This figure shows the location of the dawn and dusk of a solar eclipse observed with the Alice spectrograph on the ship New Horizons. The sunset eclipse occurred south of the “cordial” region of Pluto from the range of 30, 120 miles (48, 200 km), while the Sun rose north of the "whale tail" from the range of 35, 650 miles (57, 000 km).
Scientists working on the NASA New Horizons spacecraft watched Pluto’s atmosphere 1,000 miles (1,600 kilometers) above the surface of the planet, demonstrating that it is rich in nitrogen. This is the first observation of Pluto in the atmosphere at a height greater than 170 miles above the surface of the planet (270 kilometers).
New information was collected by Alice’s New Horizon spectrograph during the alignment of the Sun, Pluto and the spacecraft, starting about an hour after the ship’s closest approach to the planet on July 14. During an event known as a solar eclipse, the New Horizons passed through the shadow of Pluto, while the sun illuminated the atmosphere of Pluto.
This data shows how the Alice tempo has changed over time during sunrise and sunset observations. The counting speed is greatest when the angle of view of the Sun is outside the atmosphere at the beginning and at the end. Molecular nitrogen (N2) begins to absorb sunlight in the upper reaches of Pluto’s atmosphere, diminishing as the spacecraft approaches the planet’s shadow. While the eclipse is progressing, atmospheric methane and hydrocarbons can also absorb sunlight and further reduce the counting rate. When the spacecraft is completely in the shadow of Pluto, the counting rate drops to zero. As the ship emerges from the shadow of Pluto at sunrise, the reverse occurs. Analyzing the observed count rates in the opposite direction of time, it is noticed that the atmospheres on opposite sides of Pluto are almost identical. “This is just the beginning of a study about the atmosphere of Pluto,” said mission scientist Andrew Steffle of the Southwestern Research Institute in Boulder, Colorado. "Next month, all information about the blackout Alice will be sent to Earth for analysis. Despite this, we have information that gives an idea that Pluto's atmosphere is above its surface."