The Panspermia Hypothesis on Trial
The physical location of this test site is a suitcase-sized platform called EXPOSE-R2, bolted to the outer hull of the Zvezda module of the International Space Station (ISS). Its intellectual home is the German Aerospace Center (DLR) in Bavaria, at the coordinates provided. Here, scientists planned the BIOMEX (Biology and Mars Experiment) mission to test a component of the panspermia hypothesis: could life survive an interplanetary journey inside a rock? The idea, called lithopanspermia, suggests that microbes could be blasted off a life-bearing planet by an asteroid impact, travel through space shielded by the rock, and seed another world.
On August 18, 2014, Russian cosmonauts attached the EXPOSE-R2 facility to the ISS. Inside its compartments were more than 600 biological samples and 150 organic compounds. The samples were not floating freely; many were dried onto, or mixed with, simulated Martian and lunar soil to mimic the conditions inside a meteorite. For 533 days, the platform exposed its microscopic passengers to the unrelenting harshness of Low Earth Orbit—a vacuum, extreme temperature swings, and solar radiation of solar and cosmic radiation. The mission was a direct test of life's absolute limits.
Earth's Toughest Inhabitants
The passengers for this orbital trial were selected from Earth's most resilient organisms, known as extremophiles. The cargo included bacteria, archaea, lichens, fungi, mosses, and algae. One important subject was the cyanobacterium Chroococcidiopsis, a desert-dwelling microbe known for its incredible resistance to radiation and dehydration. Spores of the bacterium Bacillus subtilis were also included, known for their near-indestructible dormant state.
Some samples were exposed directly to the vacuum and unfiltered sunlight. Others were placed under filters that simulated the thin atmosphere of Mars, letting through a specific spectrum of UV radiation. For 469 days, the samples endured the full force of solar radiation. The total UV dose was hundreds of times higher than on Earth's surface. After their trial by vacuum and fire, the samples were retrieved by cosmonauts and returned to Earth on a Soyuz capsule on June 18, 2016.
Back in labs across Europe, the analysis began. The results were startling. Many of the organisms survived. The cyanobacteria, shielded by a thin layer of simulated Martian soil, repaired their radiation-damaged DNA after being rehydrated. Archaea, single-celled organisms isolated from arctic permafrost, were successfully reanimated. Biofilms—communities of bacteria encased in a protective slime—proved especially durable, with the outer layers sacrificing themselves to protect the cells within. The experiments demonstrated that if shielded by just a few millimeters of rock, microbial life could indeed withstand the rigors of a trip from, for example, Mars to Earth.