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Planetary science

Volcanoes on Quaoar?

Quaoar, a large body in the Kuiper belt, has crystalline water ice on its surface, yet conditions there should favour amorphous ice. Does this mean that resurfacing has taken place — perhaps even volcanism?

Our planetary system does not end at Pluto. Hundreds of bodies exist in the Kuiper belt, which extends outwards from Pluto, sharing the same plane as the planetary orbits. The largest known bodies in the Kuiper belt are not much smaller than Pluto, and some have similar dynamics to that planet. Although the existence of the Kuiper belt had long been hypothesized, the first Kuiper-belt body was discovered only in 1992, by Jewitt and Luu1. These same authors now propose2 that Quaoar, the largest known of these bodies, has crystalline water ice on its surface and possibly also ammonia (see page 731 of this issue). The presence of crystalline ice is surprising, because it is widely believed that its formation requires a temperature of around 100 K or more — substantially higher than the surface temperature of these bodies. The precise temperature required, however, is not known and may not be the same in laboratory experiments as it is in space. Yet it might be that we are seeing evidence for ‘planetary’ processes such as volcanism within these bodies.

The discovery and characterization of the Kuiper belt is among the most important developments in planetary science in the past decade3. As is usual with the discovery of new bodies (inside or outside the Solar System), the initial excitement focused on the dynamical implications: why do they occupy these orbits and how did they form? Some orbital migration may occur, but it is likely that these bodies never experienced much higher surface temperatures than the ambient conditions provided by the Sun (temperatures of about 50 K or less). Quaoar, discovered in 2002 by Trujillo and Brown, is the largest body to be found in our system since the discovery of Pluto in 1930. It has a radius of about 650 km, roughly half that of Pluto. The composition and nature of the surfaces of these bodies are difficult to determine, yet such characteristics may be important for understanding their history. Colour and brightness (albedo) can be informative, but spectroscopy at near-infrared wavelengths is the preferred technique of investigation.

Water is the most abundant condensed material in the Universe, and it should form the ‘bedrock’ for solid bodies in the outer Solar System. This does not necessarily mean that the water would be readily observed; it could be hidden beneath a mantle of other material. Still, it is not surprising that Jewitt and Luu2 observed the distinctive spectroscopic feature of water ice. But water ice that forms and remains at very low temperatures would be expected to be amorphous — that is, lacking the periodic structure of crystalline water ice. This is because the highly coordinated architecture of a crystal lattice is difficult to establish when molecules are added to a substrate at very low energy (temperature). At higher temperature, amorphous ice rearranges to crystallize into the ordered, thermodynamic ground state (and releases latent heat as it does so).

More controversially, Jewitt and Luu claim evidence for ammonia ice. The slight dip they see in the spectrum of reflected light around a wavelength of 2.22 micrometres is a subtle characteristic, and seems to be part of a broader spectral feature that is imperfectly understood. Brown and Trujillo have observed the crystalline water-ice feature independently (personal communication), and they also report a putative feature at 2.22 micrometres — but they favour methane or some other explanation for the shape of the spectrum in this region. Irrespective of whether one believes the evidence for ammonia — and it would indeed survive for only a limited time on the surface — it is likely that this molecule is present to some extent in the internal make-up of Quaoar: ammonia is present in interstellar space and is a natural (although possibly minor) carrier of nitrogen in the Universe. This raises the intriguing possibility of water–ammonia volcanism.

For any Kuiper-belt object with a radius greater than about 200 km, the time taken for heat to diffuse through the body is longer than the lifetime of long-lived radioactive elements — the same elements that provide most of Earth's geothermal heat flow. These bodies are expected to contain sufficient rocky material, half or more by mass, for their temperature to easily exceed 200 K deep down during their evolution. An ice mixture that contains both water and ammonia would begin to melt at a temperature of about 175 K. The melt produced would be one-third ammonia and two-thirds water, and it would be much less dense than the surrounding ice–rock mixture. It could percolate upwards and perhaps segregate to form a fluid-filled crack. Even in the low gravity of Quaoar, a crack of a few kilometres in length would create a hydrostatic ‘head’ exceeding several atmospheres of pressure, probably sufficient to enable the crack to propagate to the surface, or near to it. All of these processes are directly analogous to basaltic volcanism on Earth and other terrestrial planets. The cooling lava would contain crystalline water ice and crystalline ammonium hydrate, which could become exposed by infrequent impacts on the surface of Quaoar.

This is speculative and might be unnecessary. There may be non-thermal processes that make crystalline ice and do not depend on the presence of a massive body with a warm subsurface. Crystalline ice has been observed in the disks around newly forming stars4 (but we do not know the thermal history of this dust). Evidently, more observations are needed, both of our Solar System and of more distant targets. Data from the infrared space observatory Spitzer5, currently in orbit, are expected to help. More laboratory data are also in order. But we should not exclude the possibility that planetary processes such as volcanism could occur in volatile-rich bodies at these outer reaches of the Solar System.

References

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Stevenson, D. Volcanoes on Quaoar?. Nature 432, 681–682 (2004). https://doi.org/10.1038/432681a

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