Rod Yazzie, Crownpoint Institute of Technology; Jesse Yoburn, Franklin & Marshall College
Determining absolute ages on Mars may have to wait for multiple sample returns, as absolute ages can only be accurately determined through radiometric dating, similar to what has been done for rocks on Earth and from the Moon. Surface-feature dating is thus entirely relative until a sample is retrieved from a specific location and dated radiometrically. Because relative and absolute ages of rocks on the Earth and Moon are well constrained, and relative ages on Mars can be determined via superposition and cross-cutting relationships, we can attempt to relate relative Martian ages to absolute lunar ages through crater dating. Crater-counting techniques have been evolving since the 1960s, when scientists determined surface ages for the Moon by comparing crater size and frequency to North American craters (these dates were later found to be almost exactly correct through rock samples from the lunar missions). Lunar craters were then used as a guide for studying craters on the surface of Mars (Hartmann, 1966).
Our study area on Mars is located between 176 ° and 190 ° West, and 2 ° and 21 ° North in the Elysium Planitia region, surrounding Orcus Patera (Figure 1). The area was chosen for its wide range of geologic features, varied topography, good satellite-image coverage, and possible interest as a site for future Mars missions. Nearby features include the Medusae Fossae formation, numerous young lava flows (Hartmann and Berman, 2000), remnants of cratered terrain, and possible recent fluvial activity.
In this paper we first identify and map the units in our area using characteristics such as albedo variation and observed texture. Next we determine the relative ages of these units using cross-cutting relationships and superposition. Third, we test this stratigraphy by using crater counts to assign an absolute age to each unit. Finally, we integrate all relative age information, including absolute age approximations, in order to develop a well-constrained understanding of the regional geologic history.
In conclusion, our absolute age calculations resulted in ages older than what we expected, although they agreed with our relative age predictions. The relative ages were placing the region in the early Hesperian to late Amazonian epochs, while the absolute ages were placing the region in the middle Noachian to late Amazonian. A difference that was expected due to the separate techniques used in determining the ages.
We found that Hartmannís method of crater counting gave us a better range of ages than Neukum and Wiseís method, especially when the area being studied had a small range of crater diameters. Our data showed that areas with a wide range of craters were older than areas with all of its craters nearly the same size, which was statistically logical. Overall, our study area was shown to have a mixture of very young and very old surfaces; the rough and knobby area on the West side was dated to over 3.5 billion years old, and the smooth region to the South and East of Orcus Patera was found to be just over 500 million years old.