The main goal of this site is to advocate for a regional approach in planetary studies focused on the morphology of the planetary surfaces rather than the standard approach based on interpretation of lithostratigraphic units. The former approach was developed for use with the geologically simple Moon, and is less useful when applied to the more dynamic environment of Mars. Lunar geologic units are identified by morphology, which is assumed to be diagnostic of origin and mainly unchanged by later evolution. Because the environment of Mars, especially its capacity for erosion by eolian, fluvial and periglacial processes, is so different from the lunar case, discrimination of surface geologic units by texture is more difficult and potentially confusing.
The first full geologic mapping of Mars, at 1:25,000,000 scale, was based on Mariner 9 images (Scott and Carr 1978). Since then the resolution of satellite images has been continuously improved from about 1 km/pixel (Mariner 9) to less than 1 m/pixel today. Although very rough global topographic maps were compiled from Mariner 9 data and earth-based radar data in the early 1970s, there was no reliable database until Mars Global Surveyor (MGS) arrived at the planet in 1997. A high resolution global digital elevation model (DEM) dataset was released in the late 1990 based on the MGS MOLA (Mars Orbiter Laser Altimeter) instrument. It has a 160-m spot size and a ranging precision that varies from ~2 m up to ~30 m, depending on surface slope (Smith et al. 1999).
As a consequence, studies of the Martian surface have been focused on progressively smaller areas over time, as higher resolution data resolved ever-smaller details. This allowed the mapping of regional to local geologic sequences and the refining of ages of smaller deposits or landforms. Three main issues complicate these efforts:
(1) The planetary geologic methods describe the surface morphologies as stratigraphic units. Delineation of successive stratigraphic series uses visual similarities among deposits scattered across Mars and crater counts on larger areas than that which characterize individual landforms seen on higher resolution images. But at higher resolution, images are more likely to reveal smaller erosional textures than the primary depositional textures which reveal unit origins.
(2) Lunar-type stratigraphic markers (large impact basin ejecta blankets that formed hemisphere-scale markers almost instantaneously) are unknown on Mars and contacts among different stratigraphic units produced by smaller impact events are often eroded, buried or exhumed by morphological processes (Wilhelms 1990).
(3) Primary impact craters are eroded, buried or exhumed by geological and morphological processes in ways not seen on the Moon, and counts are also affected by the presence of secondary craters, both of which have consequences in either under or over estimation of the age of medium to small-size deposits, and in developing serious difficulties in evaluating the crater retention on those deposits (McEwen and Bierhaus 2006, Malin et al. 2006 and references therein). The view of Mars today is of a planet with a continuous history of surface modification, which is very different from the essentially dead lunar surface.
Nowadays there is a gap between the scale of study in former geologically-based approach, and new high-resolution imagery that describe more accurately the processes on Mars. This gap refers to the hierarchy of resurfacing events (high- magnitude events – impact cratering, volcanism and tectonism subordinate the low magnitude events – the exogenic events), to the scale of study (a geologic sequence incorporates many medium-sized landforms), and to the temporal extent of different processes that shaped the Martian interior and surface. For instance, a fluvial morphology superposed on a mid-Noachian cratered deposit is considered as old as the cratered material, about 4.4 to 4.1 billion years old, without distinguishing the different ages of landforms of very different scales. As an example of this point, consider the terrestrial case, where at the continental-scale satellite images and maps show cratons, ocean basins and mountain belts. At regional-scale terrestrial maps reveal watersheds, folds and faults, and at the local-scale maps or images show small gullies on a hillside, slumping on an undercut river bank, slumps on a hillside or arctic polygonal terrain. In the Martian case the highland/lowland dichotomy, large impact basins and volcanic provinces such as Tharsis are visible at a planetary scale. At intermediate-scales individual impact craters, volcanoes and large outflow channels are seen, and at the local-scale small gullies, dunes, wind streaks and landslides can be mapped. Thus, the geologic processes seen at different scales are very different, and so are the timescales involved, ranging from a billion years to tens of millions of years, and down to a few years or even months. So on Mars, current approaches of mapping and dating based on low resolution images and new methods based on very high resolution images are different due to the data scale.
References:
Malin, M. C., Edgett, K. S., Posiolova, L., McColley, V., Shawn, M., Dobrea, N., Eldar, Z. 2006. Present-day impact cratering rate and contemporary gully activity on Mars. Science, 314. DOI: 10.1126/science.1135156.
McEwen, A.S. and Bierhaus, E.B. 2006. The importance of secondary craters to age constraints on planetary surfaces. Annual Rev. of Earth and Planetary Sciences, 34, pp. 535-567.
Scott, D.H. and Carr, M.H. 1978. Geologic map of Mars., M Series 1:25M, I-1083 US Geological Survey, Reston, VA, USA
Smith, D. E. and 18 collaborators 1999. The global topography of Mars and implications for surface evolution. Science, 284, pp.1495-1503.
Wilhelms, D. E. 1990. Geologic mapping., in Planetary Mapping, ed. by R Greeley and R. M. Batson (New York Cambridge Univ. Press. 1990), pp. 208-260.