I should point out that there is no evidence for panspermia in that link, either; it does refer to the theory of panspermia and to a number of phenomena which might or might not make panspermia possible.
Interplanetary panspermia seems possible, although it does require that microbes can be transferred through interplanetary space without being sterilised by solar and cosmic radiation. This only puts the origin of life back one step. lnterstellar panspermia requires that life arose by abiogenesis on a planet within our solar system.
On the other hand interstellar panspermia is incredibly unlikely, since stars are mind-bogglingly far apart and extremely small targets. Interstellar panspermia is so unlikely that it will have happened only a few times in the history of our galaxy, if that. Once again, this only puts the origin of life back one step at most.
Mild evidence:* scientists at the German Aerospace Centre in Cologne designed experiments using the Russian FOTON satellite. They mixed bacterial spores with particles of clay, red sandstone, Martian meteorite or simulated Martian soil to make small lumps a centimeter across. The lumps were then exposed via the satellite to outer space. After two weeks of exposure, researchers found that nearly all of the bacterial spores mixed with red sandstone were able to survive. Another study showed that bacterial spores could survive the extreme conditions of outer space for six years if they were protected from extraterrestrial solar UV radiation. This would be possible if the spores traveled within comets or meteorites.
One study focused on the heat radiation emitted from Halley’s Comet’s dust particles as the comet approached the sun. The particles’ radiation fingerprint corresponded surprisingly well to that of bacteria heated to elevated temperatures – no material other than bacteria matched the observed spectrum. As comets are known to have collided with Earth at different points in the past, this observation presents an interesting argument for panspermia. While this study does not provide conclusive evidence for presence of life in outer space, it does raise the possibility that our galaxy may be littered with bacterial spores.*
How exactly are these microbes supposed to get into comets?
There is a small chance that meteorites expelled from Earth could contain viable microbes, but the chances are very small that they will reach another world and survive. Interplanetary panspermia may be possible, and it would not surprise me to find that Earth life has infected one or more other bodies in our solar system. Or conversely we may find that life originated on another body in our solar system, and was bought here by impacts.
But this still requires an abiogenesis event to have occurred somewhere in our solar system- there is practically no chance that life could persist in a meteor long enough to reach another star.
Same way they get into the middle of mud or dirt clods.
Another idea, called lithopanspermia, is that bacteria could travel through space protected in rock. Many rocks contain small empty spaces called vesicles that can house bacteria colonies (solidified lava is often quickly colonized by bacteria living in vesicles). Bacteria could thus travel between planets within rock inside asteroids and comets. These asteroids or comets would thus provide both a means of protection and a means of transport.
Some forms of life can stay in a state for indefinite periods.
A meteorite blasted off from the surface Mars about 15 million years ago was found in Antarctica in 1984 by a team of scientists on an annual United States government mission to search for meteors. The meteor was named Allan Hills 84001 (ALH84001). In 1996 ALH84001 was shown to contain structures that may be the remains of terrestrial nanobacteria…A meteorite blasted off from the surface Mars about 15 million years ago was found in Antarctica in 1984 by a team of scientists on an annual United States government mission to search for meteors. The meteor was named Allan Hills 84001 (ALH84001). In 1996 ALH84001 was shown to contain structures that may be the remains of terrestrial nanobacteria.
How many samples have been examined specifically to locate them? It’s pure conjecture on my part, but if panspermia is possible, I would imagine it would be done by certain “trans-life” things like virus, that can exist in the vacuum of space.
I am quite familiar with the concept of lithopanspermia; it involves small rocks (meteoroids) which are expelled from Earth (or from another life-bearing world) which then travel though space containing viable microbes, and eventually hit another world. Asteroids are not involved in this concept, neither are comets. Note that the chances of a meteoroid hitting a comet are much smaller that that of hitting a planet.
If they were ‘terrestrial nanobacteria’ they didn’t come from Mars. But very few experts nowadays believe that the structures in ALH84001 were living creatures at all, let alone ‘terrestrial nanobacteria’ which is a class of organisms that does not seem to exist.