On Jan 4, 4:11 am, James Annan <[email protected]> wrote: > Phil Hays wrote: > > On Sun, 2010-01-03 at 09:27 -0800, Alastair wrote: > > >> Hansen seems to believe that Venus had oceans, but that is now seeming > >> unlikely. It is now thought that Earth's oceans were formed from > >> impacts from Main Asteroid Belt Comets. > >>http://www.solstation.com/stars/asteroid.htm > >> These are icy outer asteroids which had there orbits distorted by > >> Jupiter. As their orbits became more elliptical, they collided with > >> Earth, which protected Venus and kept it dry. The runaway state of > >> Venus's atmosphere must be due to CO2, not H2O! > > > A scale model might help to visualize. > > > If the Earth is a peppercorn, then the Sun is a 20 cm ball about 26 > > meters away. Get a peppercorn and a ball, and make a scale model of the > > Earth/Sun system. If you please, you could also add Venus another > > peppercorn 19 meters from the Sun, and the Earth's Moon, a bit of gravel > > 2 cm away from the Earth. Now imagine that there are comets coming from > > the outer solar system, in the neighborhood of Jupiter, another 110 > > meters beyond the Earth. > > > Just how many of these comets will the Earth prevent from hitting Venus? > > I don't know much about planetary evolution, but I've read enough to > know that there are some pretty counterintuitive (to me) things going > on, mostly due to the amazingly long time scales involved. > > In this case it doesn't seem too implausible that if the comets are > slowly nudged into increasingly elliptical orbits, they will almost > invariably get caught by the Earth before the orbit becomes sufficiently > distorted to reach Venus. But I'm just guessing really. > > James
Yes, as the major axis increases due to the gravitational pull of Jupiter, the minor axis decreases until it reaches 1AU, then crash! The major axis rather than the minor will increase because that is when the asteroid is on the corresponding part of the ellipse that it is nearest Jupiter, and so Jupiter has a greater gravitational effect. A sort of positive feedback! BTW I misunderstood the term "Main Belt". It refers to the whole of the Asteroid Belt, not a part of it. It is used to distinguish objects in the Asteroid belt from those in the Kuiper and Oort belts. However, that does not negate the theory. The outer asteroids in the Main Belt tend to be icy, and they are the ones that are most affected by Jupiter, so it is logical that they would be the cause of the Late Heavy Bombardment. http://en.wikipedia.org/wiki/Late_Heavy_Bombardment Note, this is all 21st Century science, dating back only to 1999. Cheers, Alastair. -- You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/globalchange
