Occurrence
The colored rain of Kerala began falling on July 25, 2001, in the districts of Kottayam and
Idukki in the southern part of the state. Yellow, green, and black rain was also reported.
[2][3][4] Many more occurrences of the red rain were reported over the following ten days, and then with diminishing frequency until late September.
[3] According to locals, the first colored rain was preceded by a loud
thunderclap and flash of light, and followed by groves of trees shedding shriveled grey "burnt" leaves. Shriveled leaves and the disappearance and sudden formation of
wells were also reported around the same time in the area.
[13][14][15]It typically fell over small areas, no more than a few square kilometers in size, and was sometimes so localized that normal rain could be falling just a few meters away from the red rain. Red rainfalls typically lasted less than 20 minutes.
[3] Each
milliliter of rain water contained about 9 million
red particles, and each
liter of rainwater contained approximately 100
milligrams of solids.
Extrapolating these figures to the total amount of red rain estimated to have fallen, it was estimated that 50,000 kilograms (110,000 lb) of red particles had fallen on Kerala.
[3]
[edit]Description of the particles
The brownish-red solid separated from the red rain consisted of about 90% round red particles and the balance consisted of debris.
[5] The particles in
suspension in the rain water were responsible for the color of the rain, which at times was strongly colored red. A small percentage of particles were white or had light yellow, bluish gray and green tints.
[3] The particles were typically 4 to 10
µmacross and spherical or oval.
Electron microscope images showed the particles as having a depressed center, suggestive of biological cells. At still higher magnification some particles showed internal structures.
[3]
[edit]Chemical composition
Several groups of researchers analyzed the chemical elements in the solid particles and different techniques gave similar results. The particles were composed mostly of
carbon and
oxygen with lesser amounts of
hydrogen,
nitrogen,
silicon,
chlorine and
metals.
The samples of water were brought to the
Centre for Earth Science Studies (CESS) in India, where they separated the suspended particles by filtration. The
pH (acidity) of the water was found to be around 7 (neutral). The
electrical conductivity of the rainwater showed the absence of any dissolved
salts. Sediment (red particles plus debris) was collected and analyzed by the CESS using a combination of ion-coupled plasma
mass spectrometry, atomic absorption spectrometry and wet chemical methods. The major elements found are listed below.
[5] The CESS analysis also showed significant amounts of
heavy metals in the raindust, including
nickel (43
ppm),
manganese (59 ppm),
titanium (321 ppm),
chromium(67ppm) and
copper (55 ppm).
CESS analysis: Major elements present in the dried sediment
| Element | Al | K | Mg | Ca | Na | Fe | Si | C | P |
|---|
| Weight % | 1.00 | 0.26 | 1.48 | 2.52 | 0.49 | 0.61 | 7.50 | 51.00 | 0.08 |
|---|
Louis and Kumar used
energy dispersive X-ray spectroscopy analysis of the red solid and showed that the particles were composed of mostly carbon and oxygen, with trace amounts of silicon and iron
[3] (see table below).
Elemental composition of red particles by EDXRF analysis
| Element | Weight % | Atomic % | Standards |
|---|
| C | 49.53 | 57.83 | CaCO3 |
| O | 45.42 | 39.82 | Quartz |
| Na | 0.69 | 0.42 | Albite |
| Al | 0.41 | 0.21 | Al2O3 |
| Si | 2.85 | 1.42 | Quartz |
| Cl | 0.12 | 0.05 | KCl |
| Fe | 0.97 | 0.24 | Fe |
A
CHN analyzer showed content of 43.03% carbon, 4.43% hydrogen, and 1.84% nitrogen.
[3]
J. Thomas Brenna in the Division of Nutritional Sciences at
Cornell University conducted carbon and nitrogen isotope analyses using a scanning electron microscope with X-ray microanalysis, an elemental analyzer, and an isotope ratio (IR) mass spectrometer. The red particles collapsed when dried, which suggested that they were filled with fluid. The
amino acids in the particles were analyzed and seven were identified (in order of concentration):
phenylalanine,
glutamic acid/
glutamine,
serine,
aspartic acid,
threonine, and
arginine. He concluded that the results were consistent with a marine origin or a terrestrial plant that uses a
C4 photosynthetic pathway.
[16]
[edit]Official report
Initially, the
Centre for Earth Science Studies (CESS) stated that the likely cause of the red rain was an exploding meteor, which had dispersed about 1,000 kg (one ton) of material. A few days later, following a basic
light microscopy evaluation, the CESS retracted this as they noticed the particles resembled spores,
[17] and because debris from a meteor would not have continued to fall from the stratosphere onto the same area while unaffected by wind. A sample was, therefore, handed over to the
Tropical Botanical Garden and Research Institute (TBGRI) for
microbiological studies, where the spores were allowed to grow in a medium suitable for growth of algae and fungi. The inoculated Petri dishes and conical flasks were incubated for three to seven days and the culture was observed under a microscope.
In November 2001, commissioned by the
Government of India's Department of Science & Technology, the Center for Earth Science Studies (CESS) and the Tropical Botanical Garden and Research Institute (TBGRI) issued a joint report which concluded that:
[5][17]
The color was found to be due to the presence of a large amount of spores of a
lichen-forming
alga belonging to the genus
Trentepohlia. Field verification showed that the region had plenty of such lichens. Samples of lichen taken from
Changanacherry area, when cultured in an algal growth medium, also showed the presence of the same species of algae. Both samples (from rainwater and from trees) produced the same kind of algae, indicating that the spores seen in the rainwater most probably came from local sources.
The site was again visited on August 16, 2001 and it was found that almost all the trees, rocks and even lamp posts in the region were covered with
Trentepohlia lichen, and estimated that the extent of lichen in the region is sufficient to generate the quantity of spores seen in the rainwater.
[5] Although red or orange,
Trentepohlia is a
Chlorophyte green alga which can grow abundantly on tree bark or damp soil and rocks, but is also the photosynthetic
symbiont or photobiont of many lichens, including some of those abundant on the trees in Changanacherry area.
[5] The strong orange colour of the algae, which masks the green of the
chlorophyll, is caused by the presence of large quantities of orange
carotenoid pigments. A
lichen is not a single organism, but the result of a partnership (
symbiosis) between a fungus and an alga or cyanobacterium.
The report also stated that there was no meteoric, volcanic or desert dust origin present in the rainwater and that its color was not due to any dissolved gases or pollutants.
[5] The report concluded that heavy rains in Kerala -in the weeks preceding the red rains- could have caused the widespread growth of lichens, which had given rise to a large quantity of spores into the atmosphere. However, for these lichen to release their spores simultaneously, it is necessary for them to enter their reproductive phase at about the same time. The CESS report noted that while this may be a possibility, it is quite improbable.
[5] Also, they could find no satisfactory explanation for the apparently extraordinary dispersal, nor for the apparent uptake of the spores into clouds. CESS scientists noted that "While the cause of the color in the rainfall has been identified, finding the answers to these questions is a challenge."
[17] Attempting to explain the unusual spore proliferation and dispersal, researcher Ian Goddard proposed several local atmospheric models.
