The mystery of the Indian red rain
(See a short introductory video)
Let us begin the story of the mysterious "red rain" with the abstract of the article by Godfrey Louis and A. Santhosh Kumar :
"A red rain phenomenon occurred in Kerala, India starting from 25th July 2001, in which the rainwater appeared colored in various localized places that are spread over a few hundred kilometers in Kerala. Maximum cases were reported during the first 10 days and isolated cases were found to occur for about 2 months. The striking red coloration of the rainwater was found to be due to the suspension of microscopic red particles having the appearance of biological cells. These particles have no similarity with usual desert dust.
An estimated minimum quantity of 50,000 kg of red particles has fallen from the sky through red rain. An analysis of this strange phenomenon further shows that the conventional atmospheric transport processes like dust storms etc. cannot explain this phenomenon. The electron microscopic study of the red particles shows fine cell structure indicating their biological cell like nature. EDAX analysis shows that the major elements present in these cell like particles are carbon and oxygen. Strangely, a test for DNA using Ethidium Bromide dye fluorescence technique indicates absence of DNA in these cells.
In the context of a suspected link between a meteor airburst event and the red rain, the possibility for the extraterrestrial origin of these particles from cometary fragments is discussed."
The colored rain started suddenly during a period of rainfall in the state Kerala (in southwestern India). The main red rain (about 70% of the total colored rain reports) fell on the surface of state Kerala on 27 and 28 July 2001, some smaller amounts (about 15%) during the next week, and once more some stronger red rainfall (about 10%) on 23 September 2001. It was a rainy period between 10 July and 26 August 2001 in Kerala. One has noted a rainfall almost every day. About a half of those days have measured a comparative rainfall amount as on the two days with the red rain. Thus it was not the rain alone that was extraordinary during these two days of 27 and 28 July. Extraordinary was only the showers containing the red specks which vaguely resembled red blood cells. The scientists were agreed on two points: the red particles looked like cells (at least superficially), and no one was sure what they are. "It is a mystery from where the rain clouds have picked up such a large quantity of pure red particles", wrote Louis and Kumar in their paper four years after the rainfall. The red rain of 2001 is still eluding explanation today, at the beginning of 2012.
"These particles have much similarity with biological cells though they indicate the absence of DNA", Luis and Kumar added. These cell-like particles do not have any flagella as found in many algae cells. But the particles can grow and reproduce if placed in heat (see further below).
The conventional atmospheric transport processes like dust storms etc. cannot explain the origin of the red particles. "The red particles were uniformly dispersed in the rainwater," they wrote. "In a few places the concentration of particles were so great that the rainwater appeared almost like blood." When the red rainwater was collected and kept for several hours in a vessel, the suspended particles have a tendency to settle to the bottom of the vessel causing a color reduction for the red rainwater. A typical red rain lasted from a few minutes to less than about 20 minutes, they added. Alive or dead, the particles have some staying power.
What do we know about the red particles? What could they be? Where is their origin? There are still today, more than a decade after the rain has fallen, the main questions open for an investigation and hypotheses.
What do we know?
Colored rain had been reported in Kerala as early as 1896 and several times since then . The red rain 2001 started in the State Kerala during a period of normal rain, which indicate that the red particles are not something, which accumulated in the atmosphere during a dry period and washed down on a first rain.
The geographical area of all reported cases forms an elongated ellipse of size 450 km by 150 km along the Kerala-state coast of the Arabian Sea, partly on land and partly on sea (between 9 and 12,6 degree latitude North). The maximum numbers of reported cases were reported from in and around Kottayam and Pathanamthitta districts, forming an abrupt southeastern boundary of the ellipse. The remaining reported rainfalls decrease gradually towards the northwest of the ellipse. Another characteristics of the red rain were its highly localized appearance. It usually occurs over an area of less than a square kilometer to a few square kilometers. Many times it had a sharp boundary, which means while it was raining strongly red at a place a few meters away there were no red rain. There were a few cases of yellow colored rain and rare unconfirmed cases of other colors like black, green, gray etc. Colored hailstones were also reported. However, in majority of the cases the color of the rain was red. The particles are very stable against decay with time. Even after storage in the original rainwater at room temperature without any preservative for about four years between the rainfall and the Louis` and Kumar`s publication, no decay or discolouration of the particles could be found.
The particles look like one-celled organisms and are about 4 to 10 µm large, somewhat larger than typical bacteria. "Under low magnification the particles look like smooth, red coloured glass beads. Under high magnifications (1000x) their differences in size and shape can be seen," wrote Louis and Kumar. Shapes vary from spherical to ellipsoid and slightly elongated. These cell-like particles have a thick and coloured cell envelope, which can be well identified under the microscope. Electron microscopy study show that the red rain particles have fine-structured membranes. There are no flagella or filamentous structures attached to the outer surface these cells. In a large collection only a few were found to have broken cell envelopes.
