Always entertaining Sterling, and as a consequence, forces mathematically challanged schlubs like yours truly onward and "upward" [I think] through to your conclusions. Jerry Flaherty ----- Original Message ----- From: "Sterling K. Webb" <[EMAIL PROTECTED]> To: <meteorite-list@meteoritecentral.com> Sent: Monday, January 29, 2007 4:47 PM Subject: Re: [meteorite-list] Average size of craters across the solarsystem?
Hi, List, The "head-on" collision is a real rarity because of its improbability. But all things happen given time enough. A comet rounding the Sun on a highly eccentric orbit could have its perihelion at the orbit of your planet and be traveling in the opposite direction. If the comet was hyperbolic (like SWAN M4), it would be moving at just above the local escape velocity from the Sun at that orbital distance. The combined collision velocities would be 2.414 times the orbital velocity of the planet. (Escape velocity is 1.414 times orbital velocity.) For Mercury, that would be 115.64 km/sec. For the Earth, 72.94 km/sec. But the "head-on" is unlikely. A bit more likely is "crossing paths," when a body in a highly eccentric orbit smacks us upside the planet, a right-angle collision. The velocity there is that of the comet plus the acceleration of the Earth's gravity on it, a complicated sum that depends on the precise details, but likely an impact velocity of ~ 54 km/sec for the Earth. For smaller random bodies, the minimum impact velocity is the velocity gained by falling down the larger body's gravity well, the escape velocity of the planet, if the falling body started "at rest" at a distance from the planet. But few things are "at rest" in the solar system. "Common" objects (like most meteoroids) have terminal velocities of 15 to 25 km/sec, 11.2 km/sec gained from falling to the Earth and the rest is what they approached with. At Mercury, things are likely to be approaching much faster, so velocities, as a statistical matter, are likely higher. The energy of the collision goes up by the square of the velocity, so, yeah, speed counts more than mass. [This is an excursion.] A standard "BB" weighs 0.12 gram. If it fell in through the Earth's gravitation field, its energy would be 15,053 Joules, or 0.00178 pounds of TNT: the explosion of about a gram of TNT. Hmmm. Let's shoot that BB out a rail gun at 500 km/sec (quite achievable); now it has the impact energy of 14.33 pounds of TNT. Heck, let's boost it up 1% of the speed of light. Impact energy? 2400 TONS of TNT. Just stand off a few billion miles and fire three pound iron balls at a planet at 1% of light speed. 25 Megaton impacts. Call it the Tunguska Cannon. Luckily, gravity wells are natural limiters of speed; go too fast and you're out of here. The other end of the speed problem comes up a lot in modeling "giant" collisions, like the formation of the Moon by the "impact" of a Mars-size body on the Earth. It just doesn't work if the two big bodies are moving very fast relative to each other when they smack or even just graze. Escape velocity is way too fast. They have to "kiss" at only 1 or 2 km/sec or even less. How the heck does that happen? The only way is if the two bodies are in very similar orbits with similar velocities and are perturbed gently into each other. How do you get a Proto-Earth and a Mars Mass (or two) into the same orbit? I would propose that the Proto-Moon was a big Earth-orbit Trojan that was perturbed out of its Lagrangian resonance and drifted along the orbit until we met up with each other. > would a 10 cm object hitting Mercury at top velocity > not make a larger crater with Mercury's larger velocity? Yes, on average, but there's so much variation in mass and speed circumstances that averages don't mean much. Callisto is not massive nor does it have a high orbital velocity, yet the Valhalla Basin is 4000 km. The impactor was probably pretty good sized! The fact that little Mars has so many big basins suggests multiple big impactors. I listed the biggest hits because impactors follow a statistical distribution of sizes and energies, a power law with a variable coefficient. The size and number of the biggest ones is a good indicator of the size of the impactor population. I would propose that Mars had a bigger impactor population than other planets. (I told you this was a bad neighborhood.) And the "saturation" (means new craters just destroy old craters) of most cratered surfaces wipes out the fine details of the cratering population, so that all cratering populations look alike after a while. All the airless rocky bodies have more or less the same statistical distributuon of craters. There's always enough rocks to go around. The big basins are the only clues left. As for the three meteorites spotted on Mars, well, any object that makes it safely to the surface of any planet is a lucky bird, a one in 100,000, one in a million, and that phenomenon is really unrelated to the "big picture." Meteorites are improbabilities. They most likely have a) low approach velocities, b) low angles of incidence to the atmosphere, c) an initial or intermediate aerodynamic shape, and d) luck (again). That the rovers have now spotted three meteorites on Mars in such a tiny land area of the planet as they survey suggests there may be a lot of meteorites on Mars. Why? Using our terrestrial minds, our first thought is that lots of meteorites must fall. But it ain't necessarily so. The surface could just be very old, quite undisturbed. Frankly, I'm beginning to get suspicious about Mars. Dating the surface is "entangled;" so much human expectation is involved. Thanks to the wonderful rovers and orbiters, we've had the opportunity to watch Mars for these past years. (By we, I mean us poor schlubs with computers going to the websites.) OK, we've got THREE new sub-striations in gullies in five years, and... Anything else? Mars is a big place. Something must be happening, says our Earthly expectation. Mars is as big as the Earth! Before somebody dashes to correct me, here's what I mean: the land area of Earth is 148,939,100 km² and the land area of Mars is 144,798,465 km² because ALL of Mars is land area. Anyway, as I watch the surface, I'm starting to get the impression that most of the surfaces we see are old, really OLD. Has that iron meteorite been sitting there for a thousand years? A million years? A billion years? Our terrestrially trained minds want to say, "A billion years? That's silly!" But is it? A mud flat, a dune field, pebbles on the ground. On Earth, they are transient phenomena; take your eyes off them, something happens to them. But on Mars? Maybe the Red Planet is really the Dead Planet. (So depressing; give me back those canals!) We are SO invested; we find a new scratch in the side of a gully where there are thousands of scratches in one gully among the ten thousand gullies and we want to party all night! Doesn't mean the gullies haven't looked pretty much the same for the last 1.4 billion years. Sterling K. Webb --------------------------------------------------------------------------- ----- Original Message ----- From: "Darren Garrison" <[EMAIL PROTECTED]> To: <meteorite-list@meteoritecentral.com> Sent: Sunday, January 28, 2007 11:25 PM Subject: Re: [meteorite-list] Average size of craters across the solarsystem? On Sun, 28 Jan 2007 22:41:40 -0600, you wrote: >Hi, > > The biggest craters are multi-ringed; they are >big enough that they are called "multi-ringed basins" >or just "basins." Properly, I suppose we should >call them "impact features" rather than craters. Not so much the biggest craters, but I'm wondering if all craters tend to be larger-- for example (no attempt at accurate figures here) if a 10 cm object hitting the moon at the top valocity for an object hitting the moon (a "head on" collision" made a crater 5 meters across, would a 10 cm object hitting Mercury at top velocity not make a larger crater with Mercury's larger velocity? And wouldn't Mars' slower speed mean for "lighter" hits than for the moon (or Earth)? Which could factor into how iron meteorites are surviving to be found on the surface of Mars by the rovers, even though Mars' thinner atmosphere means less loss of speed? http://www.sjsu.edu/faculty/watkins/orbital.htm ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list