Discovery quadrangle
The Discovery quadrangle lies within the heavily cratered part of Mercury in a region roughly antipodal to the 1550-km-wide Caloris Basin.
Because of its small size and very early segregation into core and crust, Mercury has seemingly been a dead planet for a long time—possibly longer than the Moon.
As on the Moon and Mars, sequences of craters and basins of differing relative ages provide the best means of establishing stratigraphic order on Mercury.
Over all of Mercury, the crispness of crater rims and the morphology of their walls, central peaks, ejecta deposits, and secondary-crater fields have undergone systematic changes with time.
The Discovery quadrangle includes some of the most distinctive relief-forming material on the planet, the hilly and lineated terrain unit mapped by Trask and Guest.
Most craters within this material appear to predate its formation, and their ages cannot be estimated: their rims have been disrupted into hills and valleys identical to those of the hilly and lineated unit; the floors of some of these degraded craters contain hummocky plains material that resembles the hilly and lineated unit, except that the hills are fewer and lower.
Joint-controlled mass movements were most likely responsible for the fact that many craters of all ages have polygonal outlines, and some linear joints may have provided surface access for lavas that formed the intercrater plains.
Vostok transects and foreshortens the crater Guido d'Arezzo, which suggests that arcuate scarps are compressional tectonic features (thrust or high-angle reverse faults).
Melosh and Dzurisin[11] have speculated that both arcuate scarps and the global mercurian lineament pattern may have formed as a result of simultaneous despinning and thermal contraction of Mercury.
Both facts can most easily be accounted for by the presence of an iron core, possibly liquid, roughly 4,200 km in diameter, overlain by a silicate crust a few hundred kilometers thick.
The postulated volcanic origin of a substantial fraction of the Mercurian plains also implies a thick silicate crust, and thereby supports the existence of a large iron core.
Melosh and Dzurisin[11] have pointed out the similarity between this predicted tectonic pattern and that observed on Mercury, and they have proposed that the global system of lineaments and arcuate scarps, which is well developed in the Discovery quadrangle, formed in response to early, simultaneous planetary contraction and tidal despinning.
Formation of intermediate and smooth plains materials may have been abetted by the c3 and c4 crater- and basin-forming events that opened up temporary magma conduits.
One of the latest large impacts was the Caloris event, which occurred on the other side of the planet from the Discovery quadrangle and which may have initiated formation of the hilly and lineated material within it.
