Late and heavy: Did a fifth rocky planet, long subsumed by the Sun, create the bombardment that pockmarked the surface of the Moon?
Credit: NASA
NEW YORK: The fiery demise of a fifth rocky planet in our Solar System might have led to a flurry of asteroid impacts that pockmarked the Moon and Earth billions of years ago.
The Late Heavy Bombardment (LHB) is a relatively brief period, about 3.9 billion years ago, when wayward space projectiles heavily pelted the Moon and inner planets. Craters from that chaotic time are still visible on the Moon, but have been erased from Earth, where the crust is continually recycled.
All shook up
Try as they might, astronomers have not yet been able to pin down a cause for the bombardment. Some experts have postulated that a shuffling up of the arrangement of the planets in their youth may have been responsible. One popular theory is that the outward migration of a young Neptune perturbed rocky bodies in the distant Kuiper Belt, causing some to veer into the inner Solar System.
But John Chambers, an astrophysicist at the Carnegie Institution in Washington DC, now says the size distribution of craters on the Moon better match asteroids from the Asteroid Belt, located beyond the orbit of Mars. And he thinks the misbehaviour of a long-lost, fifth rocky planet called 'Planet V' was the trigger that upset the gravitational balance of the belt and ejected some of its inhabitants. Our Solar System currently contains four rocky planets: Mercury, Venus, Earth and Mars.
Using a new computer model detailed in a recent issue of the journal Icarus, Chambers provides the most compelling evidence yet that a hypothetical Planet V could have existed for hundreds of millions of years before minute gravitational tugs from Mars and Jupiter destabilised its orbit, causing it to fall into the Sun.
"Before it was lost, its orbit would have moved across the Asteroid Belt for quite a long period of time, scattering asteroids as a result," Chambers said. "My model would predict that it's only asteroids, and not comets, that caused the impacts, and that the asteroids would tend to come from the inner asteroid belt."
Unknown planet
Planet V's orbit was between that of Mars and the Asteroid Belt, Chambers predicts, and it may have been smaller than Mars but larger than our Moon. "If it was bigger than Mars, then Mars should have been the one that was lost," he said. "If it's smaller than the Moon, that's not really big enough to disturb the Asteroid Belt much."
David Kring, a planetary scientist at the U.S. Lunar and Planetary Institute in Houston, Texas, agrees that the Planet V hypothesis is interesting because it suggests an inner Solar System origin for the impacting debris. But he added that Chamber's "model relies on the invention of an unknown planet... so it is a hypothesis that will need to be rigorously tested with the geologic record."
One such test could be done if scientists had more lunar rock samples, he said. It was using these samples, returned by NASA's Apollo missions, that scientists were able to date the Late Heavy Bombardment in the first place.
Planet V wandered or detonated?
A wandering planet V could have caused the LHB, but what if several craters from the LHB on the Moon are investigated for remnants of the impactors - and they're all chemically identical? That would indicate that the LHB was caused by remains of a planet V. Could planet V have detonated?
Astronomer Tom van Flandern actually thinks several planets exploded in our solar system's history - could he be correct? Another possibility is that "twin" Trojan planets formed either side of Jupiter's orbit and were set into motion that perturbed them into a collision course with each other.
Many things are possible when it comes to wandering planets - even the survivors might be launched into wandering orbits. For example, Mercury has a small probability of becoming "unchained from the Sun" over the next billion years due to chaotic amplification of its orbital eccentricity.
Detonation...no
No. Planets don't "detonate." What's going to explode?
And even if they could, short of hitting another planet composed of antimatter, there's no way to get a planet that large (~3,000 miles diameter) to blow up with enough force to avoid reforming afterwards - it takes just oodles of energy to get most of the mass of a planet to reach that planet's escape velocity.
I mean, look, Earth was hit by a planet the size of Mars, and the best it could do was "lose" (and not very far) the mass of the Moon. The rest, right back into Earth (Mark II).
"Kryptons" only happen in comics.
Detonating planets
Hi David
Normally planets don't detonate - which has been a problem with Tom van Flandern's theory since he proposed it in the 1970s. It's not an original idea - even Oort thought the asteroid belt and the Oort Cloud of comets might be remnants of a planetary detonation. But planets certainly wander and they can collide, which well and truly packs enough punch to shatter worlds if they're similar in mass.
Earth-Moon systems are one possible outcome. Look at the track record of the inner Solar System - Earth collided with Theia; Venus probably collided with two objects; Mercury lost its crust, and even Mars has a hemispherical asymmetry perhaps due to a mega-impactor.
Here's my theory - Earth and Venus formed, and both formed with L4/5 planets accreting in the same orbits. But the Trojan planets got too big and started wandering around their orbits - Venus's Trojans collided with each other. One became Mercury, the other blew away Mercury's mantle, and big bits probably smacked into Venus, eventually. Earth's Trojans were Theia and Mars.
But that's just one scenario - other extremes are possible. Most simulations assume a thousand Moon-sized bodies have formed out of the dust-disk and the simulation lets them orbitally evolve and collide until planets in stable orbits result. Surprisingly they do form, even eventually looking like our own inner planets.
Detonation of a planet?
I was curious about the concept myself, and evidently a geophysicist with some good credentials has given it some thought - J. Marvin Herndon (nuclearplanet.com) proposes that under the high earth core temperatures and pressures, an actual ongoing uranium fission reaction is the driving force not only for geothermal forces but for the geomagnetic field. Some research tends to support this theory, however nobody has currently been able to devise an experiment to detect signature particles from a fission reaction that deep below ground. Herndon's theory also tends to support the possibility that planets can indeed "explode" - and both Uranus and Jupiter radiate far more energy than they absorb from the sun - whether this is indeed from a core fission reaction it is not known - however the math says it's possible. For a slightly more biased slant, check out Dr. Tom Chalko's article about the subject at http://nujournal.net/core.pdf - he backs his opinions up with math but the article gets a little emotional in places for a "scientific paper"...
Have fun - oh, and check out http://www.thesurfaceofthesun.com - it's an interesting theory. No - I'm not trying to advertise for these folks, just thought the sites were interesting...
Of course we did!
Yeah, we used to have another planet. We called it Pluto.
A Rock Giant?
I reckoned many years back, that before the sun consolidated/ expanded to its full scale in the period of accretion, that a `rock giant' may have existed between Jupiter & mars. & that the combo of increased gravitational force from the sun (transcending the sub rotational forces BEFORE it became large enough to exert a FULLY rotational orbit bringing to an end the period of accretion) would have caused an inbalance between its unstable internal pressures (unlike a gas giant nowhere else to go) & the INCREASED forces exerted on it by the FULLY rotational orbit.
That what happened next was a process of REVERSE accretion (aided by massive collisions that could only take place ONCE the sun has expanded to the point that the SUB rotational phase of planetary development had ended.
Trouble with THIS idea, is that you would expect a lot of metal debris.
This notion of an exploding planet does however seem to match Mercury. which has a heavy metal core, which implies that after the explosive period (loss of matter) it had found equilibrium between internal & external pressures.
Simplify
Mars and another planet, had an interplanetary war. Both had extremely
powerful weapons. Both sides lost. The result was the surface of Mars being devestated, and the other planet was pulverized.
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a cool story. i gess you know alot about earth.
did mars used to be a moon?
why is one side of mars extremely cratered and the otherside not? perhaps mars was a moon of a exploding planet and earth picked up the life and water from it.