There are areas on Mars where exploration is prohibited, and the reasons are rooted in international agreements and planetary protection concerns. NASA and the China National Space Administration (CNSA) have both sent rovers to Mars to investigate whether life could exist or has existed on the Red Planet, our next-door planetary neighbor.
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Although missions explore intriguing regions that might once have hosted life, certain zones on Mars are restricted by space treaties. These areas, called "special regions," along with lesser-categorized "uncertain regions," are deemed unsafe for exploration despite their scientific appeal.
During the height of the 20th-century space race, the United States, the Soviet Union, and, for reasons not always clear, the United Kingdom helped shape the UN Outer Space Treaty of 1967. This landmark agreement forbids signatory nations from laying claim to celestial bodies, establishing military bases on them, or deploying nuclear weapons in space. (Note: these rules have not prevented ongoing discussions about nuclear approaches to space activities, including debates about the Moon.)
The treaty also carries other crucial provisions. A central aim is to maximize the chances of discovering life beyond Earth while avoiding the risk of transporting Earth-originating life to other worlds, and preventing any contamination of Earth from extraterrestrial matter introduced during exploration. This balance is essential because a false positive for life could mislead science, and reversing contamination to Earth's biosphere is also a major concern.
To this end, the UN Outer Space Treaty includes language about conducting space research and exploration in a way that minimizes harmful contamination and environmental changes on Earth that could arise from introducing extraterrestrial material.
As understanding of Mars has grown, guidelines have become more granular about where exploration can occur. The Committee on Space Research (COSPAR), under the International Council for Science, has played a pivotal role in defining special regions and in recommending protective measures before spacecraft enter those areas.
COSPAR explains that high-priority science goals—such as the search for life and characterizing Mars’ organic environment—could be compromised if Earth microbes carried by spacecraft were to proliferate on Mars. This is why certain areas on Mars are designated as special regions and subjected to strict planetary protection requirements prior to any entry by a spacecraft.
The concept of a special region describes locations where conditions might support microbial growth based on current biological understanding. In particular, these are places that could be warm and wet enough to sustain microbes potentially transported from Earth.
A special region includes places that are interpreted to have a high potential for the existence of extant Martian life.
In practice, no currently reachable Martian locations have been officially designated as special regions. However, there are zones known as uncertain regions that could qualify as special regions with further study. Examples include recurring slope lineae (RSL)—seasonal, dark streaks observed on Martian slopes—initially suspected to indicate flowing water. Later research suggested that some of these features may result from dry, granular flows. More recently, subsurface oceans of liquid water have been proposed beneath Mars’ surface, though they lie between 11.5 and 20 kilometers (7.1 to 12.4 miles) below the surface and would be exceedingly difficult to access or contaminate without advanced, large-scale drilling technologies.
This year, researchers announced possible biosignatures found by the Perseverance rover in the Bright Angel formation at Cheyenne Falls. Beyond ruling out alternative explanations, scientists must also consider the possibility of Earth-derived contamination, underscoring the critical importance of planetary protection and thorough decontamination before future missions to other planets and moons.
Guidelines suggest evaluating any landing site to determine whether it should be classified as a special region or an uncertain region before proceeding with a mission.
Although some advocate loosening these restrictions to reduce costs or accelerate the search for extraterrestrial life, the protections are in place for strong reasons. Life on Earth is already proving to be remarkably hardy, thriving in extreme conditions that resemble Martian environments in many ways.
A recent study argues that claims of relaxing bioburden controls could hinder accurate life detection and forward contamination control during robotic exploration. Such easing could be self-limiting and would not benefit future human exploration, as it risks underestimating real costs and risks.
Given these considerations, the prudent course may be to leave potential special regions undisturbed until methods prove that resilient Earth life cannot survive transport to Mars. This approach helps protect both scientists in their search for genuine Martian life and future explorers who might one day set foot on the planet.
