Abstract DGP2026-73 |
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Underestimation of planetary contraction due to obscuration of shortening structures by surface roughness: The case of Mercury
Estimating Mercury’s radial contraction is key to understanding its geodynamic evolution, as the radial contraction reflects the net cooling of the interior and growth of the solid inner core. Mercury displays shortening structures such as wrinkle ridges and lobate scarps, as surface expression of the radial contraction. Based on the mapping of shortening structures, the total radial contraction of Mercury has been estimated to be 1–7 km [e.g., 1, 2], providing constraints for thermochemical evolution models. However, whether all shortening structures have been preserved to present-day and can be successfully is yet to be investigated, as any lack of detection can lead to the underestimation of Mercury's radial contraction.
A key to this question lies in the heterogeneous distribution of shortening structures on Mercury, which is at odds with the expected isotropic global contraction. The spatial heterogeneity cannot be fully explained by observational bias in the MESSENGER observation or mantle dynamic pressure distribution [3]. An alternative hypothesis is the obscuration of shortening structures by extensive ejecta from impact basins younger than tectonic landforms and/or the incomplete detection due to other topographic features. This hypothesis may be tested by analyzing short-baseline topographic roughness as a proxy of surface freshness.
In this study, we compare surface roughness of Mercury with the distribution of shortening structures. Based on the global topography model of Mercury and laser altimetry from MESSENGER, we first generate a surface roughness map at 10-km baseline using statistics of topographic curvatures [4]. Our roughness map is then compared to a global tectonic catalog [5].
Our comparison reveals a spatial anti-correlation between roughness and the distribution of shortening structures. High-roughness regions, such as fresh crater ejecta, exhibit fewer shortening structures. Separated comparison within the smooth and cratered plains statistically confirms that roughness at shortening structures is lower than the average across the whole plains. This anti-correlation may be attributable to the masking effect of shortening structures caused by extensive ejecta from young craters, incomplete detection of tectonic landforms in rough areas, and/or lower porosity at heavily cratered (i.e., rough) terrains.
Regardless of the underlying cause, this anti-correlation demonstrates that roughness-related processes substantially affect the strain record. Compressional strain at low-roughness regions is less affected by obscuration and could be thus representative of the actual radial contraction. Upon comparing between roughness and compressional strain maps [6], we found that that Mercury’s radial contraction may have been underestimated by up to 30 %, providing new insights into the planet’s thermal evolution and interior composition. In addition, the roughness-induced obscuration is also relevant to other planetary bodies, particularly on the Moon, as higher surface roughness of lunar highlands may also substantially obscure the tectonic record.
[1] Byrne et al. (2014), Nat. Geosci., 7, 4, 301-307.
[2] Watters (2021), Commun. Earth Environ., 2, 1, 1–9.
[3] Watters et al. (2021), GRL, 48, 17, e2021GL093528.
[4] Nishiyama et al., under review in PSJ.
[5] Klimczak et al. (2025), EPSL, 658, 119331.
[6] Broquet & Andrews-Hanna, submitted to JGR: Planets.