Two Kinds of Panspermia

Caleb A. Scharf offers an interesting argument against interstellar panspermia:

You and I, or fluffy bunnies and daffodils are all unlikely candidates for interplanetary or interstellar transferral. The sequence of events involved in panspermia will weed out all but the toughest or most serendipitously suited organisms. So, let’s suppose that galactic panspermia has really been going on for the past ten billion years or so – what do we end up with? …

Life driven by cosmic dispersal will probably end up being completely dominated by the super-hardy, spore-forming, radiation resistant, chemical-eating, and long-lived but prolific type of critters. …

The problem, and the potential paradox, is that if evolved galactic panspermia is real it’ll be capable of living just about everywhere. There should be stuff on the Moon, Mars, Europa, Ganymede, Titan, Enceladus, even minor planets and cometary nuclei. Every icy nook and cranny in our solar system should be a veritable paradise for these ultra-tough lifeforms, honed by natural selection to make the most of appalling conditions. So if galactic panspermia exists why haven’t we noticed it yet? (more)

I see two rather different interstellar panspermia scenarios:

  1. Space-centered – As Scharf says, life might mainly drift from one harsh space environment to another. Yes sometimes life would fall onto and then prosper on someplace like Earth, but being poorly adapted to space such planet life would contribute less to future space life. Under this scenario life must on average grow in common space environments, and so we should see a lot of life out there in such environments.
  2. Planet-centered – Alternatively, space life might usually die away, and only grow greatly in special rare places like planets (or perhaps comets). In this scenario the progress of life would alternate between growth on planets (or comets) and then decay in space. A similar scenario plays out when seeds like coconuts drift between islands in the ocean – seeds die away during ocean journeys, and then multiply on islands. In this scenario life would be adapted both to grow well on planets, and to decay as slow as possible in space.

Scharf’s argument weighs against a space-centered scenario, but not a planet-centered scenario. Of course there is actually a range of intermediate scenarios, depending on how wide a range of environments let life grow.

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  • VV

    I don’t get why people take interstellar panspermia seriously. Intuitively, its much more parsimonious to hypothesize that life just appeared on earth than that it appeared somewhere else, then got jettisoned in space, survived for decades at least in an exceptionally harsh environment (with temperatures violently alternating between freezing cold and boiling hot, no atmosphere, no liquid water, UV solar radiation, cosmic rays) and then fell on this planet, survived atmospheric entry, and happened to find a compatible chemical environment.

    It’s already implausible inside the solar system, and much more at interstellar level, as stars are needles in the galactic haystack, solar radiation pressure deflects away small incoming objects and larger meteorites would reach the earth so fast that they will almost always burn during atmospheric entry.

    (A quick googling turns up this:…68H)

  • Robin Gane-McCalla

    I don’t buy this argument, by this logic there should be human babies everywhere somebody jerks off.

    The lack of life everywhere isn’t evidence against panspermia, it’s evidence that that the interstellar sperm being sent out need a specific environment to create life, which is how sperm works for many animals.


  • Where did we look for life? I know we’re preparing to look for it on mars, but the only place I know that we’ve already looked is Earth, and it’s everywhere that there’s water.

  • rapscallion

    If finding extra-terrestrial life in harsh environments is evidence for panspermia, it’s absence has to be evidence against panspermia. Can’t have it both ways.

  • dmytryl

    Isn’t life quite dominated by spore forming, radiation resistant, et cetera lifeforms anyway? How is his view affected by this?

    Furthermore, no matter how adapted, as long as we aren’t speaking of something silicon based straight out of science fiction, if there’s no nutrients and no water and temperature is outside of certain range, life can’t reproduce.

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