The early results from the New Horizon Mission have challenged assumptions in relation to the dwarf planet Pluto
, confirming early ideas of radiogenic heating being dominant in icy bodies of the outer solar system [1
]. Pluto is one of several 1000 km-sized bodies that are now seen as the largest amidst millions of trans-Neptunian objects (TNOs) occupying the frigid outer reaches of our planetary system[3
]. They populate the misnamed Edgeworth-Kuiper Belt - misnamed because it is a toroidal distribution and very thick, extending beyond Pluto for several times its distance from the sun. The large outer moon of Neptune
, Triton, is a “twin” of Pluto, and both are by no means dead bodies.
Pluto’s surface shows clear evidence of restructuring, activity and mobility on timescales under millions of years as can be inferred from the surprising absence of meteorite impact craters (Figure 1). Its radius of ~1200 km and low density 1.9 g cm-3 imply a composition of water ice plus mineral and carbonaceous particles, much like comets.
, Pluto has surface ices and a thin atmosphere with relatively small masses of methane and nitrogen. Surface temperatures of ~40 K in full sunlight ensure the ices sublimate to maintain a tenuous atmosphere, but also facilitate photolytic reactions that lead to loss of gases from the low surface gravity (escape velocity of 1.2 km/s).
The high mountains and deep chasms discovered by the New Horizon Mission evidencing tectonic activity point to interior energy sources, as predicted by Czechowski & Leliwa-Kopysty´nski [4
]. Whilst a small amount of tidal heat is produced in the Pluto-Charon
binary system [4
], radioactive decays are a much more significant heat source. Depending on the admixture of radioactive elements, melting of ices is likely to occur due to heat they generate [1
]. Two short-lived nuclides that have been considered in the context of comets are 26Al (half-life 0.74My) and 60Fe (half-life 1.5My). A limiting factor for heating by these short-lived nuclides is the time taken for the bodies to accrete following a supernova injecting radioactive nuclides into the solar nebula. If the process takes much longer than several million years, these radioactive heat sources would have become extinct.
Unlike comets, Pluto and its slightly larger twin Triton are big enough for longer-term radiogenic heating by 238U (half-life 4.5Gyr) and 232Th (half-life 14Gyr) serving to melt ices in their interiors. With (U+Th)/(C+N+O) ~10-8 by mass (solar system abundances) and given the total energy released from the fission of U + Th to be ~1014 J/kg we have an average heat release per unit mass of ~106 J/kg. When this is compared with ~3.34 x 105 J/kg appropriate for the heat of fusion of water ice, the implication is that a substantial mass fraction of the planet’s water-ice content would be converted to liquid water/slush on energetic arguments alone[5
To determine whether an interior liquid core can persist long enough in a body such as Pluto or Triton we need to consider the rate of heat transport through an outer frozen ice shell. For a body with a spherical liquid core of radius 1000 km and temperature 273 K surrounded by a concentric frozen shell of ice of outer radius 1200 km and surface temperature 40 K, it is readily shown that the characteristic timescale for heat loss and refreezing of the interior is of the order of 1010 yr [6
]. Yabushita [2
] and Bar-Nun and Owen [7
] have performed more detailed numerical integrations to conclude that icy spheres larger than 100 km would have their slushy liquid interiors transformed, at least partially re-crystalizing their amorphous ice and mobilizing dissolved volatile gases. Any interior gases including CO and CH4 (sublimation temperatures ~25 K, 31 K respectively at normal pressure) diffuse outwards through pores. These gases would escape to the atmosphere and to space if not sealed in by surface layers of H2
O-ice or captured as H2
Long-term evolutionary models were also calculated for mainly H2
O-ice TNOs by Choi et al. [8
]. Sarid and Prialnik [9
] elaborated on this approach, adding pore-diffusion, but ignored the phenomenon of ice-convection which enhances heat transfer and loss, but also promotes the outward diffusion and escape of gases.
Convection of outer ice-shells (turnover time ~108 yr for a 200 km frozen outer layer) is responsible for the formation of the surface features on several icy satellites of Jupiter and Saturn. Czechowski and Leliwa-Kopystýnski [4
] reviewed modeling of the tectonic activity driven by internal heating, by tidal stresses or radiogenic sources. The new images of Pluto with ice-mountains and planet-scale rifts verify that this is happening there too.
The major surprise is the presence of widespread methane ice on Pluto’s surface, much covered by semi-transparent nitrogen-ice, which is especially thick at the poles (Figure 1: bottom). Methane is not expected to be abundant in the solar nebula material from which Pluto was formed (likewise not abundant in ‘pristine’ comets), and any small initial compliment of methane would have been largely lost through the diffusion and ice convection processes. We therefore infer an internal source of methane venting preferentially through equatorial “cracks”. This applies likewise to Triton [10
], with the difference that Triton has been subject to strong tidal stresses during its capture by Neptune – these enhanced the heating, causing rupture and turnover of its ice shell. Methane being weaker on Triton, but still surprisingly abundant, is possibly due to its faster loss.
On Mars, the methane source (that is now well attested) is explained as either biogenic –subsurface methanogens in the permafrost - or mineral serpentisation. Pluto lacks the rocks for a mineral source, so it may be argued that a biogenic source is indicated. On the panspermia hypothesis the material from which Pluto and Triton condensed would have included a small number of viable microbes. In its earliest history, when the interior ice first reached a temperature ~270 K due to radioactive heating, the initial number of microorganisms (including methanogens) would have increased exponentially on a short timescale with interstellar organics serving as nutrients. Some of this biogenic methane would have been trapped within the 100 km-scale ice shell and released more slowly than the 108 year ice-convection time for planet-scale cracks [11
Alternatively the methane source in Pluto could be an indication of on-going microbiology - methanogens relatively close to the surface that are still active because the tectonic overturning brings them close to unused organic and mineral nutrients of the interstellar dust presumed incorporated in all TNOs. Organic material was discovered to be abundant in the dust from Halley’s Comet (Giotto/Vega missions of 1986) and would provide the essential nutrients for microbial life in the absence of light. Similar organic material was found recently to dominate the very dark surface of the Rosetta comet 67P/C-G, and this may turn out to be so for Pluto as well. Far from writing off this region of our solar system as barren and lifeless its TNOs may turn out to be reservoirs of microbial life, just on our doorstep. The New Horizons mission over-fulfills expectations in initiating astrobiological exploration of the new worlds of dwarf planets.