As humans explore the solar system, the tantalizing possibility of discovering extraterrestrial life continues to pop up. But the goal of most scientists is to discover existing life on another world, not to accidentally bring it there from Earth. Whether travelers are robotic probes or human astronauts, scientists are increasingly faced with the challenge of preventing the contamination of alien environments. But if we uncover still-evolving alien life on another world, can we even justify going there?

Scientific and ethical questions

There are both important scientific and ethical reasons why cross-contaminating another planet or asteroid doesn't sit well with many.

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Scientifically, “you don’t want to find yourself in the position of equivocating about whether you found something left behind by a previous probe versus something which truly represents a separate generation of life,” says James H. Beall, a senior consultant in the Space Sciences Division at the Naval Research Laboratory in Washington, D.C., and a member of the faculty at St. John’s College in Annapolis, Maryland.

That’s not to dismiss the ethical concerns, though. Beall says that should humans find signs of extraterrestrial life, we will have to face tests about whether we’re there for exploitation, preservation, or a mixture of both that's more akin to animal husbandry.

“The way in which these kinds of things [extraterrestrial life] are kept by us and preserved by us — whether they reveal the original state and original evolutionary paths — is very important," he says. "Not just from a scientific point of view, but also as far as the kind of regard we ought to have for the remarkable complexity of the world."

Though no extraterrestrial microbial life has yet been found, Beall says, “Evolution seems to take place on almost any environment on Earth where it can.” The presence of extremophiles — organisms that thrive in difficult environments such as the high-pressured waters of the Mariana Trench or the desiccated sands of the Atacama Desert — suggests that life can evolve and prosper in surprising places.

What’s to say that couldn’t also be the case beyond Earth? Microbes might be the cause of methane on Mars. They might explain the mysterious behavior of the unknown absorbers in Venus’ atmosphere. Or perhaps microorganisms inhabit the subsurface ocean of Europa. For this reason, we already take some precautions to avoid polluting other worlds with Earth-based life, such as decontaminating rovers and landers before they venture out into the solar system. But is that really enough?

Guidelines, but no enforcement

Humans have been concerned about these issues for decades. The Outer Space Treaty of 1967, for example, called on its member nations to avoid “harmful contamination” of the Moon and other celestial bodies. The Committee on Space Research of the International Council for Science, established in 1958, maintains a Planetary Protection Policy that aims to address the specifics of fulfilling this charge.

For example, in an August 2019 report, the Panel on Planetary Protection described its ongoing study of the ESA’s JUpiter ICy moons Explorer (JUICE) mission, which is scheduled to launch in 2022 to explore the solar system's largest planet and its moons, as well as NASA’s Europa Clipper mission, which plans to investigate Jupiter's smallest Galilean moon after launching in 2023. The panel has also called for scientific inquiry into how contamination could travel through the atmosphere of Mars in advance of proposed human missions to the Red Planet, raising questions about how much one human settlement or exploration site could impact the environment as a whole.