Abstract | The surface observations performed during the MESSENGER mission revealed that smooth plains occupy ~27% of the surface of Mercury and are potentially formed by volcanic resurfacing. Comparing the two hemispheres, the plains area within the northern hemisphere is approximately 7 times larger than that within the southern hemisphere. This asymmetric distribution may reflect the differences in the thermal condition in the mantle and/or the ease of magma eruption.
In this study, we estimated the magma eruption fluxes within the two smooth plains interior to impact basins, the Rembrandt basin in the southern hemisphere and the Caloris basin in the northern hemisphere. By comparing the volcanic activities, we discuss the factor that may cause the asymmetric distribution of smooth plains.
The estimation of the resurfacing ages and the basin formation ages were conducted based on the crater measurements using the surface images acquired by MDIS/MESSENGER. We adopted the crater chronology model of Le Feuvre and Wieczorek (2011) (Mercury, porous-target). The observed crater size-frequency distributions hold kinks, suggesting the preferential removal of smaller craters by multiple eruptions. The thicknesses of the surface lava layers were estimated from the crater diameter corresponding to the kinks. Also, the total thicknesses of the volcanic materials were investigated using the enhanced color map. By classifying all the craters into two groups by the color properties, blue craters and yellow craters.
The basin formation ages were estimated as 3.93 ± 0.06 and 3.94 ± 0.04 Gy, while the volcanic resurfacing ages were from 3.87 ± 0.04 to 3.76 ± 0.01 Gy and 3.88 ± 0.03 to 3.74 ± 0.01 Gy, and the magma eruption fluxes were estimated as 5.2–12.3 km/Gy and 9.3–15.1 km/Gy, respectively. The magma eruption flux is at most 3 times larger in the Caloris basin than in the Rembrandt basin.
Based on the gravitational data, the crustal thickness model has been proposed. Considering the difference in basin diameter, the Caloris-forming impact excavated ~2 times deeper than the Rembrandt-forming impact. The crust beneath the Caloris basin is likely to be ~1.8 times thinner, which might make the ascent of magma to the surface easier, comparing with the Rembrandt basin. Thus, the magma production quantities in the mantle beneath these two basins should not differ by a factor of <3.
We finally discuss the spatial variations in the abundance of heat-producing elements in the mantle, which can be an energy source for volcanic activities. |
