In humanity's future expansion into space, Mars is widely regarded as the most promising candidate for human habitation. As a planet with a solid surface like Earth, it has long inspired scientists and engineers to explore its potential. In reality, however, Mars is an inhospitable world, dominated by a thin atmosphere, intense radiation, and severe cold.

The concept of transforming such an environment into one capable of sustaining life is known as terraforming. By altering the climate of the entire planet, terraforming seeks to create conditions closer to those of Earth. Yet the idea involves more than simply constructing a habitable environment from scratch. Traces of ancient water flows have been discovered on Mars, suggesting that the planet may once have possessed a warmer climate and substantial quantities of liquid water. It is even possible that conditions once existed in which microbial life could emerge and survive. In this sense, the terraforming of Mars may also be understood as an attempt to restore a lost environment from the planet's distant past. Realizing such a vision, however, would require immense amounts of energy and time, while also presenting a wide range of technological challenges.

With this in mind, the Obayashi Project Team explored an approach based not on transforming the entire planet, but on recreating an Earth-like natural environment within a limited portion of the Martian landscape through a concept known as paraterraforming (*1). A giant canopy would enclose a crater, within which gases and water would be collected from the Martian atmosphere and subsurface. Life forms would then be introduced in stages, gradually greening the environment and transforming it into a living ecosystem. In doing so, the project would also serve as the setting for a grand experiment—one that reminds us how extraordinary Earth's natural environment truly is: a world where rain falls, winds blow, forests grow, and animals nurture life.

*1　A form of partial or pseudo-terraforming in which an Earth-like environment is created within a limited area of a planet rather than over an entire planet. The prefix para- implies something analogous to or modeled after the original concept of terraforming.

Since the 1980s, OBAYASHI CORPORATION has been envisioning a future in which humanity lives in space from a practical and technical perspective, integrating cutting-edge science with construction technologies. These visions have included concepts such as the Lunar City 2050, Mars Habitation 1 (a Mars settlement plan), the space-city concept Space Nut II to be constructed at a Lagrange point (*2), and the Space Elevator, a 100,000-km tower connecting Earth and space (*3).

Mars Habitation 1 envisions the construction of a settlement equipped with farms capable of maintaining food self-sufficiency. In this vision, life on Mars in 2057 would be largely self-sufficient, with a baby born on Mars and a population of 500 living there. The vision also includes regular interplanetary transport services between Earth and Mars via large shuttle spacecraft.

Since ancient times, humanity has regarded the red-glowing planet Mars with a sense of awe, speaking of it symbolically as Mars, the god of war, while at the same time harboring a deep fascination with it.

The idea of intelligent life on Mars began to capture the human imagination at the end of the 19th century, after the astronomer Giovanni SCHIAPARELLI observed linear formations on Mars that he called canali ("channels"). The term was mistakenly translated into English as "canals," giving rise to the theory that vast artificial waterways built by intelligent beings crisscrossed the Martian surface. This theory was enthusiastically promoted by the American astronomer Percival LOWELL. Mars came to be imagined as a world inhabited by intelligent life, inspiring countless works of science fiction ranging from stories of extraterrestrial invasion and interplanetary conflict to tales of adventure. For humanity, Mars became the nearest "other world," a place that inspires both awe and fascination.

That dream darkened in 1965, however, when Mariner 4 spacecraft of the United States' National Aeronautics and Space Administration (NASA) approached Mars and transmitted images of a barren, crater-covered landscape. Mars proved to be a world far too hostile for life: an atmosphere less than 1 % as dense as Earth's, composed primarily of carbon dioxide, and an average surface temperature of -60 ℃. The "canals" astronomers believed they had seen were ultimately revealed to be optical illusions caused by the limits of telescope resolution.

Yet humanity did not abandon its dream. As the prospect of intelligent life on the planet receded, a new possibility began to take shape: perhaps Mars itself could be transformed into a world habitable by humans. From this idea emerged the new dream of terraforming—reshaping the Martian environment into a habitable world for humans. The concept involved thickening the atmosphere, warming the planet, maintaining water in its liquid state, and allowing natural processes to gradually green the landscape. Although such ambitions would require immense spans of time and energy far beyond current technological capabilities, scientists have continued to explore them in earnest.

In the meantime, traces suggesting that water once flowed across the Martian surface have been discovered, leading researchers to consider the possibility that Mars may once have possessed a warm environment much like that of Earth. Significant quantities of water likely existed there, forming seas and lakes, and dramatically altering the composition and structure of the Martian surface through hydrological activity. The possibility that life may once have emerged on Mars has also been raised. However, owing in part to the planet's relatively weak gravity, Mars is thought to have gradually transformed into the dry and desolate world we know today.

This project focuses on the restoration of a temperate Mars and the creation of new terrain through paraterraforming. In recent years, organizations including NASA, Japan Aerospace Exploration Agency (JAXA), the European Space Agency (ESA), as well as countries such as China and the United Arab Emirates, have advanced technologies for future crewed exploration and sample-return missions designed to bring Martian soil and rock samples back to Earth. At the same time, private companies have also begun pursuing missions to Mars. Humanity's first steps onto our neighboring planet are no longer confined to the realm of fantasy.

Against this backdrop, we sought not simply to transplant Earth's environment to Mars in pursuit of human interests, but to envision an approach that would make use of the planet's own inherent characteristics. Our aim was to create a natural environment suited specifically to Mars and, ultimately, to apply the knowledge gained through efforts to restore and green the Martian landscape toward the restoration of Earth's environment as well. Moreover, conventional terraforming studies have rarely addressed the process of greening. This perspective constitutes one of the defining features of our project.

Terraforming inevitably raises ethical questions, including the alteration of extraterrestrial environments and the potential impact on possible forms of non-terrestrial life. This project, however, is intended solely as an exploration and examination of possibilities from scientific and engineering perspectives.

*2　A stable point in space where the gravitational forces of celestial bodies are in equilibrium. Such regions are considered well suited to the construction of large-scale space structures.

*3　Specifically, the following concepts: "Moon" published in Obayashi Quarterly (OQ) No. 25 (1987), "Mars" in OQ No. 33 (1990), "Lagrange Point" in OQ No. 41 (1996), and "Tower" in OQ No. 53 (2012).