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Interstellar panspermia reconsidered
Abstract
The absence of free-living microorganisms simpler than bacteria on Earth
is evidence that life did not originate on Earth, but immigrated. The
question then arises as to whether life was imported from a point of
origin in our solar system, most likely Mars, of whether the solar
system was seeded from interstellar sources. The search for fossil or
extant prebacterial organisms (prebacteria) on Mars can resolve this
question. However, to understand the likelihood of interstellar
panspermia, we also need to consider whether the Earth itself has served
as an efficient source for the spread of microorganisms. Close
encounters with other stars due to random stellar motion occur with a
frequency of 1/20 Myr, in fair agreement with the observed frequency of
major impact events and mass extinctions. Such events are estimated to
eject unsterilized material into interstellar space at a time-averaged
rate of 10 tonnes per year. A number of mechanisms for the interstellar
dissemination of bacteria along with this material are considered. It is
shown that transmission of microbial life from one solar system to
another is highly probable.
... Impact cratering is a dominant geological process in the Solar system that played an important role in the formation and early evolutionary process of constituents in the solar system (e.g., Barlow, 2014;Li et al., 2018;Maruyama and Ebisuzaki, 2017). It also plays an active role in the crustal evolution of young terrestrial planets (O'Neill et al., 2017), initiating impact-driven subduction, localized lithospheric thinning, mantle upwelling and associated tectonic activity on the Hadean Earth (O'Neill et al., 2017;Santosh et al., 2017), in supplying chemical ingredients for the evolution of life on Earth (Zubrin, 2001;Gladman et al., 2005;Maruyama et al., 2018aMaruyama et al., , 2018b and also in modifying the terrestrial life evolutionary process through impact-induced mass extinctions (Schulte et al., 2010;Alvarez, 2003;Chatterjee et al., 2006). It has been recognized that the formation of Moons around Earth's (Canup, 2012;Cuk and Stewart, 2012) and Pluto's (Canup, 2005(Canup, , 2011 orbits are possibly linked with large impact with planets. ...
Impact cratering is a dominant geological process in the Solar system and is one of the frontier themes in planetary research. Here we explore the possible dependency of surface gravity and crater geometry in the natural and theoretical prediction from scaling factor of crater diameter using morphometric analysis. Theoretical model predicts a negative correlation between final crater diameter (D fr )and gravitational acceleration of the target body (g). In our study, an inner solar system body reveals consistent correlation that demonstrates a fundamental relationship between D fr and g, although such process may not be applicable if we consider outer solar system bodies. Our study indicates that impact-cratering process is primarily controlled by objects associated with asteroids belts. We also demonstrate that crater-diameter scaling can be used as the proxy for extra-terrestrial impact.
... Impacts played a major role in the formation and early evolution of solar system bodies, including the formation of moons around Earth (e.g., Hartmann and Davis, 1975;Benz et al., 1986Benz et al., , 1987Canup and Esposito, 1996;Canup, 2012;Cuk and Stewart, 2012) and Pluto (Canup, 2005;Stern et al., 2006;Canup, 2011), possible contributions to the sizeable planetary obliquity of Uranus (Safranov and Zvyagina, 1969;Harris and Ward, 1982;Korycansky et al., 1990) and volumetrically large core size of Mercury (Benz et al., 1988), and the creation of large impact basins which influenced planetary shape (e.g., Wilhelms and Squyres, 1984;Andrews-Hanna et al., 2008;Matsumoto et al., 2010;Burke et al., 2012;Jaumann et al., 2012;Jutzi et al., 2013;Kreslavsky et al., 2013;Zuber et al., 2013) and geologic evolution (Schultz and Gault, 1975;Williams and Greeley, 1994;Bruesch and Asphaug, 2004;Lü et al., 2011;Meschede et al., 2011). Impacts may have helped deliver the needed chemical ingredients and energy for life to arise on Earth (e.g., Chyba, 1993;Davis and McKay, 1996;Zubrin, 2001;Gladman et al., 2005) and create environments utilized by life forms (Newsom, 1980;Newsom et al., 1986;Cockell et al., 2003;Pope et al., 2006;Lindgren et al., 2010;Osinski et al., 2013) but also have redirected the evolution of terrestrial life through impactinduced mass extinctions (e.g., Alvarez et al., 1980;Chyba, 1993;Alvarez, 2003;Tanner et al., 2004;Schulte et al., 2010). Impact craters form by the explosive release of energy which occurs when an object moving at high velocities ("hypervelocities"; generally a few to a few 10s of km s −1 ) collides with the surface of another solar system body. ...