The particles seem to lack a nucleus. Chemical tests indicated they also lacked DNA. Nonetheless, Louis and Kumar wrote that the particles show "fine-structured membranes" under magnification, like normal cells. The outer envelope seems to contain an "inner capsule," they added, which in some places "appears to be detached from the outer wall to form an empty region inside the cell. Further, there appears to be a faintly visible mucus layer present on the outer side of the cell. ... One characteristic feature is the inward depression of the spherical surface to form cup like structures giving a squeezed appearance," which varies among particles, they added. "The major constituents of the red particles are carbon and oxygen," they wrote. Carbon is the key component of life on Earth. "Silicon is most prominent among the minor constituents" of the particles. Other elements found were iron, sodium, aluminum and chlorine.
The pH (acidity) of the water was found to be around 7 (neutral), which is the pH for normal rain water. The electrical conductivity of the rainwater showed the absence of any dissolved salts .
In another study  to culture this microbe Louis and Kumar reported that it was optimally replicating at an extreme high temperature of 300 °C in hydrothermal condition and could metabolize inorganic and organic compounds including hydrocarbons. Reproduction process of this new organism was identified as a special kind of multiple fission process and the original red rain cells were identified as the resting spores of this microbe. These findings now rules out the possibility that these cells are common algal or fungal cells. Considering the ability of this organism to replicate at extreme high temperature of 300 °C and the fact that ordinary biomolecules cannot stand this temperature, Louis and Kumar proposed that these cells possibly represent a new kind of biology. It was also speculated that these cells might have alternate type of biomolecules, which could withstand extreme high temperatures. However the biomolecular constituents of these cells are yet to be identified .
Where did they come from?
One easy assumption is that the red particles "got airlifted from a distant source on Earth by some wind system, but this leaves several puzzles." Louis and Kumar added however that it is also possible to explain the phenomenon by assuming the meteoric origin of the red particles. The red rain phenomenon first started in Kerala after a meteor airburst event, which occurred on 25th July 2001 near Changanacherry in the Kottayam district. This meteor airburst is evidenced by the sonic boom experienced by several people during early morning of that day. The first case of red rain occurred in this area few hours after the airburst. An examination of the red rain data shows that more than 85% of the red rain cases occurred during the first 10 days after the airburst event. "This points to a possible link between the meteor and red rain. If particle clouds are created in the atmosphere by the fragmentation and disintegration of a special kind of fragile cometary meteor that presumably contains a dense collection of red particles, then clouds of such particles can mix with the rain clouds to cause red rain," they wrote. The pair proposed that while approaching Earth at low angle, the meteor traveled southeast above Kerala with a final airburst above the Kottayam district. "During its travel in the atmosphere it must have released several small fragments, which caused the deposition of cell clusters in the atmosphere." They consider the following possibility: "This delayed time distribution for the first few days can be accounted as due to the slow settling of the microscopic red rain particles in the atmosphere, with a settling rate of a few hundred meters per day. For this the meteor disintegration is expected to provide a vertical distribution of particles spanning over a few kilometers above the rain clouds. The remaining 15 % of the isolated delayed red rain cases occurred with a delay of up to 60 days, which presumably also reflect gradual settling of the particles in the upper atmosphere."
As to the biological nature of the red particles the wrote: "The optical microscope images also support the idea that these transparent red particles are similar to biological cells. The clear presence 14 Red rain phenomenon of Kerala.. of carbon as shown by the elemental analysis indicates the organic nature of these particles. While these particles have striking morphological similarity with biological cells, the test for DNA gives a negative result, which argues against their biological nature. ... Are these cell-like particles a kind of alternate life from space?"
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 .
Kerala red rain is not a unique recent event. A study has been published showing a correlation between historic reports of colored rains and of meteors. The author of the study, Patrick McCafferty, stated that sixty of these events (colored rain), or 36 %, were linked to meteoritic or cometary activity. But not always strongly. Sometimes the fall of red rain seems to have occurred after an airburst, as from a meteor exploding in air; other times the odd rainfall is merely recorded in the same year as the appearance of a comet. The red colored rain occurred in Vietnam on July 29 in 2002 and the colored rain occurred in Colombia, on 31 July 2008. Yellow rain was also reported in Kerala in the month of July 2008 on days 18th, 25th and 29th. Kerala and Colombia are near to equator but are on diametrically opposite side of the Earth. Meteoric origin is the most likely explanation for the appearance of similar kind of colored rains on subsequent days on either side of the Earth..
What kind of cells are they?
Growth experiments by Godfrey Louis and A. Santhosh Kumar  show standard features of microbial growth. The cells show increased growth rate at higher temperatures as expected from a thermophilic organism. Thus red rain microbes exhibits one fundamental property of life that is their ability to grow and multiply when supplied with food under proper growth temperature. However the contradicting facts are: that DNA and RNA could not be detected in these cells; that the cells grow at temperatures much above the currently known upper temperature limit of life and at temperatures like 300°C conventional biomolecules cannot possibly survive so life is not expected at such high temperatures. Louis and Kumar conclude that these contradictions will vanish if the red rain microbes have an alternate biochemistry which appears increasingly possible now.
 "The Red Rain Phenomenon Of Kerala And Its Possible Extraterrestrial Origin" by Godfrey Louis and A. Santhosh Kumar, School of Pure & Applied Physics, Mahatma Gandhi University; 1 January, 2006 (from arXiv.org Website) (http://www.bibliotecapleyades.net/ciencia/ciencia_kerala03.htm)