The Red Rain of Kerala: When Blood-Coloured Water Fell for Two Months
The washing was the first thing to betray it. In the last week of July 2001, women in the hill villages around Kottayam and Changanassery, in the south of the Indian state of Kerala, pulled their laundry from the line and found white cotton freckled with pink, as though someone had flicked a brush of thin blood across the drying shirts. Rain had fallen while the clothes hung out, and the rain had come down coloured.
In the days that followed the same rose coloured stain settled on rooftops and courtyards, pooled in the ruts of red dirt lanes and, according to residents, tinted the water standing in wells. People stood in their doorways and watched water the colour of watered wine fall out of an ordinary monsoon sky. The question everyone asked was the practical one. What is in it?
It was not a single freak shower. By the official reckoning the red rain began on 25 July 2001 and returned in fits and starts until 23 September, a span of almost two months. It did not fall everywhere at once. It arrived in sudden, localized cloudbursts over one district, stopped, and then broke red again somewhere else days later, scattered across the southern districts, with Kottayam and Idukki worst affected.
Two further details were reported at the time and have stayed attached to the case ever since. First, not all of the coloured rain was red. In places the downpours came down tinted yellow, green, even black. Second, a handful of residents insisted that the very first fall had been heralded, in the hours before it, by a loud clap like a thunderbolt and a flash of light, with no storm to account for either, as though something overhead had burst.
When scientists finally trapped the rain and put it under the microscope, the numbers were arresting. Each millilitre of the red water held something on the order of nine million particles, tiny spheres and ovals between about four and ten micrometres across, most of them a deep brick red, packed so densely that a sample looked like diluted blood. Working from the rainfall totals and the particle counts, one estimate put the total mass that came down over those two months at roughly fifty thousand kilograms of red matter. Fifty tonnes of microscopic somethings, delivered out of the clouds over a stretch of Kerala, cloudburst by cloudburst, for eight weeks.
Under the electron microscope the particles were more elaborate than a smudge of dust. Many showed a faintly depressed centre and a hint of internal structure, a wall around a core, the kind of architecture associated with living cells rather than with grains of mineral. Chemical analysis found them built mostly of carbon and oxygen, the signature of something organic. None of that was on its own proof of anything exotic, but it was enough to make careful observers look closer.
The event drew national attention while it was still going on, and samples were collected both by state scientists and by university researchers in Kottayam, which is why more than one laboratory ended up holding material from the same rainfalls. That detail matters later, because the disputes about the red rain are disputes between laboratories working on the same substance.
The official investigation reported quickly. In November 2001 a joint team from the Centre for Earth Science Studies and the Tropical Botanical Garden and Research Institute, working under a commission from India's Department of Science and Technology, concluded that the colour was due to a vast quantity of spores of Trentepohlia, a lichen forming alga.
Trentepohlia is not an exotic import. It grows all over the humid south in rust orange crusts on tree bark, damp rock and old walls, and although it is an alga, its cells are so loaded with orange and red carotenoid pigments that they completely mask the green of the chlorophyll beneath. That is why a heavy growth of it looks less like pond scum than like a smear of rust or dried blood.
The investigators did more than match the particles by eye. They cultured them, and the particles grew, in the dish, into ordinary Trentepohlia. When they went out into the affected districts they found the algae abundant on the trees, the same species raining down as was crusting the bark overhead. In plain terms, the rain had somehow gathered up a colossal local bloom of algal spores and rained them back down.
That identification has held up under later testing. Work reported around 2012 and 2013, including a study by Rajkumar Gangappa and Stuart Hogg and separate analyses associated with the astrobiologist Chandra Wickramasinghe, examined the red cells again and identified them firmly as Trentepohlia. Those studies detected DNA in the cells. One strand of that later work went further and suggested that the particular strain was closely related to a European species of the alga, which would give the spores a thoroughly earthly, if well travelled, pedigree.
It helps to remember that coloured rain is not in itself a marvel. Europe has recorded blood rain for centuries, and the culprit there is almost always mineral rather than biological: fine reddish dust lifted from the Sahara by desert winds, carried north, and washed out of the sky by ordinary storms to leave cars and rooftops filmed in orange. Against that long backdrop, what made Kerala unusual was not the colour but the cause, a living cause rather than a dead one, a rain reddened by biology instead of geology.
Nor was Kerala's version a one off freak. Reddish rains carrying the same kind of spores have been reported in the region since 2001, and comparable coloured rains have fallen elsewhere in South Asia. The event of 2001 was the largest and longest documented example, but it belongs to a recurring family of local phenomena rather than standing alone.
So the basic account is settled and it is a modest one. For two months in the monsoon of 2001, rain over southern Kerala came down carrying an extraordinary load of spores from a common red alga that grows on the bark of the region's own trees, in quantities large enough to stain laundry, rooftops and well water the colour of blood.
Conclusions and Open Questions
The identity of the red particles is not seriously in dispute. What they were, and where the pigment came from, was answered in 2001 and confirmed a decade later. What has never been closed is the mechanism, and it is around that gap that the competing readings of the event grew.
The most famous of them is the panspermia hypothesis, proposed in 2006 by two physicists at Mahatma Gandhi University in Kottayam, Godfrey Louis and A. Santhosh Kumar, in a peer reviewed journal. They argued that the red cells were biological but were like nothing on Earth. They reported that the particles had a cell wall and inner structure but appeared to contain no DNA at all, that they were extraordinarily tough, and, most sensationally, that the cells could be induced to replicate at temperatures around 300 degrees Celsius, far hotter than any terrestrial life can endure. From this they built a hypothesis reaching back to the old idea of panspermia. The red cells, they held, had not grown on Kerala's trees at all but had come from space, seeded into the high atmosphere by a fragmenting comet and washed down in the monsoon, with the reported bang and flash as the airburst of that disintegrating body. The claim travelled far beyond India.
Its weakness is that the two central laboratory claims never survived contact with other laboratories. Neither the absence of DNA nor the replication at 300 degrees was independently reproduced or confirmed in the peer reviewed literature, and both remain, in effect, assertions from a single group. The later work described above found the opposite: DNA was present, and the cells matched a known terrestrial alga. That evidence pulled the red cells back down out of orbit, and the panspermia reading survives today only at the scientific fringe.
That does not make the mechanism question go away, and this is where the ordinary explanation is thinner than it looks. Trentepohlia grows in thin films on bark and stone. It is not obvious what atmospheric machinery scrapes its spores loose by the tonne, lofts them thousands of feet into monsoon cloud, holds them there, and then releases them cloudburst by cloudburst for eight weeks rather than in one great dump. The 2001 report named the organism convincingly. It was far thinner on how so much of it came to be airborne at once and stayed airborne so long.
Some argue that the answer is an unlucky coincidence of ordinary things: a bumper season of algal growth loading the bark with far more spores than usual, a heavy release just as pre monsoon heat built powerful updrafts, and a long stalled run of storms wringing out the same charged air over the same districts. Each piece is plausible. The weakness is that none of it appears to have been modelled end to end for the 2001 event, and unquantified is not the same as understood.
Two things remain genuinely unexplained. The first is that full path from bark to cloud to street, which no one appears to have reconstructed. The second is the yellow, green and black rain reported alongside the red, which sits awkwardly with a single species of orange red alga. The reported bang and flash are also unaccounted for by the algal explanation, whatever their true cause.
So the open questions are narrower than they were in 2001, and stranger for it. How does a humble crust of algae on the bark of a region's trees climb, in its uncounted trillions, into the monsoon sky? What kept it there for two months? What produced the other colours? We know what fell on Kerala. We still do not know how it got up there.