Slashdot Mirror
Mars Polar Cap Mystery Solved
Matt_dk writes "Scientists are now able to explain why Mars' residual southern ice cap is misplaced, thanks to data from ESA's Mars Express spacecraft (the same probe running the 'Mars Webcam'). It turns out the martian weather system is to blame. And so is the largest impact crater on Mars — even though it is nowhere near the south pole. Like Earth, Mars has frozen polar caps, but unlike Earth, these caps are made of carbon dioxide ice as well as water ice. During the southern hemisphere's summer, much of the ice cap sublimates, a process in which the ice turns straight back into gas, leaving behind what is known as the residual polar cap. The mystery was that while the winter cap is symmetrical about the south pole, the residual cap was offset, and scientists couldn't figure out why."
You seem to be confusing two different effects of a large asteroid impact - initial and long term.
...
One of the initial effects will be a huge cloud of dust, blocking almost all sunlight for years at at time - this is what caused the mass extinction at the end of the cretaceous. Others include giant tsunamis (if it crashes into a liquid), 'rain' of molten rock and ash, earthquakes, ect
However, on Earth, there are few long term effects, none of which include altering global weather patterns long term. This is due to our planet's ever-changing surface, with new land being created, land being destroyed, and of course continental drift. Mars has no active volcanism and no continental drift, therefore a surface feature which would be rapidly (in geological terms) altered on Earth would last for a very, very long time on Mars.
Another thing is that we have a much thicker atmosphere, reducing the size of the asteroid before impact, AND the probability of it hitting solid ground as opposed to water is about 30/70.