Some panspermia papers by Wickramasinghe and coworkers at Cardiff University on the red rain phenomenon.

I'd love the panspermia hypothesis to be true. But none of this stuff helps. Case not proven, sorry.

Top down or bottom up?

The facts: there are things that look like microbes in the stratosphere and advocates of panspermia try to argue that these are more likely to be arriving space-travellers than uplifted terrestrial organisms. But why are these likely to be visitors from the vacuum? Well apparently there's just so many of them:

"atmospheric transport may raise terrestrial particles into the stratosphere above the 16km tropopause, but if the particles rise above 20km to 40km the fraction is likely to drop off substantially, at least in comparison with particles descending from spacecraft or space sources."

But consider this: if there are formerly terrestrial organisms up there, why presume they are recent accidental additions? Isn't it more likely that the biosphere is bigger than you presume than that the stratosphere is constantly contaminated from above? Why can't life, which occupies black smokers on the deep sea floor and underground ice lakes and every other extreme environment on Earth where we've taken the time to look - why can't life make it to the stratosphere? The authors actually acknowledge this:

"In recent years the limits of microbial life on the Earth have expanded to encompass an extraordinarily wide range of habitats: geothermal vents, the ocean floor, radioactive dumps and antarctic soil, eight kilometres underneath the Earth's crust, to name but a few . The long-term survivability of bacteria has also been extended from 25-40 million years (Cano and Borucki, 1995) to a quarter of a billion years the case of a bacterium entrapped in a salt crystal (Vreeland et al, 2001)."

but seem unaware of its consequences for their theory. They think it means bacteria are tough enough for space, when it equally (or more!) means they're tough enough for the stratosphere. And if the idea that the biosphere extends to the stratosphere seems a little far-fetched, well fine - aren't you at least bound to check that option too before you claim that anything found up there is drifting down from space? If life has colonized the stratosphere, it will be growing and dividing and increasing its biomass up there - which might be why you see more than you would expect if you presume the only terrans up there are unfortunate strays.

I'm only saying this: it is their duty to consider all alternative hypotheses, and it took me five minutes to think of that one.

Crossing the line

The authors have already dispensed with any theory of native stratosphereans by assuming that the tropopause is a strong barrier against ambitious terrestrial microbes en route to the stratosphere:

"Convection currents lead to mixing of ground level particulates in the air that can be carried relatively easily into the troposphere, but temperature inversions beyond 15 km lead to barriers through which very few aerosols can penetrate. Whenever rare events such as volcanic eruptions loft particles above 30 km, particles larger than a few microns fall back quickly to the ground under gravity. The isothermal temperature regime between 15 and 25 km effectively stops the ascent of particulates, and the rapidly rising ambient temperature gradient at higher levels makes the upper stratosphere almost impervious to the transport of aerosols from the ground."

This presumes much and overlooks lots (and doesn't define "few" in the right context[59]). It doesn't take a volcanic eruption to hoist stuff up there: storms will do. Many tropical storms are violent enough to shove material through the tropopause via convective overshoot. Some conformations of high-relief terrain can lift troposheric air through the tropopause too (this is how some glider pilots can make it into the stratosphere even though thermal plumes don't extend so high). Mundane cloud microbes could be carried up by these processes and would therefore inevitably be in the stratospheric samples. Moreover, this is not a new phenomenon: even if it is impossible for life to sustain itself in the stratosphere there is plenty of incoming, upwelling supply of microbes to fill the sample chambers (and, if it is possible to earn a microbial living in the stratosphere, they will have been there billions of years).

There's another way to get into the stratosphere, one where all an organism has to do is stand still. The tropopause can descend far enough for decidedly earth-bound life to find itself in the stratosphere if on sufficiently high terrain. The summit of Everest is only a few kilometres shy of the tropopause on an average day and there are occasional low pressure systems in the troposphere that can pull down the tropopause enough for the Chomolangma's summit to poke through into the stratosphere. This has killed numerous climbers on Everest. Microbes, on the other hand, are better at adapting.

There is no data to support the theory that any of Wickramasinghe's red rain or high-altitude samples contain anything other than homegrown microbes. But they should keep looking. Because, if it were true, that'd be amazing!