First Signs of Life - Reality of Panspermia and Pathogens from Space
As a brief respite from our work on the influenza pandemic I decided upon impulse to take a closer look at the visual extinction curve of starlight, the way that starlight is extinguished by dust at visual wavelengths. This, it would be recalled, is where my researches into this whole subject began in 1961. There were many unresolved problems that had to be solved. Over the wavelength range from 7000 to 3000 ˚ A , the extinction (dimming ratio on a logarithmic scale) was approximately inversely proportional to the wavelength, and this was the case in whatever direction one looked. Such an invariance of behaviour was difficult to reconcile with the grain models we had discussed so far involving mixtures of silicate or organic grains along with graphite and iron. In all these non-biological grain models the visual part of the extinction curve had to come mostly from the dielectric (non-absorbing) component of the mixture, and to get the correct shape of the extinction curve, grain radii had to be fairly sharply fixed. This condition could not be relaxed as long as one stayed with grain materials such as ice, organics or silicates which have visual refractive index values 1.5, 1.6, respectively. In other words, the solutions obtained so far in all these cases are highly parameter sensitive, and therefore not very satisfactory. I discovered that one way to relax the size constraint is to reduce the value of the refractive index, below 1.3. A value closer to n=1.15 would be nearly optimal from this point of view, for the reason that the extinction efficiency of spheres with this refractive index can remain closely proportional to inverse wavelength over a wide range of wavelengths. But what material could possess such a low value of n? When I searched the relevant handbooks of physical constants it soon became obvious that, with the exception of solid hydrogen, there was no other homogeneous solid material that had the desired property. Solid hydrogen was of course ruled out because we had found earlier that it could not survive under normal interstellar conditions.