ac detective
What made the rain red in India? Isotopic analysis points to a terrestrial origin for the unusual organic particles that colored the rain like blood.
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COURTESY OF J. THOMAS BRENNA
Analysis in an IR mass spectrometer the samples came from—an asteroid, a arly in the morning on July 25, revealed a nitrogen value of –5.9‰, well planet, or a moon. Because the isotopic 2001, the state of Kerala, India, within the range of terrestrial samples. began to experience one of the strangest variability of carbon and nitrogen are The carbon value was –16‰, which is well known in the terrestrial record, dephenomena in all of meteorological sciconsidered a high isotope ratio for a bitermining whether samples are extraterence—red rain and hail. Red rainwater ological sample but is consistent with a covered a geographic area of 450 3 150 restrial is a matter of observing isotope marine origin or a plant that uses a C4 ratios outside of the expected values. km. Witnesses reported hearing loud thunder or perhaps a sonic boom photosynthetic pathway. (This a few hours before the rain began. pathway is less common than the Isolated reports of the blood-colC3 one, which most plants use.) Brenna then hydrolyzed a samored rain continued for 2 months. Godfrey Louis, a physicist at the ple in acid to liberate amino acids. Cochin University of Science and Molecular MS identified seven in Technology (India), collected order of concentration: Phe, Glu/ samples to find out where this Gln, Ser, Asp, Thr, and Arg. strange precipitation came from. Brenna’s unpublished concluHe calculated that 50,000 kg of sions are that the isotope ratio is red particles fell from the sky. consistent with a terrestrial origin In January 2006, Louis and a and that the particles are largely colleague, Santhosh Kumar at the organic, not mineral. The particles Mahatma Gandhi University have a hard shell and an interior of (India), published a study of this unknown composition. He also phenomenon in the peer-reviewed found that the shell encasing the Light micrograph of a red rain smear at 10003. The samjournal Astrophysics and Space Sciparticles and the red color are reple was stored in a vial without preservatives at room ence (302, 175–187). They specusistant to dissolution by organic lated that the red rain might have temperature for 4 years without noticeable degradation. solvents. “My chemical tests are an extraterrestrial origin—the disconsistent with a terrestrial origin Louis sent two 15-mL samples to J. integration of a small comet or meteor for this material. This doesn’t mean it is Thomas Brenna in the division of nutrifragment entering Earth’s atmosphere. not extraterrestrial, but it certainly does tional sciences at Cornell University. Upon microscopic examination of not support it.” Brenna agreed to conduct carbon and the rain, Louis found brown and red nitrogen isotope analyses using a scancell-like particles that were 4–10 µm in But what are they? ning electron microscope with X-ray mi- Nevertheless, some interesting questions diameter and resembled human blood. croanalysis, an elemental analyzer, and Analysis with an energy-dispersive X-ray remain. Are they terrestrial spores? If so, analyzer revealed that the cells consisted an isotope ratio (IR) mass spectrometer. why did the red rain occur over the state of Kerala in 2001 and not again Brenna examined a sample with a mainly of carbon and oxygen. To deepsince? How did so many spores become light microscope at 10003. “The oben the mystery, when Louis stained the airborne simultaneously? samples with ethidium bromide to jects were observed to fluoresce spontaLouis is hesitant to accept Brenna’s check for traces of DNA, he found neously under various wavelengths of irisotopic analysis as the final blow to his none. After he published his theory, radiation,” he says. “No interpretation dozens of newspapers and magazines, extraterrestrial hypothesis. “It appears has been attached to this observation.” including New Scientist (2006, 2541 that this red rain organism is showing Next, he studied a sample on a scanmore preference for 14 N than any other ning electron microscope with X-ray [March 4], 34), reported on his findings. They were even the subject of a microanalysis. The red particles colorganism,” he notes. “Also, I am eager BBC documentary in November 2006. lapsed when dried, which suggested that to know the content of P in the cells they were filled with fluid. They were because P is involved with DNA.” And bounded by a 200 –300-µm-thick “cell” so far, he adds, “no evidence for DNA A terrestrial origin wall and were composed mainly of carhas been found.” a Researchers who study meteorites often use isotopic analysis to determine where bon and oxygen. —Barry E. DiGregorio 3238
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© 2007 AMERICAN CHEMICAL SOCIETY
