Panspermia is at the best a hypothesis.
We have a good idea of the chemistry of abiogenesis on Earth. The sequence of reactions is almost obvious.
The problem is When and Where.
Latest stages (assembly of prebiotic compounds) could have occurred inside the rock itself, around deep ocean thermal vents, in geysers, etc.
However, the earliest stages (non-organic to prebiotic) are more likely to have happened elsewhere, for example in comets.
The concept of panspermia is that wherever, whenever life appeared, it steadily contaminated planets since then, either by the way of living organisms, or chemical bricks of life.
Bricks of life are everywhere in space, and as soon as environmental conditions are favorable, life appears as easily as resistant bacterias infect a hospital.
- The earliest fossil of bacteria colony found on Earth is also one of the oldest rocks found.
- Amino acids have been found in interplanetary dust, and in comets (Glycine - see missions Star Dust, Rosetta, Tanpopo, etc.) and in meteorites (Adenine, Guanine and related organic molecules).
- The Solar System outer cloud of comets (Oort) is believed to extend to half the distance to the nearest stars (the 3 α Centauri stars), making the exchange of comets between stars plausible.
- Extremophile organisms can handle a wide range of conditions on Earth and even outer space conditions.
- An estimation of 1 to 2 billion bacteria spores from Earth, having traveled on not well sterilized spacecrafts, are now surviving on Mars, waiting for better conditions. An example of accidental panspermia.
- Panspermia doesn't solve the question of abiogenesis, it only moves it to billions of other planets
- Amino acids in space might not come from a previous form of life, they can form naturally in comets.
- Experiments of outer space conditions exposure of micro-organisms always result in the quick death of the organisms. (However, most can survive years if protected from UVs)