Harvard University Professor Dr. Andrew Knoll speaks on the scientific intrigue of Mars, his involvement with the Mars Exploration Rover Opportunity, and the future of Mars investigation

Mars. The Red Planet, the fourth from our sun. Named after the Roman god of war, Mars has intrigued humans for thousands of years. The peoples of ancient China, Egypt, and Babylonia studied it in detail, using only their naked eyes to plot its precise course through the sky. In 1610, Galileo observed Mars with the first crude telescope, enabling a shift in Martian study from mere tracking to actual mapping of its surface.

At the turn of the twentieth century, amateur astronomer Percival Lowell sealed Mars’s fate as preferred planet with his observations that Mars appeared to harbor artificial canals built by intelligent beings. “That set off a firestorm of interest in the press,” Andrew says, “and spawned a large number of books, of which H.G. Wells’s War of the Worlds is the most lasting.”

Books and comics set on Mars indeed tally in the hundreds, and movies about Mars far outnumber those set on other planets, or even the moon [1]. Even a chocolate bar was named after it. No other planet has captured the heart of the public like Mars has.

At Harvard, Fisher Professor of Natural History Dr. Andrew Knoll let this passion for Mars sneak right into his everyday work. For the last fifteen years, Andy has been working with a team of NASA scientists, interpreting data and images sent back by the Mars rover Opportunity.

The Early Years

Unlike most young people who lived through the Space Race, Andy was never enamored with science fiction or the stars while growing up. As a kid, Andy says he was “much more interested in fossils and the history of life.” When he was in elementary school, and his older brother was in junior high, Andy was brought along on a field trip that their father was chaperoning. The class ventured to a 400 million year old Devonian era rock outcrop, hunting for fossils. “My brother could care less,” Andy remembers, “but I thought it was the most exciting thing, to hit a rock, open it up, and see the remains of something that lived millions of years ago.”

“It never occurred to me that one could do that kind of thing for a living.”

Humans have always been curious about their history, the origins of life, and whether or not they are alone in the universe. For space scientists, it is this inherent interest in life that makes Mars the most intriguing of our neighboring planets. Unlike Mercury or Venus, Andy explains, “Mars appears to be near the habitable zone around our sun and may have been within the habitable zone in the past.”

Also known as the “Goldilocks Zone,” the habitable zone is the area around a star in which a planet with enough atmospheric pressure could hold liquid water on its surface. Andy says, “For life as we understand it, we know of no environment where life is present on Earth where water is not. Water is the medium for life, and it’s more than just a circumstantial medium.” In other words, liquid water seems to be the true bare necessity of life [2].

For decades, the question of whether Mars held any water at all remained unanswered. Finally in 1965, the Mariner 4 spacecraft passed within 6,000 miles of Mars, sending back pictures of a cratered surface, debunking Lowell’s hypothesis that Mars featured constructed canals [3]. Mariner 9 was the first spacecraft to be successfully placed in orbit around the Red Planet. By the early 1970s, the photos sent back from Mariner 9 made it clear that although the planet lacks constructed waterways, Mars’ surfaces does contain “channel systems carved by running water.” Andy explains that this mission “helped to move us from speculation to observation.”

The Viking rovers were sent to Mars to search for micro-organisms in the mid-seventies, when Andy was a graduate student of the “father of Precambrian paleontology,” Elso Barghoorn. While the Viking rover was searching for microbial life among the rocks, Andy was honing his skills at doing just that: learning to interpret ancient rocks for signs of single-celled microbes such as bacteria and their more extreme dwelling cousins, the archaea.

Viking never found life, but Andy did. Over the next twenty years, Andy became an expert in geology and natural history of ancient life on Earth. Finally, in 1996, Mars beckoned.


“That year, David McKay and colleagues published a paper claiming that meteorites from Mars contain evidence of past Martian life,” Andy recalls. The study sparked instant controversy, and despite being eventually proven false, the United States government quickly held a conference to discuss new strategies for Mars exploration. Because of his decades of geology and paleobiology experience, Andy was one of the scientists invited to the summit.
There, Andy met renowned NASA scientist Dr. Steve Squyres, who was, at the time, just a “young guy at Cornell.” Steve was putting together a research proposal for a Mars Exploration Rover (MER), and encouraged Andy to apply for a position on the Opportunity science team. “In the long run, it was the ability to read the rock record and interpret the signatures of microbial life in sedimentary rocks that earned me the invitation to join the MER rover team.”

NASA funded two different proposed missions, and Opportunity was one of them. Opportunity was launched on July 7th, 2003, just a few weeks after its twin, Spirit.

Most people do not truly appreciate the precision required to land a rover in the landing zone on Mars. Andy did the math once, and says the task “scaled almost exactly to hitting a golf ball from Boston and [sinking] a hole in one in San Francisco.”

Dr. Andrew Knoll poses in front of a scale model of Opportunity, the day that Opportunity landed on Mars. Credit: NASA

Opportunity’s landing site was in Eagle crater of Meridiani Planum, an area that was studied by previous Mars fly-by missions, and thought to contain the mineral hematite. On Earth, hematite is often found in hot springs or pools of water, so finding hematite on Mars would indicate a wet past.

Once the rover was successfully on Martian soil, the project essentially became one of remote sensing geology.

