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- Title
Viscous Relaxation of Oort and Edgeworth Craters on Pluto: Possible Indicators of an Epoch of Early High Heat Flow.
- Authors
McKinnon, William B.; Bland, Michael T.; Singer, Kelsi N.; Schenk, Paul M.; Robbins, Stuart J.
- Abstract
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. Plain Language Summary: 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‐to‐100 million years or more long ago. If true, such high heat flows have important implications for Pluto's early evolution and that of its ocean. Key Points: Two large (greater than 100 km across) proximate ancient impact craters on Pluto show evidence of substantial viscous relaxationNumerical relaxation models for these craters offer a window into Pluto's early heat flow, which we show to be different for each craterHeat flows elevated (by ≲50%) above radiogenic steady‐state and reduced thermal conductivity of cold (brittle, cracked) water ice are required
- Subjects
PLUTO (Dwarf planet); IMPACT craters; SURFACE temperature; STRESS relaxation (Mechanics)
- Publication
Journal of Geophysical Research. Planets, 2023, Vol 128, Issue 9, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2023JE007831