Alien Rain

From the print edition of New Scientist:
http://www.newscientistspace.com/article/mg18925411.100.html

Here is the original article:
http://arxiv.org/ftp/astro-ph/papers/0312/0312639.pdf

I'VE heard plenty of tales about freak weather that are strange, but
nonetheless true. In August 2000, a shower of sprats, dead but
conveniently still fresh, fell from the skies onto the English port of
Great Yarmouth just after a thunderstorm. A torrent of live toads
pelted a Mexican town in June 1997. And in 2001, 50 tonnes of alien
life forms rained down from the clouds over India.

Actually, I'm not sure that the alien story is true. But it is
surprisingly persistent. I first saw it in 2003 in a scientific paper
written by Godfrey Louis, a physicist working in the Indian state of
Kerala, on the country's southern tip. He described how, during two
months in 2001, red rain fell sporadically right across the state. No
one could explain it, but after lengthy studies of red particles in
the rainwater, Louis came to the extraordinary conclusion that they
were alien microbes that hitched a ride to Earth on a comet.

To most people, that would sound eccentric at the very least. It
looked as if the idea would quietly wither on the vine. Then in
January this year, it turned out that Louis's theory is still alive
and kicking, and soon to roll off the press in a reputable peer-
reviewed journal. I sent a preprint to several researchers, who
despite voicing mixed opinions almost all agreed about one thing: the
red particles Louis describes look biological.

"If they're not living cells, I don't know what they are," said
Milton Wainwright, a microbiologist at the University of Sheffield,
UK. "Maybe this is the beginning of something amazing." Another
scientist simply commented: "Sounds like bullshit to me." That was
it - I could resist this weird and controversial story no longer.

The saga dates back to 25 July 2001, when red rain fell in a district
of Kerala called Kottayam. Over the following two months, red rain
fell sporadically there and in other Kerala districts, gradually
tailing off over time. The local newspapers buzzed with eyewitness
reports. People found their clothes stained by red raindrops.
Although these usually had a mild red tint, sometimes the colour was
so strong that witnesses compared it to blood. Usually, the red rain
would fall for less than 20 minutes.

Louis, a solid-state physicist at Mahatma Gandhi University in
Kottayam, was intrigued and decided to study the rain with his
student Santhosh Kumar. The pair compiled more than 120 reports of
the rain from local newspapers and other sources, and gathered
samples of the red rain from spots more than 100 kilometres apart
(see Map).

Under the microscope, they could see red particles 4 to 10
micrometres wide with an average density of about 9 million particles
per millilitre. When they dried the samples they found that each
cubic metre of rainwater contained about 100 grams of the red stuff.
Louis suggests 5 millimetres of red rain would typically have fallen
over a square-kilometre area during each of about 100 downpours. That
would make 500,000 cubic metres of water in total, containing a
staggering 50 tonnes of red particles.

What could they be? One possibility was that fine red sand had blown
over Kerala from some distant desert. Sand can travel amazingly far.
In July 1968, for instance, fine grit in raindrops left parts of
southern England coloured rusty red. The sand had blown from the
Sahara inside a massive high-pressure system before falling in a rain
shower.

But under the microscope, the red particles that rained on Kerala
were clearly not sand. Electron micrographs show that they are shaped
like biological cells. "They don't look anything like sand, they look
biological," says Monica Grady, a meteorite expert at the UK's Open
University in Milton Keynes. The cells, if that's what they are, are
mostly cup-shaped and have a thick wall.

One type of analysis shows their chemical make-up is about 50 per
cent carbon and 45 per cent oxygen by weight, along with traces of
other elements such as sodium and iron. That's consistent with the
components of a biological cell, according to Jeffrey Walker, a
molecular biologist from the University of Colorado in Boulder. But
although many of the cells have some kind of detached inner capsule,
there is no visible cell nucleus, and tests for DNA that Louis
carried out came back negative.

