Viscous Relaxation of Oort and Edgeworth Craters on Pluto: Possible Indicators of an Epoch of Early High Heat Flow

• Published in: Journal of geophysical research. Planets 2023-09, Vol.128 (9)
• Main Authors: McKinnon, William B., Bland, Michael T., Singer, Kelsi N., Schenk, Paul M., Robbins, Stuart J.
• Format: Article
• Language: English
• Subjects: Atmospheric models; Bowing; Craters; Diameters; Flattening; Geological history; Geology; Ground plane; Heat; Heat flow; Heat transfer; Heat transmission; Ice; Ice cover; Icy satellites; Lithosphere; Mars; Mars probes; Mathematical models; Numerical models; Oceans; Planetary probes; Pluto; Pluto (dwarf planet); Probes; Rheological properties; Surface temperature; Thermal conductivity; Viscosity
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2023JE007831

Impact craters, with their well‐defined initial shapes, have proven useful as heat flow probes of a number of icy bodies, provided characteristics of viscous relaxation can be identified. For Pluto's numerous craters, such identifications are hampered/complicated by infilling and erosion by mobile volatile ices, but not in every case. Large craters offer relatively deep probes of rheological structure, and on Pluto two large old craters in a major dark (volatile‐ice free) region are probably the best examples for possible viscous relaxation: Oort (115‐km diameter) and Edgeworth (140‐km diameter). They are similar in size, location, and apparent age (morphological preservation), but may or may not be coeval. Edgeworth is particularly shallow and its floor appears bowed up above the original ground plane, a classic hallmark of viscous relaxation in which viscosity decreases sharply with depth. Oort is less relaxed, but may be somewhat younger and less affected by an early epoch of high heat flow. Finite element calculations show that this heat flow would have to have been substantial to explain Edgeworth's upbowed floor by viscous relaxation, several times steady‐state radiogenic values for present‐day surface temperatures. We expect Pluto's brittle ice lithosphere to be fractured and porous, however, markedly reducing thermal conductivity and increasing temperatures at depth and relaxation for a given heat flow. We find that most relaxation occurs well within 100 Myr after impact for Edgeworth and Oort, and focus attention on a temporal (and/or regional) epoch of elevated heat flow, possibly tied to the serpentinization of Pluto's rocky core. It has been said that Pluto in many ways resembles Mars, a planet with a rich and complex geologic history but one whose major present‐day activity is driven mainly by its atmosphere and mobile, volatile ices. Evidence points to a present‐day subsurface ocean, but the thickness of Pluto's surface ice shell and the heat flow that sustains it are poorly constrained. Geophysical analyses generally indicate low heat flows, in keeping with theoretical expectations, but elevated heat flows in the deep geologic past are not ruled out. One line of evidence concerns the viscous relaxation (slow flattening) of old, large craters (>100 km across) on Pluto. Here, we use numerical models to show that this flattening is consistent with elevated heat flows (several times present‐day values, and possibly much more) persisting over 10‐