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| How The Man In The Moon Was Born |
| 2005-05-26 |
| A jarring game of planetary billiards in the early years of the Solar System created the Man in the Moon, astrophysicists believe. Rocks picked up by the Apollo astronauts suggest that the basins which appear to form the Moon's "face" when seen from Earth resulted from a massive flurry of impacts by space rocks some four billion years ago. But explaining this event, called the Late Heavy Bombardment, has been a matter of controversy. If the dating is right, the lunar bombardment occurred around 600 million years after the Sun burst into light and the planets start to form, building up from clusters of primitive dust. By that time, the Solar System should - in theory - have been a relatively calm place. Most of the Solar System's construction debris should have settled in stabilised orbits or been mopped up by the planets, sucked in by gravitational pull. So how could this bombardment of the Moon have arisen? The answer may lie in a novel theory that may also explain two other strange features of the Solar System. Writing on Thursday in the British weekly journal Nature, scientists say the key is in the formation of the Solar System's two giant planets, Jupiter and Saturn, around 4.5 billion years ago. If there was a time when Saturn completed exactly one orbit of the Sun for every two orbits made by Jupiter, the two planets would trigger a phenomenon called mean-motion resonance, they say. Under this, one planet's gravitational perturbations affect the other, rather like two giant ships which pass close to another, causing waves that make both vessels bobble and slightly change course. In the case of Jupiter and Saturn, the resonance eventually had an enormous knock-on effect. It edged the two planets into wider orbits around the Sun, elongated and tilted their orbital planes and, in turn, eventually scattered the two outermost large planets, Uranus and Neptune. When Neptune was flung outwards, it in turn scattered an orbiting cloud of rocky debris towards the Sun, some of which smacked into the Moon. The planetary migration unfolded over hundreds of millions of years, and this explains why the Late Heavy Bombardment occurred relatively late in the history of the Solar System. If this theory of planetary movement, backed by high-powered computer models, is right, it would also explain why the giant planets have stabilised into eccentric orbits around the Sun instead of neat circular ones. And it would also provide the answer as to why their inclinations - the tilt of their orbital planes - are so much larger than those predicted by the conventional hypothesis of planetary formation. |
| Posted by:Spavirt Pheng6042 |
| #13 Well, 2x4, it's kind of hard to show you planets at all; we have a hard time imaging planets outside the solar system at all, mostly we just infer their existance from gravitational effects. the ones we do see are usually anomalous jupiter-sized planets close to the primary, and that's more a selection effect of the instruments than any sort of valid population measurement. There are also pictures of bright objects in protoplanetary discs that may be planets, or may be brown dwarves. Noone can really tell. |
| Posted by: Phil Fraering 2005-05-26 23:54 |
| #12 It's surprising how well it holds together Well, let me modify it a tad: "It's surprising how well it seems to holds together" Cuz it doesn't. Just google "scientists are puzzled" and pick those related to astronomy, for the last 4 or so years. A good theory predicts a lot of stuff. But once you are puzzled one time too many, introducing additional epicycles to fix it--that does not a good theory make. It just shows how well the current paradigm is entrenched. |
| Posted by: twobyfour 2005-05-26 21:01 |
| #11 BTW, the '70s show "Cosmos" is out on Bittorrent. Just showed by Estonian girlfriend the first ep. She's hooked. It's surprising how well it holds together 30 years hence. Even with Carl's wife spouting out in the first ep about how the Soviet Union and the United States held "the world hostage" during the Cold War. No moral equivalency there. |
| Posted by: Scott 2005-05-26 19:42 |
| #10 As above so under, rjschwarz? Entertaining. I'll chalk it up as 'dust bunny proof of planetary accretion'. Kidding. |
| Posted by: twobyfour 2005-05-26 15:22 |
