Panspermia refers to the notion of the "seeds" of life being dispersed across the universe. This idea takes several forms. In the now-discredited notion of a steady-state universe of infinite size and age, life would have been present throughout the universe, forever. In the current view, life cannot be older than the Big Bang. In fact, its origin could not predate the formation of elements heavier than hydrogen and helium in the first generations of stars.
In the panspermia scenario, the origin of life would take place at least once and possibly several times and would be spreading outwards from its origin(s), through dust, comets and bits knocked off planets carrying something like bacterial spores. This idea was popularized by Svante Arrhenius and the torch is presently being carried by the British astrophysicist Fred Hoyle and his associates.
True panspermia relates to life being carried at least between stellar systems within the same galaxy. The time scales for such travel by physical matter (as opposed to electromagnetic radiation such as light) seem incredibly vast and the whole process extremely haphazard. Directed panspermia refers to the idea that an advanced life form may chose to deliberately spread life, in which case the time scale for filling an entire galaxy can conceivably be as low as 20 million years, a mere cosmic eye blink. In this case we might justifiably ask, as Enrico Fermi did, "Where is everybody?" He will examine this question in more detail in the last lesson.
The idea that close planets such as Earth and Mars may trade life hardly qualifies as panspermia. We know that rocks are being knocked off the Moon and Mars and making their way to Earth in less than 20 million years. We estimate that certain direct trajectories could take only a few years from Mars to Earth. The reverse journey is a bit more difficult, in that Earth has greater gravity and has a thicker atmosphere, which both retard outgoing projectiles. In addition, Mars is both "upstream" (away from the Sun) and has a smaller gravitational field to capture projectiles with. Nevertheless, we expect that these two planets have been "trading spit" for billions of years. There is some evidence that bacteria might survive sealed in salt on Earth in a spore state for millions of years. Thus, if sheltered behind a millimeter of rock they should easily survive a trip between planets. It will be no surprise to discover Earth-like life on Mars - the only question will be: did it originate there or here? Or was it brought to both planets from elsewhere?