The Alice instrument – a lightweight (4.4 kg), low-power (4.4 Watt) imaging spectrograph aboard NASA’s New Horizons – has observed Pluto’s nitrogen-rich atmosphere as far as 1,000 miles (1,600 km) above the dwarf planet’s surface. The spacecraft has also discovered a region of ionized nitrogen tens of thousands of miles beyond the planet.
This figure shows how New Horizons’ Alice instrument count rate changed over time during the sunset and sunrise observations. The count rate is largest when the line of sight to the Sun is outside of the atmosphere at the start and end times. Molecular nitrogen starts absorbing sunlight in the upper reaches of Pluto’s atmosphere, decreasing as the spacecraft approaches the dwarf planet’s shadow. As the occultation progresses, atmospheric methane and hydrocarbons can also absorb the sunlight and further decrease the count rate. When the spacecraft is totally in Pluto’s shadow the count rate goes to zero. As the spacecraft emerges from Pluto’s shadow into sunrise, the process is reversed. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
The Alice spectrograph gathered valuable new data during a carefully designed alignment of the Sun, Pluto, and the spacecraft starting about an hour after the spacecraft’s closest approach to the dwarf planet on July 14, 2015. During the event known as a solar occultation, New Horizons passed through Pluto’s shadow while the Sun backlit the atmosphere of the dwarf planet.
“This is only the beginning for Pluto atmospheric science. Next month, the full Alice occultation dataset will be sent to Earth for analysis. Even so, the data we have now show that Pluto’s atmosphere rises higher above its surface, in relative terms, than does the Earth’s,” Dr Andrew Steffl from the Southwest Research Institute, a scientist for the New Horizons mission.
Beginning 90 minutes after closest approach, New Horizons’ SWAP instrument observed a cavity in the solar wind between 48,000 miles (77,000 km) and 68,000 miles (109,000 km) downstream of Pluto.
The data revealed this cavity to be populated with nitrogen ions forming a ‘plasma tail’ of undetermined structure and length extending behind the planet. Similar plasma tails are observed at terrestrial planets like Venus and Mars.
In the case of Pluto’s nitrogen atmosphere, escaping molecules are ionized by solar UV light, ‘picked up’ by the solar wind, and carried past the dwarf planet to form the plasma tail discovered by New Horizons.
Prior to closest approach, nitrogen ions were detected far upstream of Pluto by New Horizons’ Pluto Energetic Particle Spectrometer Science Investigation instrument, providing a foretaste of Pluto’s escaping atmosphere.
Plasma tail formation is but one fundamental aspect of Pluto’s solar wind interaction, the nature of which is determined by several yet poorly constrained factors. Of these, perhaps the most important is the atmospheric loss rate.
“This is just a first tantalizing look at Pluto’s plasma environment,” said Dr Fran Bagenal from the University of Colorado, Boulder.
Artist’s concept of the interaction of the solar wind with Pluto’s atmosphere. Some of the molecules that form the atmosphere have enough energy to overcome Pluto’s weak gravity and escape into space, where they are ionized by solar ultraviolet radiation. As the solar wind encounters the obstacle formed by the ions, it is slowed and diverted (depicted in the red region), possibly forming a shock wave upstream of Pluto. The ions are ‘picked up’ by the solar wind and carried in its flow past the dwarf planet to form an ion or plasma tail (blue region). Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
“We’ll be getting more data in August, which we can combine with the Alice and Rex atmospheric measurements to pin down the rate at which Pluto is losing its atmosphere,” he said.
“Once we know that, we’ll be able to answer outstanding questions about the evolution of Pluto’s atmosphere and surface and determine to what extent Pluto’s solar wind interaction is like that of Mars.”
