QCD Discoveries Shed New Light on Northern Lowlands Geological age
Figure 1. The upper images a and b show the radargrams of two locations, where the red dotted lines trace the location of the potential quasi-circular depressions. The lower images c and d show the location in white of the inferred buried basins on a MOLA color-coded shaded relief map of western Acidalia Planitia.1
Evidence of buried impact via the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) radar sounder has helped revolutionize scientist’s understanding of the northern lowland plains and the Martian hemispherical dichotomy in general. Determining the geological age of the lowland crusts is crucial for reconstructing the dichotomy timeline. Detecting these buried features has forced scientists to reassess their prior interpretations about the lowlands age, as well as bringing researchers one step closer to deciphering the dichotomy mystery.
Crater densities are used to determine the relative age of planetary surfaces, where high crater density correlates to an old surface and low crater density equates to a relatively young surface. The smooth, flat, relatively crater-free northern lowlands fit the profile of a young geologic surface, especially in contrast to the ancient heavily cratered southern highlands. Finding evidence of buried basin depressions as depicted in the Figure 1, implies the underlying lowlands crust is not as young as previously thought, but ancient like the highlands1.
MARSIS radar sounder transmits radio waves into the subsurface at four frequencies between 1.3 MHz and 5.5 MHz that reflect off of differing compositional layers and structures1,2. The MARSIS data revealed buried impact basins with diameters ranging from 130 km to 470 km covering 14% of the northern lowlands1. The high frequency of these basins greater than 200 km in diameter denotes that the lowlands crust is incredibly old 1, far older than initially determined.
The Martian dichotomy between the northern lowlands and the southern highlands is one the largest unknown pieces of Mars’ geological evolution. How did such a global scale event occur? What triggered the dichotomy? When did it happen? These questions guide geologist’s research on the subject. Not knowing the age of the lowlands crust opens the possibilities for when the dichotomy could have occurred. Proving the lowlands crust ancient constricts the dichotomy event to the early part of Mars’ geological evolution, specifically restricting it to the early Noachian period3. Though multiple dichotomy-producing mechanisms proposed require the event to occur early in Mars’ geological history, this requirement does thin the crowd of hypotheses.
Dr. Watters research on the dichotomy extends beyond his participation with MARSIS, and involves working with the dichotomy big-picture more closely through his studies on particular segments of the dichotomy boundary. This work entails determining the characteristics of the lithosphere underneath this boundary. By modeling the lithosphere as both continuous and broken, and then comparing the model data to an actual long-wavelength topographic profile of the modeled region scientists discovered the region correlated more closely with a broken lithosphere setting3. A broken lithosphere implies tectonic dynamics played a role in the dichotomy creation3, though what type of role is still being determined. For more information on the context and results of the lithosphere studies, see Dichotomy.
Understanding the context for the dichotomy is one of Dr. Watter's prerogatives, and through the MARSIS and lithosphere projects he is slowly doing just that. Continued MARSIS application and modeling should lead to even more dichotomy deciphering in the future.
Images taken in January 2008 by MESSENGER of Caloris Basin show a complex pattern of deformation unlike that found in any other basin in the solar system. High-resolution images indicate evidence of volcanic vents around the inner margin of Caloris basin2. Impact craters show evidence for embayment, Dr. Watters and other researchers are confidant that the interior smooth plains of Caloris are volcanic on origin.
Caloris Basin’s geologic history is different from its lunar counterparts2, the mare basins. In contrast to lunar maria, the interior smooth plains of Caloris are higher in albedo than the underlying basin material2. This correlates to a difference in spectra between the Moon and Mercury basin materials, where lunar basins exhibit high ferrous silicate content and Caloris basin silicates had relatively low amounts of FeO1,2.
1. Watters et al. (2006), MARSIS radar sounder evidence of buried basins in the northern lowlands of Mars.
2. Watters et al. (2007), Hemispheres Apart: The Crustal Dichotomy on Mars.
3. Watters and McGovern. (2006), Lithosphere Flexure and the Evolution of the Dichotomy Boundary on Mars
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