Fortieth anniversary of Viking invasion of Mars

Today marks one of biggest moments in the history of space exploration, perhaps second only to the Apollo 11 landing that occurred on the same day in 1969. On July 20, 1976, the unmanned Viking 1 lander became the first spacecraft to successfully land and operate on the surface of Mars. More than just a technological achievement, this feat completely altered our view of the Red Planet in a way that kept future planetary exploration from being seriously curtailed, and paved the way for an ambitious new generation of Mars missions.

As the nuclear-powered Viking 1 touched down on the surface of Mars in 1976, there was a strange atmosphere of elation and pessimism back on Earth. The elation was understandable. Since 1960 there had been 24 missions launched by the United States and the Soviet Union to study Mars, but only four had been completely successful. Now NASA had placed a working probe on the Martian surface intact, functional, and reporting back to Earth.

The pessimism was something more recent. The expectations of many scientists and members of the public had been severely dampened in previous years and Mars was thought to be a dead planet.

Dead Mars

There's a cliché question in popular science that asks, "Are we alone?" In other words, is Earth the only inhabited planet in the universe? Could life exist elsewhere? Could it be in our own Solar System? These are questions that we still ask, but the premises behind them are very different today than they were in 1976. In fact, the widely accepted answer before Viking 1 was, "Yes, we are alone. Very much so."

The reason for this depressing statement is that the previous 16 years had not been very good for those seeking extraterrestrial life – so much so that the very thing that was meant by "life" was fundamentally altered.

It wasn't that many decades before the first Sputnik that the idea of life existing or having once existed on Mars was taken very seriously, but for a different definition of life than we harbor hopes of finding today. When we talk about life on Mars in 2016, we mean microbes – some sort of bacteria or virus, perhaps remaining as nothing more than fossils. Around 1900, life meant real live Martians building a planet-wide network of canals to bring water from the poles to irrigate farmlands and keep their ancient, dying civilization alive.

This rather romantic vision that inspired H.G. Wells and Edgar Rice Burroughs was tempered over time, and by 1960 the Martians of popular imagination were replaced with the notion of a dead world that was a cross between the Gobi Desert and the top of Mount Everest. But "dead" meant a landscape where the air isn't too thin and is dotted by scrub vegetation and some primitive insect life. Even the most pessimistic scientist's definition of "dead" allowed for a few mosses and lichens.

Mosses and lichens? Anyone who found those on Mars today would get an automatic Nobel Prize and a large basket of fruit.

Two things killed this remarkably lively image of Mars. The first was NASA's Mariner 2 mission to Venus in 1962. If Mars had a rival for harboring life, it was eternally cloud-shrouded Venus, which every sci-fi aficionado knew was a giant primeval swamp inhabited by dinosaurs. What Mariner 2 found instead was a planet with an atmosphere like a pressure cooker, temperatures high enough to melt lead, and sulfuric acid rains for good measure.

This odds killer was bad enough, but then on July 15, 1966, the Mariner 4 probe flew by Mars. As it did so, it sent back the best images yet of the Red Planet – a landscape pockmarked by craters and an atmosphere much thinner and drier than previously thought.

By the time Mariner 9 became the first spacecraft to successfully orbit another planet in 1971 and mapped the surface of Mars in detail, the verdict was all but certain. Mars was a very, very dead world with only the hope that the simplest of microorganisms might still survive – though no one was betting much on even that outside of the very optimistic few.

The Viking Program

NASA's follow up to Mariner, the Viking program, began in 1968 and was tailored for three prime objectives:

• To carry out a high-resolution orbital survey of Mars and return images from the surface

• Study the chemical composition of the planet's atmosphere and the surface

• Look for evidence of life

To achieve these goals, NASA built two identical spacecraft, so there would be a back up on Mars in the event of the other failing. Christened Viking 1 and Viking 2, each probe consisted of a solar-powered orbiter and a nuclear-powered lander.

Orbiter

Acting as mothership for the lander, orbital survey craft, and communication relay with Earth, the Viking orbiter (PDF) was based on the successful Mariner 9 design, though with a modified engine, frame, and power system to accommodate the lander. The orbiter carried a pair of vidicon imaging cameras, an infrared spectrometer for water vapor mapping, and infrared radiometers for thermal mapping.

Propulsion for the orbiter was provided by a liquid-fueled rocket engine burning monomethylhydrazine and nitrogen tetroxide, which produced 297 lb of thrust. Attitude control was courtesy of 12 nitrogen gas thrusters.

Power for the orbiter and for the lander on the way to Mars came from eight solar panels holding 34,800 solar cells that generated 620 watts of electricity. Back up power for maneuvering and eclipses came from two nickel-cadmium 30-Ah batteries.

