by Rod Pyle
And there is a third item, which for now is a fantasy but may not stay that way. The MAHLI instrument (microscopic imager) was not designed to look for microfossils, and in any event it would be sheer serendipity to stumble across some. But it could detect a huge collection of them—say a preserved colony of microscopic creatures—or a larger life-form, such as a snail. And where might the record of such a colony be found? In sedimentary layering—the preserved remains of an ancient seabed or the like. And this brings us back to an item in the opening chapter of this book—stromatolites.
Stromatolites are rock layers that have a shape that can—sometimes—identify ancient life-forms. They often indicate microbial colonies or mats, and on Earth they can date back as far as 3.5 billion years, about as far back as you can see evidence of anything biological. They come in many shapes and sizes and can be found all over the planet. But they can also be chimerical, which is to say that a stromatolite might indicate life—biological activity—or it might not.
This was made clear to me in a profoundly visual way at Caltech during one of my meetings with Grotzinger. It helped to illustrate why his undergraduate classes would be popular.
Fossils and rocks adorn much of Grotzinger's office. He showed me a number of stromatolites, ranging from flat slabs with knobby textures to more rectangular rocks with a cross-section that showed the same knobby layer from the edge. I said, “Oh, so this is what microbial life looks like when preserved as a fossil.” We have all seen fossilized shells, plants, and dinosaur bones, but it was the first time I had knowingly held microbial fossils in my hand. They are distinguishable only because they grew into a colony large enough to see and preserve.
Of course it's not that simple—that would be too easy.
He handed me another rock—a rectangular, white one about five or six inches long, three inches wide, and three inches high. The cross-section showed more of the stromatolites in profile. It looked just like the other stromatolites.
I looked at him and waited for the punch line. “That's not a result of biology.” He said.
It turns out that what I was holding in my hand was a chunk of brake lining from a railcar. When the stony material used in this lining gets hot enough (as it does through the friction of slowing a train), the heating creates wavy lines in the “rock” as chemicals separate, change, and create visible divisions. And they look a whole damn lot like the other stromatolites, the ones that I had just held and that resulted from biological creatures. It was an enlightening moment.
This is an extreme example. There are lots of other nonbiological activities that can cause rocks to look like the result of living processes. On Earth it's often extremely difficult to discern whether or not the rock you are looking at is a result of biology or nonbiological activities in the ancient past. This differentiation can be a challenge on Earth; you can imagine how difficult it would be on Mars.
So in addition to designing the spacecraft, building it, launching it, landing it, and figuring out where to go once you are there; once you have located sedimentary layers, and even if you find stromatolites, you may still not know how they formed—critters or no critters? This is where geochemistry comes into play. If you can find the structures, and if you can find a biosignature within them, then you may have some indication of life on Mars.
Of course, the scientists will argue over that, too. It's what they do.
The seminal Hollywood fabrication about biological pollution from outer space may be The Invasion of the Body Snatchers, a seldom-seen black-and-white potboiler about nasty creatures from space that invade Earth and exterminate people, growing exact (but nefarious) copies of them in giant seed pods. Today the film is seen as a by-product of Cold War hysteria that produced a number of such fantasies. A remake from the 1970s did little to improve on the basic idea. Alien species coming to Earth rarely bode well (Spielberg's E.T. excepted).
Many other movies, TV shows, and books have expanded on the invasion theme (often with human-sized, rubber-masked alien beings), but the point is this: you don't want evil things coming to your planet. When the time came to start sending spacecraft to the surface of Mars, it occurred to some within NASA that it might not be such a great idea to make the same mistake in reverse, either.
The issue of the contamination caused by space exploration was outlined as early as 1958 by the International Astronautical Federation, and in 1967 the United Nations issued a report with the weighty title “Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Bodies.” Along with many other proclamations, it included wording about keeping other planetary bodies safe from earthly infestation, specifically that all countries signing the treaty “shall pursue studies of outer space, including the moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination.” Exactly how this would be accomplished was left to other experts.
When the Viking program was being planned, contamination of Mars became a central concern. Letting earthly bacteria loose there to reproduce, mutate, and eventually devour future astronauts was not on anybody's hit parade. More important to that particular mission, the life-science people did not want to fly all the way to Mars, land, sample soil, put it into their billion-dollar spacecraft, and end up finding life, only to realize that it was actually something that hitched a ride from Earth. They had to figure out a way to sterilize the landers.
Their solution may have been a case of overkill, but if so, it is overkill that has continued to some degree to this day. There was—and continues to be—a lot of debate over how careful we need to be in this regard. Many scientists believe that the surface of Mars is so hostile, given the high radiation, extreme temperatures, and the nasty chemicals in the soil, that few, if any, earthly life-forms could persist for long once they arrived. But as your mother probably told you, one can never be too careful.
After the Vikings were built, they were subjected to various indignities to sterilize them as well as 1970s science knew how, just short of destroying their innards. In fact, about ten percent of the mission's budget, $100 million, was diverted to this endeavor. They called it Planetary Quarantine.
