Twenty years ago, America celebrated its Independence Day by landing several thousand invaders on the surface of Mars.

On July 4, 1997, the Pathfinder spacecraft touched down in a northern lowland called Chryse Planitia carrying a small rover named Sojourner—as well as a large amount of stowaways in the form of earthly microbes.

Did any of these microbes survive and reproduce, establishing themselves as Earth’s first colonists on a distant world? Highly unlikely, NASA assured us at the time, noting that scientists believed “it would be difficult to sustain and cultivate life on Mars.”

That remains true today. In the years since Pathfinder, scientists have cataloged more than a dozen factors—from radiation to soil toxins—that make the red planet a death trap for most terrestrial organisms.

But, as Jurassic Park famously pointed out, life often finds a way. Biologists have been discovering all kinds of organisms on Earth capable of thriving in extreme environments, such as frozen Arctic tundra and desiccated deserts. Also, other Mars probes and rovers have found areas on the red planet, designated by NASA as special regions, where environmental conditions could support the growth of hardy microorganisms.

TIL Mars hasn't received an "Earth vaccine," so if too much earthly biological material gets transferred from the scientists to the rovers to Mars, those Earth germs could have devastating effects—not only on the success of NASA's missions but to the red planet as a whole.

National Geographic Channel is currently airing MARS, a global event series, on Monday nights at 9/8 Central. Join the journey at MakeMarsHome.com. #CountdownToMars

And on September 27, SpaceX CEO Elon Musk will unveil his plans for sending humans to Mars at a global space conference being held in Mexico. The excitement over this pending announcement overshadows a worrisome dilemma: The special regions where Earth life could take hold are also the areas where we would most likely find indigenous Martian life. That means—unless we are very, very careful—we could ruin our chances of discovering extraterrestrial organisms just by going to look for them. (Read "SpaceX Plans to Send Spacecraft to Mars in 2018.")

“It’s like looking for stars when the sun's out,” says NASA’s Catharine Conley. “If you want to find Mars life, you have to get rid of the signals of Earth life so that you can see it.”

Keeping Mars Martian As scientists find more Earth microbes that can survive extreme environmental conditions, NASA has identified regions of the red planet where exploration must be extremely cautious. Areas with shallow ice beneath the surface or slopes that seem to show periodic flows of water are potential havens for terrestrial life. Recurring slope lineae— features possibly formed by liquid water Continuous ice about one foot below the surface No shallow ice observed Discontinuous ice about 16 feet below the surface Continuous ice about one foot below the surface 2000 mi SCALE AT THE EQUATOR 2000 km MATTHEW W. CHWASTYK, NG STAFF SOURCES: NASA; MARS EXPLORATION PROGRAM ANALYSIS GROUP; McEWEN ET AL., 2014; OJHA ET AL., 2014; STILLMAN ET AL., 2016 Keeping Mars Martian As scientists find more Earth microbes that can survive extreme environmental conditions, NASA has identified regions of the red planet where exploration must be extremely cautious. Areas with shallow ice beneath the surface or slopes that seem to show periodic flows of water are potential havens for terrestrial life. Continuous ice about one foot below the surface Discontinuous ice about 16 feet below the surface No shallow ice observed 0° 0° EQUATOR Continuous ice about one foot below the surface 1000 mi Discontinuous ice about 16 feet below the surface 1000 km SCALE AT THE EQUATOR Recurring slope lineae— features possibly formed by liquid water Exploration zones— NASA-identified potential landing sites MATTHEW W. CHWASTYK, NG STAFF SOURCES: NASA; MARS EXPLORATION PROGRAM ANALYSIS GROUP; McEWEN ET AL., 2014; OJHA ET AL., 2014; STILLMAN ET AL., 2016; LINDSAY E. HAYS, NASA/JPL

For the past 10 years at NASA, Conley has had the difficult, sometimes unappreciated job of keeping Mars clean. She is the head of the Office of Planetary Protection, which is tasked with preventing alien organisms from getting loose in Earth’s ecosystem, as well as keeping humans from inadvertently seeding other planets with terrestrial life.

Of course, scientists have been worrying about planetary protection since the very beginning of the space age. With the launch of Sputnik in 1957, it seemed only a matter of time before the United States and the Soviet Union would begin sending spacecraft to the moon, Venus, and Mars.

Some researchers immediately saw this as an unprecedented opportunity to find and study organisms that had evolved under completely different circumstances—possibly revealing alternative models of life that we would never be able to find on Earth. But those same scientists also worried that biological contamination could ruin this endeavor.

