6,003 research outputs found
Formation of bi-lobed shapes by sub-catastrophic collisions: A late origin of comet 67P/C-G's structure
The origin of the particular shape of a small body like comet
67P/Churyumov-Gerasimenko (67P/C-G) is a topic of active research. How and when
it acquired its peculiar characteristics has distinct implications on the
origin of the solar system and its dynamics. We investigate how shapes like the
one of comet 67P/C-G can result from a new type of low-energy, sub-catastrophic
impacts involving elongated, rotating bodies. We focus on parameters
potentially leading to bi-lobed structures. We also estimate the probability
for such structures to survive subsequent impacts. We use a smooth particle
hydrodynamics (SPH) shock physics code to model the impacts, the subsequent
reaccumulation of material and the reconfiguration into a stable final shape.
The energy increase as well as the degree of compaction of the resulting bodies
are tracked in the simulations. Our modelling results suggest that the
formation of bi-lobed structures like 67P/C-G is a natural outcome of the low
energy, sub-catastrophic collisions considered here. Sub-catastrophic impacts
have the potential to alter the shape of a small body significantly, without
leading to major heating or compaction. The currently observed shapes of
cometary nuclei, such as 67P/C-G, maybe a result of such a last major shape
forming impact.Comment: Astronomy & Astrophysics, accepted pending minor revision
Determination of the relative resistance to ignition of selected turbopump materials in high-pressure, high-temperature, oxygen environments, volume 1
Advances in the design of the liquid oxygen, liquid hydrogen engines for the Space Transportation System call for the use of warm, high-pressure oxygen as the driving gas in the liquid oxygen turbopump. The NASA Lewis Research Center requested the NASA White Sands Test Facility (WSTF) to design a test program to determine the relative resistance to ignition of nine selected turbopump materials: Hastelloy X, Inconel 600, Invar 36, Monel K-500, nickel 200, silicon carbide, stainless steel 316, and zirconium copper. The materials were subjected to particle impact and to frictional heating in high-pressure oxygen
Critical core mass for enriched envelopes: the role of H2O condensation
Context. Within the core accretion scenario of planetary formation, most
simulations performed so far always assume the accreting envelope to have a
solar composition. From the study of meteorite showers on Earth and numerical
simulations, we know that planetesimals must undergo thermal ablation and
disruption when crossing a protoplanetary envelope. Once the protoplanet has
acquired an atmosphere, the primordial envelope gets enriched in volatiles and
silicates from the planetesimals. This change of envelope composition during
the formation can have a significant effect in the final atmospheric
composition and on the formation timescale of giant planets.
Aims. To investigate the physical implications of considering the envelope
enrichment of protoplanets due to the disruption of icy planetesimals during
their way to the core. Particular focus is placed on the effect on the critical
core mass for envelopes where condensation of water can occur.
Methods. Internal structure models are numerically solved with the
implementation of updated opacities for all ranges of metallicities and the
software CEA to compute the equation of state. CEA computes the chemical
equilibrium for an arbitrary mixture of gases and allows the condensation of
some species, including water. This means that the latent heat of phase
transitions is consistently incorporated in the total energy budget.
Results. The critical core mass is found to decrease significantly when an
enriched envelope composition is considered in the internal structure
equations. A particular strong reduction of the critical core mass is obtained
for planets whose envelope metallicity is larger than Z=0.45 when the outer
boundary conditions are suitable for condensation of water to occur in the top
layers of the atmosphere. We show that this effect is qualitatively preserved
when the atmosphere is out of chemical equilibrium.Comment: Accepted for publication in A&
ARISTOTELES: A European approach for an Earth gravity field recovery mission
Under contract of the European Space Agency a system study for a spaceborne gravity field recovery mission was performed, covering as a secondary mission objective geodetic point positioning in the cm range as well. It was demonstrated that under the given programmatic constraints including dual launch and a very tight development schedule, a six months gravity field mission in a 200 km near polar, dawn-dusk orbit is adequate to determine gravity anomalies to better than 5 mgal with a spatial resolution of 100 x 100 km half wavelength. This will enable scientists to determine improved spherical harmonic coefficients of the Earth gravity field equation to the order and degree of 180 or better
A hit-and-run Giant Impact scenario
The formation of the Moon from the debris of a slow and grazing giant impact
of a Mars-sized impactor on the proto-Earth (Cameron & Ward 1976, Canup &
Asphaug 2001) is widely accepted today. We present an alternative scenario with
a hit-and-run collision (Asphaug 2010) with a fractionally increased impact
velocity and a steeper impact angle.Comment: 11 pages, 2 figures, in press in ICARUS note
Do solar decimetric spikes originate in coronal X-ray sources?
