1,773 research outputs found
The great dichotomy of the Solar System: small terrestrial embryos and massive giant planet cores
The basic structure of the solar system is set by the presence of low-mass
terrestrial planets in its inner part and giant planets in its outer part. This
is the result of the formation of a system of multiple embryos with
approximately the mass of Mars in the inner disk and of a few multi-Earth-mass
cores in the outer disk, within the lifetime of the gaseous component of the
protoplanetary disk. What was the origin of this dichotomy in the mass
distribution of embryos/cores? We show in this paper that the classic processes
of runaway and oligarchic growth from a disk of planetesimals cannot explain
this dichotomy, even if the original surface density of solids increased at the
snowline. Instead, the accretion of drifting pebbles by embryos and cores can
explain the dichotomy, provided that some assumptions hold true. We propose
that the mass-flow of pebbles is two-times lower and the characteristic size of
the pebbles is approximately ten times smaller within the snowline than beyond
the snowline (respectively at heliocentric distance and
, where is the snowline heliocentric distance), due to ice
sublimation and the splitting of icy pebbles into a collection of
chondrule-size silicate grains. In this case, objects of original sub-lunar
mass would grow at drastically different rates in the two regions of the disk.
Within the snowline these bodies would reach approximately the mass of Mars
while beyond the snowline they would grow to Earth masses. The
results may change quantitatively with changes to the assumed parameters, but
the establishment of a clear dichotomy in the mass distribution of protoplanets
appears robust, provided that there is enough turbulence in the disk to prevent
the sedimentation of the silicate grains into a very thin layer.Comment: In press in Icaru
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
Dynamic response studies on aggregation and breakage dynamics of colloidal dispersions in stirred tanks
Aggregation and breakage of aggregates of fully destabilized polystyrene latex particles in turbulent flow was studied experimentally in both batch and continuous stirred tanks using small-angle static light scattering. It was found that the steady-state values of the root-mean-square radius of gyration are fully reversible upon changes of stirring speed as well as solid volume fraction. Steady-state values of the root-mean-square radius of gyration were decreasing with decreasing solid volume fraction as well as with increasing stirring speed. Moreover, it was found that the steady-state structure and shape of the aggregates is not influenced by the applied stirring speed
A Method to Constrain the Size of the Protosolar Nebula
Observations indicate that the gaseous circumstellar disks around young stars
vary significantly in size, ranging from tens to thousands of AU. Models of
planet formation depend critically upon the properties of these primordial
disks, yet in general it is impossible to connect an existing planetary system
with an observed disk. We present a method by which we can constrain the size
of our own protosolar nebula using the properties of the small body reservoirs
in the solar system. In standard planet formation theory, after Jupiter and
Saturn formed they scattered a significant number of remnant planetesimals into
highly eccentric orbits. In this paper, we show that if there had been a
massive, extended protoplanetary disk at that time, then the disk would have
excited Kozai oscillations in some of the scattered objects, driving them into
high-inclination (i > 50 deg), low-eccentricity orbits (q > 30 AU). The
dissipation of the gaseous disk would strand a subset of objects in these
high-inclination orbits; orbits that are stable on Gyr time scales. To date,
surveys have not detected any Kuiper Belt Objects with orbits consistent with
this dynamical mechanism. Using these non-detections by the Deep Ecliptic
Survey (DES) and the Palomar Distant Solar System Survey we are able to rule
out an extended gaseous protoplanetary disk (R_D > 80 AU) in our solar system
at the time of Jupiter's formation. Future deep all sky surveys such as the
Large Synoptic Survey Telescope (LSST) will all us to further constrain the
size of the protoplanetary disk.Comment: 10 pages, Accepted to A
Did the Hilda collisional family form during the late heavy bombardment?
We model the long-term evolution of the Hilda collisional family located in
the 3/2 mean-motion resonance with Jupiter. Its eccentricity distribution
evolves mostly due to the Yarkovsky/YORP effect and assuming that: (i) impact
disruption was isotropic, and (ii) albedo distribution of small asteroids is
the same as for large ones, we can estimate the age of the Hilda family to be
. We also calculate collisional activity in the J3/2
region. Our results indicate that current collisional rates are very low for a
200\,km parent body such that the number of expected events over Gyrs is much
smaller than one.
