candidate among others would be the isotope of potassium, 40 K, with a half-life of 1.3 billion years( the biggest contributor today to the Earth’ s radiogenic heat source). Potassium is also possibly more common within Pluto’ s rocky core( see Figure 9) than it is in Earth, as it is relatively volatile and would have boiled away more rapidly from the solar nebula in the early Earth’ s neighbourhood. And even if the heat source is relatively weak today, with an estimated radiogenic heat flux of roughly 3 mW per square metre as compared to about 80 mW m-2 for the Earth, it’ s thought that nitrogen ice, being a good thermal insulator, could store sufficient heat at depth to initiate convection.
Figure 9: Pluto’ s density is 1.860g / cm 3. This would imply a mixture of rock and ices. The rocky core contains radioactive isotopes which decay over time, producing a weak internal heat source. There is some speculation that faulting of Pluto’ s surface, which is indicative of past expansion of the dwarf planet, may be evidence for a residual subsurface liquid water layer some 100 to 180 km thick at the core – mantle boundary, like the sub-surface oceans postulated for Europa, Ganymede or Titan. Credit: NASA / JHUAPL / SwRI
Pits, blades, haloes and gunk
Pluto’ s atmosphere may well be replenished by this convective activity. Elsewhere,‘ much of what we see … can be attributed to surface-atmosphere interactions and the mobilization of volatile ices by insolation.’ These have, for instance, produced striking landforms on Sputnik Planum and Tombaugh Regio. At such low surface atmospheric pressures as on Pluto( about 10μbar – microbars – 100,000 less than Earth’ s, the equivalent of being at an altitude of 80 km on Earth) transfer of volatiles from the surface( ice) to the atmosphere( gas) currently takes place directly by sublimation. This leaves the terrain pitted with holes a few hundred meters wide by a few tens of meters deep. In some areas these are aligned( Figure 10), but the reason for this is not yet clear.
Interactions with the atmosphere may also explain the bizarre, socalled‘ bladed’ terrain in Tartarus Dorsa, east of Tombaugh region( Figure 11), described as‘ texturally‘ snakeskin’-like, owing to their …. scaly raised relief.’ Typically a few hundred metres high and spaced a few kilometres apart, these long, steep, tightly-packed ridges align from north to south. Current theories to explain them include erosion from evaporating ices, deposition of methane ices, or even structures formed from primitive methane clathrates, with origins dating back to the proto-solar nebula.
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