| #9 twobyfour, you ask a good question but consider a couple of things that are easily observable that have a similar effect. Check out the dust bunnies that develop beneath beds. These are created by the accumulation of smaller particles smacking together and sticking together. Gravity and glue are not involved, just motion, friction,and time. If the dust bunnies were travelling in circles they'd be even more effecient in colliding with each other. The planets are just giant dust bunnies that have now sucked up the leftovers so they can no longer grow bigger. |
| Posted by: rjschwarz 2005-05-26 14:59 |
| #8 RC, soo, where's da planet? |
| Posted by: twobyfour 2005-05-26 14:52 |
| #7 That being said... whould someone please show me a planet forming from an accretion disk or savage dust clusters? http://www.eso.org/outreach/press-rel/pr-2004/pr-12-04.html This system complies perfectly with a newly forming high-mass star surrounded by a huge accretion disc and accompanied by an energetic bipolar mass outflow. http://antwrp.gsfc.nasa.gov/apod/ap980423.html These separate radio images reveal three dusty debris disks surrounding three bright, young, nearby stars - evidence for solar systems in formation HTH. HAND. LTUG. |
| Posted by: Robert Crawford 2005-05-26 07:49 |
| #6 A problem I see is related to twobyfour's comment: simulations need to be initialized, and it appears that it was initialized with Jupiter and Saturn already accreted into planets with the postulated orbital characteristics. However, would not the pre-accreted proto-Jupiter and proto-Saturn non-planetary conglomerates ALSO possess the SAME orbital characteristics, and their gravitational interactions be the same (albeit reduced in strength)? I don't know if anyone sees what I'm seeing: the planets had to accrete from the disk of material postulated to have been the the beginning of our solar system. Gravitational forces pulled the majority of the mass of the stuff into what would become the sun, while smaller lumps that randomly appeared in the disk became larger via gravitational attraction of smaller lumps of material to them. The lumps that eventually became Jupiter and Saturn HAD to have had the postulated orbital characteristics some time BEFORE becoming planets for them to have them AFTER becoming planets. Why didn't this effect affect the pre-planetary Jupiter and Saturn masses? Final kiss of death: This sounds like something Immanuel Velikovsky would have come up with. |
| Posted by: Ptah 2005-05-26 04:46 |
| #5 2x4: This baby has all the signs of being helpfully dumbed-down for Joe Sixpack. "Formed" instead of "accreted", "primitive" instead of "primordial", etc, etc... |
| Posted by: mojo 2005-05-26 01:59 |
| #4 Phil, the model states that Jupiter was migrating, not that it does so at the present. So, the point about main belt and asteroids may be moot. OTOH, there may be another mechanism that affects planetary orbits at any point in planet's life and then ... riot. BTW, the obliquity and tilt seem to suggest rather 'recent' event, as revolving planets tend to smooth out their orbit towards more circular in time. |
| Posted by: Sobiesky 2005-05-26 01:28 |
| #3 Mojo, it may have been "savage" dust. ;-P That being said... whould someone please show me a planet forming from an accretion disk or savage dust clusters? haven't seen one such case yet, and since there are billyons and billyons of stars, as Carl Sagan used to say, there must be one now somewhere. |
| Posted by: twobyfour 2005-05-26 00:52 |
| #2 ...the lunar bombardment occurred around 600 million years after the Sun burst into light and the planets start to form, building up from clusters of primitive dust. By that time, the Solar System should - in theory - have been a relatively calm place. Sounds like the theory is wrong. Change it. That's called "science". And: "primitive" dust? |
| Posted by: mojo 2005-05-26 00:39 |
| #1 I tihnk they're neglecting a couple possibilities. If Jupiter's migrating, the asteroids in the main belt are migrating too. But the resonances that Jupiter's swept clean in the main belt are migrating at different rates than the asteroids are, and they're going to get disrupted. In short, something that was skirting the edge of a Kirkwood Gap is suddenly going to be _in_ the gap, in a resonance with Jupiter, and disrupted. I know I need to double-check the math on this. I didn't know that many people were doing work on Jupiter possibly moving around in the early solar system. |
| Posted by: Phil Fraering 2005-05-26 00:15 |