A crimson downpour
Between July 25 and September 23, 2001, the southern Indian state of Kerala experienced a bizarre meteorological phenomenon: sporadic, heavy downpours of red-colored rain. The rain was often highly localized, sometimes falling in areas of just a few square kilometers, with normal rain observed only meters away. These crimson showers, which stained clothes pink, typically lasted less than 20 minutes. Locals in the Kottayam and Idukki districts, where the phenomenon was concentrated, reported hearing a loud thunderclap and seeing a flash of light just before the first red rain fell. In the aftermath, some trees reportedly shed shriveled, grey "burnt" leaves.
Initial analysis of the rainwater revealed it was colored by microscopic red particles. Each milliliter of rainwater contained an estimated nine million of these particles. The total amount of solid red material that fell over Kerala was calculated to be around 50,000 kilograms. These particles were mostly spherical or oval, measuring between 4 and 10 micrometers across. Chemical analysis showed they were composed primarily of carbon and oxygen, with trace amounts of silicon and iron. The Centre for Earth Science Studies (CESS) in India initially suspected the particles were fallout from a meteor burst, but this was soon questioned because the rain continued to fall over the same area for two months, which would be unlikely for atmospheric debris.
Alien life or terrestrial algae?
A more controversial explanation emerged from physicists Godfrey Louis and Santhosh Kumar of Mahatma Gandhi University. They proposed that the particles were extraterrestrial microorganisms. Linking the phenomenon to the sonic boom heard on July 25, they suggested a comet had disintegrated upon entering Earth's atmosphere, releasing the cells. Their 2006 paper in the journal Astrophysics and Space Science pointed to the particles' cellular appearance under an electron microscope. Most startlingly, their tests using ethidium bromide, a fluorescent dye that binds with DNA, indicated the absence of genetic material. This led them to speculate that the cells were a new form of life with an alternative biochemistry.
However, the scientific consensus eventually settled on a terrestrial origin. A study commissioned by the Government of India concluded the particles were airborne spores from a locally abundant green alga of the genus Trentepohlia. This alga is known to grow on trees and rocks and can produce reddish-orange spores. Later independent studies confirmed the presence of DNA in the red rain particles. In 2015, further analysis identified the specific species as Trentepohlia annulata. This species was previously unrecorded in India, with researchers suggesting the spores may have been transported by wind over vast distances, possibly even from Europe. While the extraterrestrial hypothesis is now dismissed by the scientific community, the debate over how such a massive quantity of spores became airborne and concentrated in the rain continues.