A dramatic event early in the history of the Solar System– called the late heavy bombardment, also nicknamed the lunar cataclysm– may have caused planets to migrate in our solar systemThere may have been a dramatic event early in the history of the Solar System–the intense bombardment of the inner planets and the Moon by planetesimals during a narrow interval between 3.92 and 3.85 billion years ago, called the late heavy bombardment, but also nicknamed the lunar cataclysm.
A group of physicists simulated the migration of Saturn and Jupiter. When the orbits of these giant planets reached the special condition of Saturn making one trip around the Sun for every two trips by Jupiter (called the 1:2 resonance), violent gravitational shoves made the orbits of Neptune and Uranus unstable, causing them to migrate rapidly and scatter countless planetesimals throughout the Solar System. This dramatic event could have happened in a short interval, anywhere from 200 million years to a billion years after planet formation, causing the lunar cataclysm, which would have affected all the inner planets.
The Lunar Cataclysm
Many lunar rocks formed by melting during huge impact events, which we call “impact melt breccias,” have ages that fall into a narrow time interval, between 3.92 and 3.85 billion years. Some scientists have concluded that the apparent clustering of ages record an intense bombardment of the Moon. They called it the “lunar cataclysm” and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9 billion years ago. More recent work on lunar samples and lunar meteorites generally confirms that there is a dearth of ages for impact melts older than 3.9 billion years
Dramatic Dynamics of the Early Solar System
We normally think of the orbits of the planets as unchanging and reliable. Oh sure, stray asteroids and comets can come close to Earth and even hit it, maybe causing extinctions of numerous species. But not the planets. They stay put. We can rely on them. Well, it turns out that early in the history of the Solar System, the planets may have roamed, especially the giants Jupiter, Saturn, Uranus, and Neptune. Jupiter seems to have migrated in towards the Sun while the others wandered away from the Sun. When their orbits reached certain simple relationships to each other, serious gravitational pushing and pulling happened that violently destabilized the orbits of Uranus and Nepture, thus flinging millions of leftovers from planet formation (asteroid-to-moon-sized planetesimals) throughout the Solar System. Planets moved and craters formed, possibly in a dramatic, chaotic, messy moment of geologic history.
This is the story of the early Solar System being developed by physicists and astronomers. Current theory says that migrating planets are a natural consequence of planet formation. Such migration would be accompanied by changes in the inclination and eccentricity of the outer planets, increasing from nice, co-planar circular orbits to those that are inclined to the ecliptic (Earth’s orbital plane) and not exactly circular (they follow elliptical paths).
After the planets were constructed the Solar System still teemed with the detritus of planet formation–planetesimals that had not accreted to a planet. These stunted planets were indiscriminately hurled around by gravitational interactions with the outer planets. These interactions, called dynamical friction, caused changes in the orbits of the outer planets, too. Jupiter moved inward toward the Sun as Saturn, Neptune, and Uranus migrated outward. As Saturn drifted outward it eventually reached a resonance with Jupiter, in which it orbited the Sun once for every two orbits of Jupiter around the Sun. This is called a 1:2 resonance. This special timing of orbits of these two giant planets caused their gravitational interactions to pump up the motions of Neptune and Uranus. Tsiganis and his colleagues calculate, for example, that the orbits of Neptune and Uranus might have become highly elliptical and Neptune’s orbital distance from the Sun doubled, sending it into a zone peppered with planetesimals. The planetesimals were scattered by Neptune’s substantial gravity field, sending them all over the place, including into the inner Solar System to bombard the rocky planets and the Moon. The situation is so dynamic that Neptune started closer to the Sun than Uranus, but ended up farther away. See the animation below.
In this dynamical simulation of the late heavy bombardment, the Sun is in the center, the colored circular rings represent the orbits of the four giant planets, and the green dots represent the disk of planetesimals between 15.5 AU and 34 AU. Watch the state of the planetary system at different times, starting at t=100 million years. Saturn and Jupiter migrate slowly, reaching 2:1 resonance. This scatters Neptune and Uranus. Their extreme migrations scatter planetesimals in a short time interval–a cataclysm. Credit: Gomes, et al
Read more from NLSI’s Bill Bottke by downloading his presentation: “New Views on the Lunar Late Heavy Bombardment”.
posted by: Soderman/NLSI Staff