Our model not only naturally explains the LHB, but also reproduces the observational constraints of the outer Solar System 9. Previous work 9 explains the current orbital architecture of the planetary system by invoking an initially compact configuration in which Saturn's orbital period was less than twice that of Jupiter.
This abrupt transition temporarily destabilized the giant planets, leading to a short phase of close encounters among Saturn, Uranus and Neptune. As a result of these encounters, and of the interactions of the ice giants with the disk, Uranus and Neptune reached their current heliocentric distances and Jupiter and Saturn evolved to their current orbital eccentricities 9. Thus, our goal is to determine if there is a generic mechanism that could delay the migration process.
In previous studies 9 , 10 , 11 , 12 , planet migration started immediately because planetesimals were placed close enough to the planets to be violently unstable. Although this type of initial condition was reasonable for the goals of those studies, it is unlikely. Planetesimal-driven migration is probably not important for planet dynamics as long as the gaseous massive solar nebula exists. The initial conditions for the migration simulations should represent the system that existed at the time the nebula dissipated.
Thus, the planetesimal disk should contain only those particles that had dynamical lifetimes longer than the lifetime of the solar nebula. In planetary systems like those we adopt from ref.
Stable Trojans of the planets have been removed from this computation. Each vertical bar in the plot represents the average lifetime for those 10 particles. A comparison between the histogram and the putative lifetime of the gaseous nebula 20 argues that, when the latter dissipated, the inner edge of the planetesimal disk had to be about 1—1.
In all cases, the disk had a surface density equivalent to 1. The outer edge of the disk was varied so that the total mass of the disk was 35 M E. A comparison between a and b shows that a disk that naturally should exist when the nebula dissipated would produce a MMR crossing at a time comparable to that of the LHB event.
The four panels correspond to four different snapshots taken from our reference simulation. In this run, the four giant planets were initially on nearly circular, co-planar orbits with semimajor axes of 5. The dynamically cold planetesimal disk was 35 M E , with an inner edge at In this configuration, the initial speed of migration would be dependent on the rate at which disk particles evolve onto planet-crossing orbits.
The time at which Jupiter and Saturn cross their MMR depends on: 1 their initial distance from the location of the resonance, 2 the surface density of the disk near its inner edge, and 3 the relative location of the inner edge of the disk and the outer ice giant. On the basis of the above arguments, we initially performed a series of eight simulations where the location of the inner edge of the disk was set as the unique free parameter Fig.
As expected, we found a strong correlation between the location of the inner edge and the time of the MMR crossing. For disks with inner edges near We found that we can delay the resonant crossing to 1. Therefore, we can conclude that the global instability caused by the MMR crossing of Jupiter and Saturn could be responsible for the LHB, because the estimated date of the LHB falls in the range of the times that we found.
Figures 2 and 3 show the evolution of one of our runs from the first series of eight. Initially, the giant planets migrated slowly owing to leakage of particles from the disk Fig. After the resonance crossing event, the orbits of the ice giants became unstable and they were scattered into the disk by Saturn. They disrupted the disk and scattered objects all over the Solar System, including the inner regions. The solid curve in Fig. Such an influx spike happened in all our runs.
The average amount of material accreted by the Moon during this spike was 8. Each planet is represented by a pair of curves—the top and bottom curves are the aphelion and perihelion distances, Q and q , respectively. The subsequent interaction between the planets and the disk led to the current planetary configuration as shown in ref.
We have offset the comet curve so that the value is zero at the time of MMR crossing. Although the terrestrial planets were not included in our cometary simulations, we estimated the amount of material accreted by the Moon directly from the mass of the planetesimal disk by combining the particles' dynamical evolution with the analytic expressions in ref. Estimating the asteroidal flux first requires a determination of the mass of the asteroid belt before resonant crossing.
The flux was then again determined by combining the particles' dynamical evolution with the analytic expressions in ref. The dashed curve shows a simulation where class 2 particles dominate. The above mass delivery estimate corresponds only to the cometary contribution to the LHB, as the projectiles originated from the external massive, presumably icy, disk.
However, our scheme probably also produced an in flux of material from the asteroid belt. As Jupiter and Saturn moved from MMR towards their current positions, secular resonances which occur when the orbit of an asteroid processes at the same rate as a planet swept across the entire belt Astronomy Day. The Complete Star Atlas. Our planet may have been last pummeled by asteroid impacts longer ago than previously thought, explaining why life began to form almost 4 billion years ago.
Asteroids may have stopped pummeling Earth some million years earlier than scientists thought, giving life more time to evolve. The solar system once experienced a meteor shower of epic proportions: Asteroids whizzed around the inner planets, crashing down in a rain of fire that left their surfaces scarred for billions of years. Astronomers typically call this period the Late Heavy Bombardment. But exactly when that fiery assault happened has been a matter of intense debate.
