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In search of signs of life on Mars with China's sample return mission Tianwen-3

The origin of life is one of the most fundamental and enduring questions of mankind and one of the three greatest Origin Questions in the natural sciences. Recently, China officially launched its Mars Sample Return (MSR) mission, Tianwen-3, marking a significant step forward in planetary exploration. The mission aims to bring Martian samples back to Earth, where advanced laboratory instruments will be employed to conduct comprehensive analyses, seeking to determine whether life ever existed—or may still exist—on Mars.

Professor Yiliang Li, an astrobiologist from the Department of Earth Sciences at The University of Hong Kong (HKU), serves as a core member of the Tianwen-3 scientific team and a co-author of a recently perspective article in Nature Astronomy outlining the mission's objectives. His role mainly involves leading an HKU group that is working on the selection of the landing site for the Tianwen-3 MSR mission.

Is there life on Mars?

Earth is the only planet we know that harbors life. Research traces the origin of life on Earth dates back to approximately 3.8 billion years ago, about 700 million years after the formation of our solar system. Drawing on theoretical, experimental, and observational approaches, scientists believe that Earth's evolution during its first 700 million years made it a planet capable of producing life and being habitable. However, definitive evidence is still lacking as to whether life on Earth arose solely through indigenous evolution.

Like Earth, Mars lies within the habitable zone of our solar system. Research suggests that Mars once had a dense atmosphere and a warm, moist climate early in its history, making it suitable for the emergence and development of microbial life. From an astrobiological perspective, the early Martian environment was conducive to the survival of many of the so-called extremophiles found on Earth.

The mission: Bringing Mars to Earth

The key to China's MSR mission lies in identifying Martian materials most likely to preserve evidence of past or present life. To achieve this, Chinese scientists must conduct extensive research before launching the rockets. This includes searching for regions on Mars where liquid water was likely present in the planet's early history, areas rich in essential metallic nutrients, and sites where traces of Martian microbial activity could potentially be preserved for billions of years. While this article outlines the fundamental framework for these studies, the search for promising sampling sites on Mars remains an ongoing and active endeavor.

The MSR mission, scheduled for launch in 2028, involves two separate rockets:

1. A lander, which will land on the Martian surface to collect samples.
2. An orbiter, which will wait in Mars' orbit to receive the samples and bring them back to Earth.

The lander will drill 2 meters underground—a critical depth because the surface of Mars is bombarded with radiation and corrosive chemicals that can destroy any signs of past or present life. Below this hostile surface layer, valuable organic materials may still be preserved. The samples will be transferred to the orbiter and then flown back to Earth for detailed analysis using sophisticated instruments not available on Mars.

Advancing planetary exploration frontiers

The article further highlights that the greatest challenge in returning Mars samples to Earth lies not in the formidable technical or scientific obstacles, but in quarantining and monitoring required once these extraterrestrial materials arrive—a process known as planetary protection.

As China is poised to become the first country to return potentially biologically active planetary material, including potential life forms, from beyond Earth, the potential risk such substances might pose to terrestrial life, including humans, is a major concern. To address this, China plans to construct a specialized facility on the outskirts of Hefei, its renowned scientific hub, where Martian samples will undergo comprehensive biochemical and pathological testing under strict isolation from Earth's environment.

Only after it is conclusively determined that the samples contain no active biological agents or substances that could threaten Earth's biosphere will they be released to designated laboratories for in-depth scientific analysis.

China's upcoming Mars sample return mission represents the next research goal following the successful deployment of the Zhurong rover on Mars in 2021. With this achievement, China became the second country—after the United States—to successfully land and operate a rover on the Martian surface. In 2020, several countries and entities announced ambitious goals for close-up and in-situ exploration of Mars by around 2030. Ultimately, only China's plan has made significant progress and been realized thus far.