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) Northern surface much smoother, displays far fewer craters, implies a resurfacing event somewhat later in Mars history (Hesperian)
How do we explain this? Endogenic Processes –Mantle Convection Requires long-wavelength pattern of heat loss Upwelling in one hemisphere, downwelling in other 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) –Plate Tectonics Transform faults, subduction zones along dichotomy boundary Problems: Relatively little evidence. Difficult to line up expected geological features using a transform fault model (A.D. Fortes, 1999) Exogenic Processes –Impacts! Zhong & Zuber, 2001 A.D. Fortes, 1999
Anatomy of an Impact 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.
Observing remnants of Impacts An impact will lead to the following evidentiary features: Circular crater Ejecta rock and dust in surrounding region Brecchiated (shocked) rock in basin Increased gravity anomaly (“mascon”) due to thin crust and isostatic compensation Crustal magnetization may be different from surrounding region. Meteor Crater, AZ, ~1km across Manicougan Crater, Canada, ~72 km across Chicxulub Crater, Yucatan, ~200 km across
Early Suggestions: Giant Impact Wilhelms and Squyres (1984) –Suggest a single giant impact event in early Martian history could account for dichotomy –The impact would have created the “Borealis basin”, D>7700 km. –Identified 5 features (massifs A-D) that could be seen as remnant pieces of an ancient crater rim. Mainly mountains with steep slopes. –Impact occurs, crustal material redeposits on basin periphery (southern highlands). Loss of mass is at least partly compensated by isostatic uplift of more dense mantle material, leaving depression. –Partly filled later in history by lavas rising through weakened lithosphere.
Giant Impact (contd.) –Basin has diameter of 7700km, 130 deg of Lat. Centered at 50 o N, 190 o W –Used Diameter-energy scaling relation based on experimental data and impacts of spacecraft on Moon D=KE a h(g) 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) –For impactor with density of 3 g/cc and velocity of 24km/s (orbital velocity of Mars), an object of diameter 600 km could have created Borealis Basin. If velocity is 12 km/s, you’d need a body of 950 km diameter. Reasonable sizes for bodies near Mars orbit at end of accretion. (Hartmann & Davis, 1977; Wetherill, 1985)
Proposed “Borealis” impact basin overlaid on MOLA map. Accounts for: Topographic depression Correlation of massifs not related to other basins Abrupt scarps between highlands and lowlands Extrusion of lava into northern lowlands Problems: One impact alone can’t explain entire dichotomy
Qualification: Multiple Impacts Frey and Schultz (1988) –Tested the Borealis basin hypothesis by trying to fit observed impact basins to a 1/D^2 distribution (empirically derived for Mercury and Moon). – Doesn’t work for Borealis basin (Diameter > 7000km), since there would be too many “missing” impact craters of enormous size in order to fit the distribution. –IE: would expect 47 more craters with D > 1000km than we actually observe. –Works better if Chryse basin (Diameter ~ 4300km) is the largest of impacts, and multiple overlapping impacts caused the dichotomy. –IE: would only expect 7 more craters >1000km than we observe. Fewer “missing” craters. A single giant impact is unlikely to have caused the entire dichotomy, but multiple overlapping large impacts might. Multiple large impacts also capable of depositing more heat and causing more crustal thinning than single.
Multiple 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. Chryse Borealis Utopia Olympus Mons
Evidence of “hidden” impact basins McGill (1989) –The distribution of knobs and partially buried structures in the Utopia Planitia area of the northern plains indicates a very large (D ~ 4600km) ring surrounding what is inferred to be an impact basin. –“Knobs” are small, isolated hills standing above material in the plains of the northern highlands. Clusters of these knobs have been inferred to represent remnants of ancient impact crater rings that protrude up through the younger plains material.
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.) Sparse Knobs Elysium Mons Dense Knobs Fractured flat terrain
Viking Gravity measurements indicate a coincident mascon.
Relative Age of Utopia materials (McGill, 1989)(Zuber, 2001) -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.
