A gravity survey of Type 2 coronae on Venus

Abstract

[1] Coronae are roughly circular tectonic features believed to form as a result of smallscale mantle upwellings. We examine the admittance signature for all the 32 Type 2 coronae (defined as having less than 50% complete fracture annuli) that are both well resolved in the gravity data and have fairly well constrained lithospheric parameters. We use top and bottom loading models to derive values of elastic thickness and crustal thickness or apparent depth of compensation. Both Cartesian and spatiospectral methods are used to calculate admittance. Overall we find reasonable agreement between the two methods. The shape of the filter used in the spatiospectral method [Simons et al., 1997] provides a means of isolating the signature of coronae that occur in close proximity to other geologic features. The narrow filters used in the Cartesian approach are useful for identifying transitions between different types of compensation, as well as for estimating error. We obtain a wide range of elastic thicknesses, from 25 to 80 km. Larger values are derived from bottom loading models, which have not been used in most prior studies of Venus. Fifteen of the coronae have elastic thickness values between 0 and 20 km and are probably isostatically compensated. Estimates of crustal and elastic thicknesses and apparent depth of compensation obtained for Type 2 coronae span the range obtained for Venus globally. Neither the thickness of the elastic lithosphere nor the crust appear to control whether Type 1 coronae, Type 2 coronae, or volcanoes form over small-scale mantle upwellings.

Effects of a low‐viscosity lower crust on topography and gravity at convergent mountain belts during gravitational instability of mantle lithosphere

“…Analyses of Venusian gravity and topography fields also confirm the prevalence of positive admittances and the frequent incidence of isostatic equilibrium on Earth's sister planet [Hoogenboom and Houseman, 2006;Smrekar et al, 2003]. Although negative admittance has been observed for Titan [Hemingway et al, 2013], this signature is at very long wavelengths (spherical harmonic degree ≤ 6) and is interpreted to arise from variations in the thickness of the outer relatively rigid icy shell, without any clear indication that this represents an unstable stratification.…”

Gravity and Topography of the Terrestrial Planets

“…However, the majority of these investigations have fit only the admittance or coherence functions, but not both simultaneously (e.g., Anderson and Smrekar, 2006;Barnett et al, 2000Barnett et al, , 2002Hoogenboom et al, 2005Hoogenboom et al, , 2004Lawrence and Phillips, 2003;McKenzie, 1994;McKenzie and Nimmo, 1997;Simons et al, 1994Simons et al, , 1997Smrekar, 1994;Smrekar et al, 2003;Smrekar and Stofan, 1999). The robustness of the inverted parameter values, the fidelity of the gravity model, and the appropriateness of the loading model are all difficult to assess in these studies.…”

Effects of lithospheric properties on the formation of Type 2 coronae on Venus

“…Additionally, a study of the gravity and topography for all of the resolvable Type 2 coronae (32 out of 107) has been completed [ Smrekar et al , 2003]. The elastic thickness and the crustal thickness or the apparent depth of compensation are derived by modeling the admittance, the ratio of the gravity to topography in the spectral domain.…”

“…Of the 107 Type 2 coronae, 32 are both well resolved in the gravity and display well‐behaved admittance signatures [ Smrekar et al , 2003]. A summary of these results is given in Table 1.…”

“…Each compensation mechanism produces a different relationship between gravity and topography that can usually be distinguished by modeling the admittance spectrum. As discussed in more detail by Smrekar et al [2003], a combination of loads can be present and influence the derived parameters. Most (15 out of 16) of the coronae fit with a top‐loading model have T e ranges that include 0 km.…”