First, the machines were built in a clean-room environment—white gloves, breathing masks, little hats to cover the hair, bunny suits, and booties. The room's air was processed to make it as sterile as possible. But this can never catch everything.
The components and final assembly were cleaned obsessively. And I don't mean the type of mop-down they do each night at the local McDonald's. I mean alcohol and tiny paintbrushes and sterilized Q-Tips in every nook and cranny of the spacecraft. But that's not all.
They were then sealed in giant ovens and baked at 257°F for thirty hours; five hours was considered sufficient to kill just about anything so, in a tribute to their thoroughness, they cooked the landers six times as long as needed. The goal was to reduce the number of bacteria, or spores, to no more than roughly 300 per square yard, or about 300,000 total spores per spacecraft. This remains the goal for planetary protection even today. Note that we are talking about spores here, things that can survive harsh conditions and potentially reproduce. Active microorganisms are thought to be insufficiently hardy to survive the Martian environment (or the harsh transit from Earth), but nobody was taking any chances.
The landers were then sealed up and not touched again prior to launch. In all, during the cleaning and sterilization processes, six thousand tests were performed to ensure that the probes would be as clean as science could make them. As far as anyone knows to this day, nothing survived the trip.
This may seem like vast overkill, and some in the scientific community would agree with that sentiment. It's tough on the machines, it's expensive to do, and it limits what can be built and flown. But it's worth mentioning that in 1969 when Apollo 12 landed on the moon, part of its mission was to retrieve parts of a robotic lander that had set down nearby three years earlie
r. The moonwalkers snipped off parts of the lander's camera, bagged them, and brought them home. Upon examination, biologists were surprised to find bits of streptococci virus still living on the pieces. Once subjected to the harsh vacuum of space and the unrelenting radiation of the lunar surface, they had merely taken a nap for three years until they came back to Earth. So the concerns about contaminating Mars (or other destinations) are not entirely misplaced.
In a slight diversion, let's consider NASA's Galileo spacecraft. It was sent off to explore Jupiter and its moons in 1989, arriving in 1995. The bus-sized spacecraft was assembled, like all of JPL's machines, in clean-room conditions, but it had never been subjected to the rigors of sterilization since it was designed to operate only in space. As part of its mission, it took many looks at Jupiter's moon Europa, and over time it became clear that there is probably a vast ocean of liquid water below the surface of the barren, icy moon. Where there is liquid water there could be life. When the mission wound down, NASA was concerned about unintended contamination of the potentially fertile moon by the massive spacecraft if it happened to crash on Europa. This contributed to the decision to send it slamming into Jupiter's dense atmosphere, where there was deemed to be little to no risk of meaningful contamination. You can never be too careful.
Then in the 1990s came the Pathfinder mission. Though it was operating on a lean budget, it was going to land on Mars and hence needed to be sterile. It was subjected to extreme cleaning similar to the Viking protocols but was not baked. There are many issues with baking, but the primary constraint is in the materials used on the spacecraft. Pathfinder was to be a highly cost-effective mission, so they used as many off-the-shelf components as possible. In general, these are far more susceptible to damage than items specifically created for spaceflight or military uses. Think of it in terms of setting your oven to 250°F and leaving your cell phone in it for a few hours. The result would be a thoroughly baked, and likely inoperable, cell phone.
Once Pathfinder arrived on Mars, the Sojourner rover sat atop the lander for two days. There were checks to be performed, but this was also a chance to allow the severe radiation that bombards Mars—including ultraviolet light—to kill anything remaining on the rover, especially the wheels, since they would come in direct contact with the soil. Later tests determined that even brief exposure to such conditions would exterminate about 90 percent of anything that might have hitched a ride.
When the Mars Exploration Rovers were being prepared for flight, similar wipe-downs were performed to kill anything that could be reached with alcohol-soaked swabs and sprays. Additionally, they used a method called “heat shock,” baking it for fifteen minutes at almost 180°F. Again, the 300,000-spores benchmark was used. As Laura Newlin, JPL's planetary-protection lead for MER later said in a May 2003 press release, “Keeping the spacecraft as clean as possible before, during and after launch is very important for any science instruments searching for organic compounds on the surface of other planets. Up to 300,000 spores are allowed on the exposed surfaces of the landed spacecraft; that many spores would fit on the head of a large pin.”
Then, along came the MSL project. While not a true life-science mission, the Curiosity rover would be carrying instruments of unprecedented sensitivity, so sterile conditions were critical.
The usual clean-room conditions were utilized. Frequent cleaning and wipe-downs were performed on all exposed surfaces. Components that could tolerate baking, such as parachutes, thermal blanketing, and other parts were baked at up to 230°F.
But in an innovative solution to the cleanliness of delicate parts, the rover's interior—which contains incredibly sensitive and somewhat-delicate components—was sealed. Venting of the main body of the rover, to equalize pressures, was allowed only through highly efficient and dense filters to keep anything living inside.