Louis rules out a distant terrestrial source for the mysterious
particles, because the red rain was concentrated over Kerala for two
months despite changes in climate and wind patterns. Could the cells
instead be local pollen or fungal spores washed off trees and houses
by the rain? Louis says no, because red rain was collected in buckets
placed in wide-open spaces. Equally, he says, the red particles can't
be pollen or spores from the ground that accumulated in the
atmosphere, because the rain would then have been red at the start of
a shower; often the colour came later.

Instead, he links the coloured rain to a meteor airburst. During the
early hours of 25 July 2001, just hours before the first red rain
fell, several people in the Kottayam district heard a loud sonic boom
that made their houses rattle. Louis has interviewed some of those
who heard it, and concluded that it was too loud to have been an
ordinary thunderclap. It's possible that an incoming meteor exploded
in the atmosphere.

Louis then takes a large leap and suggests the meteor was a fragment
of a comet harbouring microbes from space. He thinks that is the only
explanation for the red rain pattern. The meteor flew over Kerala
from north to south, he suggests, shedding fragments and alien
microbes in the upper atmosphere, before finally exploding over
Kottayam district. There, some of the red microbes mixed with rain
clouds and fell fairly quickly, while the rest gradually settled into
the clouds and fell in rain over the following weeks.

"Yes, it is an extraordinary claim, but I have to report what I
observe," says Louis. "We are not able to explain it by assuming a
terrestrial object." The red particles look like biological cells, he
stresses, but contain no DNA. They could therefore be exotic, alien
life forms unknown to science.

Far-fetched? Certainly sounds it. But the idea would undoubtedly have
appealed to the late University of Cambridge astronomer Fred Hoyle,
champion of the "panspermia" theory. With Chandra Wickramasinghe of
Cardiff University, UK, Hoyle developed the idea that life on Earth
evolved from microbes that fell to its surface on a comet. In this
picture, primitive life forms could be ubiquitous throughout the
universe, peppered among the planets and the stars.

Philosophically, panspermia has a certain appeal. It could resolve
the genuine puzzle about why life arose on Earth so fast. The solar
system began its life some 4.5 billion years ago as a hostile
interplanetary war zone, with rocky missiles pelting everything in
sight. Around 3.9 billion years ago, the Earth suffered a
particularly violent bombardment that pulverised its crust. Yet
carbon isotopes in ancient rocks hint that primitive microbes were
thriving just 50,000 years later - a blink of an eye in evolutionary
terms. Panspermia allows off-the-shelf microbes to arrive on the
newly hospitable Earth. This waves away the apparent paradox that the
Earth is the only place in the entire cosmos where we've found signs
of life. From experience, astronomers assume that if something has
occurred once in the universe, it's probably occurred many more
times - we just haven't seen it yet.

Panspermia developed a touch of giggle-factor when Hoyle and
Wickramasinghe blamed extraterrestrial viruses for flu epidemics. But
it has come back into fashion of late, and proponents argue there's
plenty of evidence for it. Experiments have shown that some tough
bacteria can survive for years in space, despite the extreme cold and
high levels of radiation. Others have proved that some of these bugs
could survive the high-speed collisions that they would experience if
they slammed into the Earth on a comet.

The idea of primitive microbes flying around the solar system in its
early days is not as wild as it seems. "Most of the rocks near the
surface of the Earth are shot through with microbial life. It would
be a fairly simple thing for a little piece of the crust to be
ejected and life survive and land somewhere else," says Walker. On
balance, he says, he'd bet that life began here on Earth. But he
wouldn't be that surprised if evidence emerged that life started
somewhere else and was delivered to Earth by a hunk of space rock.

Extraordinary claims
In 1996, Martian meteorite ALH 84001 caused a furore when some
scientists claimed that it harboured fossil bugs. The case was never
proved. "But the most interesting information that we gathered from
that meteorite was that when the rock was ejected from Mars and
travelled to the Earth, the temperature of the interior never
exceeded something like 50 C," says Walker. "Plenty of microbes can
survive that, especially spores."

All in all, it seems that panspermia could work. Now Louis thinks the
red rain of Kerala provides evidence that it actually does. His new
report on the subject, which will appear in Astrophysics and Space
Science in the next few months, is impressive in its detail,
according to Wainwright. "Everything in the paper is done correctly,
there's nothing wacky about it," he says. Grady says it is "very,
very thorough indeed".

