Visible from the Gulf of Mexico and rising about 18,500 feet above sea level, this snowcapped volcanic cone is the highest mountain in Mexico. McKay has visited the peak of Pico de Orizaba a dozen times to study the mountain’s tree line, which is the world’s highest.
Here, pine trees grow at elevations above 13,000 feet. If Mars turns out to be devoid of life, McKay advocates “terraforming” it — altering its climate and atmosphere to make it suitable for Earth-like life. “If you want to get serious about life on Mars, you want to make trees,” which he says are important for producing oxygen on massive scales. “Every habitable world should have them. If we ever attempt to change the climate of Mars, the trees of Pico de Orizaba can tell us how much we’d have to warm Mars in order to have trees growing near its equator.”