Receiving particular scrutiny were the items that would come in direct contact with Mars. As noted, the parachute and other items were baked to sterilization. But the wheels were a concern, and the drill bit was also a huge worry—it would be providing sample material to the instruments on board, and besides apprehension about contaminating the planet, they wanted to avoid false readings caused by non-Martian organisms.
But before launch there was a lapse in the protocols. It was with regard to the drill bits, a critical part of the sampling system. The design had called for the bits to be sterilized and sealed, and not to be touched by anyone or anything until after the rover landed and needed them. But the robotic arm held the drill head, which would not be able to function without a drill bit. The engineers worried that the drill's ability to grab a bit might somehow be impaired, so why not simply place one in the drill before launch? So, during the final phases of preparation, MSL technicians opened the sterile box and attached one of the bits to the drill mechanism. This way, if there was a failure to grab a bit, at least one would be available. It made eminent sense…unless you are the guardian of the cosmos regarding spacecraft sterility.
And there is such a person. It's currently Catherine “Cassie” Conley, NASA's Planetary Protection Officer. Shortly before launch, and well after anything meaningful could be done, she discovered the lapse of sterile procedures.
She continued the tale in a NASA statement: “As the Planetary Protection Officer for NASA, I am responsible for ensuring that the United States complies with Article 9 of the Outer Space Treaty…which specifies that planetary exploration should be carried out in a manner so as to avoid contamination of the bodies we are exploring throughout the solar system, and also to avoid any adverse effects to Earth if materials are brought back from outer space.”
She repeated the challenge of interplanetary sterility: “For Mars, ‘clean’ in terms of spacecraft surfaces, regarding biological contamination, is that there should be fewer than 300 heat-resistant bacterial spores per square meter of spacecraft surface. There's an additional requirement for internal bacterial spores, inside a circuit board or inside the glue that's been used to attach two things together. But for surfaces, which are what you worry about for spacecraft that haven't crashed, it should be fewer than 300 per square meter, if you're going to a place on Mars that isn't given special protection. If you want to go to a place where there might be liquid water on Mars—a ‘special region,’ as it's called—it should be reduced by an additional 4 orders of magnitude, by some kind of treatment like baking in a dry-heat oven.”
The reason the MER rovers had not been baked was that they were not expected to travel into “special regions”—no Martian beaches, lakes, or bayous. “It's extremely difficult to build a spacecraft that can tolerate several days of baking,” she continued. “The cleaning procedure is something that a lot more materials can withstand than the baking, so in order to allow missions to go to Mars without such a stressful treatment, it was decided in the early 90s that, depending on what you were doing, it was okay to just do the Viking pre-sterilization levels.”
When managers for MSL changed the process regarding the drill (that is, opening the covered and sterilized spacecraft and moving the drill bit from the rack to the drill), the memo somehow took a slow route to Conley's desk. It arrived “very late in the game,” she said. So, while the drill placement was performed within a very clean environment, it was not up to NASA's planetary-protection specs. It was certainly an inadvertent oversight, but an oversight nevertheless. “That's where the miscommunication happened,” said Conley. “I will certainly expect to have a lessons-learned report that will indicate how future projects will not have this same process issue. I'm sure that the Mars exploration program doesn't want to have a similar process issue in the future. We need to make sure we do it right.” Hmmm. Point made.
Given the scope of the MSL mission and the complexity of the rover, this seems a small breach. But when you are charged with one sweeping mission—keeping other planets safe from earthly contaminants—you need to worry about these things. Upon further review, the issue turned out to be a smal
l one. As far as they could see from orbit, Gale Crater did not have any potentially life-harboring ice within ten feet of the surface, the closest allowable for this mission's level of sterility. “That reinforced the reasonableness of not having the drill bits sterilized, because there's unlikely to be ‘special regions’ in the Gale Crater landing site,” Conley said. And while the process was not exactly within spec, she noted that it was the cleanest lander since Viking—which is pretty damn clean.
There are issues beyond MSL. The next Mars rover, the Mars 2020 mission, may include the capability to “cache” samples, that is, to take the best bits of Mars they can find and situate them for later pickup and transport to a lander. A subsequent lander would gather them, and then rocket them back to Earth—a sample-return mission, currently the fondest hope of the robotic Mars exploration community.
But planetary contamination can go both ways. What if some nasty, human-melting, zombie-creating megabug came back hiding in the Martian rocks and dirt? In fact, this concern was first dealt with during the Apollo lunar landings. As far-fetched as it seemed at the time, there was concern that some…thing…from the moon might escape from NASA's labs and devastate life on Earth. So when the early moonwalkers splashed down in the ocean after returning from their mission, they were forced to don biological-containment suits—basically the opposite of what biohazard workers wear—to contain any such nasties their clothes or bodies might be carrying. The moon rocks were in sealed metal boxes and then whisked off to be placed in equally sealed laboratory quarantine. Once the astronauts got back to Houston, they spent three weeks inside a customized, sealed travel trailer. Blood samples were taken and tests were run. The air exhausted from the trailer was filtered and examined. Nothing was found.