An Evolving Model The problem with the single Giant Impact hypothesis (Borealis) is that it cannot account for the irregular shape of the northern lowlands by itself. The problem with the Multiple Large Impact hypothesis (Utopia + Chryse + Isidis, etc.) is that there should be towering piles of ejecta between the large basins (as there is around Hellas). It at least seems reasonable that multiple impacts are involved, since we’ve found multiple basins. Further discovery of northern plain impact basins would be helpful, but seemed difficult before MOLA.
Crustal Thickness Zuber (2001) –MGS topography and gravity measurements show that, in general, the Martian crust is thinner in the northern lowlands than in southern highlands. - 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.
Quasi-Circular Depressions Frey, et al. (2002) –MOLA reveals a very large population of Quasi-Circular Depressions (QCD’s): many shallow (~100’s of meters vs. ~1.6 km for a 50 km diameter visible impact crater) basins in Northern Lowlands which have no visible expression in Viking or MOC images. –Frey, et al. interprets these newly discovered objects as impact craters which have been long buried by the northern plains material.
–Out of 644 QCD’s larger than 50 km, only about 90 are visible impact craters. –But they are well distributed, supporting the idea that they are impact basins. –Based on Garvin et al. 2000, overlying cover must not exceed 5-6 km or basins would not reveal any relief. Most are probably buried under ~1.5 km of younger crust.
QCD’s imply very old Lowlands –Frey et al compares crater counts of the QCD’s in the lowlands to QCD’s and visible craters in the highlands, again using the empirical 1/D^2 law to fit. –Finds that visible lowland impacts lie on lower line, consistent with Hesperian resurfacing –But there are more lowland QCD’s than highland visible impacts! –Implies that the buried lowlands are about the same age as the highlands. The Northern Lowlands were formed during the early Noachian, and have been around for most of the history of Mars! –Constrains formation of dichotomy to mechanisms that operate quickly and early. Large scale impacts near end of accretion (early Noachian) fit the bill. –QCD’s superimposed on Isidis/Utopia. Thus Utopia and Isidis pre-date QCD’s, and are some of the oldest features on Mars. –If internal mechanisms like mantle convection caused lowland formation, they had to have operated very early (ie: before Utopia)
Conclusions The case for the northern lowlands being caused by impacts has been criticized, but not ruled out. Recent evidence supporting the idea that the lowlands have been low since the early Noachian lends favor to possibility of large impacts as a cause. (Frey et al 2002) Remains a viable option for creation of the hemispheric dichotomy. As in many cases in science, the real solution is probably a combination of multiple mechanisms.
References Wilhelms, D. E. & Squyres, S. W. The Martian hemispheric dichotomy may be due to a giant impact. Nature 309, 138–140 (1984). Tanaka, K. L., The stratigraphy of Mars, Proc. Lunar Planet. Sci. Conf. 17, J. Geophys. Res. Suppl., 91, E139–E158, 1986. Frey, H. V., and R. A. Schultz, Large impact basins and the mega-impact origin for the crustal dichotomy on Mars, Geophys. Res. Lett., 15, 229–232, 1988. McGill, G. E., Buried topography of Utopia, Mars: Persistence of a giant impact depression, J. Geophys. Res., 94, 2753– 2759, 1989. Smith, D. E., et al., The global topography of Mars and implications for surface evolution, Science, 284, 1495–1502, 1999. Fortes, A.D., Origin of the Martian Hemispheric Dichotomy. Department of Geological Sciences, University College, London. 1999. Garvin, J. B., S. E. H. Sakimoto, J. J. Frawley, and C. Schnetzler, North polar region craterforms on Mars: Geometric characteristics from the Mars Orbiter Laser Altimeter, Icarus, 144, 329– 352, 2000. Zhong, S., and M. T. Zuber, Degree-1 mantle convection and the crustal dichotomy on Mars, Earth and Planetary Science Letters, 189, 75– 84, 2001. Zuber, M.T., et al., The crust and mantle of Mars. Nature 412, 220 – 227 (2001) Frey, H., et al., Ancient lowlands on Mars., Geophys. Res. Lett, Vol. 29, No. 10, 2002.