A research program was designed (1) to map regional lithological unitsof the lunar surface based on measurements of spatial variations in spectralreflectance, and, (2) to establish the sequence of the formation of suchlithological units from measurements of the accumulated affects of impactingbodies.
Spectral reflectance data were obtained by scanning luminancevariations over the lunar surface at three wavelengths (0.4µ, 0.52µ, and0.7µ). These luminance measurements were reduced to normalized spectralreflectance values relative to a standard area in More Serenitotis. Thespectral type of each lunar area was identified from the shape of itsreflectance spectrum. From these data lithological units or regions ofconstant color were identified. The maria fall into two major spectralclasses: circular moria like More Serenitotis contain S-type or red materialand thin, irregular, expansive maria like Mare Tranquillitatis contain T-typeor blue material. Four distinct subtypes of S-type reflectances and two ofT-type reflectances exist. As these six subtypes occur in a number of lunarregions, it is concluded that they represent specific types of material ratherthan some homologous set of a few end members.
The relative ages or sequence of formation of these more units wereestablished from measurements of the accumulated impacts which haveoccurred since more formation. A model was developed which relates theintegrated flux of particles which hove impacted a surface to the distributionof craters as functions of size and shape. Erosion of craters is caused chieflyby small bodies which produce negligible individual changes in crater shape.Hence the shape of a crater can be used to estimate the total number of small impacts that have occurred since the crater was formed. Relative ages of a surface can then be obtained from measurements of the slopes of the walls ofthe oldest craters formed on the surface. The results show that different mariaand regions within them were emplaced at different times. An approximateabsolute time scale was derived from Apollo 11 crystallization ages under anassumption of a constant rote of impacting for the last 4 x 10^9 yrs. Assuming,constant flux, the period of mare formation lasted from over 4 x 10^9 yrs toabout 1.5 x 10^9 yrs ago.
A synthesis of the results of relative age measurements and of spectralreflectance mapping shows that (1) the formation of the lunar maria occurredin three stages; material of only one spectral type was deposited in eachstage, (2) two distinct kinds of maria exist, each type distinguished bymorphology, structure, gravity anomalies, time of formation, and spectralreflectance type, and (3) individual maria have complicated histories; theycontain a variety of lithic units emplaced at different times.
