Mars
I. Survey of major geologic features.
A. Crustal dichotomy: In most places defined by a fairly abrupt change of about 5 km in elevation, the northern 40% of the planet sits lower than the southern 60% and the two regions have very different properties.
1. Southern Highlands:
a. Heavily cratered, typically with an estimated production age of 3-4 Ga.
b. High elevation relative to Northern Plains.
c. Contains several large impact basins that were later filled by volcanic and sedimentary deposits.
d. In many places permeated by networks of apparently water-cut valleys.
2. Northern Plains:
a. Contains all the tall shield volcanoes on the planet.
b. Much lower crater density, with volcanic plains units spanning a range of apparent ages from 0-3.5 Ga.
B. The Tharsis region - A huge topographic bulge exists just north of the equator in the western region of Mars (10N, 120W); although parts of Tharsis extend into the Southern Highlands, Tharsis is centered in the Northern Plains and largely contained within them. Some features associated with Tharsis are:
1. Olympus Mons - the largest volcanic structure in the solar system. It is a shield volcano about 550 km in diameter and rising 25 km above the surrounding plains.
2. Arsia, Pavonis, and Ascraeus Mons - three large volcanoes in a SW-NE line a few hundred km SE of Olympus.
3. Alba Patera - A large volcanic structure NE of Olympus that bears some similarity to coronae structures on Venus; its appearance is unlike the other Tharsis volcanoes.
4. There are 8 more volcanoes in the 100-km diameter range. Most of these appear to predate the larger volcanoes.
Valles Marineris - A huge canyon system that trends ESE and is radial to the Tharsis bulge. It appears to be a system of graben whose origin is related to uplift of the Tharsis region. The canyon system is 4000 km long and in places 700 km wide and 7 km deep.
Landslides have obviously occurred extensively in canyons (and valleys) throughout geologic history.
Layered deposits occur in some of the canyons. It is unclear whether they share an origin with the wall deposits, or if they were deposited after the canyon formed. If the latter, it is unclear whether they are of volcanic or sedimentary origin.
6. In addition to Valles Marineris there are numerous other fractures and graben radial to the Tharsis bulge.
C. Elysium region - A smaller Tharsis-like region (large volcanoes on a crustal swell, radial fractures) about 6000 km west and 1000 km north of Tharsis.
D. Chaotic terrain and outflow channels - In some regions huge catastrophic floods appear to have flowed from the southern highlands into the lowlands, particularly at Chryse basin, an ancient impact basin that forms the highlands-lowlands boundary (20N, 40W). The source regions are highly disrupted areas called chaotic terrain, and the floods carved huge channels called outflow channels.
E. Polar caps - there are ice caps on both the north and south poles of Mars. The north polar cap is larger and definitely contains a substantial fraction of water ice, while the south polar cap is apparently mostly carbon dioxide, at least at the surface. They are surrounded by dunes and contain layered deposits of alternating ice and sand.
F. Eolian features occur and may or may not have had a major erosional effect.
1. Dunes are common and large dune fields occur in some basins.
2. Wind streaks occur in some regions.
Large, sometimes near-global dust storms can occur.
Dust devils have been observed and leave temporary marks on the surface.
G. Recent activity also includes the mini dust avalanches that produce dark streaks, and the gullies.
II. General geologic history:
A. Formation of the cratered highlands. (Formally known as the Noachian period).
1. Impact basins - like the moon, at the tail end of accretion there was a rapid drop-off in the cratering rate.
2. It appears that volcanism was occuring on a widespread basis during and after the end of accretion, forming intercrater plains and filling basins.
The atmosphere was apparently denser and warmer, and dendritic valley networks formed among the highlands.
Recent discovery: Underlying a fairly shallow cover, it appears that the northern plains have just as many craters as the southern highlands, indicating that except for this shallow cover it is just as old as the southern highlands.
Recent discovery: The load associated with the Tharsis region created a shallow, thousands of kilometers wide trough around this region as a result of the flexural response of the lithosphere. This trough, combined with the global dichotomy, controls the drainage pattern of the valley networks. This means that Tharsis is a very old feature.
Recent discovery: There are some weird magnetic bands preserved in the southern highlands, which means that early in Martian history a global magnetic field existed. These bands are interrupted by the big impact basins in the south, so they must be very old. What the bands mean is not understood.
B. After global dichotomy formed:
Formation of sparsely cratered lava plains began. These plains generally occur below the global dichotomy (in the north) except for fill of a few southern impact basins.
Recent discovery: Lots of layering is observed in many places where there are cuts (like Valles Marineris). This and the extreme flatness of the northern plains suggests that much of the plains may be sedimentary and not volcanic in nature.
Volcanism has continued up to the present or near-present. Recent discovery: With high-resolution MOC imagery, some volcanism is so young that the flows are void of even small craters.
Fracturing associated with Tharsis started about 3 Ga ago.
The volcanic centers associated with Elysium and Tharsis started forming very early. After about 2.5 Ga most of the action was on Tharsis, and Olympus Mons appears to have the most recent activity.
Valles Marineris is mostly developed by about 2.5 Ga. The outflow channels develop about this time also.
Recent discovery: The high-resolution MOC imagery shows that there are active erosional processes going on currently. Some of these, the "gullies", may be indicative of occasional running water on the surface.
III. Ideas for the origin of the crustal dichotomy:
A. Endogenic (internal origin)
1. Crustal thinning underneath the northern hemisphere by convective forces
Main problem - numerical simulations of convection have not demonstrated physical plausibility.
Advantages:
Parts of the boundary do not appear to be formed by impact.
Some faulting on the scarp boundary appear to post-date heavy bombardment.
Plate tectonics, where lowlands are equivalent of Earth's ocean basins, and highlands are continents.
Problems:
magnetic banding is observed, but in the highlands.
other than alignment of 3 big Tharsis volcanoes, no really plate-tectonic looking features
Does not go well with recent discoveries that ancient crater surface underlies the northern lowlands
Advantage is that it can produce a relatively uniform crustal thickness for the lowlands.
C. Exogenic (extraterrestrial origin)
1. A single giant impact
Problems:
Dichotomy boundary is not a circle
No apparent ejecta blanket
The purported rim does not appear to have thick crust
Advantage - a single large impact is plausible
2. Multiple impacts
Problems:
Still can't fully explain shape of boundary
Why were the impacts clustered into northern hemisphere?
Some areas in northern lowlands outside proposed multiple impacts
Advantage - can produce a more irregular boundary