To explore Mars, two different teams of people are needed. “One is a team of engineers who make things happen, and then there are the teams of scientists that develop a sense of what we would like to do.” There are daily meetings where the engineers say what is possible, and the scientists say what they want to research. Then the teams work together to develop missions that can yield the most scientific knowledge without compromising the rover.

Luckily, they didn’t have to move the rover much at all to find something interesting.

“Opportunity landed in a region shown by orbital imagery to contain a great deal of the iron oxide mineral hematite,” Andy says. Because of the color filters on Opportunity, the blue-tinged hematite were fondly referred to as “blueberries.” Just like on Earth, hematite rarely forms over large areas in the absence of liquid water. With this discovery, Opportunity provided solid evidence that water once flowed on Mars.

Andy Knoll at an early MER press conference, discussing the formation of hematite “blueberries” in Eagle Crater. Credit: Hap McSween

Blueberries were only the beginning. “Opportunity measured the first stratigraphic section on a another planet,” Andy adds. By cutting through multiple layers of rock, Opportunity was able to show that the waters at Meridiani Planum were “acidic, as well as oxidizing, and that the landscape was generally arid.” Scientists went from possessing only knowledge of Mars’ current geology, to suddenly holding a picture of what the surface had been like in the past, as well.

“Particularly, early in the mission,” he explains, “every day brought evidence of features that not only had never been seen on Mars but had never been imagined. Solving the riddles posed by these observations was genuinely exciting, as exciting as anything I’ve done in my scientific career.”

After getting stuck in soft sediment, Spirit ceased communication in 2010, but Opportunity is still functioning and returning data years after the team thought the rover’s batteries would fail. To clean off the solar panels manually, the rover would have needed an extremely precise windshield wiper that could withstand the harsh conditions of Mars. The team had not been counting on Martian weather to provide regular dust storms to clean off the solar panels. “Fortunately, Mars was helpful in this regard.”

But despite having an extra nine years to search, Opportunity still has not unearthed anything even remotely resembling a fossil. Andy Knoll would know, since the standards used to distinguish rock from fossil are named after him. Andy explains how the “Knoll Criterion” came about like this: whenever scientists pick metrics to identify life that are based on the life we find on Earth, some hopeful people insist that life may be different on another planet, and so those metrics may not apply. However, Andy counters, “modern Mars exploration is geological principles and practices exported to another planet.” So, he inverted the approach to identifying fossils, explaining “we should really understand what chemical and physical patterns can be generated by physical processes alone.” Then, using process of elimination, anything that can not be explained by chemistry and physics is likely to be biological, whether on Earth, Mars or elsewhere.

Future Mars Exploration

Andy believes that some areas on Mars that might still hold clues to past microbial life. The current atmosphere is too harsh and dry to support life, but “there is an interval between three and four billion years ago where there is a lot of evidence for erosional channels and deltas – chemical evidence for rock-water interactions.” If Andy could go anywhere on Mars, it would be to rocks that were formed during that time period. The environment around those young rocks may have been able to support certain kinds of bacteria, and allow them to grow and reproduce.

It all comes back to liquid water. “Was water a conspicuous and persistent feature, or were there intermittent wet patches?” Andy asks. “That is one thing that Curiosity hopes to learn.”

Andy was invited to be on the science interpretation team for Curiosity, an even larger rover that landed on Mars in 2012. He declined the offer, however, in order to focus his research back on Earth, where life has definitively flourished for billions of years. That’s not to say he did not feel a pang of jealousy recently, when the team asked him to review the first findings by Curiosity. One of his former graduate students, Linda Kay, is even on the team, exploring new areas on Mars that Opportunity will never see.

In the next few decades, NASA plans to launch sample return missions to bring back slices of Mars to Earth. Studying Martian rocks on Earth will be especially informative, as there are harsh limits to the instruments that can be sent on a Mars rover. A scanning electron microscope (SEM), for example, takes extremely detailed photos of samples by scanning them with a focused beam of electrons. Using an SEM could tell researchers a lot about the make-up of Martian rocks, but an SEM takes up far too much space and has far too much mass to be sent along with a rover. Plus, SEM samples require preparatory work that would be quite difficult to perform remotely. “We will learn things when we return samples that we are unlikely to know about for a very long time by doing in situ science,” Andy says.

Of course, some people would take mankind’s exploration one not-so-small step further – making a one-way trip to the planet themselves, with no chance of returning to Earth. The goal of the Mars One Foundation is to establish a permanent human settlement on Mars in 2023. The project website says the plan is to put a number of astronauts on Mars, “where they will live and work for what will likely be the remainder of their lives” [4].

Andy offered his thoughts on the matter.

He said he knew Mars enthusiasts that would definitely jump at the opportunity to walk on Mars. However, “There is something about the ethics of consigning someone to death for the privilege of seeing Mars up close that is a bit bothersome,” Andy admits. “Other than that, what can I say?”

Cat Adams is a graduate student in Organismic and Evolutionary Biology.


[1] “Mars in fiction.” http://en.wikipedia.org/wiki/Mars_in_fiction

[2] “Life’s Little Essential.” http://www.pbs.org/wgbh/nova/evolution/liquid-of-life.html

[3] “A Chronology of Mars Exploration.” http://history.nasa.gov/marschro.htm

[4] Mars One Foundation. http://mars-one.com/en/

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