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*** Cosmic Ancestry preprint of
Astrophysics and Space Science v 268 p 369-372, 1999
***

THE ASTONISHING REDNESS OF
KUIPER-BELT OBJECTS What'sNEW
by
N.C. Wickramasinghe and F. Hoyle
School of Mathematics, Cardiff
University
PO Box 926, Senghennydd Road
Cardiff CF2 4YH, UK

E-mail: [email-masked]

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.

______________

Look not thou upon the wine when it is red, when it giveth his
colour in the cup,…
At the last it biteth like a serpent, and stingeth
like an adder. — Proverbs, xxiii. 31
______________

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, 54th 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 1

Reflectivity ratios for comets and asteroids

Object

Reflectivity Ratio,

R(6500A)/R(4500A)

Comets, period < 20 yr

1.26

Comets, period > 35 yr

1.11

P/Halley (Period 76yr)

1.00

D-type Asteroids(Mean)

1.16

Kuiper-Belt Red Class

2.50

Table 2

Reflectivity ratio, f , for laboratory systems

Laboratory system

Reflectivity Ratio,

R(6500A)/R(4500A)

Pyroxene

1.58

Olivine

1.63

Ripe pear

3.67

Ripe peach

4.15

Irradiated organics

3.30 decreasing with dose to
1.0

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.

What'sNEW
2000,
October 25: Red and grey Kuiper-belt objects have different orbits.
2000,
January 5: The spectrum of Pluto's moon, Charon, shows crystalline ice.

1998, March 21: Two distinct classes of Kuiper-belt objects
-- the story that promped this article.

References

Andronico, G., Baratta, G.A., Spinella, F. and Strazzulla, G.:
1987, Astonon.Astrophys 184, 49-51

CRC Handbook of Chemistry and Physics, 54th ed: CRC
Press, 1973

Hoover, R. B., Hoyle, F., Wickramasinghe, N. C., Hoover, M. J.
& Al-Mufti, S.: 1986, Earth, Moon, and Planets, 35,
19-45

Hoyle, F. and Wickramasinghe, N. C.: 1983, Living
Comets, Univ Coll. Cardiff Press

Hoyle, F. and Wickramasinghe, N. C.: 1997, Life on Mars? The
case for a cosmic heritage Clinical Press, Bristol

Larson, H.P. and Fink, U.: 1977, Applied Spectroscopy,
31, 386

Tegler, S. and Romanishin, W.: 1998, Nature, 392,
49-51

Tholen, D. J, Cruikshank, D. P, Hartman, W. K, Lark, N, Hammel,
H. B. & Piscitelli, J. R.: 1986, Proc. 20th ESLAB Symposium on the
Exploration of Halley's Comet, Heidelberg 27-31 October 1986, ESA
SP-250, Vol . III, 503-507

Wallis, M.K. and Wickramasinghe, N.C.: The Observatory,
112, 228-234

Acknowledgement
This work was supported by a grant from Acorn
Enterprises LLC, Memphis, TN.

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