The ice giant Neptune is the most distant and least explored planet in our solar system – correspondingly little is known about it and the processes that shape it. A surprising development on the planet has now been revealed by evaluations of 17 years of infrared observations. They reveal that Neptune has cooled by around eight degrees since 2003 – even though it was experiencing the peak of its southern summer during that time. Even more amazing is a rapid heating of Neptune’s south pole, which warmed by 11 degrees in just two years from 2018 to 2020. With seasons lasting 40 years, astronomers have yet to explain such a rapid and drastic change.
Neptune is the outermost planet in our solar system. It orbits around 30 astronomical units away from the sun and receives correspondingly little light and heat from our central star: the average temperature on the ice giant is therefore minus 201 degrees Celsius. It is all the more astonishing that this planet is not a frozen world, but still has a very dynamic atmosphere. The fastest storms in the solar system rage on it and its clouds can appear and disappear again within a few days or remain visible for months or years. These rapid changes are also surprising because Neptune needs 165 years to orbit the sun and its seasons change at a leisurely pace of around 40 years.
Infrared telescopes as planetary thermometers
Little is known about how the seasons change temperatures and atmospheric processes on Neptune. Because to measure its temperature from Earth, you need high-resolution mid-infrared telescopes that can act as a kind of thermal imaging camera. “Such spatially sufficiently high-resolution infrared observations have only become possible in the last two decades,” explain Michael Roman from the University of Leicester and his colleagues. Only since then have powerful telescopes such as the Very Large Telescope (VLT) of the European Southern Observatory ESO and the Gemini South telescope in Chile, as well as the Gemini North telescope and the telescopes of the Keck Observator in Hawaii been able to record the temperatures of distant Neptune more precisely. NASA’s Spitzer Space Telescope has also repeatedly targeted the planet with its infrared optics.
Roman and his team have now evaluated this data and thus reconstructed the temperature development of Neptune over the course of the last 17 years. Their data record the development of temperatures in the upper troposphere and lower stratosphere since 2003 and thus also the development since the summer solstice in the southern hemisphere in 2005. “Our data cover less than half of a season of Neptune, so nobody expected to see big and rapid changes,” says co-author Glenn Orton of NASA’s Jet Propulsion Laboratory (JPL). According to models, however, the southern hemisphere and especially the south polar region of Neptune should gradually warm up in the second half of summer. In addition, there could be increasing photochemical reactions in the stratosphere.
(Video: ESO/ M. Roman)
Colder instead of warmer
Analysis of the infrared data, however, did not reveal what was expected – on the contrary: instead of warming, Neptune cooled as the summer progressed, recognizable by the decreasing brightness of the planet in the infrared. The temperature of the lower stratosphere fell by around eight degrees Celsius from 2003 to 2018, the team determined. “This change was unexpected because since we’ve been observing Neptune since the beginning of early summer, we would expect temperatures to slowly increase, not decrease,” says Roman. The astronomers see a possible explanation for this development in the interaction of temperatures with photochemical processes: “While methane absorbs sunlight and thus warms the atmosphere, the photochemically formed hydrocarbons such as ethane and acetylene are strong infrared emitters that contribute to cooling the stratosphere.” , explains the team.
Also surprising was a change observed at Neptune’s south pole between 2018 and 2020. Accordingly, the south polar region of the ice giant, which is marked by a polar vortex, has warmed by a drastic eleven degrees in these two years. “These rapid changes are surprisingly rapid for a seasonal variation, especially given that the South Pole has been constantly in the sun since 1963,” the researchers write. “Apparently there are additional processes at work in Neptune’s atmosphere that take place within the seasons and on regional and global scales.” However, it is not yet clear which processes these are. Astronomers hope that new telescopes such as NASA’s James Webb Space Telescope and the European Southern Observatory’s (ESO) Earth-based Extremely Large Telescope (ELT) will be able to unveil the reasons for the amazing developments on Neptune.
Source: Michael Roman (University of Leicester, UK) et al., Planetary Science Journal, doi: 10.3847/PSJ/ac5aa4