Mars sample return: Could Red Planet life contaminate Earth?

Conceptual drawing of a Mars Sample Retrieval Lander. (Image credit: NASA)

The robotic return of pristine samples from the surface of Mars has been a holy grail goal of Red Planet investigators for many, many years.

Over that time, strategies for returning Mars collectibles have ranged from "grab and go" acquisition from the surface to dust collection in the atmosphere to on-planet scientific selection by specially equipped robots — a task now underway by NASA's Perseverance rover, which is wheeling around Jezero Crater.

NASA and the European Space Agency (ESA) are now orchestrating a Mars sample return (MSR) campaign, which will scoop up geologic and atmospheric samples gathered by Perseverance for return to Earth in the early 2030s.

Shipping Martian bits and pieces to Earth is a daunting and multibillion dollar task. Having our planet on the receiving end of material that might contain Mars life is deemed a "low risk" affair in terms of ecological and public safety — but that risk is not zero.

In photos: NASA's Mars Perseverance rover on the Red Planet

NASA's Perseverance Mars rover is busily studying Jezero Crater, caching collected samples of the Red Planet for later return to Earth. (Image credit: NASA/JPL-Caltech)

High anxiety

Could returned Mars samples be biological "hot property" that sparks public anxiety about creepy-crawlies from that world chomping away at Earth's biosphere?

The idea of hauling back the goods from the Red Planet may resonate in some people as a reality replay of Michael Crichton's 1969 novel "The Andromeda Strain," which was made into a 1971 sci-fi film that dramatized the idea of alien organisms infecting Earth.

For its part, NASA has recently requested public remarks on the scope of a draft environmental impact statement for the agency's proposed MSR efforts. That report is currently scheduled to be released for public comment later this year.

For NASA, the scientific study of returned Mars rock, dirt, and atmosphere could well answer a key question: Did life ever exist on the Red Planet? "Only by bringing the samples back can we truly answer the question by using the most sophisticated, state-of-the-art labs, at a time when future generations can study them using techniques yet to be invented," the agency's MSR website explains.

NASA's National Environmental Policy Act approach highlights a multifaceted Mars sample-return program. (Image credit: NASA)

Low likelihood

Over the past few decades, several different panels of scientific experts from the United States and around the world have tackled the question of whether samples from Mars could present a danger to Earth's biosphere. 

"The reports from these panels have found an extremely low likelihood that samples collected from areas on Mars like those being explored by Perseverance could possibly contain a biological hazard to our biosphere," a NASA MSR fact sheet states.

But there's also the potential issue of forward contamination of Mars — hauling Earth life to that globe and, potentially, fouling the nest with our own biota.

Life on Mars: Exploration and evidence

Mars sample drop off at Earth, with Red Planet collectibles to land in the Utah desert. (Image credit: NASA/JPL-Caltech)


In October 2021, a Space Studies Board report from the prestigious U.S. National Academies of Sciences, Engineering and Medicine identified criteria that could allow robotic missions to certain locations on Mars to be carried out with less restrictive "bioburden" requirements, which are designed to prevent the unintentional transport of Earth-based microbes to Mars.

At the request of NASA, a committee on planetary protection wrote the consensus study report "Evaluation of Bioburden Requirements for Mars Missions," identifying criteria for determining whether robotic missions to certain areas of Mars could be subject to less stringent bioburden requirements.

As the committee reported, the harsh conditions on much of the surface of Mars, including the ultraviolet radiation environment, scarcity of persistent liquid water, and humidity-and-temperature cycles, "make survival, growth, and proliferation of terrestrial organisms on the surface unlikely."

However, the report also said that Earth organisms delivered to the surface of Mars might be able to survive and be transported by wind or robotic devices to some regions of the Martian subsurface where such organisms could grow and proliferate. 

These below-surface regions include caves where water ice deposits and salt and brine deposits might exist, and the deep subsurface, where underground aquifers are hypothesized to exist. "These areas might also be where evidence of indigenous Martian organisms is most likely to be found," the report's authors wrote.

An extraterrestrial abode for present-day life? The Red Planet, as seen by Europe's Mars Express. (Image credit: ESA/D. O'Donnell – CC BY-SA IGO)

Take precautions

Some researchers and exploration advocates may assume that any Earth microbes that travel to Mars with rovers, landers and their landing gear will be killed quickly by the harsh environment of the Red Planet's surface. 

But that's not necessarily the case, said John Rummel, a former and founding chair of the panel on planetary protection of the Committee on Space Research, an international confab of experts. He previously worked at NASA headquarters (1986 to 1993 and 1998 to 2008) as the space agency's senior scientist for astrobiology and as NASA's Planetary Protection Officer. 

"Meanwhile, those spacecraft cover and shade Earth organisms, and may even bury them alive below the first few inches of the Martian surface where Mars return samples will be collected," Rummel told, referring specifically to landing gear such as backshells that hit the red dirt hard. "As such, a sample return mission must take precautions against microbial tourists — Earth organisms making the round trip and masquerading as Martians when the samples are analyzed."

