Pluto and Charon's Topographic Variance Spectra from Limb Profiles

The Long Range Reconnaissance Imager instrument (LORRI) on the New Horizons spacecraft unveiled Pluto and Charon's varied surfaces [1, 2]. LORRI images were used to determine the radius and flattening of Pluto and Charon from the body edges (i.e. limb profiles) [3]. While [3] found that both worlds are nearly spherical, they left the full analysis of limb profile topography for future work. We used the methods detailed in [3] to obtain preliminary limb profile topography for a larger set of images to analyze properties over a wider range of wavelengths.

Beyond specific shorter wavelength features like graben, we also investigated the topographic variance spectra for Pluto and Charon. Fitting a power law to the results gives a power law slope of ~-2. This matches results from other worlds in the solar system [4]. We also investigated if a piecewise power law improves the fit to the variance spectra of both bodies [5]. While the result for Pluto shows no significant improvement, Charon shows a break in slope at a wavelength of 200 ± 90 km. As with the Saturnian satellites [6], this break in slope may correspond to the characteristic flexural parameter of Charon's crust.

Under a Cartesian assumption [7], this flexural parameter implies an elastic thickness of 25 ± 15 km. However, if the lithospheres of Pluto and Charon are unbroken, we also need to account for curvature. Using the method detailed by [8], we can determine the expected topographic variance spectrum taking membrane stresses into account. Pluto's lack of a change in slope sets a lower limit on the crustal elastic thickness; a break is expected to be visible for elastic thicknesses < 30 km. Under a reasonable range of parameters for Charon, however, the break and change of slope cannot be explained by invoking membrane stresses, perhaps indicating a fractured lithosphere [9].

References: [1] Stern S.A. et al. (2015) Science, 350, aad1815. [2] Moore J. M. et al. (2016) Science, 351, 1284-1293. [3] Nimmo F. et al. (2017) Icarus 287, 12-29. [4] Ermakov A.I. et al. (2018) JGR: Planets 123, 2038-2064. [5] Main I.G. et al. (1999) GRL 26.18, 2801-2804. [6] Nimmo et al. (2011) JGR, 115, E10008. [7] Turcotte, D.L., & Schubert, G. (2014). Geodynamics. Cambridge university press. [8] Turcotte, D.L., et al. (1981). JGR: Solid Earth, 86(B5), 3951-3959. [9] Beyer, R.A. et al. (2019). Icarus. 323, 16-32.