Lander

The late Carl Sagan compared the lander to either a grasshopper or an amoeba, depending on how you looked at it. It was made of machined aluminum and titanium and shaped like a fat triangle with its points cut off and laid on its side.

Attached to this core were three landing legs that when extended gave the lander a width of 2.2 m (7 ft), but were also designed to fold up for the flight to Mars. The function of the legs was to absorb the impact of landing without jarring the delicate scientific instruments or avionics. As a result, the legs were a complex system of springs, shock absorbers, and crushable aluminum honeycombs. At the end of each leg was a mushroom-shaped footpad, one of which would become world famous when it was the subject of the first image from Mars.

All of the lander's components had to be shielded not only against dust and radiation, but also against the extreme cold of the Martian night as well as the several hundred degree fluctuations in temperature experienced on the voyage to Mars, and the high g forces and vibrations of lift off and landing. In addition, the lander had to prevent paint chips, metal filings, gases, or other bits of the spacecraft from escaping to contaminate the Martian environment.

The lander wasn't just an instrument station. It was also a proper spacecraft with its own propulsion system for the last stage of the landing maneuver. This consisted of three liquid-fueled engines that throttled to up to 600 lb of thrust to slow the craft down for a relatively gentle landing. To prevent scorching the landing site, the engines had 18 nozzles to disperse the exhaust.

As to computers, the landers used a Guidance, Control, and Sequencing Computer (GCSC) made of two Honeywell HDC 402 24-bit computers with 18K of plated-wire memory. By today's standards, that would be like a modern Mars lander being run by the electronics of a one-dollar digital watch.

But the important bits of the Viking 1 lander were its working tools. These consisted of a meteorology boom, an S-band low-gain antenna, a folding high-gain antenna dish, the robotic sampling arm, the laboratory module, and the cameras.

All of these were powered by two nuclear radiothermal generators (RTG). Each weighing 30 lb (13.6 kg), these were powered by plugs of plutonium and generated 30 Watts of electricity at 4.4 volts. Between them, they would continually provide the lander with over 35 Watts of power to run the electronics and keep the spacecraft from freezing at night. In addition, there were four nickel-cadmium batteries to help with peak loads.

Unlike the orbiter, the lander needed special protection for the voyage to Mars. Because of the heat and stress of reentry, the lander needed to be sealed inside an aeroshell with an ablative heat shield. In addition, Viking 1 had to be protected against bringing any Earth microorganisms, so the spacecraft and its aeroshell were sealed in a bioshell and baked at 111° C (232° F) for 40 hours.

Mission to Mars

On August 20, 1975, Viking 1 lifted off from Cape Canaveral, Florida atop a Titan IIIE/Centaur rocket and arrived in Mars orbit on June 19, 1976. Since 1976 was the United States' Bicentennial year, the original plan was for Viking 1 to land on July 4 – Independence Day. Unfortunately, because the images from previous missions were of relatively low resolution and previously selected landing sites proved to be too rough, the orbiter had to carry out a new survey to find a suitable landing area and the landing date was put back.

On July 20 at 08:51 GMT, the lander, still in its metal and plastic cocoon, separated from the orbiter. After the bioshell was jettisoned, the Lander executed a deorbit burn and plunged into the Martian atmosphere. Slowed by the heat shield, the spacecraft approached subsonic velocities in the thin air. At an altitude of 6 km (3.7 mi), the aeroshell blew away and the parachute deployed.

The lander descended to a height of 5,000 ft (1.5 km) when its descent engines kicked in and the parachute broke loose. Guided by radar, Viking 1 continued to descend for 40 seconds before, a short distance from the ground, the engines cutout and the spacecraft dropped in the ⅓ Earth gravity of the planet to land with a slight jolt.

The time was 11:53 GMT and the place was Chryse Planitia (22.48° N 49.97° W) near the Martian equator. Not wasting a moment, the lander's computer activated the onboard cameras. These two cone-like devices began to build up an image line by line like a high-definition color fax machine within 25 seconds of touching down.

In four minutes, the first image ever seen clearly from Mars had been transmitted to Earth. It was a black and white picture (seen at the top of this article) of one of the lander's footpads sitting on pebble-strewn ground. Another seven minutes and a 300-degree panoramic scene of the terrain around the spacecraft arrived.

It wasn't until the next day that the first color images were transmitted. At first, these were rendered with a blue sky, but after calibrating the cameras with the test card mounted on the Lander, the true reddish haze of the Martian atmosphere was seen for the first time.

