The cold layer is colder than any part of Earth's atmosphere, despite Venus' reputation for hotness and its nearness to the sun. Around 125 kilometers (77 miles) above the surface, the atmosphere plunges to –175º C (-283º F). Scientists used the European Venus Express orbiter to figure this out, and they did it by staring at the terminator, the dividing line between day and night on the planet. Sunlight filtering through the atmosphere revealed concentrations of carbon dioxide molecules. Then scientists combined this data with information about atmospheric pressure at various altitudes, and were able to figure out the temperature.