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At such speeds, the projecticle explodes on impact and carves out a round bowl-shaped depression on the surface. How can you distinguish an impact crater from a volcanic crater?Volcano craters are above the surrounding area on mountaintops while the craters from impacts are below the surrounding area with raised rims.Most of the impact basins---craters measured in hundreds of kilometers---were made during this time.It is noteworthy that about the time the heavy bombardment ended, life took hold.Victoria Crater on Mars as seen by the Mars Reconnaissance Orbiter. It is a simple crater that has been partially filled in with sand---note the sand dunes on its floor.If you download the full-resolution version of this picture from the link on the picture, you will be able to see the rover, Opportunity next to the top left crater rim.
Even Venus with its thick atmosphere has impact craters, though they all have diameters measured in kilometers because smaller projecticles burn up in its atmosphere.Careful studies of how the craters overlap other craters and other features can be used to develop a history or sequence of the bombardment on the moons and planets. Worlds with less volcanism or erosion or tectonic activity in their history will retain more impact craters since the planet formed.Worlds with more geological or erosional activity will have newer surfaces or craters that have been so worn away as to be unrecognizable. Terrestrial planets have hard surfaces that can be re-shaped by several different processes: impact cratering, volcanism, erosion, and tectonics. Links to external sites will be displayed in another window.The oldest fossil evidence of ancient organisms dates back to 3.5 billion years ago and evidence for biological activity based on isotopic ratios of carbon date back to about 3.85, even up to 4.2 billion years ago, though the carbon isotope ratio evidence is controversial.The number of craters per unit area on a surface can be used to determine an approximate age for the planet or moon surface if there is no erosion.The size of the craters having central peaks depends on the gravity of the planet or moon: on the Moon craters larger than about 60 kilometers in diameter have central peaks while the crater diameter on the Earth needs to be larger than just 1 to 3 kilometers.Impact cratering was especially prevalent for the first several hundred million years after the planets formed as the planets swept up left-over material.The craters on all of the moons except Io, Mercury, and most of the ones on Mars are from impacts.The kinetic energy of the impacting meteorite or asteroid is converted into heat, sound, and mechanical energy---the projectile explodes on impact.