"Buried Ancient Lowlands on Mars: Laser Altimetry, Hidden Craters, and Primordial Crustal Dichotomy"
The outstanding problem in the crustal evolution of Mars
is the origin of the martian crustal dichotomy: how
and when the separation into what are now ancient cratered
highlands and young lowland plains occurred. Explanations for this
fundamental feature of the martian crust range from purely exogenic (one
or more giant impacts) to purely endogenic (single-cell mantle convection;
sub-crustal erosion; plate tectonics). Mars Orbiter Laser Altimeter (MOLA)
data have revealed a very large population of quasi-circular depressions
(QCDs) in the northern lowlands of Mars. Most of these are probably buried
impact basins. If true, the lowlands are extremely old (even though
the plains which cover them are relatively young), and likely date from
the earliest period of martian history. This new constraint on the
possible “when” the crustal dichotomy formed helps limit the possible “how’s”.