As we count down to the much-anticipated landing of NASA's six-wheeled Mars Science Lab (MSL) on Aug. 5/6th, it's noteworthy that 36 years ago today mankind made the first successful touchdown on the Red Planet.
The nuclear-powered Viking 1 lander settled down in a burst of retrorocket fire on a smooth circular plain close to the great volcanic Tharsis Bulge on July 20, 1976. Four billion years ago this region may have been a water-filled bay on Mars.
Viking's first black-and-white image (above) of a footpad resting on an alien planet transfixed the world.
Viking 1 was shutdown in 1982, but its legacy is as alive as ever today. Viking 1, and its sister robot, Viking 2, were the only two spacecraft ever dispatched to Mars with miniature onboard biological laboratories that performed the first in-situ experiments to find extraterrestrial life.
Though sending such a payload to what was then a largely unknown planet seemed premature, it does reflect NASA's aggressive spirit of exploration from the glory days of the 1960s and early 70s.
One out of three independent miniature experiment labs aboard the Vikings yielded positive results, as established by the rules of its builders.
The Viking lab measured a rapid increase in oxygen, carbon dioxide and some nitrogen when a soil sample was saturated with liquid nutrients that astrobiologists thought would be tasty to Mars microbes. The out-gassing from the damp soil was like an Alka Seltzer tablet bubbling away. This reaction did not happen in samples that were sterilized by heat as a control. The second Viking lander recorded similar results 4,000 miles away.
However, the findings were dismissed almost immediately because no organic compounds were detected on the Martian surface by another Viking instrument. The building blocks of life as we know it apparently weren't there. It was like hearing music, but not finding the orchestra.
The apparent false positive in the Labeled Release experiment was attributed to peculiar properties of the Martian soil. Hydrogen peroxide in combination with other chemicals in the Martian surface had been theorized to produce false life signals. But lab experiments on Earth have never precisely duplicated the Viking data.
In 2007 Joop Houtkooper of the University of Giessen proposed that the test procedure inside Viking's onboard lab actually killed Mars microbes. He hypothesizes that they had a water-hydrogen peroxide metabolism that could not tolerate exposure to the "chicken broth" mix the soil sample were soaked in to nurture metabolic reactions. Instead, soaking microbes with water, as the Viking experiments did, would destroy them! The outgassing measured by Viking was the result of cell membranes disintegrating, he suggested.
In 2008, NASA's Phoenix lander found perchlorates in the Martian soil. These salts are powerful enough to combust organics. Tiny trace chemicals found in the Viking experiment were dismissed as contaminants from Earth. But they could really have been combusted organic compounds leftover from contact with perchlorates in the Martian soil.
In 2012, a numerical analysis of the Viking data was used to conclude the reactions measured in Viking's Labeled Release experiment were too complex to be simply oddball surface chemistry, and instead were evidence of biological activity.
The Viking debate aside, we've learned a lot more about Mars since 1976. With each mission the circumstantial evidence for life has ratcheted up. Today we know that Mars has the energy, water and the chemical resources for supporting life.
Mounting geologic evidence points to Mars starting out as a habitable planet. But it grew colder and drier as its water froze, much of the atmosphere was ablated away by the solar wind, and a protective magnetic field fizzled away. Darwinian evolution should have ensured that primeval life, perhaps spawned in a great polar ocean, would find innovative ways to adapt and survive on a slowly dying world.
The MSL will continue this methodical search for life. The nuclear powered, Volkswagen-sized rover will look for environments that might support life and again look for organic compounds in the soil.
However, the rover does not have a biological lab to try again to culture any Mars microorganisms in an onboard Petri dish.
It's intriguing that very small traces of methane, perhaps from Martian organisms, have been discovered in Earth-based observations of Mars' atmosphere. What's more, the European Space Agency's Mars Express Orbiter detected traces of formaldehyde in the atmosphere. This could be the byproduct of the oxidation of methane by microbes, or instead geologic processes. If the MSL detects methane it will be able to measure the isotope ratios, which would allow astrobiologists to distinguish between a geochemical versus a biological origin of the gas.
Whether or not Viking actually had alien microbes incubating in its chasse, this first and only attempt at detecting life off Earth is a sobering foreshadowing of what's in store for finding life on extrasolar planets.
The prolific work of NASA's Kepler space observatory, and other exoplanets searches, have left astrobiologists confident that finding life elsewhere in our galaxy is a matter of when, not if.
Clearly we are rapidly converging on evidence for lots of Earth-sized planets in the habitable zones around their stars. But convincing scientist that these target worlds are inhabited is a very tall order.
Based on the Viking experience, I would predict that we are decades away from a solid answer. That's because, short of a sample return mission with the goal of bringing Mars microorganisms back to Earth and locked away in an "Andromeda Strain"-type quarantine lab, we will never definitely know if Mars has life. It's imaginable a future rover might have an onboard microscope to send back image of microbes squirming around.
All we can do for a world light-years away is to chemically sniff out its atmospheric composition from an immense space telescope at least 10 times sharper and more sensitive than the Hubble Space Telescope.
We've collected a snapshot of the signature of Earth's biosphere as measured from space. Our planet is unusually abundant in oxygen, methane and other biotracers of life. If we are extraordinarily lucky we will find a planet with almost the same spectral fingerprint. That will offer very compelling evidence for extraterrestrial life.
But anything less than that scenario will be debated by scientists for a long time. To be convinced of life existing light-years away, scientists will have to eliminate all simpler non-biological explanations, as has been the case for the Viking data.
I submit that to convince everyone, it will require a direct visit by artificially intelligent probes that are distant descendants from Viking. They will land on exotic planets to peruse an extraterrestrial Serengeti of alien creatures.
However, that reality is very far into the future and no doubt will cost so much that such an endeavor will be unaffordable.
This is why I believe extraterrestrial intelligent species are motivated to send radio or optical signals between the stars. They have to worry about their budgets too.
Frankly, this is one of the reasons I am so enthusiastic about the SETI effort, which regrettably is struggling along on a fraction the cost of space missions. If we detect a signal someday it will be like getting the card that sends you straight to the GO square on the Monopoly board game.
The discovery would immediately answer three fundamental astrobiological questions: (1) whether life exists in space, (2) whether extraterrestrial life has followed Darwinian evolution beyond microbes, (3) whether intelligent life is a condition of the universe.
Image credit: NASA