Searching for Life in the Solar System ... And Beyond

A Research Discussion Meeting
London, UK - 31 October 1996


Ramon D Wolstencroft, Royal Observatory, Edinburgh

Prior to Viking's search for life on Mars in 1976 a group of us at the University of Hawaii had measured the optical circular polarisation (Q) of the main planets. Having discovered that Q for planets with negligible atmospheres was well described by models of rough surface scattering and that Q was a well-behaved, anti-symmetric function of planetary latitude and phase; we considered if a small component of circular polarisation arising from the circular dichroism of an optically active (e.g. organic) material might also be detected in the case of Mars. This was not totally impractical as the rough scattering element vanishes at opposition whereas the optically active (OA) element stays constant. We therefore carried out observations of Mars during the 1971 opposition period in the spectral range 4000A to 8300A with a precision in Q of 3x10(-6). If the two hemispheres were identical in optical properties and had no OA elements then Q(N)=-Q(S). However Q(A)=0.5[Q(N)+Q(S)] was not zero and this N-S asymmetry could be explained either (a) as an asymmetry in optical properties(n,k) or (b) as an upper limit to the optical activity of 0.3x10(-5) at 4700A-7000A and 1.3x10(-5) at 7000A-8000A.

Although we are aware in 1996 that the presence of any optically active relics of organic life on Mars is extremely unlikely, nevertheless this approach illustrates how the remote detection on other planets might proceed in very favourable circumstances, given the major strides in instrumentation and telescopes in the past 20 years. Laboratory measures of the reflected light at the peak of the chlorophyll a band near 6600A yields Q=0.1% which indicates the level of sensitivity and a real coverage that would allow remote detection. Perhaps the best way to prepare for such a prospect might be to make a survey of the circular polarisation signatures of planet Earth.