Other than Monday, the week here has been devoted to the outer planets, and before I leave that subject, I want to work in the findings of a team of astronomers looking at the early history of the asteroid belt. Recent numerical simulations suggest that many of the objects found in the ‘main belt’ between the orbits of Mars and Jupiter actually formed far out in the Solar System, moving inward during a violent spasm of planetary evolution.
That points to an early system that, at particular times, underwent upheaval caused by a rearrangement of the gas giant planets. This is the so-called Nice model, so named because much of the work on it was performed at the Observatoire de la Côte d’Azur in Nice. The model proposes that the gas giant planets migrated to their present positions long after the protoplanetary gas disk had dissipated, playing a role in the Late Heavy Bombardment of the inner planets some 3.9 billion years ago, and producing many other effects, including the formation of the Oort Cloud and the Kuiper Belt.
What’s interesting for our view of the asteroid belt is that the ‘main belt’ asteroids between the orbits of Mars and Jupiter range widely in their composition, from igneous rocks to mixtures of rock and ice. And while it’s long been assumed that these asteroids formed in place out of a primordial disk that experienced chemical changes, the new simulations suggest that many asteroids formed in the outer system and, at the time of the Late Heavy Bombardment moved inward to their present positions.
Image: The asteroid belt lies in the region between Mars and Jupiter. The Trojan asteroids lie in Jupiter’s orbit, in two distinct regions in front of and behind the planet. Credit: Lunar and Planetary Institute.
The LHB was clearly not limited to the Earth, but devastated the Moon and other planets as well. Kleomenis Tsiganis (Aristotle University, Thessaloniki) notes the evidence for this idea in the asteroid belt, which the team used in its modeling:
“Some of the meteorites that once resided in the asteroid belt show signs they were hit by 3.5 to 3.9 billion years ago. Our model allows us to make the case they were hit by captured comets or perhaps their fragments. If so, they are telling us the same intriguing story as the lunar samples, namely that the solar system apparently went berserk and reconfigured itself about 4 billion years ago.”
This is a new view of the asteroid belt, one that will need follow-up through studies of meteorites, asteroids and the moon. Needless to say, data we can also gain from missions to the Kuiper Belt, like the Haumea orbiter we’ve been discussing, would materially benefit this analysis. The paper is Levison et al., “The Contamination of the Asteroid Belt by Primordial Trans-Neptunian Objects,” Nature 460 (16 July 2009), pp. 364-366 (abstract).
This is a significantly different theory from the one we were taught years ago that the asteroid belt is a “failed planet” because Jupiter’s gravitational pull prevented the primordial dust ring between Mars and Jupiter from coalescing into anything larger than the largest asteroid there, or that- perhaps- Jupiter’s pull tore that planet apart early in its life. If there’s still any validity to that theory, perhaps it means the “missing” planet would have been quite a bit smaller than previously presumed if some/most of the objects in that region migrated there long after the planets were essentially done forming.
The new theory also gives us extra incentive to explore the asteroid belt if it does contain objects from the Oort cloud, particularly those objects that are still (mostly) pristine because they haven’t collided/interacted with any inner objects. While exploration of the Oort cloud is still an essential goal (among other reasons, to help us “field test” technologies for future interstellar missions,) it’s exciting to think that- in a way- the Oort cloud is much closer than we ever thought and already within our technological grasp.
This theory just means that there are a lot more volatiles in the asteroid belt than previously thought. This is beneficial for O’neill space colonization.
Asteroids Were Born Big
Authors: Alessandro Morbidelli (CASSIOPEE), William Bottke, David Nesvorny, Harold F. Levison
(Submitted on 15 Jul 2009)
Abstract: How big were the first planetesimals? We attempt to answer this question by conducting coagulation simulations in which the planetesimals grow by mutual collisions and form larger bodies and planetary embryos. The size frequency distribution (SFD) of the initial planetesimals is considered a free parameter in these simulations, and we search for the one that produces at the end objects with a SFD that is consistent with asteroid belt constraints.
We find that, if the initial planetesimals were small (e.g. km-sized), the final SFD fails to fulfill these constraints. In particular, reproducing the bump observed at diameter D~100km in the current SFD of the asteroids requires that the minimal size of the initial planetesimals was also ~100km. This supports the idea that planetesimals formed big, namely that the size of solids in the proto-planetary disk “jumped” from sub-meter scale to multi-kilometer scale, without passing through intermediate values.
Moreover, we find evidence that the initial planetesimals had to have sizes ranging from 100 to several 100km, probably even 1,000km, and that their SFD had to have a slope over this interval that was similar to the one characterizing the current asteroids in the same size-range.
This result sets a new constraint on planetesimal formation models and opens new perspectives for the investigation of the collisional evolution in the asteroid and Kuiper belts as well as of the accretion of the cores of the giant planets.
