Viruses can survive in hot spring water, such as that found in the Grand Prismatic Spring in Yellowstone National Park, at temperatures reaching as high as 93 degrees Celsius. Credit: David Monniaux Enlarge

In HG Wells' 'The War of the Worlds', the invading Martians were beaten by that most unassuming of combatants – the common cold. Could the reverse happen and alien viruses pose a threat to human astronauts when they land on Mars? This intriguing question is asked by Dale Griffin in a new paper for the journal Astrobiology, who also asks whether our first evidence of extraterrestrial life could come in the shape of viruses.

Biologists do not consider  to be alive. They are smaller than bacteria (20-300 nanometers compared to 500-1500 nanometers) and cannot replicate on their own – instead they must invade a host cell and use its genetic tools to aid its replication. Yet viruses completely dominate the planet – hypochondriacs might tremble at the fact that there are ten million trillion trillion viruses existing on Earth right now, with a tenth of them found in the oceans. Given their total dependence on cellular  in order for them to replicate, it is little surprise that wherever life is found on our planet, viruses are right there with them.

Griffin, a microbiologist at the US Geological Survey at St Petersburg in Florida, thinks that we can expect to find a similar situation where life exists on other planets. "I would think that the evolution of  on another planet would be very similar to what occurred on Earth," he says. "If this is the case and cellular life is present on any given planetary body then I would expect viruses to also be present in superior numbers."

Yet he doesn't believe that astrobiologists have fully cottoned onto this fact and that exo-viruses are just as viable a topic for speculative study as extraterrestrial cellular life. The reason for this may be partly because the study of viruses on Earth beyond those that cause illness in humans and animals has really only taken off in the last decade or so. As Stedman alludes to, studying viruses is not easy.

"It is only recently in the historical microbiology record that we've had the molecular tools that have allowed us to determine the number and the extent of the diversity of viruses on Earth," says Griffin. Part of the problem is that viruses on Earth have for the most part evolved symbiotic relationships with specific hosts – it's the reason you usually cannot catch a cold from a dog, for example, as the viruses that cause colds in dogs have developed to work with canine cells, and vice versa. So to really study viruses in detail one must culture a , usually a bacterium, in the laboratory and for many viruses we have not identified their host(s). This has slowed research into the vast range and diversity of viruses on Earth considerably. Says Professor Chris Impey of the University of Arizona, who has written several books on astrobiology, "Because of the difficulty of culturing most bacterial species, we're still ignorant of the full range of symbiotic relationships between bacteria and viruses."

Now that things are changing, Griffin thinks it is time to consider extraterrestrial viruses. Stedman agrees. "Life on Earth is clearly heavily influenced by viruses," he says. "The jury is still out on whether viruses are essential for life, but certainly life on Earth would be very different without viruses. Finding life in the absence of viruses would surprise me, but would be very interesting."

Around two billion years from now the Earth would have warmed, boiling away the oceans and eradicating all forms of life. However Griffin suggests that the microbial world might be the last state of life to be found on our planet, before the Sun grows so hot that, they too, are snuffed out. Credit: Detlev Van Ravenswaay/SPL

The key point, Griffin argues, isn't that viruses will exist where life also exists, as we will have noticed that life long before we find the accompanying viruses, but rather that multiple host types of viruses could be found at both the beginning and the end stages of life on a planet.

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