QUESTION: How big are the largest impact craters on Mars and Earth? ANSWER from Geoff Briggs on January 31, 1997: The craters on Mars range all the way from giants like Hellas -- which is termed an impact BASIN because it is so large (about 1000 miles across) -- all the way down to craters a few yards across. The martian atmosphere burns up infalling meteors that would otherwise make even smaller craters (as is the case for the Moon which has no atmosphere). Note that on Earth the thicker atmosphere burns up many meteors that would otherwise make sizeable holes. On Venus with its heavy atmosphere even fewer meteors reach the surface. Most of the heavily cratered terrain is found in the southern hemisphere which has been accumulating impacts since the beginning of martian history (over 4 billion years). Cratering was most intense near the beginning. The giant basins formed during that time. Smaller impacts are still occuring. The northern hemisphere has had its heavily cratered terrain destroyed by geologic processes not fully understood (including floods of volcanic lava) so that the northern plains have accumulated craters for less time and do not have the record of the heavy early bombardment. There is a book devoted to this subject available from the NSTA -- "Craters" by William Hartmann. ANSWER from Internet: The URL http://seds.lpl.arizona.edu/nineplanets/nineplanets/mars.html lists Hellas Planitia in the southern hemisphere of Mars as the largest impact crater. It is 2000 km in diameter and 6 km deep. The URL http://gdcinfo.agg.emr.ca/cgi-bin/crater/crater_table?e contains a table of known impact craters on earth. It shows the largest as the Vredefort crater in South Africa with a diameter of 300 km, with an age of 2,081,000,000 years. The URL http://bang.lanl.gov/solarsys/tercrate.htm#intro shows the Chicxulub crater, Yucatan Peninsula, Mexico as also being 300 km in diameter. This crater is 64.98 million years old, and some scientists believe it is the result of the impact of a 10-20 km diameter asteroid which may have caused the demise of the dinosaurs. ANSWER from Bruce Jakosky on November 24, 1997: The biggest impact crater on Mars is the Hellas basin. It's a large impact basin (meaning that it is larger than a crater), about 1500 km across. It's one of the oldest features on Mars, and has been heavily eroded by subsequent processes, including later impacts.