Abstract:The recently reported extreme redness of a class of Kuiper-belt objects could be yet another indirect indication of extraterrestrial microbiology in the outer solar system.

The existence of an ancient reservoir of cometary-type objects in stable circular orbits lying beyond the orbit of Neptune is now beyond dispute. Tegler and Romanishen (1998) have recently made the remarkable discovery that these so-called Kuiper-belt objects include some that are exceedingly red. Accurate photometic studies using CCD techniques have revealed two distinct classes of such objects. One class is comprised of objects with surface colours that are only very slightly redder than the sun, whilst the other contains objects that are said to be "the reddest objects of the Solar System". The fact that the distribution of colour amongst these objects does not correlate with heliocentric distance indicates that the intensity of solar radiation does not play an important role in the colouring process.

The so-called reddest objects have a B-V colour excess relative to the Sun typically of ~ 0.65 mag, and a V-R colour excess of ~ 0.4. This implies that the ratio of reflectivity at the wavelengths 4500A and 6500A is

f = R(6500A)/R(4500A) » 2.5 (1)

Table 1 compares this value with reflectiviy ratios extracted from the data of Tholen et al. (1986) for a representative set of comets and D-type asteroids. From Table 1 we see that the surfaces of comets and asteroids fall significantly short of meeting the condition implied by (1). Table 2 sets out experimentally determined values of the same ratio f for several different types of laboratory materials (CRC Handbook of Chemistry and Physics, 54^th ed., 1973; Larson and Fink, 1977). We note from here that some mineral surfaces could come close to satisfying (1), but by far the best candidates for producing redness are naturally occuring pigments as typified by the data for 'ripe pear' and 'ripe peach'.

Table 2 also includes data for irradiated hydrocarbon mixtures (Andronico et al., 1987). The relevant values of f range from 3.3 to 1, decreasing with increasing radiation dose beyond a certain point. Generally similar results are reported for irradiation with high-energy photons rather than nucleons. In all cases colours ranging from 'yellow' to 'brown' can be generated under carefully controlled conditions, and with precisely chosen cut-off values of radiation doses. On the basis of such laboratory data one could thus conclude that prolonged exposure to high-energy radiation, as occurs in interplanetary space, would lead eventually to the appearance of a grey or neutral colour. One might try to retrieve the case for radiation colouring by invoking meteorite and micrometeorite impacts. Such impacts, it could be said, arrests this greying process by continually exposing a pristine cometary surface that will be subject only to brief interludes of irradiation. But it is clear from Table 1 that the colours of real comets exposed to the interplanetary environment do not bear testimony to such an effect. Indeed Halley's comet and other long-period comets that spend most of their time in the outer regions of the solar system have mostly neutral colours, whilst the shortest period comets show reddening, albeit to a minor degree. From Table 2 it is clear that the reflectivity ratio given by (1) is consistent with the presence of highly absorptive organic chromophores (pigments) that have their absorption peaks distributed over green to red wavelengths.

Table 2

Reflectivity ratio, f , for laboratory systems

For many years the present authors have maintained that red colorations of planetary ices, for example the surface of Europa, could most plausibly be explained on the basis of biological pigments (Hoyle and Wickramasinghe, 1983, 1997; Hoover et al., 1986). Such pigments will be continually regenerated and brought up to the surface as long a biological activity persists. Suitable candidates for such pigmented microorganisms could be found among the Antarctic snow-ice algae Chlamydomonas, and diatoms. These organisms, which produce brownish and reddish colorations throughout the polar regions, might well serve as an analogue for the colours of icy bodies in the Kuiper belt. It may be relevant in the present context that diatoms are able to replicate and to carry out photosynthesis beneath an ice crust, operating at light levels of less than 1% that at the surface (Hoover et al., 1986).

We have argued elsewhere that radioactive heat sources present in primordial solar material would inevitably produce melting of ices in the interiors of comets (Hoyle and Wickramasinghe, 1983; Wallis and Wickramasinghe, 1992). The larger objects amongst the comets, giant comets with radii greater than, say 50km, may also be appropriate representations of Kuiper-belt objects. Such objects could retain interior lakes beneath an ice crust for timescales that may even exceed the age of the solar system. Anaerobic bacterial activity in subsurface lakes, leading to the build-up of high-pressure gas pockets, could cause sporadic cracking of an overlying ice layer. And this in turn leads to the transport of biological pigments to the surface.

The classes of red and grey Kuiper-belt objects discovered by Tegler and Romanishen could thus mark out a simple distinction between objects that are biologically active from those that are not. In objects where biological activity has ceased the red pigments would rapidly degrade to become grey.

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