However, if scientists have a favourite quote, it's this one,
popularised by Carl Sagan: "extraordinary claims require
extraordinary evidence". I'm hearing it a lot in discussions about
the red rain of Kerala. Grady thinks Louis and Kumar have jumped to
the extraterrestrial conclusion far too quickly. "They seem to prefer
the most bizarre explanation they could find," agrees Charles Cockell
at the Open University, who studies the microbiology of extreme rocky
environments.

What other explanations are there? Wainwright likens the red cells to
spores from a rust fungus, or possibly pollen or algae. With
Wickramasinghe and others, Wainwright has shown in balloon
experiments that winds can carry microbes from the ground to high
altitudes. Particles the size of those in the red rain could soar
several kilometres above the Earth's surface. The dimpled shape could
easily have arisen when the cells collapsed in the microscopy
process. If that were true, he says, then the only mystery concerns
the lack of DNA. "You wouldn't expect spores, microbes or algae not
to have any DNA," he says. The simplest explanation is that Louis's
experiments missed it, so Wainwright wants to repeat the tests. If
the cells do turn out to contain DNA, then there is no great
mystery. "I'd kind of relax if there was DNA there," says Wainwright.

If there is no DNA, Wainwright argues, the cells might be something
extraordinary. He speculates, like Louis, that the lack of DNA might
point to some kind of exotic life form, although he admits it would
be paradoxical for cells without DNA to be classed as "living".

Cockell argues that there could be a simpler explanation - the red
particles are actually blood. "They look like red blood cells to me,"
he says. The size fits just right; red blood cells are normally about
6 to 8 micrometres wide. They are naturally dimpled just like the red
rain particles. What's more, mammalian red blood cells contain no DNA
because they don't have a cell nucleus.

It's tough to explain, however, how 50 tonnes of mammal blood could
have ended up in rain clouds. Cockell takes a wild guess that maybe a
meteor explosion massacred a flock of bats, splattering their blood
in all directions. India is home to around 100 species of bats, which
sometimes fly to altitudes of 3 kilometres or more. "A giant flock of
bats is actually a possibility - maybe a meteor airburst occurred
during a bat migration," he says. "But one would have to wonder where
the bat wings are."

Walker agrees that the particles in the red rain look uncannily like
red blood cells. He says a simple test for haemoglobin could resolve
this quickly. "If they believe they aren't red blood cells, then they
need to explain how they've managed to eliminate that possibility,"
says Cockell. "I would have thought some more basic biochemical
analysis of these cells would be worthwhile, and that should identify
it, whatever it is."

"It's a pity that they don't realise this is interesting without all
the extraterrestrial hype," Cockell adds. "How might you get blood
into rain? I don't think anyone has observed an event where they've
seen an animal ripped apart and its blood distributed in clouds. In
some ways, that whole process is far more interesting than what Louis
is trying to prove." For blood cells to survive would be astonishing:
normally they would be destroyed within minutes if kept in rainwater,
unless the salinity was the same as inside the blood cell.

In the next few weeks, the mystery of Kerala's red rain may finally
be solved. Louis sent samples to Wickramasinghe's lab in Cardiff last
month. As New Scientist went to press, he and Wainwright were still
analysing them.

If they can't explain the origin of the samples, then the suggestion
that they are alien life will gain credence. In that case, someone
will have to verify an observation that Louis made which even he
finds astonishing: that the cells replicate. In earlier unpublished
papers, Louis says he cultured the red rain cells in unconventional
nutrients, such as cedar wood oil, and showed that these DNA-devoid
microbes divide happily at a temperature of 300 C. Louis admits he
left these claims out of his latest paper because he thought they
would be considered "too extraordinary".

Extraordinary is an understatement: if the cells really do replicate
we'll have found the first evidence of extraterrestrial life. In the
end, though, I didn't find any scientist willing to bet that the red
rain of Kerala contained aliens. But everyone agreed it's a cracking
good story that's crying out for a proper explanation. "I think
you've got to be intrigued," said Wainwright. "If you're not
intrigued, then what are you doing in science?"