Comments: Icarus (2009) in press
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0907.2512v1 [astro-ph.EP]
Submission history
From: Morbidelli Alessandro [view email] [via CCSD proxy]
[v1] Wed, 15 Jul 2009 07:55:19 GMT (76kb)
http://arxiv.org/abs/0907.2512
Order statistics and heavy-tail distributions for planetary perturbations on Oort cloud comets
Authors: R. S. Stoica, S. Liu, Yu. Davydov, M. Fouchard, A. Vienne, G.B. Valsecchi
(Submitted on 13 Jul 2009)
Abstract: This paper tackles important aspects of comets dynamics from a statistical point of view. Existing methodology uses numerical integration for computing planetary perturbations for simulating such dynamics. This operation is highly computational. It is reasonable to wonder whenever statistical simulation of the perturbations can be much more easy to handle.
The first step for answering such a question is to provide a statistical study of these perturbations in order to catch their main features. The statistical tools used are order statistics and heavy tail distributions. The study carried out indicated a general pattern exhibited by the perturbations around the orbits of the important planet. These characteristics were validated through statistical testing and a theoretical study based on Opik theory.
Comments: 9 pages, 12 figures, submitted for publication in Astronomy and Astrophysics
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0907.2123v1 [astro-ph.IM]
Submission history
From: Marc Fouchard [view email]
[v1] Mon, 13 Jul 2009 10:10:07 GMT (1564kb)
http://arxiv.org/abs/0907.2123
I always subscribed to the idea that much of the asteroid belt are the bits and pieces of a planet that never formed. I was strengthened in this belief by the fact that Bode’s Law predicts a planet right where the asteroid belt and Ceres is located.. and given that, Bode’s Law matches planetary positions up to Uranus.
This is interesting though.. would this provide better opportunities for space mining?
Hi All
Explains all the Main Belt comets discovered in recent years. Would make the Belt an even more inviting region to colonise ala Marshall Savage’s “Millennial Project” style, using water as the main mass-shielding material against cosmic rays.
But could we ever migrate into raw space en masse the way he imagined? There’s certainly a multitude of small bodies to inhabit Out There – the NEOs are just the driest and tiniest of the lot. The Jupiter Trojans are believed to be more abundant than the Main Belt, for example, and the Edgeworth-Kuiper Belt more so again.
A paper appeared today on the arxiv.org server which deduces a population of co-orbital NEOs from the statistics of impactors on the Moon. There’s an asymmetry in rayed crater frequency between the leading and trailing hemispheres of the Moon, but too many to be explained by the classical NEO population. Thus we might be seeing evidence of an Earth-Sun Trojan population being perturbed our way. Will be very interesting to see what the paired STEREO probes observe when they arrive in Earth’s twin Trojan points…
Asymmetric impacts of near-Earth asteroids on the Moon
Authors: Takashi Ito, Renu Malhotra
(Submitted on 17 Jul 2009)
Abstract: Recent lunar crater studies have revealed an asymmetric distribution of rayed craters on the lunar surface. The asymmetry is related to the synchronous rotation of the Moon: there is a higher density of rayed craters on the leading hemisphere compared with the trailing hemisphere. Rayed craters represent generally the youngest impacts.
The purpose of this paper is to test the hypotheses that (i) the population of Near-Earth asteroids (NEAs) is the source of the impactors that have made the rayed craters, and (ii) that impacts by this projectile population account quantitatively for the observed asymmetry.
We carried out numerical simulations of the orbital evolution of a large number of test particles representing NEAs in order to determine directly their impact flux on the Moon.
The simulations were done in two stages. In the first stage we obtained encounter statistics of NEAs on the Earth’s activity sphere. In the second stage we calculated the direct impact flux of the encountering particles on the surface of the Moon; the latter calculations were confined within the activity sphere of the Earth.
To represent NEAs’ initial conditions, we considered two populations: one is the currently known NEAs, and the other is a synthetic population created by debiasing the orbital distribution of the known NEAs. We find that the near-Earth asteroids do have an asymmetry in their impact flux on the Moon: apex-to-antapex ratio of 1.3-1.4. However, the observed rayed crater distribution’s asymmetry is significantly more pronounced: apex-to-antapex ratio of ~1.67.
Our simulations suggest the existence of an undetected population of slower (low impact velocity) projectiles, such as a population of objects coorbiting with Earth.
Comments: 15 pages, 6 figures, submitted to Astronomy & Astrophysics
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0907.3010v1 [astro-ph.EP]
Submission history
From: Takashi Ito [view email]
[v1] Fri, 17 Jul 2009 08:04:15 GMT (4937kb)
http://arxiv.org/abs/0907.3010