Rummel pointed to findings from earlier Space Studies Board reports in 1997 and 2009, respectively:

"Although current evidence suggests that the surface of Mars is inimical to life as we know it, there remain plausible scenarios for extant microbial life on Mars — for instance, in possible hydrothermal oases or in subsurface regions," the 1997 report states.

"Some living species on Earth have been shown to survive under conditions of extreme radiation, subfreezing temperatures, high salinity, extremely high and low pH and cycles of hydration to dehydration present on Mars today," the 2009 report says

The 2009 study also made the assessment, Rummel said, that "samples returned from Mars by spacecraft should be contained and treated as though potentially hazardous until proven otherwise. No uncontained Martian materials, including spacecraft surfaces that have been exposed to the Martian environment, should be returned to Earth unless sterilized."

Related: The search for alien life (reference)

Accidents do happen, as shown by NASA's Genesis mission back in September 2004, which crash-landed on Earth. (Image credit: USAF 388th Range Squadron, Genesis Mission, NASA)

New dilemma?

So, as usual, enigmatic Mars offers up some perplexing issues.

Barry DiGregorio, director for the International Committee Against Mars Sample Return, senses a new MSR dilemma: "Should we ask NASA/ESA to accelerate their planned return of Martian soil and rock samples directly to Earth as soon as possible?"

The reason to do so, DiGregorio told, is spurred by China's successful touchdown on Mars in May 2021 with its Zhurong rover, part of the nation's Tianwen-1 mission. Zhurong continues to wheel across the Red Planet's Utopia Planitia today. 

"They will certainly attempt to bring the first Mars samples to Earth for study in their 'Biosafety Level 4 Lab' — like the one in Wuhan — before NASA and the ESA," DiGregorio said.

In fact, Tianwen-1 is seen as a precursor to attempt a sample return effort at the Red Planet around 2030, according to Zhang Rongqiao, chief designer of the Tianwen-1 mission. 

Toss in for good measure the confidence-building success of China's Chang'e 5 lunar sample return in December 2020. That mission made use of a robotic orbiter, lander, ascent vehicle and Earth return capsule. That hardware combo, in a sense, mimicked an Apollo-like scenario that succeeded in rocketing back to Earth select specimens of lunar bits and pieces.

Some of the landing gear of NASA's Perseverance Mars rover, as imaged by the space agency's Ingenuity helicopter. Could Earth microbes have been transported to Mars in the wreckage? (Image credit: NASA/JPL-Caltech)

A new space race?

"In light of this new space race to return the first Mars samples," DiGregorio said, "the question now becomes: Who should we trust more with bringing them back as soon as possible, NASA/ESA or China?"

Obviously, if the samples contain alien life, DiGregorio said, "the first country that studies these samples will have a tremendous advantage over others to study its properties for use in trying to determine the origins of life itself, medicinal uses for human ailments and of course, possibly the use in bioweapons research. I think the COVID-19 pandemic has opened the eyes of everyone as to the power of mighty microbes and how careful[ly] you have to examine them."

And things can go awry in hauling space specimens to Earth. 

Take, for instance, NASA's Genesis mission, which collected samples of the solar wind and returned them to Earth in September 2004. Genesis' sample return capsule entered Earth's atmosphere as planned, but onboard gravity switches were oriented incorrectly and did not activate. The installation error resulted in the probe's drogue parachute not deploying, shattering many of the Genesis solar wind collectors on impact within the Utah Test and Training Range. The crash contaminated many of the samples, but some remained intact, allowing scientists to study them.

Life on Mars: Exploration and evidence

Perseverance rover confirms existence of ancient Mars lake

Bringing pieces of Mars to Earth: How NASA and Europe will do it

What's your view?

You can read more about NASA's National Environmental Policy Act process and the proposed NASA-ESA Mars sample return program here and here, respectively.

If you have anything you'd like to say about Mars sample return, either pro or con, you can log it in the Federal Register, but you need to act fast: the comment period ends on May 15. Hit the blue ‘Comment" button on this page.

Leonard David is author of the book "Moon Rush: The New Space Race," published by National Geographic in May 2019. A longtime writer for, David has been reporting on the space industry for more than five decades. Follow us on Twitter @Spacedotcom or on Facebook.

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Leonard David
Space Insider Columnist

Leonard David is an award-winning space journalist who has been reporting on space activities for more than 50 years. Currently writing as's Space Insider Columnist among his other projects, Leonard has authored numerous books on space exploration, Mars missions and more, with his latest being "Moon Rush: The New Space Race" published in 2019 by National Geographic. He also wrote "Mars: Our Future on the Red Planet" released in 2016 by National Geographic. Leonard  has served as a correspondent for SpaceNews, Scientific American and Aerospace America for the AIAA. He has received many awards, including the first Ordway Award for Sustained Excellence in Spaceflight History in 2015 at the AAS Wernher von Braun Memorial Symposium. You can find out Leonard's latest project at his website and on Twitter.