After a long period of neglect, the hypothesis of interstellar panspermia has gained new consideration in recent years, due to a series of theoretical and observational developments. In this chapter, I briefly outline why this possibility should not be dismissed, especially in regions of the Galaxy with higher stellar density than average. Furthermore, I give some motivations for taking the mechanism into account when developing theoretical models of the distribution of life in the Galaxy (such as in studies of the galactic habitable zone) and in drawing implications from the results of future searches for biosignatures in exoplanets. This theoretical work should be complemented by experimental studies, in order to assess the concrete feasibility of panspermia with higher confidence.
It is shown that a mechanism involving only random motion of the sun with respect to the surrounding star field can account for the ~1 per 25 Myr characteristic frequency of large cometary impacts on Earth. In the proposed mechanism, the sun travels through the Oort Cloud of an encounter star, most typically a Type M dwarf, while the dwarf flies through the Oort cloud of our Sun. As a result, Oort Cloud objects from our Solar System are precipitated in large numbers to impact planets in the dwarf star system, while the dwarf's Oort Cloud objects are destabilized to impact planets in our Solar System. It is shown that it is this exchange of Oort cloud object between stellar systems, rather than the precipitation of Oort Cloud objects within a stellar system, that can account for the apparent periodicity of mass extinctions. Because the sun is more massive than ~90% of stars, its Oort cloud extends further, resulting in it delivering about a factor of three more bombardments on other solar systems than our Solar System receives. About 60% of the bombardments on our Solar System are found to be delivered by Type M dwarfs, about 20% by type K dwarfs, with the remaining 20% being delivered by stars of type G or larger. Foreign star Oort cloud objects can be captured by our Sun at typical ranges of 10 AU, resulting in a cometary approach to perihelion within a few years. It is found that assuming an effective Oort Cloud radius of 40 000 AU for a star of solar mass, increasing in size with the square root of the mass, accounts for the observed characteristic frequency of mass extinction events on Earth, given the local stellar number density of 0.003 stars per cubic light year. The frequency of mass extinction events in other solar systems would increase or decrease in linear proportion to the local stellar number density. It is shown that this exchange of materials between solar systems during close stellar encounters could be an important mechanism for spreading life throughout the galaxy. Implications for the evolution of life on Earth and in other solar systems are discussed.
We present some new ideas about the possibility of life developing around sub-giant and red giant stars. Our study concerns the temporal evolution of the habitable zone. The distance between the star and the habitable zone, as well as its width, increases with time as a consequence of stellar evolution. The habitable zone moves outward after the star leaves the main sequence, sweeping a wider range of distances from the star until the star reaches the tip of the asymptotic giant branch. If life could form and evolve over time intervals from $5 \times 10^8$ to $10^9$ years, then there could be habitable planets with life around red giant stars. For a 1 M$_{\odot}$ star at the first stages of its post main-sequence evolution, the temporal transit of the habitable zone is estimated to be of several 10$^9$ years at 2 AU and around 10$^8$ years at 9 AU. Under these circumstances life could develop at distances in the range 2-9 AU in the environment of sub-giant or giant stars and in the far distant future in the environment of our own Solar System. After a star completes its first ascent along the Red Giant Branch and the He flash takes place, there is an additional stable period of quiescent He core burning during which there is another opportunity for life to develop. For a 1 M$_{\odot}$ star there is an additional $10^9$ years with a stable habitable zone in the region from 7 to 22 AU. Space astronomy missions, such as proposed for the Terrestrial Planet Finder (TPF) and Darwin should also consider the environments of sub-giants and red giant stars as potentially interesting sites for understanding the development of life.
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