...Or Hit It With A Rock?

"How about just enough to start 'climate change'..any guestimate on how big and fast an impact would need to be - presumably would also depend on where it impacted?? Posted by pmn1 at Space.com" Any SINGLE impact to "...start 'climate change...'" to a significant degree would have to be something in the vicinity of the Cretaceous-Tertiary Event (AKA "bye-bye, dinosaurs!"), initiating a notorious period of mass extinction on Earth. Better make sure no-one's on the surface of Mars if you're dropping one of those! It left the approximately 300km wide Chicxulub impact basin in northern Yucatan, Mexico. It's believed that 65 million years ago, a 10 kilometer diameter asteroid penetrated Earth's crust at a speed of 15 to 20 kilometers per second; the kinetic energy equalled the energy of 300 million nuclear weapons and created temperatures hotter than on the sun's surface for several minutes. Incredibly, Mars has taken much bigger hits than this and survived (leaving the 2,300 km Hellas impact basin) - but probably lost its atmosphere in the process. Before we seriously consider a martian bombardment, we should consider the possibility of unintended side-effects. As previously pointed out, a really massive collision can entirely destroy a planet - where DID that asteroid belt come from...? A smaller, but still sizable collision might not destroy the planet, but could have undesired effects elsewhere on the planet's surface, not just at the impact site - check out the diagram here: http://books.google.ca/books?id=kAup0TOL09gC&pg=PA135&lpg=PA135&dq=impact+outgassing&source=bl&ots=218_rgLq6_&sig=qeYdV3FxlRk_i2v-XMJbeTiIRF8&hl=en&ei=pq-jSdXHAYmMsAPIjdmjAg&sa=X&oi=book_result&resnum=8&ct=result#PPA542,M1 and the text reads: "...an antipodal relationship between the Hellas impact basin and the Alba Patera volcanism on Mars was pointed out....the convergence of seismic waves was intense enough to fracture the Martian crust." Also consider the immediate cooling effect of the dust clouds in the atmosphere, along with other unpleasantness: http://www.lanl.gov/quarterly/q_spring03/asteroid_text.shtml