Gravity and tectonic patterns of Mercury: Effect of tidal deformation, spin‐orbit resonance, nonzero eccentricity, despinning, and reorientation

Abstract

We consider the effect of spin‐orbit resonance, nonzero eccentricity, despinning, and reorientation on Mercury's gravity and tectonic pattern. Large variations of the gravity and shape coefficients from the synchronous rotation and zero eccentricity values, J2/C22 = 10/3 and (b − c)/(a − c) = 1/4, arise because of nonsynchronous rotation and nonzero eccentricity even in the absence of reorientation or despinning. Reorientation or despinning induces additional variations. The large gravity coefficients J2 = (6 ± 2) × 10−5 and C22 = (1 ± 0.5) × 10−5 estimated from the Mariner 10 flybys cannot be attributed to Caloris alone since the required mass excess in this case would have caused Caloris to migrate to one of Mercury's hot poles. Similarly, a large remnant bulge due to a smaller semimajor axis and spin‐orbit resonance can be dismissed since the required semimajor axis is unphysically small (<0.1 AU). Reorientation of a large remnant bulge recording an epoch of faster rotation (without significant semimajor axis variations) can explain the large gravity coefficients. This requires initial rotation rates ≳20 times the present value and a positive gravity anomaly associated with Caloris capable of driving ∼10°–45° equatorward reorientation. The required gravity anomaly can be explained by infilling of the basin with material of thicknesses ≳7 km or an annulus of volcanic plains emplaced around the basin with an annulus width ∼1200 km and fill thicknesses ≳2 km. The predicted tectonic pattern due to these despinning and reorientation scenarios, including some radial contraction, is in good agreement with the lobate scarp pattern observed by Mariner 10. We also predict that lobate scarps will follow a NE–SW orientation in the eastern hemisphere and a positive gravity anomaly (of a few hundred mGal) associated with Caloris.

The tectonics of Mercury: The view after MESSENGER's first flyby

“…Dombard and Hauck (2008) drew attention to the possibility that early tidal despinning and global contraction led to a population of northsouth-oriented thrust faults that, although rendered unrecognizable by heavy bombardment, may have been reactivated by subsequent global contraction as the planet continued to cool. Matsuyama and Nimmo (2009) emphasized the possible importance of polar wander resulting from the formation and infilling of the Caloris basin and found that the orientations of thrust faults predicted for a combination of such polar wander with contraction and despinning are, under some assumptions, in broad agreement with those observed by Mariner 10 as well as with limited information on the planet's long-wavelength gravity field. Thomas et al (1988) suggested that lithospheric stresses from thermal contraction may have interacted with stresses associated with the formation of the Caloris basin, resulting in Caloris-radial thrust faults, but few lobate scarps in areas imaged by Mariner 10 are radial to the Caloris basin.…”

“…Extension in Mercury's polar regions predicted by tidal despinning has not been observed (Solomon, 1978;Melosh and McKinnon, 1988;Schubert et al, 1988;Watters et al, 2004). Stresses from tidal despinning and contraction may have been coupled if the two processes overlapped in time (Pechmann and Melosh, 1979;Melosh and McKinnon, 1988;Dombard and Hauck, 2008;Matsuyama and Nimmo, 2009;Watters and Nimmo, in press). Dombard and Hauck (2008) drew attention to the possibility that early tidal despinning and global contraction led to a population of northsouth-oriented thrust faults that, although rendered unrecognizable by heavy bombardment, may have been reactivated by subsequent global contraction as the planet continued to cool.…”

“…Models for the lithospheric stresses that produced the observed distribution of tectonic features viewed by Mariner 10 have involved some combination of global contraction accompanying interior cooling, tidal despinning, and stresses related to the formation and modification of the Caloris basin (Strom et al, 1975;Solomon, 1976Solomon, , 1977Solomon, , 1978Solomon, , 1979Cordell and Strom, 1977;Melosh, 1977;Melosh and Dzurisin, 1978;Pechmann and Melosh, 1979;Melosh and McKinnon, 1988;Thomas et al, 1988;Schubert et al, 1988;Phillips and Solomon, 1997;Hauck et al, 2004;Dombard and Hauck, 2008;Matsuyama and Nimmo, 2009). Patterns of lithospheric stress for each of these models predict specific spatial and temporal distributions of contractional faults and their associated lobate scarps.…”

Reorientation of Vesta: Gravity and tectonic predictions

“…The shape coefficients have both equilibrium and remnant rotational figure contributions, c 2m EQ and c 2m RR , respectively [Matsuyama and Nimmo, 2009]. In a reference frame aligned with the present rotation axis, the only non-zero equilibrium coefficient is c 20…”

“…The degree-2 gravity field is expected to be dominated by contributions arising from the equilibrium and remnant rotational figures, and the large impact basin near the south pole. The degree-2 gravity coefficients can be written as [Matsuyama and Nimmo, 2009;Matsuyama and Manga, 2010] …”