Exploration of Jupiter disadvantages for colonization because of the severe radiation conditions prevailing in Jupiter's magnetosphere and the planet's particularly deep gravitational well. Jupiter would deliver about 36 Sv (3600 rem) per day to unshielded colonists at Io and about 5.4 Sv (540 rems) per day to unshielded colonists at Europa, which is a decisive aspect due to the fact that already an exposure to about 0.75 Sv over a period of a few days is enough to cause radiation poisoning, and about 5 Sv over a few days is fatal. Ganymede is the Solar System's largest moon and the Solar System's only
Geological history of Mars of Paleozoic, Mesozoic, and Cenozoic. The same methodology was later applied to the Moon and then to Mars. Another stratigraphic principle used on planets where impact craters are well preserved is that of crater number density. The number of craters greater than a given size per unit surface area (usually million km) provides a relative age for that surface. Heavily cratered surfaces are old, and sparsely cratered surfaces are young. Old surfaces have a lot of big craters, and young surfaces have mostly small craters or none at all. These stratigraphic concepts form the basis for the Martian geologic timescale.