Note that the ridged plains partially bury many of the old Noachian-aged craters. These ridged plains form the base of the younger Hesperian System (pictured right).
Image is THEMIS IR mosaic, based on similar Viking photo shown in Tanaka et al. The lower stratigraphic boundary of the Noachian System is not formally defined.
MOLA colorized relief map of Noachis Terra, the type area for the Noachian System.
Note the superficial resemblance to the lunar highlands.
Units of similar age are grouped globally into larger, time-stratigraphic (chronostratigraphic) units, called systems.
Periods are measured in years, determined by radioactive dating.The system was conceived originally to encompass rock units dating back to the formation of the crust 4500 million years ago.However, work by Herbert Frey at NASA’s Goddard Spaceflight Center using Mars Orbital Laser Altimeter (MOLA) data indicates that the southern highlands of Mars contain numerous buried impact basins (called quasi-circular depressions, or QCDs) that are older than the visible Noachian-aged surfaces and that pre-date the Hellas impact.Many of the large impact basins on the Moon and Mars formed at this time.The Noachian Period is roughly equivalent to the Earth’s Hadean and early Archean eons when the first life forms likely arose.