The Red Rain of Kerala
Later explainedSummary
For two months in 2001, rain fell on Kerala coloured red, staining clothes. A government study identified the cause within months: spores of a locally abundant alga. Two physicists later argued the particles were extraterrestrial cells with no DNA. The DNA was found in 2013, in a peer-reviewed journal, and the coloured rain has come back several times since.
What is documented
The event. Coloured rain fell sporadically over Kerala between 25 July and 23 September 2001. It was predominantly red. Yellow, green and black rain were also reported. Roughly 124 separate incidents were reported across the state's districts. Some 85 per cent of the red rainfall occurred within ten days of 25 July.
The particles. Each millilitre of rainwater contained on the order of nine million red particles. The total mass that fell has been estimated at around 50,000 kg.
The sonic boom. A few hours before the first fall, residents of Changanassery, in Kottayam district, reported a sonic boom and a flash of light. This is the origin of the meteor hypothesis. No meteor was observed, no fragments were recovered, and no meteoric material was found in the fallout.
The government study. In November 2001 the Centre for Earth Science Studies and the Tropical Botanical Garden and Research Institute, commissioned by the Government of India's Department of Science and Technology, issued a joint report. Their finding: the colour was caused by a large quantity of spores of a lichen-forming alga of the genus Trentepohlia.
The report was not an assertion. It was a piece of fieldwork. On 16 August 2001 investigators returned to the area and found the trees, the rocks and the lamp posts covered with Trentepohlia, in quantities sufficient to account for the spore load. They took lichen samples from the Changanassery area, cultured them in growth medium, and grew the same alga that was in the rainwater.
The alga. Trentepohlia is a green alga. Its chlorophyll is masked by orange and red carotenoid pigments, which is why an organism in the green algae looks like blood when it falls in bulk. It grows abundantly on bark, damp rock and masonry throughout the region.
The recurrence. Coloured rain has fallen on Kerala again, including in 2006, 2007, 2008 and 2012. Botanists have found the same Trentepohlia spores every time.
Leading explanations
Airborne algal spores (established). The 2001 government study, supported by the recurrence of the phenomenon and the repeated identification of the same organism, is the accepted account. What is not fully settled is the meteorology: exactly what lofted so large a spore mass into the clouds at once. That is a question about local atmospherics, not about the identity of the particles.
Cometary panspermia (proposed, refuted). From 2003, Godfrey Louis and A. Santhosh Kumar, physicists at Mahatma Gandhi University in Kottayam, argued that the particles were extraterrestrial cells delivered by a disintegrating comet, that they reproduced at 300 °C, and that they contained no DNA. The work was published as a preprint and subsequently in Astrophysics and Space Science.
The hypothesis has not survived. The bolide model requires, at minimum, an observed meteor, a coherent link between the bolide and its alleged fallout, and meteoric debris in that fallout. None of the three was ever produced. The claim of reproduction at 300 °C has not been replicated. And the central claim, the absence of DNA, was directly refuted: Gangappa and Hogg, publishing in Microbiology in 2013, found DNA in the red rain cells. It had not been detected earlier because the stain had not penetrated the cell wall.
What the popular version gets wrong
"The cells had no DNA." They had DNA. The negative result was a staining failure, and the DNA was demonstrated in a peer-reviewed microbiology journal in 2013. This is the single most repeated error about the case, and it is the one that the entire extraterrestrial reading rested on.
"A comet did it." A comet is a one-off. The coloured rain has recurred at least four times since 2001, and every time the same terrestrial alga has been found in it. Whatever else may be true, a disintegrating comet does not return every few years to the same Indian state.
"Scientists were baffled." The identification took a few months, was based on going to the site and looking at the trees, and has been confirmed on every recurrence since.
"Red rain means a red organism." Trentepohlia is a green alga. Its green pigment is masked by orange carotenoids. The rain looked like blood because of a pigment, not because of a genre.
Current status
Later explained. The record is unusually clean: an anomalous event, an unglamorous and correct answer arrived at by fieldwork, a spectacular alternative that drew far more attention, and a peer-reviewed refutation of that alternative's central claim. The entry is kept because the explanation is the interesting part, and because it is a compact demonstration of how a settled case can be made to look open by repeating a finding that was withdrawn.
Sources
- Centre for Earth Science Studies and Tropical Botanical Garden and Research Institute. Joint report on the coloured rain phenomenon in Kerala, commissioned by the Department of Science and Technology, Government of India, November 2001.
- Sampath, S. et al. (2001). Coloured rain: a report on the phenomenon. Indian Journal of Radio and Space Physics, 32, 158-165.
- Louis, G. and Kumar, A. S. (2003). "New biology of red rain extremophiles prove cometary panspermia." arXiv:astro-ph/0312639 (preprint).
- Louis, G. and Kumar, A. S. (2006). "The red rain phenomenon of Kerala and its possible extraterrestrial origin." Astrophysics and Space Science.
- Gangappa, R. and Hogg, S. I. (2013). "DNA unmasked in the red rain cells of Kerala." Microbiology, 159(1), 107-111.
Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.