Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have produced a detailed map of intensity and abundance of gaseous hydrogen cyanide in the upper stratosphere of Neptune, the eighth and farthest-known solar system planet from the Sun.
This picture of Neptune was produced from the last whole planet images taken through the green and orange filters on the narrow angle camera of NASA’s Voyager 2 spacecraft. The images were taken at a range of 7.1 million km (4.4 million miles) from the planet, 4 days and 20 hours before closest approach. The picture shows the Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible. These clouds were seen to persist for as long as Voyager’s cameras could resolve them. North of these, a bright cloud band similar to the south polar streak may be seen. Image credit: NASA / JPL.
Neptune is the only solar system planet not visible to the naked eye because of its extreme distance from Earth and the first predicted by mathematics before its discovery.
Nearly 4.5 billion km (2.8 billion miles) from the Sun, it orbits our star once every 165 years. Interestingly, the highly eccentric orbit of Pluto brings the dwarf planet inside Neptune’s orbit for a 20-year period out of every 248 Earth years.
The atmosphere of Neptune is made up mostly of molecular hydrogen, atomic helium and methane.
Neptune’s blue color is the result of methane in the atmosphere. Uranus’ blue-green color is also the result of atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color.
“Neptune orbits the Sun at the outermost edge, and its atmosphere is composed mainly of hydrogen and helium, like other gas planets such as Jupiter, Saturn, and Uranus,” said University of Tokyo’s Dr. Takahiro Iino and colleagues.
“What distinguishes Neptune from other gas planets is that, according to past studies, it has an abundance of gaseous hydrogen cyanide in the upper stratosphere.”
Intensity ratio (measured value versus radially averaged profile) map of hydrogen cyanide; the white ellipse at the bottom left illustrates the shape of the synthesized beam. Image credit: Iino et al., doi: 10.3847/2041-8213/abbb9a.
The astronomers analyzed the archival data gathered by ALMA on April 30, 2016.
“The peak intensity of hydrogen cyanide was spotted around the equator (approximately 1.7 ppb), while it was the most depleted around 60 degrees south (about 1.2 ppb),” they said.
“For reference, 1 ppb means that 1 hydrogen cyanide molecule is present per 1 billion atmospheric molecules.”
The belt-like distribution of hydrogen cyanide can be interpreted as either the effect of the transportation of nitrogen from Neptune’s troposphere by meridional atmospheric circulation, or an external source such as cometary collisions.
“Large atmospheric flows can trigger an inhomogeneous spatial distribution of trace molecules in the atmosphere of a planet. For example, ozone in Earth’s atmosphere is distributed in an uneven way,” the researchers said.
“Earth’s stratospheric ozone is characterized for its higher concentration at high latitudes. This is due to movement of the air from low latitudes to high latitudes in the stratosphere where ozone is produced.”
“Based on this, we assumed that different concentrations of hydrogen cyanide on Neptune would be attributed to stratospheric circulation,” they added.
“To put it in other words, upward branch is generated in the midlatitude where hydrogen cyanide is the most depleted, and then transports nitrogen molecules, a source of hydrogen cyanide, to the stratosphere.”
“Those nitrogen molecules are further transported to the equator and the south pole, while producing hydrogen cyanide by photochemical reactions in the stratosphere.”
“Our study strongly supports the possibility that massive atmospheric movement or the general circulation exists in Neptune, and induces production of hydrogen cyanide in the stratosphere,” they concluded.
A paper on the findings was published in the Astrophysical Journal Letters.
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Takahiro Iino et al. 2020. A Belt-like Distribution of Gaseous Hydrogen Cyanide on Neptune’s Equatorial Stratosphere Detected by ALMA. ApJL 903, L1; doi: 10.3847/2041-8213/abbb9a
