Sunday, December 28, 2014
So far, robotic exploration of the Martian surface reveals nothing but bad real estate – it’s cold, dry, lacking in atmospheric pressure and subject to sterilizing radiation. But below ground might be another story. In recent years scientists have found life underground on our planet in the form of organisms that might just be tough enough to survive on Mars . And so the investigation continues. Experts on Mars and the search for extraterrestrial life helped me compose a few reasons their quest is worth pursuing.
1: In experiments, scientists demonstrated that methane-producing microbes can live under Mars-like conditions.
In his lab at the University of Arkansas, biologist Timothy Kral subjects bacteria to conditions similar to the presumed sub-surface of Mars. He found that one group could survive for up to 120 days despite the extreme cold, dryness and low pressure – single-celled organisms that are called methanogens because they excrete methane.
Kral said he and other biologist have suspected since the early 1990s that if anything could survive on Mars, it would be this type of life. “They are ancient and simple organisms” he said. They flourished here on Earth before plants and animals and could remain after we are gone.
Methanogens don’t like oxygen – it’s toxic to them – so when the Earth’s atmosphere became rich in oxygen 2.4 billion years ago, they moved underground. (Methane producing organisms also live in the innards of animals, such as humans and cows, but this is an unlikely scenario for Mars.)
These organisms have simple appetites. Unlike plants, they need no sunlight. Unlike animals, they have no need to eat other living things. They live off just carbon dioxide, water and hydrogen.
Mars has carbon dioxide and hydrogen. There’s frozen water, and evidence that water had flowed on the surface in the past. With such a tantalizing possibility before them that methanogens might survive on Mars, scientists have been looking for Martian methane for years.
A few years ago, a group led by Michael Mumma at the NASA Goddard Space Flight Center announced they’d found it using remote sensing. That finding was met with skepticism, but rover that’s currently exploring Mars, called Curiosity, is equipped to get a much closer look. The Curiosity team first announced no methane earlier this year. Then they did an about face.
2: It used to be nicer on Mars: All the water on the surface of Mars is frozen, but as scientists continue to explore, evidence keeps mounting that the climate was a lot warmer several billion years ago. Geologic features look a lot like dry rivers and lake beds, suggesting it was warm enough for liquid water to flow, at least periodically. Mars is close enough to the sun that such warmth is perfectly plausible if it once had an atmosphere with sufficient carbon dioxide – a so-called greenhouse gas that prevents our planet from freezing. The crucial difference between our planet and Mars is that Earth is big enough for gravity to hold a stable atmosphere. Mars is smaller, so its atmosphere appears to have drifted off into space.
3: Life began very early in Earth’s history. Chemical traces of life indicate that it had already spread around the globe by 3.7 billion years ago, which is not long after our planet would have cooled off from an initial molten state. That suggests that life doesn’t need eons of time to form once favorable conditions arise. Mars was only nice for a short time, but it might have been long enough for life to have emerged.
4: Mars and Earth Have Exchanged Stuff. We have a number of meteorites from Mars. We know their origin because they carry a return address in the form of little bubbles of trapped gas. Scientists can tell that trapped gas matches the composition of the thin Martian atmosphere as measured by various NASA landers. “It’s clear the trade routes exist,” said Andrew Knoll KNL +0.94%, a biologist at Harvard University.
Our planet is a bigger target, and so it’s more likely life would have started there and landed here, but Earth rocks must have landed on Mars as well.
The oldest known Mars rock to have made the journey, called ALH84001 has been the subject of intense study since it was discovered in Antarctica in 1984. In the 1990s, NASA scientists announced possible signs of fossil life in this rock – a discovery that was indeed too good to be true. But there’s still a wealth of Martian history written into ALH84001, and this week scientists from theUniversity of California, San Diego used it to fill in more gaps the geologic and hydrologic history of Mars.
Knoll said it might not be easy for a bug adapted to Mars to survive on Earth. He also noted that on Earth, methanogens don’t live completely off the grid. They get “subsidies”, he said – nutrients from the rest of the food chain.
Many scientists also hold out the possibility that life had a separate and independent origin on Mars – a finding that would be even more profound and exciting than finding Earth-type life had been transported between the planets.
5: We have to look. Our understanding of the nature of life would be transformed if we had a second example. All life on Earth sprung from a common origin. Not only does all life use the same system for transferring information – DNA and its relative RNA – but many of the specific sequences of code are the same in bacteria, plants, and animals.
If life arose independently somewhere else, we don’t know if it would use DNA or something similar. To qualify as life, something would have to replicate itself, transfer some kind of information to subsequent generations, and undergo Darwinian evolution. But what kind of system might do that? Finding a second example of life, “would be a great moment,” said Harvard’s Knoll. Right now scientists can’t agree on exactly how one would construct a universal definition of life that would encompass extraterrestrials without incorporating crystals and other non-living systems that nevertheless reproduce and show order and structure.
Society may invest billions in future missions that eventually rule out life on our neighboring planet. In science, getting at the truth isn’t a failure, even if it’s not the truth we had wished for. A dead Mars might reveal to us that life is more fragile than we’d imagined. That might not be the answer we’d hoped for, but it’s worth knowing.