40 Years of (Triton) Summer

Other planets in our solar system experience seasons, too. On Triton, Neptune’s largest moon, seasons last an average of 40 years, complete with nitrogen snow. Here’s how Triton’s summer solstice compares to Earth’s.

Earth orbits the Sun on an axis that’s tilted 23.5 degrees. As a result, the Northern Hemisphere receives the most direct sunlight during the spring and summer months, while the Southern Hemisphere receives the most direct sunlight during the fall and winter months. On Earth’s summer solstice, the Northern Hemisphere receives the maximum amount of direct sunlight, making it the longest day of the year. This long summer day affords us more time to splash in the ocean’s waves and build sandcastles on the beach, more time for that backyard barbeque, and more time to enjoy the pool. But what is the summer solstice like elsewhere in our solar system? Do other planets and moons even have seasons?

Seasons in our solar system

Data from spacecraft and telescopes have revealed that Mars, Pluto, Saturn’s largest moon Titan, and Neptune’s largest moon Triton all have seasons. NASA’s Curiosity rover and the MAVEN, New Horizons, and Cassini spacecraft have taught us a lot about the seasons on Mars, Pluto, and Titan. No spacecraft has visited Triton since Voyager 2 flew past it in 1989. Nevertheless, the tantalizing images Voyager 2 sent back to Earth revealed that Triton is a dynamic world with an atmosphere, seasons, and geysers! Triton’s atmosphere is 70,000 times less dense than Earth’s and is composed of nitrogen, methane, and carbon monoxide. Triton is about 2.8 billion miles away from the Sun, and although the surface temperature there is a frigid -230 ºC, the Sun’s heat can cause seasonal changes.

What are the seasons like on Triton?

In 2010, scientists using the European Southern Observatory’s Very Large Telescope (VLT) confirmed that Triton has seasons. These seasons last, on average, 40 years. When Voyager 2 visited Triton’s southern hemisphere in 1989, it was spring. Now, summer is in full swing. The summer solstice occurred in 2000.  

With each change in season comes changes in Triton’s atmosphere and surface conditions. During the winter months, Triton’s atmosphere condenses and falls to the surface as nitrogen, methane, and carbon monoxide snow. During the summer months, when more direct sunlight hits the Triton surface, a thin surface layer evaporates to form the atmosphere once again.

What would it be like to stand on Triton during the summer solstice?

Voyager 2 captured images of geysers erupting at Triton’s south pole. These geysers may be similar to the geysers on Saturn’s moon Enceladus, with material being drawn from an internal ocean. Conversely, they may form when solar radiation heats a thin layer of nitrogen ice on Triton’s surface. This heating causes the nitrogen to vaporize, which forms geysers that extend 26,000 feet (8 km) above the surface. For comparison, the maximum height reached by Old Faithful Geyser at Yellowstone National Park is 185 feet. The peak sunlight Triton experiences during the summer solstice would ensure that the atmosphere stays intact, preventing snow showers from entering the forecast. However, the increased solar heating means that we could expect geysers to erupt quite frequently, lofting huge columns of dust and nitrogen gas into the sky. The surface of Triton would not be the safest place to be during the summer solstice—we’d have to watch our step!