The large age and the low probability of the collisional disruption lead us
to the conclusion that the Hilda family might have been created during the Late
Heavy Bombardment when the collisions were much more frequent. The Hilda family
may thus serve as a test of orbital behavior of planets during the LHB. We
tested the influence of the giant-planet migration on the distribution of the
family members. The scenarios that are consistent with the observed Hilda
family are those with fast migration time scales to
, because longer time scales produce a family that is depleted
and too much spread in eccentricity. Moreover, there is an indication that
Jupiter and Saturn were no longer in a compact configuration (with period ratio
) at the time when the Hilda family was created
The Complex History of Trojan Asteroids
The Trojan asteroids provide a unique perspective on the history of Solar
System. As a large population of small bodies, they record important
gravitational interactions and dynamical evolution of the Solar System. In the
past decade, significant advances have been made in understanding physical
properties, and there has been a revolution in thinking about the origin of
Trojans. The ice and organics generally presumed to be a significant part of
Trojan compositions have yet to be detected directly, though low density of the
binary system Patroclus (and possibly low density of the binary/moonlet system
Hektor) is consistent with an interior ice component. By contrast, fine-grained
silicates that appear to be similar to cometary silicates in composition have
been detected, and a color bimodality may indicate distinct compositional
groups among the Trojans. Whereas Trojans had traditionally been thought to
have formed near 5 AU, a new paradigm has developed in which the Trojans formed
in the proto-Kuiper Belt, and they were scattered inward and captured in the
Trojan swarms as a result of resonant interactions of the giant planets.
Whereas the orbital and population distributions of current Trojans are
consistent with this origin scenario, there are significant differences between
current physical properties of Trojans and those of Kuiper Belt objects. These
differences may be indicative of surface modification due to the inward
migration of objects that became the Trojans, but understanding of appropriate
modification mechanisms is poor and would benefit from additional laboratory
studies. Many open questions remain, and the future promises significant
strides in our understanding of Trojans. The time is ripe for a spacecraft
mission to the Trojans, to turn these objects into geologic worlds that can be
studied in detail to unravel their complex history.Comment: Chapter for Asteroids IV book (UA Press), accepted for publication,
33 pages, 10 figure
The population of Near Earth Asteroids in coorbital motion with Venus
We estimate the size and orbital distributions of Near Earth Asteroids (NEAs) that are expected to be in the 1:1 mean motion resonance with Venus in a steady state scenario. We predict that the number of such objects with absolute magnitudes H<18 and H<22 is 0.14±0.03 and 3.5±0.7, respectively. We also map the distribution in the sky of these Venus coorbital NEAs and we see that these objects, as the Earth coorbital NEAs studied in a previous paper, are more likely to be found by NEAs search programs that do not simply observe around opposition and that scan large areas of the sky.http://www.sciencedirect.com/science/article/B6WGF-4KMYG2W-2/1/a7a3dcf3c98ddc8cf0d2d1cdbde6853
Constraining the cometary flux through the asteroid belt during the late heavy bombardment
In the Nice model, the late heavy bombardment (LHB) is related to an orbital
instability of giant planets which causes a fast dynamical dispersion of a
transneptunian cometary disk. We study effects produced by these hypothetical
cometary projectiles on main-belt asteroids. In particular, we want to check
whether the observed collisional families provide a lower or an upper limit for
the cometary flux during the LHB.
We present an updated list of observed asteroid families as identified in the
space of synthetic proper elements by the hierarchical clustering method,
colour data, albedo data and dynamical considerations and we estimate their
physical parameters. We selected 12 families which may be related to the LHB
according to their dynamical ages. We then used collisional models and N-body
orbital simulations to gain insight into the long-term dynamical evolution of
synthetic LHB families over 4 Gyr. We account for the mutual collisions, the
physical disruptions of comets, the Yarkovsky/YORP drift, chaotic diffusion, or
possible perturbations by the giant-planet migration.
Assuming a "standard" size-frequency distribution of primordial comets, we
predict the number of families with parent-body sizes D_PB >= 200 km which
seems consistent with observations. However, more than 100 asteroid families
with D_PB >= 100 km should be created at the same time which are not observed.
This discrepancy can be nevertheless explained by the following processes: i)
asteroid families are efficiently destroyed by comminution (via collisional
cascade), ii) disruptions of comets below some critical perihelion distance (q
<~ 1.5 AU) are common.
Given the freedom in the cometary-disruption law, we cannot provide stringent
limits on the cometary flux, but we can conclude that the observed distribution
of asteroid families does not contradict with a cometary LHB.Comment: accepted in Astronomy and Astrophysic
- …