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All Hail William Napier
William Napier was born in 1940 and got his Ph.D. three years after his B.S., in 1966. After a career as a professional astronomer, he published his first book of fiction in 1998, at the age of 58, and published three more over the next five years.
But I still would not have heard of Napier had I not read his two brilliant 2007 Astrobiology papers, published when he was 67. The first argued comets were a likely origin of life:
A single comet of radius 10 km … contains [about] as much clay … as … early Earth. … Our Solar System is surrounded by about 1011 comets … A cometary interior provides a stable, aqueous, organic-rich environment for around 106 years.
The second showed that life could spread across a galaxy when via giant molecular clouds reliably collecting life from the stars they drift near, and then passing that life on to a few of the thousands of new stars they create. I now honor Napier by quoting lots of his detail:
The Solar System passes within 5 pc of star-forming nebulae every 50–100 million years, a distance which can be bridged by protected micro-organisms ejected from the Earth by impacts. Such encounters disturb the Oort cloud, and induce episodes of bombardment of the Earth and the ejection of microbiota from its surface. Star-forming regions within the nebulae encountered may thus be seeded by signiﬁcant numbers of microorganisms. … Dissemination of microbiota proceeds most rapidly through the molecular ring of the Galaxy. …
The Oort cloud comprises perhaps 1011 comets at distances up to 50,000 AU from the sun, with orbital periods up to ~4 Myr. The mass of the system probably lies in the range 0.1–250 Earth masses. … Surges in ﬂux (‘ comet showers ’ …) are expected owing to discrete perturbations of the Oort cloud. Giant molecular clouds have masses characteristically M~5 x 105 Msun. …. The bombardment proﬁle resulting from a grazing encounter with a [500K Msun] GMC [Giant Molecular Cloud], an event which has probably happened ﬁve to ten times in Solar System history. … [has] a distinct bombardment episode, declining with a half-width ~3 Myr. …
Bombardment episodes can be seen with some degree of statistical conﬁdence in the terrestrial impact record of the past 250 million years. … A weak periodicity does seem to be present … The periodicity may be ~36 Myr … It likely has a Galactic provenance, consistent with the Sun’s vertical motion through the Galactic disc and the tendency for nebulae to be concentrated towards the plane. …
In the inner planetary system, … a 1 m boulder would be destroyed by erosion in 19,000 to 230,000 yr. … Metre-sized boulders ejected from Earth may then be collisionally reduced to submicrometre particles on timescales of a few centuries, whereupon Solar radiation pressure will expel them from the Solar System. … The meteoroid attains a terminal velocity v ~13 km/s and travels 1.9 pc from the Solar System in 140,000 years. … A 1 mm grain with a graphite coat 0.02 mm thick would achieve this. This timescale may be compared with the half-life of deep-frozen microbiota against destruction by Galactic cosmic rays, which according to Mileikowsky et al. (2000) is ~75 000 yr for some micro-organisms. Thus a signiﬁcant fraction of microbiota would survive exposure to Galactic cosmic rays while travelling out to a few parsecs from the Solar System (arguments have been given to suggest much longer survival times). The Galactic cosmic ray count within a GMC is low and will increase the half-life taken for the biosphere radius calculation.
The Solar System may thus be surrounded by a biosphere extending out to at least ~5 pc, capable of infecting a star-forming nebula during a close encounter. One readily ﬁnds that, conservatively, ~3 x 1015 g of unshocked surface material from the Earth is injected into the GMC during the ~3 Myr of the passage. Furthermore, as this material is largely in micrometre-sized particles, … If there are 106 microorganisms per gram of terrestrial material ejected, …
Massive nebulae are scavengers, disturbing the comet clouds of star systems penetrating them and gathering up the expelled dust, which will include any microbiota ejected in unshocked planetary material. A typical GMC may have ~50 000 stars passing through it at any time. They are also sites of star and planet formation (an OB association may contain several thousand stars) and so there is a clear potential for propagating life throughout the Galaxy as a chain reaction. …
Diffusion of life throughout the Galactic habitable zone would proceed most rapidly through the molecular ring of the Galaxy and spread from there. The mean interval between signiﬁcant encounters in the ring is ~50 Myr. In the ring, only 1.12 habitable planets or their precursor material need be inoculated per encounter with a molecular cloud for panspermia to go to completion within the age of the Galaxy. Two inoculations per encounter would lead to complete dissemination in less than 400 Myr.
Some criticism of these papers here, which noted:
The [Falkowski] team recovered highly degraded microbial DNA from 8 million-year-old Antarctic ice and estimated that DNA on Earth has a half-life of only about 1.1 million years. In other words, every 1.1 million years, half of the DNA disappears.
Liquid water in comets, once considered impossible, now appears to be almost certain.
New evidence has come from the discovery of clay minerals in comet Tempel 1, which compliments the indirect evidence in aqueous alteration of carbonaceous chondrites. Infrared spectral indication of clay is conﬁrmed by modelling data.