About the only thing that has happened on many Solar System bodies since their formation is cratering. Fortunately, craters are rich sources of information on the history of planetary bodies.
Superposition is easy to apply on planetary surfaces: young craters overlap older ones.
|Archimedes, the large crater, was formed before the flooding of the Imbrium Basin by mare basalts. It is partially buried and filled by basalts. Interestingly, there is no obvious inlet for the lava - it must have erupted independently inside the crater or entered through crustal cracks.|
|The largest crater in this picture is the oldest. The large crater inside it in the upper left is obviously younger, and it also looks less degraded. The smaller craters within that crater are younger yet.|
|Mare Orientale on the Moon - 1000 km in diameter - is the archetypical multiple-ring impact basin. Perhaps a billion years after the impact, lava flooded the low-lying parts of the basin and also ponded against some of the scarps. We can tell there was an appreciable interval between the impact and flooding because there are several large craters within the basin, some flooded with lava (trust me - there won't be any pre-basin craters left after an impact big enough to produce the Orientale Basin!)|
The very youngest craters are surrounded by bright rays of fresh ejecta that stand out under high sun illumination. As the crater ages, the rays disappear as small impacts churn up the ejecta. Very old craters are overlapped by numerous younger craters. Their rims and wall terraces are rounded, softened, and obscured
|This small pit crater has a sharply-defined rim and is obviously quite young.|
|Tycho, the youngest large crater on the Moon, is still quite sharp and fresh looking.|
|The largest crater in this picture is old. It is overlapped by many small craters, its rim is rounded and indistinct.|
The formation of the planets by accretion was essentially a process of continuous cratering. Bodies with primordial crust should be crater saturated, that is, every square meter has been cratered many times over and every new crater overlaps an older one. Whenever bodies exhibit low crater densities, we immediately ask what obliterated the craters.
|An area of low crater saturation on the Moon. Apart from the two large craters, very few craters are visible.|
|This view shows ancient, crater-saturated highland terrain on the Moon (about 4.6 billion years old) and much younger and less saturated mare terrain (about 3 billion years old). If the maria were also 4.6 billion years old, they'd only have 50% more craters and would still not be as saturated as the highlands. Clearly, therefore, the rate of cratering must have decined sharply before the maria formed.|
|The "crater farm" on Venus is one of the nicest cluster of craters but note the near-total lack of craters otherwise. Venus' thick atmosphere prevents a lot of cratering, but the lack of large craters points to a widespread resurfacing of Venus.|
|This channeled terrain on Mars clearly postdates the large craters, many of which are eroded. However, the large number of fresh small craters dotting the channeled terrain shows that the channeling was a brief episode that ended a long time ago, perhaps a billion years.|
|Most of Mercury has moderate saturation and wide intercrater plains. Apparently Mercury was volcanically resurfaced late in the final accretion process, but early enough that substantial numbers of craters formed afterward. Patches of high saturation might reflect high-standing ancient crust but could also be chance clusters of craters. Note that most of the craters are fresh-looking. Several very subdued circular depressions, possibly reflecting buried craters, are also visible.|
|This view is typical of the more highly-saturated areas on Mars. The flat floors and shallow depths of some craters suggests infilling, perhaps by lava.|
|An area of highly-saturated, ancient terrain on the Moon.|
|The dull side of Callisto, opposite the spectacular impact basins. Callisto is the most cratered body in the Solar System.|
Created 6 April 1999, Last Update 10 May 1999
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