Advocates for planetary protection can cite ample historical evidence to justify their fears. In the 14th century, ships from Asia carrying flea-infested rats brought the Black Death to Europe. Years later, European explorers introduced more than two dozen diseases to the Americas that devastated indigenous populations. In more recent decades, invasive species have demonstrated how even a single type of plant or animal can wreak havoc on an entire ecosystem.

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Some in the scientific community questioned whether their Soviet counterparts, in particular, could be trusted to take adequate precautions.

“The United States and the Soviet Union are engaged in a program of space exploration regardless of whether we biologists like it or not,” Wolf Vishniac wrote in a 1964 editorial published in Science. “If we ever wish to derive any biologically signiﬁcant information from landings on other planets, then we must plan for it now while it is still possible to include the necessary biological safeguards.”

Fortunately, planetary protection measures became embedded in international law. The Outer Space Treaty of 1967—which has been signed and ratified by all spacefaring nations—obliges countries to avoid harmful contamination of the moon and other celestial bodies.

How to Bake a Spacecraft

Building on that treaty, the international Committee on Space Research has drafted guidelines for sterilizing spacecraft, depending on the type of mission.

For instance, a Category I mission—sending a spacecraft to someplace like the sun—requires no planetary protection measures. However, if a spacecraft is orbiting or flying by a planet that could sustain life—such as Mars or Jupiter’s icy moon Europa—then it’s deemed a Category III mission and requires sterilization, in case the probe accidentally crashes onto an otherwise pristine surface.

There’s no way to build and launch a spacecraft that’s entirely microbe free, but NASA has developed a multipronged approach to make the most of its anti-germ warfare.

View Images Workers in the Spacecraft Assembly and Encapsulation Facility-2 at NASA's Jet Propulsion Laboratory are covered from head to toe except for their eyes to reduce contamination of the Mars Sojourner rover. Photograph courtesy NASA

To start, all spacecraft components are limited to no more than 500,000 bacterial spores—about one-tenth as many as there are in a typical teaspoon of seawater. And the equipment that is designated for safely landing on Mars—such as a rover—is limited to no more than 300,000 spores on its exposed surfaces.

For the Curiosity rover, which landed on Mars in August 2014, the components were assembled in a clean room and routinely wiped down with alcohol. Parts that were tolerant of extreme heat were baked at temperatures ranging from 230 to 295 degrees Fahrenheit for up to 144 hours. NASA also made sure Curiosity’s heat shield would get toasty enough as it descended through the Martian atmosphere to kill most of the spores it was carrying.

These and other measures made it possible to limit Curiosity’s spore count to just 56,400 on the surfaces of the rover’s hardware. (See incredible pictures from Curiosity's four years on Mars.)

NASA took the additional precaution of excluding special regions from the list of potential landing sites, including areas of Mars that have ice near the surface. If Curiosity had crashed near frozen water, the result could have been instant primordial soup: Heat from its nuclear battery would melt the ice, providing a warm, wet environment where Earth microbes could potentially thrive.

To send a rover to explore a special region, NASA would have to characterize it as a Category IVc mission. That would require the most stringent sterilization procedures possible, used only once before by the space agency.

MARS: Behind the Scenes Get a peek behind the scenes and meet some of the international cast and crew on the set of National Geographic's global event series, MARS, premiering this November.

Before launch, the Viking spacecraft were placed in what Conley describes as “a giant casserole dish” and then heated for several days at around 230 degrees Fahrenheit. While spacecraft are more complex today than they were during the disco era, Conley says it’s still perfectly feasible to design rovers and landers that can tolerate high heat.

“For Mars missions, the biggest challenge is that the mission planners and designers choose not to include the heat-tolerance requirements up front, and it costs a lot more if you have to add those functions later,” she says.

Studies conducted a decade ago suggested that late design changes for heat tolerance would add $100 million to the price tag of a mission. But, Conley says, the cost per mission gets cheaper as the space agency becomes more adept at designing heat-resistant hardware.

Stayin’ Alive

Assuming some microbes survived this assault and made it to a Mars special region, what kinds of germs would we be letting loose on the red planet?

Mars is a hostile place for life as we know it: Scientists have identified 17 “biocidal” factors on the planet that could kill most known microbes, or at least render them dormant. For instance, given the thin atmosphere and lack of a global magnetic field, sunlight is one of the most deadly forces on the planet.

The sun’s punishing ultraviolet radiation would destroy most of the microbes on the surface of a lander or rover within a few hours. Even those on the underbelly of the rover, shaded from direct sunlight, would gradually die over the next 50 to a hundred days, exposed to UV reflecting off the planet’s surface.