The answer has big implications for the evolution of the solar system as a whole, and even for the timeline of life on Earth. Finding evidence of such a bombardment here on Earth is difficult. Our planet regularly melts and recycles its crust, destroying detailed evidence that might give us a concrete age for the period of heavy meteor impacts. Farther off, on Mercury, Mars, and the rocky or icy moons of the outer solar system, scientists are left to count craters, an imprecise dating method.
The other option is to use an objective dating method — radiometric rock dating, for instance — on bodies that have kept cleaner records than Earth. The Moon and asteroids — or the meteorite pieces of them that fall to Earth — are the most accessible.
The first really new data arrived in But when the results came back, they showed a curious, and familiar, pattern. Instead, they found no evidence of impacts before the hypothesized time of the LHB 3. But researchers still wondered how a bombardment could come so long after the Solar System formed. By the half-billion-year mark, most of the leftover debris should either have been cast out or have settled into stable zones such as the main asteroid belt, which sits between Mars and Jupiter, or the Kuiper belt beyond Neptune.
Nobody could come up with a physical reason for the unexpected drama at such a late date. A potential answer arrived in , with the emergence of what came to be known as the Nice model , after the French city where it was conceived. Computer simulations showed 4 how the massive gravitational pull of Jupiter and Saturn could have created an instability that ultimately bumped Uranus and Neptune into more distant orbits, knocked comets out of remote reservoirs and kicked asteroids out of the main belt.
The Nice model offered huge support for the LHB. Yet just when the idea of the LHB finally seemed unimpeachable, holes began to appear. This suggested that the impact that formed the crater might have knocked rocks into nearby Serenitatis, contaminating the Apollo samples picked up there. In , a reanalysis of rocks thought to have been ejected from Nectaris indicated that they were also chemically and geologically similar to Imbrium material 6.
Although none of the samples seemed to be older than 4 billion years, some were billions of years younger than that 3 , with no obvious spike around 3. And the Apollo samples held other surprises. Since , detailed study 7 of microscopic regions in the rocks has turned up ages of as much as 4. Prodded in part by these revelations, some researchers proposed 8 a longer-lasting LHB that began around 4.
But that idea had one major strike against it: some of the most ancient crystals on Earth, from the Jack Hills range in Australia, suggest 9 that the planet was a fairly clement place then, with relatively low temperatures and ample water. Others are still scrutinizing the original Apollo evidence.
Different minerals will release their argon at different temperatures. Researchers have tried to extrapolate from this behaviour, but Harrison says the complex patterns often lead them to pick essentially arbitrary ages. Cohen says that other chronometers, such as those using radioactive isotopes of rubidium and uranium, corroborate the argon ages although Harrison counters that the dates can differ by as much as million years.
Such back and forth underscores how difficult it can be to tease small clues out of extremely ancient rocks. Meanwhile, the Nice model has proved less helpful to the idea of an LHB than it once seemed. He no longer believes in the LHB, and sees many others in the field trading in the idea of a sudden asteroid deluge for that of a long, declining tail of bombardment. Even those who remain tied to the LHB have had to modify their ideas. Planetary scientist William Bottke of the Southwest Research Institute agrees that there is no longer much support for a single, short spike.
He says the best reading of the evidence, including samples from ancient Earth and radiometric dates in meteorite rocks, is a more drawn-out surge of bombardment that began around 4.
Learn more about a solar system time machine and meteorites. Earth has very little crust older than about 3. Venus experienced a massive resurfacing event sometime between million and 1 billion years ago, leaving no older crust around to study for impacts. But the evidence for a period of heavy bombardment is strong: large impacts on the terrestrial planets and our Moon, signs of shock impacts in the asteroid belt, and the age distribution of meteors found on Earth.
These all suggest that the inner solar system was bombarded with some large meteors around 4 billion years ago. But a lot of evidence points to a spike in the number of impacts around 4 billion years ago.
Learn more about exploring the Earth-Moon system. But this apparent spike could also be a result of a sampling bias. For instance, it is possible that the lunar samples collected during the Apollo missions are all just from one large impact crater on the Moon: Imbrium.
And since Imbrium is the youngest of the large impact basins, the ejecta would be on top of everything. Could it be possible that samples we collected from other locations on the Moon all happen to be ejecta from Imbrium? If so, that would explain why all the ages in the samples cluster around the age of Imbrium. There may not have been a dramatic increase in impactors. It might have been a more steady stream of impactors from the beginning of the solar system up to about 3.
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