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The Internal History of the Moon and Kuiper Belt Objects from Gravity and Topography

  • Author(s): Conrad, Jack William
  • Advisor(s): Nimmo, Francis
  • et al.

This thesis comprises two separate but interesting projects that attempt to constrain the internal history of planetary bodies. The first set attempts to interpret the Moon's internal thermal history from the relaxation state of lunar impact basins. As the Moon cools, impact structures degrade at a slower and slower rate. This can be observed in maps of lunar topography and crustal thickness. This analysis, however, was greatly enhanced by the GRAIL spacecraft mission to the Moon. In Chapter 2,I present the first relaxation analysis of the most up-to-date complete lunar impact basin catalog. With the addition of ~6 new impact basins and the re-qualification of other basins, a basin relaxation transition is clearly observed in the lunar impact record. This relaxation transition signal can be used to constrain and link lunar solidification and cooling models with impact chronology models. In that study, I find that if the lunar surface experienced a lull in basin-class impacts it must have solidified and cooled rapidly following its formation.

The second project involves two studies that try to understand the thermal history of Pluto and its moon Charon. In July 2015, the field of Kuiper Belt Objects was greatly widened with the arrival of the New Horizons spacecraft at the Pluto-Charon system. One of the major discoveries of that mission was the prevalence of extensional tectonic features on both worlds, a likely signal of a frozen-out (or possibly still freezing) subsurface ocean. In Chapter 3, I characterize the large extensional tectonics features in the encounter hemisphere on Pluto. Then by comparing the features to topographic flexure models, I was able to constrain the maximum surface heat flux experienced by Pluto. This showed that Pluto's internal evolution matches thermal models that primarily use a radiogenic heat source.

Although Chapter 3 put maximum heat flux constrains on the thermal history of Pluto, the constraints can be improved upon and expanded to include analysis of Charon's surface. In Chapter 4, I create and use limb profile topography of Pluto andCharon to understand the differences in the morphological and interior history of the two worlds. This is achieved by calculating the topographic variance spectra from limb profiles, which typically results in a single-power law spectrum. While this typical case holds for Pluto, it does not for Charon which displays a characteristic wavelength. My analysis further constrains an upper limit for Pluto's maximum surface heat flux, but it also sets a range for the absolute maximum heat flux for Charon that cannot be solely explained by radiogenic heating. This implies that an extra heat source, probably tidal heating, was necessary.

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