Patricia Straat served as co-experimenter on one of the most controversial experiments ever sent to Mars: the Labeled Release instrument on the Viking Mars landers. The experiment’s principal investigator, Gilbert Levin, insists to this day that the project found extraterrestrial life. Most scientists doubt this interpretation, but the issue has never been fully settled.

When Viking 1 and 2 touched down on Mars in 1976, each carried several instruments to study the planet and look for signs of life. The Labeled Release experiment mixed small samples of Martian soil with drops of water containing a nutrient solution and some radioactive carbon. The instrument then sampled the atmosphere of its internal chamber. If it detected the radioactive carbon, the thinking went, then microorganisms in the soil must have metabolized the nutrients and emitted the carbon. The air around a control version, in contrast, heated up to temperatures thought to kill microbes, should not have any radioactive carbon.

And that is essentially what the investigators found—yet Viking’s other experiments saw no signs of life or of the organic compounds needed to support life. Many scientists concluded that the results were too good to believe and that the findings might be explained by reactive chemicals such as perchlorates in the Martian soil.

Now Straat has published a memoir, To Mars with Love, telling the inside story of this chapter in space history (available at www.tomarswithlove.com). Scientific American spoke with Straat about the tumultuous process of planning the experiment and analyzing its results, and about the risks of Mars exploration if life does exist on the red planet.

[An edited transcript of the conversation follows.]

Were you always interested in space?

By the time I was 12, I could identify all the major constellations in the region where I grew up.

I watched the moon landing on television in 1969, and that really turned me on. At the time I was an assistant professor at Johns Hopkins University working in molecular biology and enzyme systems, which was a long cry from anything involved in space. I decided I wanted to make a career move.

Patricia Ann Straat working with the flight components of the Labeled Release instrument prior to the 1976 Viking Mission. Credit: Patricia Ann Straat and Bruce Connor

How did you get involved in Viking?

It was 1970 and Gil Levin, who designed the Labeled Release life detection experiment, heard I was considering a move and called me for an interview. At the time I was not really interested in leaving academia for private industry. But I met Gil and found him absolutely fascinating. He had just found out that his experiment had been selected by NASA to be sent to Mars, and he wanted someone to implement that experiment because he had a company to run. All my friends thought it was a suicidal career move, but it just sounded so fascinating.

What was the process like to develop the Labeled Release experiment?

It was a major, major, major effort. For the first few years we worked on refining the science and worked with the engineers developing the hardware, testing it as we went along.

The first test of all three life detection experiments in a flight instrument occurred in the fall of 1973. All three failed. That started a major crisis. It was nonstop meetings, and we worked 24/7 on fixing and testing the hardware to get it ready for launch. We were still analyzing data as the biology instrument was sent to the Cape [Cape Canaveral, Fla.] for the launch.

Viking launched in 1975 and landed on Mars in 1976. What was that like?

I went out to [the Jet Propulsion Laboratory (JPL) in] California for the landing and half expected it to crash. The whole team was there. The lander was released from the orbiter, maybe about midnight or so, and it came in by parachute. We had a large TV screen that we could watch with columns of numbers on it, and the last column showed the altitude of the lander above the surface of Mars. I clearly remember watching those numbers flash by. As the lander approached the surface, the numbers I saw were 1,300 feet, 600 feet, 50 feet, then there was absolute dead silence. I thought to myself, “Surely it crashed.” All of a sudden over the loudspeaker came “We have touchdown.” It was just so thrilling. Everybody stood still for a second and then there were cheers all over JPL. I have tried to convey this excitement in my book.

What are your memories of when the first results started coming in?

The Labeled Release experiment started on sol 10 [Viking’s 10th Martian day on the planet]. The first data came in around 7:30 in the evening. I was at the computer surrounded by Gil Levin and several other team members. I worked the keyboard and hit the print button. Then the computer printed the data points from the first nine hours of data. I looked at it and said, “Oh my God, it’s positive.” Not only was the instrument working, but the results were positive. That was quite a thrill. We rounded up the entire biology team to try to understand what it meant, because there was a press conference the next morning where we would report these results.

Were most of the team members thinking you’d found life?

Oh no. We weren’t convinced either. The experiment consisted of two parts: one was adding micronutrients to the active soil sample; the second involved heat-sterilizing a duplicate sample of the soil before adding the nutrients to theoretically kill any microbes that might be there. The difference between an active sample and the heat-sterilized control sample would define a positive response. So we had to wait for the next cycle, another 15 sols downstream, before we ran the control. The surprise was that the control was negative. That’s when the controversy really started.

The results met the pre-mission definition of a positive life response. But of course as soon as we got it everyone came up with alternative proposals to account for the results nonbiologically.

What did you think at this point?

I was pretty astounded, but very interested in these nonbiological hypotheses. What we could say at the time was that the result was consistent with a life response. I wasn’t ready to say we had a life response, especially not in view of all the objections.

We tried to think of some way to distinguish whether it was a biological or chemical response. We had heat-sterilized the control sample at 160 degrees centigrade. The suggestion came up that if we could somehow lower the sterilization temperature, we might enhance the biological explanation. If 50 degrees, say, killed the active response, that would be a strong indication that the positive response had been biological. Very few chemicals are destroyed by 50 degrees centigrade. However, we would expect such a low temperature to have a significant effect on Mars microbes because Mars is a much colder environment than Earth.

It turned out that two heaters were needed to reach 160 degrees. We estimated that using just one of them would heat the soil to about 50 degrees. When we did that on Mars, it significantly reduced the positive response. That was fairly strong evidence that the active response had been biological. However, there could be some chemical out there that does the same thing. Nobody has been able to find such a chemical, though.

Gil Levin has been outspoken in saying that the LR experiment found life on Mars. Do you agree?

Initially I didn’t. In the mid-’90s Gil decided that since nobody had found a suitable nonbiological agent, there was enough evidence to say we had discovered life on Mars. I didn’t agree with him. But when, four or five years later, more and more evidence was found for trace amounts of water on Mars, I began to agree that yes, we did find microbial life. The caveat is the lack of organic molecules. Some complex organics have been found on Mars, but they haven’t found simple organics like alanine and glycine [presumably required by life].

What do you think of the Mars missions that have followed Viking?

I’m disappointed that recent missions have not looked for life. They’ve studied the environment and its potential as a habitat. I just don’t understand it. They should have followed through with a second Viking mission to verify and further characterize the positive Labeled Release results.

Now they’re talking about Mars sample return missions and manned missions to Mars. While very exciting, I am concerned about the back-contamination problem. You can’t send people to Mars and return them without returning Mars soil to Earth. And it’s certainly a possibility that life does exist on Mars, whether or not the Labeled Release experiment found it. People really need to bear that in mind when they plan future missions. I think until we know more we should be very cautious about returning a sample.

What do you make of the continued controversy over your experiment’s findings?

I’d say it’s exciting. We might believe that we discovered life on Mars, but we won’t know the true answer for a long time. I would like to see more life detection experiments sent to Mars soon to either prove it or disprove it.