Usually, when a planet heads into its summer months, the air tends to warm up, a direct consequence of sunlight shining more directly on the planet’s atmosphere. But according to an extensive analysis of Neptune’s atmosphere, the opposite phenomenon happened as the planet’s southern hemisphere progressed through midsummer. Infrared images showed that over the course of 17 Earth years, less than half of a Neptunian summer, the planet experienced unexpected and sudden stratospheric cooling across most of the globe. The only exception was the south pole, which got inexplicably warmer over just 2 years.
“Seasons on Neptune last 40 years each with its 165-year orbital period,” explained lead researcher Michael Roman, a planetary scientist at the University of Leicester in the United Kingdom. “We expected that in response, the seasons would also [evolve] over a slow timescale, but the changes we were seeing were looking rather abrupt.… Really, we didn’t expect too much variation. None of this was expected.”
Diagnosing the planet’s chilly summer required combining every single midinfrared image of Neptune taken over the span of 17 years—a massive data reduction effort that took Roman and his team years to accomplish.
A Pandemic Quest
Set the Wayback Machine to 2020. The COVID-19 pandemic was making its first sweep around the world, and countries were going into lockdown.
“I more or less started on this paper right around the start of the pandemic,” Roman said. “It couldn’t have been done in a couple of months.”
Some past research suggested that prior to its 2005 summer solstice, Neptune’s stratosphere had been cooling for a few years. Roman and his team wanted to see whether that trend held true when a wider selection of observations, including some taken after the solstice, were added to the mix.
“The midinfrared is a very important and informative wavelength range because it allows retrieving the thermal and chemical structure of the atmosphere, despite being notoriously harder to observe than, for example, the near infrared,” explained Vincent Hue, a planetary scientist at the Southwest Research Institute in San Antonio who was not involved with this research.
Why is it so hard to see Neptune in the infrared? Because the planet is far away from Earth, it looks small even with a telescope, and it is very cold (meaning it doesn’t give off much infrared light). What’s more, to map variations across the atmosphere, the researchers needed telescopes large enough and with resolutions high enough that small, cold, distant Neptune was spread out over multiple pixels. Only a handful of ground-based telescopes in the world have infrared instruments that can do that.
The team gathered every scrap of data it could find, combing through old papers and data archives for any midinfrared images of Neptune. Many of those images were originally taken for the express purpose of studying Neptune. But their search also turned up hidden treasures: quick snapshots of the planet taken at random or for calibration during the course of other science observations.
“Sometimes observatories will just take an image quickly to center on the planet or to test something out,” Roman said. “Or when you’re doing spectroscopy, they’ll take an image to give a spatial comparison first.” Those so-called acquisition images helped fill in some of the gaps in the timeline. “I was desperate for as much data as I could get, and those provided meaningful measurements. It was great not to waste any bread crumb.”
“I remember sitting in the middle of pandemic lockdown and calibrating some of these images for the fifth or sixth time,” Roman said. He would “calibrate away from my kitchen.… It just became a quest in its own right.”
Every telescope instrument has a personality all its own, with its own strengths, weaknesses, and quirks. Roman and his team combined data from nine different instruments that used 26 different wavelength ranges. Coaxing a cohesive narrative out of those raw data was a bit like trying to herd 26 cats into a straight line. “This paper is a major data reduction and analysis work,” Hue added. “Because the characteristics of these telescopes and instruments can vary significantly from one facility to another, it is often challenging to cross compare these data sets and derive the big picture from them. This work has done an excellent job at providing a complete picture of the temporal evolution of Neptune’s thermal and chemical structure.”
All told, the researchers dug up 95 infrared images. “These observations only really go back to 2003 when the technology to do this sort of infrared imaging of the planets came to be,” Roman said. “It allowed us to finally say something about this trend over time.”
The midinfrared observations tracked changes in four molecules that represent different layers in Neptune’s atmosphere: molecular hydrogen from the tropopause, which is also a good proxy for temperature; lower-stratospheric ethane; two varieties of methane from the middle stratosphere; and methane from the upper stratosphere. The scientists also used spectra from the Spitzer Space Telescope to calibrate these images with respect to each other, which allowed them to directly compare images taken at different wavelengths.
They found that although the upper troposphere remained at a pretty stable global temperature across the entire 17 years, Neptune’s stratosphere experienced significant cooling from 2003 to 2009. As Neptune experienced its 2005 summer solstice, the temperature of its stratosphere dropped by at least 7°C.
“We would expect to see temperatures slowly growing warmer in response to it being summer on the part of the planet we’re seeing,” Roman said. “That’s why it was so surprising to see that the radiance from the planet, that indicator of its temperature, was dropping over time. It’s doing exactly the opposite of what we thought it would do.”
Then starting around 2018, Neptune’s south pole got about 11°C warmer in just 2 years. “That sort of sudden brightening was really unexpected,” Roman said, “because that’s not something you’d expect from a slow seasonal change.” These results were published in the Planetary Science Journal on 11 April.
Keep Your Eyes on Neptune
Given that the observations cover just 17 years of Neptune’s 165-year orbital period, it’s not likely that normal seasonal variations are the cause of the sudden temperature shifts, the team said. The trends that the team revealed within the stratosphere indicate significant cooling everywhere but the south pole, but the degree of cooling and warming varies depending on which molecule they track. That result suggests that the dramatic temperature changes are confined to a narrow vertical range of the atmosphere, which in turn suggests that a short-term or transient phenomenon might be at work.
The researchers offered up a few different ideas, including a change in clouds and hazes or atmospheric chemistry changes triggered by solar heating or activity. Ultimately, determining what led to the sudden stratospheric cooling and south pole warming will require additional and more frequent midinfrared observations of Neptune. With only 17 years of data, “it’s like observing the Southern Hemisphere of Earth for January and most of February and trying to say something about how it changes over the course of the year,” Roman said. “You need to observe these planets over decades to really start to understand how they change in time, and that can only really be done well from stable ground-based observatories.”
Although some of the midinfrared instruments used in this analysis have been decommissioned, there are still a few in operation that can help monitor Neptune’s atmosphere, Hue said. However, “the James Webb Space Telescope (JWST) will be a fantastic tool to observe the ice giants Uranus and Neptune in that wavelength range, as well as the forthcoming Extremely Large Telescope, planned for 2027,” Hue said. “JWST will be really at the center stage for observing Uranus and Neptune in the next years or so.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer