Convergence to panspermia

New discoveries in astronomy and biology continue to point to Panspermia as the most likely mode of the origin of life on Earth. This paper builds upon the work done by W.M. Napier, 2004 1 , and re-esti- mates the time for the seeding of the gal- axy given the latest 2013 estimates from the Kepler Mission data. We present this "thought experiment" in the form of an opin- ion paper. We calculate that, from a single point source of origin, the full colonisation of the entire Milky Way galaxy by primitive microbes will take place in a timescale less than the average age of low-mass stars in the galactic disc, 10 10 yr (i.e., 10 billion years ago; bya). Our calculation is independent of the location of the "point-source". The writers believe the probability of the point-source being Earth is infinitesimal compared with it being elsewhere in the galaxy. intense comet and asteroid bombard- ment leads naturally to the hypothesis that the impacts themselves were re- sponsible for the start-up of life on the Earth 2 . The presence of amino acids and nucleobases in the Murchison meteorite has given credence to the more restricted hypothesis that these molecules served as the components of an Earth-based primordial soup from which life was able to originate


Revisiting The Hypothesis Against which this "Thought Experiment" is Conducted
Our 2013 paper formally documents the Panspermia Hypothesis, and the propositions around which the rapidly evolving evidence should be assessed 8 .We restate this for the convenience of the reader.

The HOYLE-WICKRAMASINGHE (H-W) model of the Panspermia Hypothesis 8
Panspermia is the hypothesis that life ex-

Propagation Method #1: Lithopanspermia: Cometary Panspermia
The H-W Model sees comets as the primary carrier of the "seed of life" (defined above).
For this paper, comets are considered of two types: • Short-period comets, which live in the Kuiper Belt and have elliptical orbits in the plane of the solar system ecliptic; • Longer-period comets, which can have elliptical orbits reaching out to the Oort Cloud (but it is the parabolic Comets, which the writers believe have been involved in "sling shot" transfers from an adjacent star system).

Background
The earliest evidence of terrestrial life in the form of an enhanced carbon isotope ( 13 C/ 12 C) ratio associated with sediments at 3.82 to 4 billion years ago (bya) coinciding with an epoch of intense comet and asteroid bombardment leads naturally to the hypothesis that the impacts themselves were responsible for the start-up of life on the Earth 2 .The presence of amino acids and nucleobases in the Murchison meteorite has given credence to the more restricted hypothesis that these molecules served as the components of an Earth-based primordial soup from which life was able to originate 3 .However, all attempts to reproduce these processes in the laboratory have so far not been encouraging 4 .The improbability of accomplishing the minimal molecular arrangements, for example, in the order of amino acids in enzymes or nucleotides in DNA, by random shuffling of components has made the once unpopular idea of Panspermia appear increasingly more attractive.An origin of life occurring as a singular cosmic event on a galactic or even cosmological scale transferred via panspermic processes appears to be consistent with all the available data at the present time 5 .Lord Kelvin first suggested that life on Earth may have been carried to Earth via an impacting meteorite, and this idea has recently been recast into the modern form lithopanspermia.This has been discussed recently by several authors in relation to impacts onto a life-laden planet such as the Earth.A comet impact, like that which struck Earth 65 million years ago causing the extinction of the dinosaurs, would also have had the effect of ejecting surface rocks that contain terrestrial microbial ecologies.A fraction of this material can be shown to be able to infect embryonic planetary systems in a nearby molecular cloud 1,6 .The exchange of lifebearing ejecta would have been facilitated between planetary systems that are contained within star clusters.Numerical calculations show how this could be achieved, so panspermic opportunities are optimised in such cases 7 .

1 . 2 . 3 . 1 . 3 .
ists throughout the Universe, distributed by meteoroids, asteroids, comets and planetoids.The Hoyle-Wickramasinghe Model of the Panspermia Hypothesis defines the following propositions to guide the investigation: That dormant viruses and desiccated DNA/RNA can survive unprotected in interplanetary space (Radiopanspermia) That the seeds of life can survive protected from cosmic rays in asteroids, comets and meteors (Lithopanspermia) That the seeds of life are promulgated from solar system to solar system by a process of comet and asteroid collision with planets; matter ejection from planet to local planets and Moons; and then onwards and outwards from that solar system to an adjacent solar system In the above propositions of the Hoyle-Wickramasinghe Model, the "seeds of life" include biological microparticles such as bacteria, viruses, spores and pollen (Figure 1).This specifically includes: Desiccated and/or partially inactivated DNA/RNA 2. Live, dormant or fossilized non-cellular life (viruses) Live, dormant or fossilized cellular life (bacteria, archaea) In the more general Panspermia Hypothesis these "seeds of life" are not as clearly defined as in the Hoyle-Wickramasinghe Model.Also in the Hoyle-Wickramasinghe Model, Litho-Panspermia includes comets.It proposes that comets are the major promulgation "carrier" of the seeds of life, especially from solar system to solar system, and proposes that the center of comets is mostly water, not ice, an ideal environment for bacteria and viruses.THE THOUGHT EXPERIMENTCalculation of the Average Spacing Between Stars By March 2014, NASA's Kepler Mission had discovered 5,537 exoplanets (3,845 candidates + 1,692 confirmed).A recent revision of the size of the habitable zones of stars, taking account of atmospheric greenhouse effects, has led to new estimates of the number of potentially habitable Earth-sized planets around low mass stars in the Galaxy as about N ≈ 10 11 9 .Distributed over the entire volume of the galactic disc V ≈ 7.5 x 10 12 ly 3 (assumed to be a cylinder of radius 50,000 light years [ly] and thickness ≈ 1000 ly), this gives a mean spacing in the galaxy of (N/V) 1/3 ≈ 4 ly between stars with habitable planets.The spacing could be even closer within the birth clusters of stars.Transfer of viable microbial life across such short distances under interstellar conditions would appear to be entirely feasible.

Radiopanspermia
Figure 1 | Seeds of Life In the formation of a planetary system such as the solar system (and the proto-planetary galaxy) the first solid objects to form are the comets.These icy objects would contain the molecules of the parent interstellar cloud, and for a few million years after they condensed would have liquid water interiors due to the heating effect of radioactive decays 5 .If microbial life was already present in the parent interstellar cloud, the newly formed comets serve to amplify and promulgate it on a very short timescale.Evidence is accumulating for the existence of microbes (viruses, bacteria and other yet unidentified biological entities) in a large number of meteoroids collected on the surface of the Earth as well as in the stratosphere 8 .During the fall of 2014, there are two experiments observers can monitor where evidence could be acquired which would be inconsistent with the theory that both short and long period comets really do carry the seeds of life.The first is NASA's initiative monitoring long period Comet