007, Pierens & Nelson 2008). The terrestrial planets finished accreting much
later (Klein et al. 2009), and their characteristics, including Mars' small
mass, are best reproduced by starting from a planetesimal disk with an outer
edge at about one astronomical unit from the Sun (Wetherill 1978, Hansen 2009)
(1 AU is the Earth-Sun distance). Here we report simulations of the early Solar
System that show how the inward migration of Jupiter to 1.5 AU, and its
subsequent outward migration, lead to a planetesimal disk truncated at 1 AU;
the terrestrial planets then form from this disk over the next 30-50 million
years, with an Earth/Mars mass ratio consistent with observations. Scattering
by Jupiter initially empties but then repopulates the asteroid belt, with
inner-belt bodies originating between 1 and 3 AU and outer-belt bodies
originating between and beyond the giant planets. This explains the significant
compositional differences across the asteroid belt. The key aspect missing from
previous models of terrestrial planet formation is the substantial radial
migration of the giant planets, which suggests that their behaviour is more
similar to that inferred for extrasolar planets than previously thought.Comment: 12 pages, 4 figures + Supplementary Material 46 pages, 10 figure
Building Terrestrial Planets
This paper reviews our current understanding of terrestrial planets
formation. The focus is on computer simulations of the dynamical aspects of the
accretion process. Throughout the chapter, we combine the results of these
theoretical models with geochemical, cosmochemical and chronological
constraints, in order to outline a comprehensive scenario of the early
evolution of our Solar System. Given that the giant planets formed first in the
protoplanetary disk, we stress the sensitive dependence of the terrestrial
planet accretion process on the orbital architecture of the giant planets and
on their evolution. This suggests a great diversity among the terrestrial
planets populations in extrasolar systems. Issues such as the cause for the
different masses and accretion timescales between Mars and the Earth and the
origin of water (and other volatiles) on our planet are discussed at depth
Debris disks as signposts of terrestrial planet formation. II Dependence of exoplanet architectures on giant planet and disk properties
We present models for the formation of terrestrial planets, and the
collisional evolution of debris disks, in planetary systems that contain
multiple unstable gas giants. We previously showed that the dynamics of the
giant planets introduces a correlation between the presence of terrestrial
planets and debris disks. Here we present new simulations that show that this
connection is qualitatively robust to changes in: the mass distribution of the
giant planets, the width and mass distribution of the outer planetesimal disk,
and the presence of gas in the disk. We discuss how variations in these
parameters affect the evolution. Systems with equal-mass giant planets undergo
the most violent instabilities, and these destroy both terrestrial planets and
the outer planetesimal disks that produce debris disks. In contrast, systems
with low-mass giant planets efficiently produce both terrestrial planets and
debris disks. A large fraction of systems with low-mass outermost giant planets
have stable gaps between these planets that are frequently populated by
planetesimals. Planetesimal belts between outer giant planets may affect debris
disk SEDs. If Earth-mass seeds are present in outer planetesimal disks, the
disks radially spread to colder temperatures. We argue that this may explain
the very low frequency of > 1 Gyr-old solar-type stars with observed 24 micron
excesses. Among the (limited) set of configurations explored, the best
candidates for hosting terrestrial planets at ~1 AU are stars older than 0.1-1
Gyr with bright debris disks at 70 micron but with no currently-known giant
planets. These systems combine evidence for rocky building blocks, with giant
planet properties least likely to undergo destructive dynamical evolution. We
predict an anti-correlation between debris disks and eccentric giant planets,
and a positive correlation between debris disks and terrestrial planets.Comment: Astronomy and Astrophysics, in press. Movies from simulations are at
http://www.obs.u-bordeaux1.fr/e3arths/raymond/movies_debris.htm
Portable prehospital methods to treat near-hypothermic shivering cold casualties
Objectives To compare the effectiveness of a single-layered polyethylene survival bag (P), a single-layered polyethylene survival bag with a hot drink (P+HD), a multi-layered metalized plastic sheeting survival bag (MPS: Blizzard Survival), and a multi-layered MPS survival bag with four large chemical-heat pads (MPS+HP: Blizzard Heat) to treat cold casualties.
Methods Portable cold casualty treatment methods were compared by examining core and skin temperature, metabolic heat production and thermal comfort during a 3-h, 0°C cold-air exposure in seven shivering, near-hypothermic men (35.4°C). The hot drink (70°C, ~400ml, ~28kJ) was consumed at 0, 1 and 2 h during the cold-air exposure.
Results During the cold-air exposure, core-rewarming and thermal comfort were similar on all trials (P = 0.45 and P = 0.36, respectively). However, skin temperature was higher (10-13%, P 2.7) and metabolic heat production lower (15-39%, P 0.9) on MPS and MPS+HP than P and P+HD. The addition of heat pads further lowered metabolic heat production by 15% (MPS+HP vs. MPS, P = 0.05, large effect size d = 0.9). The addition of the hot drink to polyethylene survival bag did not increase skin temperature or lower metabolic heat production.
Conclusions Near-hypothermic cold casualties are rewarmed with less peripheral cold stress and shivering thermogenesis using a multi-layered MPS survival bag compared with a polyethylene survival bag. Prehospital rewarming is further aided by large chemical heat pads but not by hot drinks
On Solving the Coronal Heating Problem
This article assesses the current state of understanding of coronal heating,
outlines the key elements of a comprehensive strategy for solving the problem,
and warns of obstacles that must be overcome along the way.Comment: Accepted by Solar Physics; Published by Solar Physic
Search for New Physics with Jets and Missing Transverse Momentum in pp collisions at sqrt(s) = 7 TeV
A search for new physics is presented based on an event signature of at least
three jets accompanied by large missing transverse momentum, using a data
sample corresponding to an integrated luminosity of 36 inverse picobarns
collected in proton--proton collisions at sqrt(s)=7 TeV with the CMS detector
at the LHC. No excess of events is observed above the expected standard model
backgrounds, which are all estimated from the data. Exclusion limits are
presented for the constrained minimal supersymmetric extension of the standard
model. Cross section limits are also presented using simplified models with new
particles decaying to an undetected particle and one or two jets
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
- âŚ