Science

But Viking 1 wasn't on Mars just to send home holiday snaps. It had very serious science to do. Next to nothing was known directly about the planet and there were enough questions to start with to fill several lifetimes. Geology, chemistry, and even biology needed to be addressed to fill in the very large blanks.

It wasn't a very auspicious start. Viking 1's first failure was when the seismometer designed to study marsquakes jammed when the moving masses used to detect seismic waves was unlocked. However, the rest of the experiments went off much more smoothly, with one key experiment being the meteorology boom that used three hot-film anemometers for measuring wind velocity, while another instrument measured ambient air temperature.

A major component of the Viking Lander was its robotic arm. Unlike later articulated arms, this was a relatively simple device set on a rotating base. When deployed, the tubular metal arm telescoped out and a scoop would dig trenches and pick up soil samples. Once collected, the soil would be secured with a lid, then conveyed to the hopper on the lander leading to the built-in laboratory module.

Once inside, the samples were divided up for delivery to different experiments, such as the X-ray fluorescence spectrometer (XRFS). This would take the soil and bombard it with X-rays from two radioisotopes, then use counters to see how many fluorescent X-rays were emitted as a way to detect the various chemicals present.

The Search for Life

But the key question was whether life existed. In many ways, Mariner 4 was a blessing in disguise. Mars in 1976 seemed so hostile to life as we know it that NASA had to look much harder for much less obvious signs. If primitive plants had been expected, there might have been more emphasis on a visual survey. If the sort of microbes that are found in a terrestrial desert were thought to be under the surface, then something like standard Petri dishes and broth agars might have been used.

It's the sort of search that would have very quickly found life on a more Earth-like world, but on Mars it might have ended up like the classic mistake of the archaeologist who bored for soil samples at regular intervals over a site and missed an entire village because he was digging between the houses.

Since any Martian life that might exist would have to do so in conditions that make the Atacama Desert look like the Amazon rainforest, NASA decided on a more indirect approach that wouldn't look for life, but for signs of biological activity produced by that life. In other words, set up conditions for life and look for evidence of feeding, excretion, and reproduction. The Viking lander carried three experiments to do just this.

Gas Exchange (GEX)

The first of these was the Gas Exchange (GEX) experiment. In this, soil samples were immersed in a helium atmosphere and fed water and organic and inorganic nutrients. Over time, the atmosphere around the sample would be passed through a gas chromatograph for signs of carbon dioxide, oxygen, hydrogen, and methane.

Pyrolytic Release (PR)

The second experiment was the Pyrolytic Release (PR). Soil samples were subjected to light and water in an atmosphere of carbon dioxide and carbon monoxide laced with radioactive carbon-14 (C-14). The idea was that if anything was carrying out photosynthesis, it would absorb the C-14. After a few days, the samples were moved to an oven and baked at 650° C (1,200° F) to release any absorbed carbon in the form of a gas and the C-14 would be measured by radiation counters.

Labeled Release (LR)

The third experiment was Labeled Release (LR), where the samples were fed a nutrient solution tagged with radioactive C-14. If there was anything eating the nutrient, it would produce carbon dioxide or some other byproduct, which would contain traces of C-14, which could be measured.

Was Life Found?

At first, the Viking experiments seemed to confirm that Mars was as dead as disco. The gas chromatograph and soil studies found no traces of organic molecules. Worse, the GEX and PR experiments were completely negative.

However, the LR experiment gave some hope – at first. In the first test run, radioactive C-14 was given off shortly after the nutrient solution was introduced and this held true for samples taken from beneath the surface as well as those scraped from on top. This was promising, but when the experiment was rerun on the same samples a week later, nothing happened. The same held true for the third try.

If something had been alive, it would have accounted for the gas release, but if it was alive, it should have kept operating as more food was introduced. Instead, the reaction stopped and wouldn't restart. With no organic molecules and a start/stop cycle, if this was life, it wasn't like anything seen before.

What was happening in the Viking experiments is still controversial, but the current consensus is that what the lander found wasn't strange life, but strange chemistry.

Living on Earth, we tend to forget that water isn't as bland a liquid as we think. According to chemists, it's one of the most reactive substances and about as close to a universal substance as is possible. It permeates so much of our terrestrial environment that it's completely altered the planet's chemistry and made many compounds as rare as acorns at a squirrel convention.

On Mars, the surface is drier than any place on Earth, the air is a near-vacuum, and there's no magnetic field. The result is that the surface is constantly bombarded by hard ultraviolet radiation and cosmic rays. This creates all sorts of strong oxidizing molecules that on Earth would soon be destroyed on contact with water. On Mars, such molecules are common and such compounds are extremely destructive to organic molecules.

