Starship Asterisk*

dpackage wrote:The news occasionally runs stories about meteorites from Mars carrying gasses or amino acids or even primitive cellular life to other planets or other panspermia-type scenarios. I can't help being skeptical of impact-driven scenarios on other worlds where fragments that survived becoming completely molten during impact could achieve escape velocity from any world that both a) had a thick atmosphere and b) enough gravity to support liquid water and other things required for life. It seems like those two circumstances would significantly slow/lower a trajectory for anything kicked out by the impact. Is there some other thread here or elsewhere that discusses this? I guess it would have to be a near-planet-killer type impact that the the huge wave of crust material you see in science show animations acts as a catapult to smaller/looser material miles away from the impact site.

http://en.wikipedia.org/wiki/Panspermia#Lithopanspermia wrote:
<<Lithopanspermia, the transfer of organisms in rocks from one planet to another either through interplanetary or interstellar space, remains speculative. Although there is no evidence that lithopanspermia has occurred in our own Solar System, the various stages have become amenable to experimental testing.

Planetary ejection — For lithopanspermia to occur, microorganisms must survive ejection from a planetary surface which involves extreme forces of acceleration and shock with associated temperature excursions. Hypothetical values of shock pressures experienced by ejected rocks are obtained with Martian meteorites, which suggest the shock pressures of approximately 5 to 55 GPa, acceleration of 3×10⁶ m/s² and jerk of 6×10⁹ m/s³ and post-shock temperature increases of about 1 K to 1000 K. To determine the effect of acceleration during ejection on microorganisms, rifle and ultracentrifuge methods were successfully used under simulated outer space conditions.

Survival in transit — The survival of microorganisms has been studied extensively using both simulated facilities and in low Earth orbit. A large number of microorganisms have been selected for exposure experiments. It is possible to separate these microorganisms into two groups, the human-borne, and the extremophiles. Studying the human-borne microorganisms is significant for human welfare and future manned missions; whilst the extremophiles are vital for studying the physiological requirements of survival in space.

Atmospheric entry — An important aspect of the lithopanspermia hypothesis to test is that microbes situated on or within rocks could survive hypervelocity entry from space through Earth's atmosphere (Cockell, 2008). As with planetary ejection, this is experimentally tractable, with sounding rockets and orbital vehicles being used for microbiological experiments. B. subtilis spores inoculated onto granite domes were subjected to hypervelocity atmospheric transit (twice) by launch to a ∼120 km altitude on an Orion two-stage rocket. The spores were shown to have survived on the sides of the rock, but they did not survive on the forward-facing surface that was subjected to a maximum temperature of 145 °C. In separate experiments, as part of the ESA STONE experiment, numerous organisms were embedded in different types or rocks and were mounted in the heat shield of six Foton re-entry capsules. On reentry, the rock samples were subjected to temperatures and pressure loads comparable to those experienced in meteorites. The exogenous arrival of photosynthetic microorganisms could have quite profound consequences for the course of biological evolution on the inoculated planet. As photosynthetic organisms must be close to the surface of a rock to obtain sufficient light energy, atmospheric transit might act as a filter against them by ablating the surface layers of the rock. Although cyanobacteria have been shown to survive the desiccating, freezing conditions of space in orbital experiments, this would be of no benefit as the STONE experiment showed that they cannot survive atmospheric entry. Thus, non-photosynthetic organisms deep within rocks have a chance to survive the exit and entry process.>>

Ann wrote:
Svante Arrhenius from Sweden (1859-1927) believed that life spread from planet to planet by spores. He wasn't necessarily right.

http://en.wikipedia.org/wiki/Buller%27s_drop#Mechanism_of_basidiospore_discharge wrote:

[b][color=#0000FF]Schematic showing a basidiomycete mushroom, gill structure, and spore-bearing basidia on the gill margins.[/color][/b]

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<<A basidium is a microscopic, spore-producing structure found on the hymenophore of fruiting bodies of basidiomycete fungi. A basidium usually bears four sexual spores called basidiospores(; basidium literally means little pedestal, from the way in which the basidium supports the spores"). Each basidiospore is borne at the tip of a narrow prong or horn called a sterigma, and is forcibly discharged upon maturity. The propulsive force is derived from a sudden change in the center of gravity of the discharged spore. Important factors in forcible discharge include Buller's drop, a droplet of fluid that can be observed to accumulate at the proximal tip of each basidiospore; the offset attachment of the spore to the subtending sterigma, and the presence of hygroscopic regions on the basidiospore surface.

Upon maturity of a basidiospore, sugars present in the cell wall begin to serve as condensation loci for water vapor in the air. Two separate regions of condensation are critical. At the pointed tip of the spore (the hilum) closest to the supporting basidium, Buller's drop accumulates as a large, almost spherical water droplet. At the same time, condensation occurs in thin film on the adaxial face of the spore. When these two bodies of water coalesce, the release of surface tension and the sudden change in the center of mass leads to sudden discharge of the basidiospore. Money (1998) has estimated the initial acceleration of the spore to be about 10⁵ m/s². Successful basidiospore discharge can only occur when there is sufficient water vapor available to condense on the spore.

In the spikemoss Selaginella lepidophylla, dispersal is achieved in part by an unusual type of diaspore, a tumbleweed.>>