Smarter than a protozoan?

Rare Earth contends that a remarkable confluence of events -- the right chemicals, the right distance from a star, and more -- conspired to allow complex life to develop on our planet, and that we humans may be alarmingly alone in the universe.

But Brownlee and his coauthor, paleontologist Peter Ward, contend that while complex life might be rare, microbial life could be widespread in our galaxy and elsewhere. And Brownlee pointed out that where there is microbial life, there can be lots of it: Typical terrestrial soils have billions of bacteria per gram, he said.

"If there's microbial life on Mars, it must be carried to Earth," Brownlee said, adding that the new study tells us it is more likely that these microbes could arrive intact.

Caltech geobiology professor Joseph Kirschvink worked with Weiss and others to test several millimeter-thin slices of the meteorite with a device that detects tiny differences in the orientation of magnetic lines in rock.

Slices from near the meteorite's surface showed a magnetic alignment consistent with that of Earth, which was expected: After enduring the heat created by its plummet through the atmosphere, a rock's magnetic field reorients its magnetization, to be aligned to the local field, as it cools.

But deeper inside the rock, the magnetic orientation was found to be random, indicating that the interior had kept its cool. The researchers then heated a slice of the interior to figure out at what temperature it would begin to demagnetize and set itself up for reorientation. That temperature -- 105 degrees Fahrenheit (40 degrees Celsius) -- is a threshold the rock never reached until it came to the lab.

How in the world did it stay so cool inside that rock?

Well, rock isn't exactly the best heat conductor, Weiss said, so it takes a long time for heat to penetrate to the interior.

"For a potato-sized rock entering the atmosphere, the diffusion time to the center of the rock is significantly longer than the several minutes that the rock spends being heated in the atmosphere," Weiss said.

In addition, melted bits fly off the rock's surface, carrying heat away and, as we now know, salvaging the protective womb for any life that might be aboard.