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Mars Rover Investigates Possibility of Ancient Microbial Life
Riding with Robots writes "The robotic geologist Spirit, now scurrying to reach a safe haven before the harsh Martian winter sets in, has found signs that explorers say point to hot springs or fumaroles in the Red Planet's distant past. That possibility is not only interesting geologically, but potentially biologically, since those kinds of environments on Earth teem with microbial life. Meanwhile, on the other side of the planet, Opportunity continues its descent into a deep crater, where it has found other clues about the ancient waters of Mars."
It really depends on the ratio of strikes big enough to propel chunks that can escape either planet's gravity. That, and the fact that loose fragments are more likely to tumble inwards towards the sun as well, put the odds in favor of our being more hit prone, IMHO.
From long ago, perhaps, but there is an important difference in the past several (hundred? thousand?) millenia; Earth has a much thicker (pressure-wise) and thicker (altitude-wise) atmosphere than Mars does. A meteor(ite) that enters Earth's atmosphere is far more likely to burn up before impacting than one of the same size entering Mars' atmosphere. Furthermore, because Earth is so much bigger, we have a deeper gravity well, meaning you need a greater impact energy to get ejecta to reach escape velocity.
Aikon-
Not that I relish being negative, but I don't recommend getting too excited. I've seen way, way too many of these kinds press-releases -- especially about Mars and especially-especially about evidence of life there -- to get excited yet. Even if they find any evidence of life, it will almost certainly quickly be disputed by other groups and then counter-disputed by the original group and the whole thing will turn into a non-score tie. It also seems like the Martian astrobiology folks have a rather lower threshold for "exciting evidence" of life than many of the rest of us. (Maybe that's a selection effect and maybe that's just common to all scientists, each in his or her own field.)
It wasn't merely as insurance against a failure. It was also insurance against landing someplace utterly boring with one of them. Their landing method wasn't capable of getting into the places they knew were interesting, so they picked spots where they had a good chance of landing within driving distance of something that looked interesting from orbital images.
They originally thought they struck out with Spirit. Sure they had a few rocks to grind on, but they were all ejecta from the lava plains. Basalt isn't very interesting if you're looking for evidence of past water.
However, as the mission wore on, they realized they would probably have time to drive to Columbia Hills, which they in fact accomplished, and that turned out to be a trove of information. These possible fumeroles are on the far side of Columbia Hills from where they landed.
Opportunity was just lucky from the beginning. Sucker landed right inside a crater. The Spirit team was more than a little jealous.
Which gives it a good opportunity to hit any planet that's farther inward.
slingshots around the sun and back out, so I don't buy your theory.
It doesn't matter in which direction it goes first. To hit something that's farther outward, it needs to be launched with enough velocity to a) escape the first planets gravity well (good point made by some other posters here) and b) move away from the sun far enough to actually cross the orbit of one of the outward planets.
Even if something "slighshots" around the sun, it won't go farther out than its point of origin plus some distance it gets from its initial velocity. It's not a real "slingshot" maneuver, just an orbit. In order to pick up velocity (with a real slinghshot maneuver, like the ones performed by space probes), it would have to approach a planet at juuuuuust the right angle ... a fairly rare event.