In the standard solar flare scenario, a large number of particles are
accelerated in the corona. Nonthermal electrons emit both X-rays and radio
waves. Thus, correlated signatures of the acceleration process are predicted at
both wavelengths, coinciding either close to the footpoints of a magnetic loop
or near the coronal X-ray source. We attempt to study the spatial connection
between coronal X-ray emission and decimetric radio spikes to determine the
site and geometry of the acceleration process. The positions of radio-spike
sources and coronal X-ray sources are determined and analyzed in a
well-observed limb event. Radio spikes are identified in observations from the
Phoenix-2 spectrometer. Data from the Nan\c{c}ay radioheliograph are used to
determine the position of the radio spikes. RHESSI images in soft and hard
X-ray wavelengths are used to determine the X-ray flare geometry. Those
observations are complemented by images from GOES/SXI. We find that decimetric
spikes do not originate from coronal X-ray flare sources contrary to previous
expectations. However, the observations suggest a causal link between the
coronal X-ray source, related to the major energy release site, and
simultaneous activity in the higher corona.Comment: 4 pages, 3 figures, A&AL accepte
How primordial is the structure of comet 67P/C-G? Combined collisional and dynamical models suggest a late formation
There is an active debate about whether the properties of comets as observed
today are primordial or, alternatively, if they are a result of collisional
evolution or other processes. We investigate the effects of collisions on a
comet with a structure like 67P/C-G. We develop scaling laws for the critical
specific impact energies required for a significant shape alteration. These are
then used in simulations of the combined dynamical and collisional evolution of
comets in order to study the survival probability of a primordially formed
object with a shape like 67P/C-G. The effects of impacts on comet 67P/C-G are
studied using a SPH shock physics code. The resulting critical specific impact
energy defines a minimal projectile size which is used to compute the number of
shape-changing collisions in a set of dynamical simulations. These simulations
follow the dispersion of the trans-Neptunian disk during the giant planet
instability, the formation of a scattered disk, and produce 87 objects that
penetrate into the inner solar system with orbits consistent with the observed
JFC population. The collisional evolution before the giant planet instability
is not considered here. Hence, our study is conservative in its estimation of
the number of collisions. We find that in any scenario considered here, comet
67P/C-G would have experienced a significant number of shape-changing
collisions, if it formed primordially. This is also the case for generic
bi-lobe shapes. Our study also shows that impact heating is very localized and
that collisionally processed bodies can still have a high porosity. Our study
indicates that the observed bi-lobe structure of comet 67P/C-G may not be
primordial, but might have originated in a rather recent event, possibly within
the last 1 Gy. This may be the case for any kilometer-sized two-component
cometary nuclei.Comment: Astronomy & Astrophysics, accepted pending minor revision
Planet formation models: the interplay with the planetesimal disc
According to the sequential accretion model, giant planet formation is based
first on the formation of a solid core which, when massive enough, can
gravitationally bind gas from the nebula to form the envelope. In order to
trigger the accretion of gas, the core has to grow up to several Earth masses
before the gas component of the protoplanetary disc dissipates. We compute the
formation of planets, considering the oligarchic regime for the growth of the
solid core. Embryos growing in the disc stir their neighbour planetesimals,
exciting their relative velocities, which makes accretion more difficult. We
compute the excitation state of planetesimals, as a result of stirring by
forming planets, and gas-solid interactions. We find that the formation of
giant planets is favoured by the accretion of small planetesimals, as their
random velocities are more easily damped by the gas drag of the nebula.
Moreover, the capture radius of a protoplanet with a (tiny) envelope is also
larger for small planetesimals. However, planets migrate as a result of
disc-planet angular momentum exchange, with important consequences for their
survival: due to the slow growth of a protoplanet in the oligarchic regime,
rapid inward type I migration has important implications on intermediate mass
planets that have not started yet their runaway accretion phase of gas. Most of
these planets are lost in the central star. Surviving planets have either
masses below 10 ME or above several Jupiter masses. To form giant planets
before the dissipation of the disc, small planetesimals (~ 0.1 km) have to be
the major contributors of the solid accretion process. However, the combination
of oligarchic growth and fast inward migration leads to the absence of
intermediate mass planets. Other processes must therefore be at work in order
to explain the population of extrasolar planets presently known.Comment: Accepted for publication in Astronomy and Astrophysic
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