Experimental physicist Rob Sheldon offers some thoughts:
I do think comets can exchange spores and therefore comet-to-comet infection spreads life throughout the galaxy, if not the universe.
Is life rare in the solar system? Observations say that it is common. I’ve made a long list but briefly:
Mars: Observations of algae, fossils, bioengineered minerals, metabolism, diurnal cycles, biology-made gasses, organics
Venus: biological gasses
Jupiter moons: spectroscopic signatures of photopigments, organics, energy outgassing with water vapor
Saturn’s moon: organics, amino acids
Interstellar: organics, signature matched by hollow shells (diatoms), Comets: fossils, chiral amino acids and nucleotides, organics, anomalous heat generation (metabolism), biominerals, photopigments, oxygen
3. Could these be the result of a massive collision with Earth that spread contaminated life throughout the Solar system?
Perhaps, though the widespread observations mean that it had to happen a long time ago, perhaps over 3 billion years ago. Mars lost its water a long time ago, for example, so the biominerals are probably over a billion years old. In addition, only the most robust amino acids are found on comets, the others have decayed away. Likewise the racemization of the L-amino acids take time. These have a built in “clock”, which given the temperature profile, predict the age. We only have limits on the temperature–CI meteorites have never been above 100C. And given those limits, the age comes out to at least 100 Mya, if not 1Gya. So all these constraints make it less likely that life began on Earth and spread through the Solar System. And of course, interstellar observations are incompatible with that hypothesis.