But not all microbes would be doomed. A small number of them could survive solar blasting under the right conditions, says Andrew Schuerger, a University of Florida astrobiologist whose lab sits next to the Kennedy Space Center.

“If a portion of that [bacterial] community is covered over by paint or by cleaning fluid residues that were left at the time of cleaning the spacecraft during assembly, then they start getting protected,” he says. A rover might then shed viable spores in places where they could take hold, such as buried under a protective layer of soil.

“You only require half a millimeter or less of very fine grain dust to completely attenuate the UV radiation that's falling on the surface,” Schuerger adds.

In 2013, he and his colleagues tested 26 varieties of bacteria commonly found on spacecraft, incubating them in a chamber that simulated biocidal factors that are ubiquitous across the red planet: cold temperatures and a low-pressure atmosphere made mostly of carbon dioxide.

Of the 26 species tested, one was able to multiply and grow: Serratia liquefaciens, a common bacterium found on human skin, plants, and even in cheese.

Schuerger believes that, when S. liquefaciens is exposed to low pressure, specific genes activate an unknown biological mechanism that allows the organism to continue growing. His lab has sequenced and published the bacterial genome, encouraging the research community to weigh in on the mystery.

Thirst Quencher

Schuerger’s team is also expanding their tests to see how organisms respond to additional biocidal factors. Right now they’re working with simulated Martian soil, some of which is rich in salty minerals. Those salts are both exciting and worrisome, as they might allow water to exist as a liquid on the frigid Martian surface.

Back in 2009, NASA scientists were stunned when they observed droplets of water on the leg of the Phoenix lander, which touched down near the planet’s northern polar ice cap. Researchers later realized that calcium perchlorate—a type of salt prevalent on the red planet—melts ice that it touches.

And, since briny water has a lower freezing temperature than plain water, it could exist in liquid form during Mars’s warm seasons, when temperatures are above minus 10 degrees Fahrenheit. Last September, images obtained by NASA’s Mars Reconnaissance Orbiter suggested that dark streaks known as recurring slope lineae (RSL) were being formed by liquid saltwater that periodically flows downhill on steep crater walls on certain parts of the red planet.

In early September, NASA scientists announced that they might have to shift the course of the Curiosity rover due to concerns that it could contaminate one such RSL as it begins climbing the Aeolis Mons mountain next month.

From the perspective of planetary protection, Conley is also concerned about terrestrial organisms that can absorb water from the air. She recalls fieldwork she did in the Atacama Desert in Chile, which is one of the driest places on Earth, with less than 0.04 inch of rain a year.

Even in this dessicated place, she found life: photosynthetic bacteria that had made a home in tiny chambers within halite salt crystals. There’s a small amount of water retained inside the halite and, at night, it cools down and condenses both on the walls of the chambers and on the surface of the organisms that are sitting there.

Conley also warns that water contaminated with Earth microbes could pose serious problems if astronauts ever establish a base on Mars. Most current plans call for expeditions that rely on indigenous resources to sustain astronauts and reduce the supplies they would need to haul from Earth.

What if, for example, an advance mission carried certain types of bacteria known to create calcite when exposed to water? If such bacteria could survive on Mars, Conley says, future explorers prospecting for liquid water instead might find that underground aquifers have been turned into cement.

Overprotective?

If we do succeed in keeping Mars clean for future human explorers, there isn’t much we can do to prevent contamination caused by the humans themselves. “Leaks happen, mistakes happen, things get broken,” says Conley.

The Planetary Society, headed by Bill Nye, believes it would be premature to land humans on Mars before a thorough search for life has begun. That’s why the organization supports an “orbit first” approach.

“If we keep our filthy meatbag bodies in space and teleoperate sterile robots on the surface, we'll avoid irreversible contamination of Mars—and obfuscation of the answer to the question of whether we're alone in the solar system—for a little while longer,” writes Planetary Society blogger Emily Lakdawalla.

Other critics take the opposite view, saying that planetary protection is an unnecessary, costly endeavor that is slowing down efforts to explore the red planet. Mars, they argue, has already been contaminated.

Writing in the journal Nature Geoscience, a Cornell University astronomer and a Washington State University environmentalist expressed their view that meteors blasted off Earth most likely transferred terrestrial life to Mars millions or even billions of years ago. If those microbes couldn’t survive the alien environment then, the scientists say, we shouldn’t worry about organisms colonizing the planet now. And if they did survive, we can conclude that Earth life is already present on Mars.

“Therefore, it is too late to protect Mars from terrestrial life, and we can safely relax the planetary protection policies,” the scientists write.

For Conley, though, the possibility of a meteor exchange only strengthens the argument for keeping Mars clean.