Ballistic panspermia, sometimes referred to as interplanetary panspermia, is a version of the panspermia hypothesis in which it is argued that impact-expelled rocks from a planet's surface serve as transfer vehicles for spreading biological material from one planet to another within the same solar system. It requires that the microorganisms survive (1) the impact ejection process from the planet of origin; (2) traveling through the solar system; (3) landing on a planet within the same solar system.
During the 1830s Swedish chemist Jöns Jacob Berzelius confirmed that carbon compounds were found in certain meteorites "fallen from the heavens". Berzelius' finding contributed to theories propounded by later thinkers including the physician H.E. Richter and physicist Lord Kelvin (William Thomson) (1824–1907). Lord Kelvin declared in 1871, "[W]e must regard it as probable in the highest degree that there are countless seed-bearing meteoric stones moving about through space. If at the present instance no life existed upon this earth, one such stone falling upon it might, by what we blindly call natural causes, lead to its becoming covered with vegetation."
But it was the Swedish chemist and 1903 Nobel Prize winner, Svante Arrhenius, who popularized the concept of life originating from space in 1908. His theory was based on the notion that radiation pressure from the Sun and other stars "blew" microbes about like tiny solar sails, and not as the result of finding carbon compounds in stony meteorites.
With the discovery of meteorites on Earth which almost certainly came from the Moon and Mars, it has become relevant to ask a logical question: Can life forms and/or chemical precursors of life be transported thus across the far reaches of the solar system? Can one planet infect another ballistically? An analysis by M.K. Wallis and N.C. Wickramasinghe is rather warm towards this idea:
"The mass of escaping ejecta from the presumed 10-km comet that caused the 180-km Chicxulub crater, with a radius of roughly 10 km and 1 m deep, amounted to ~300 Mm3, of which one third may have been rock and 10% higher-speed ejecta that could have transited directly to Mars. It may have taken 10 Ma to impact Mars but...the probability is not exceedingly low but 0.1-1%.
"The survival and replication of microorganisms once they are released at destination would depend on the local conditions that prevail. Although viability on the present-day Martian surface is problematical, Earth-to-Mars transfers of life were feasible during an earlier 'wet' phase of the planet, prior to 3.5 [billion years] ago. The Martian atmosphere was also denser at that epoch, with several bars of CO2, thus serving to decelerate meteorites, as on the present-day Earth. Since the reverse transfer can occur in a similar manner, early life evolution of the two planets may well have been linked." (Wallis, Max K., and Wickramasinghe, N.C.; "Role of Major Terrestrial Cratering Events in Dispersing Life in the Solar System," Earth and Planetary Science Letters, 130:69, 1995)
Exchange of material between the Earth and Mars may have been common during the first 800 million years of the Solar System's existence, that is, between 4.6 and 3.8 billion years ago, when major impacts with asteroids and comets were frequent. If simple organisms arose on either world during this time – and there is tentative evidence of terrestrial microbial life dating back 3.85 billion years – they may have been transferred inside ejected rocks to the neighboring planet and formed a colony on arrival. There are even plausible reasons to suspect that life may have evolved first on Mars and then, via ballistic panspermia, spread to the Earth. Or, there may have been a regular cross-fertilization of microorganisms between the two worlds. Conceivably, Venus, too, may have been involved in the transference of life when its surface conditions were more clement than they are today. One consequence of the possibility of such cross-fertilization is that if life, or evidence of past life, were found on Mars, it would not immediately imply independent biological evolution.
Some researchers believe that proof of ballistic panspermia has already been found:
- A meteorite originating from Mars known as ALH84001 was shown in 1996 to contain microscopic structures resembling small terrestrial nanobacteria. When the discovery was announced, many immediately conjectured that the fossils were the first true evidence of extraterrestrial life — making headlines around the world, and even prompting United States President Bill Clinton to make a formal televised announcement to mark the event. However, most experts now agree that these are not a definite indication of life, but may instead be formed abiotically from organic molecules. It has not yet conclusively been shown how they formed and recent advances in nanobe research has made the find interesting again.