In 2008, NASA's Phoenix lander discovered that there are perchlorates in the Martian soil. One theory is that these highly-oxidizing salts interacted with the nutrient solutions and produced the lifelike results. This would also explain why the reactions ceased when the perchlorates were used up. However, the Viking experiments are regarded as inconclusive as to whether anything living was encountered.

Viking 2

As "Viking 1" implies, there was a Viking 2 mission. The 1970s were still the very early days of Mars exploration and the high failure rate meant that more than one spacecraft would be sent to do the same job as insurance. Identical to the first craft, Viking 2 separated from its orbiter and landed on a large plain called Utopia Planitia (Latin for "Nowhere Plain") on September 2, 1976. Its mission was the same as Viking 1 and it carried out the same life experiments with the same results.

Like Viking 1, Viking 2 was designed to operate for about 90 days, but it continued to function for far longer, holding out for 1,316 days before its batteries failed on April 11, 1980. The Viking 2 orbiter failed on July 25, 1978. Meanwhile, the Viking 1 orbiter lasted until August 17, 1980 and the lander, now called the Thomas Mutch Memorial Station in honor of the leader of the Viking imaging team, went offline on November 13, 1982 after 2,307 days when a software update accidentally caused its high-gain antenna to point away from Earth.

A New View of Mars

The Viking missions may have remained in one spot on the Martian surface, but they fundamentally changed our perceptions of Mars. They didn't find iron-clad proof of life, but the lander experiments sparked new interest and put the search for present or past life on the Red Planet at the top of NASA's planetary sciences' to-do list.

In addition, the images sent from the landers and the orbiters showed that Mars wasn't always a dead ball of dust. There were indications that vast areas of permafrost may exist under the sandy deserts and the rocky highlands and the terrain showed definite signs that ancient floods had carved deep valleys and flooded plains. There were also what looked like dried streambeds and the slopes of the Martian volcanoes looked as if they'd been carved by rainfall and water-based chemical erosion.

There were even signs that the Martian climate ran in cycles with underground volcanoes melting ice to cause floods, only to freeze again.

Later Missions

Viking 1 was not only the first functioning lander on Mars, but it held the record for longest operation on the planet until the Opportunity rover overtook it on May 19, 2010. However, the Viking program was more than a trailblazer, its findings had a major impact on later missions.

There wouldn't be another Mars landing until NASA's Mars Pathfinder mission touched down in 1997. This was more of a demonstrator mission than anything else and was intended to show that a Mars mission could be staged for a 15th of the cost of Viking. Consequently, it's experiment package was simpler than Viking's, but the inclusion of the Sojourner rover was based in part on Viking's discovery of the diversity of the Martian landscape and the necessity of going where the data is..

The next two lander attempts were failures. In 1999, NASA's Mars Polar Lander, designed to study the high latitudes of the planet, lost contact while descending toward the south pole of Mars. In 2003, ESA's Beagle 2 lander also lost contact when its solar panels failed to deploy.

This losing streak was broken by NASA's Mars Exploration Rover program, which set down the Spirit and Opportunity rovers on Mars in 2003 and 2004 respectively. These missions reflected the new philosophy that the best way to decide the question of life on Mars is to learn more about the geology and history of the planet. The idea is that by learning more about the nature of Mars will allow the proper question to be posed when it becomes time once again to look for life.

However, it wasn't until Curiosity started to roll across the Martian hills in 2012 that the question of life again became a specific goal. The nuclear-powered probe wasn't designed to look directly for life, nor for signs of it like Viking. Instead, it was and is looking for specific areas where life could have survived. Toward that end, it has been seeking out clay formations, ancient stream beds, areas of floods from billions of years ago when Mars was much wetter, and minerals that could only be formed in the presence of water.

The Future

All of this work over the past two and more decades would have been impossible without Viking. It lifted the veil what was believed to be a dead, never-been-born planet that was little more than a cosmic artillery range and started science on a path of new understanding. It became a world that was once again worth serious study, one that we now know to be very different in the past, with a surface featuring oceans, lakes, and rivers.

At the moment, there are seven missions operating on and around Mars, with more on the way. This next generation of Mars exploration won't just be state sponsored, but will include the first privately funded missions using privately developed spacecraft. It's a generation that is seriously considering not just the first manned mission to the Red Planet, but even colonization with permanent settlements where only robots roam today.

It was the success of Viking that made it all possible. By carrying out not one, but two landings on Mars, lessons were learned that now make multiple-probe deep space missions a thing of the past, while the data sent back turned Mars into a known, well-mapped world that real questions could be asked about. But most of all, it is ironic that Viking's first search for life has ushered in a time when the answer to the question of whether there is life on Mars will be "yes" – even if that life turns out to have been born on Earth.