Origin of The Martian Hemispheric Dichotomy: The Case for Impacts. David Galvan ESS 250. The Facts. Lowlands average 3-4 km lower than southern highlands. North Pole 6km lower than South Pole. Southern highland surface more heavily cratered, older (Noachian)
Problems: Mars has a relatively large core, difficult to produce long-wavelength pattern.
But, for plausible conditions for early Mars, mantle convection could account for a dichotomy developed over ~1 Gyr from early Noachian into Hesperian. (Zhong & Zuber, 2001)
Relatively little evidence.
Difficult to line up expected geological features using a transform fault model (A.D. Fortes, 1999)
Zhong & Zuber, 2001
A.D. Fortes, 1999
1. Object impacts crust of planet
2. Impactor releases its kinetic energy, compressing and melting the crust. Rocks, dust, and vaporized particles are ejected. Compressional shock is sent downward through crust.
3. Shock wave fractures crust, then rebounds toward surface as a rarefaction wave, uplifting the crater floor and rim. May cause faulting, which leads to concentric sets of rings.
Manicougan Crater, Canada, ~72 km across
Meteor Crater, AZ, ~1km across
Chicxulub Crater, Yucatan, ~200 km across
Where D is diameter of crater, E is kinetic energy of impactor, K and a are constants, and h(g) gives dependence on surface gravity. (Developed to describe craters on Moon by Housen et al., 1979)
“Borealis Basin” Impacts
The “Missing” basins in the Chryse distribution could fit into the southern highlands, while the missing basins in the Borealis distribution could not.
-Frey and Schultz suggest a more thorough examination of southern highlands looking for large craters.
Fractured flat terrain Impacts
Basin centered view of the inferred Utopia impact basin. Red indicates dense knobby terrain; Orange indicates sparse knobby terrain; and brown indicates 'polygonal‘ or fractured flat terrain from the Hesperian outflow. The yellow dot is Elysium Mons. The 3300km and 4715km rings are indicated. (Fortes, 1999 after McGill, 1989.)
Utopia Impact Basin Impacts
-Tanaka (1986) considers the Isidis basin to be Lower Noachian “basement”.
-Isidis basin interrupts outer Utopia ring
-Thus, Utopia impact must have occurred in early Noachian.
Borealis Impact Basin
(Wilhelm & Squyres, 1984)
Utopia Impact Basin
(Schultz & Glicken, 1979)
Isidis Impact Basin
North Polar Basin
Chryse Impact Basin
(Schultz et al, 1982)
-It is worth noting, however, that the crustal thickness dichotomy does not exactly match the topographical dichotomy (ex: Arabias Terra).
-Zuber states that, if the northern depression formed as a result of impacts, it must have occurred very early in martian history (IE: before Utopia) and subsequent processes must have modified the topographic & crustal thickness signatures.