As a book-dazzled kid growing up in St. Louis, I had the good fortune to be surrounded by books from previous generations, and specifically those belonging both to my father and my half-brother, who had died long before I was born. Among these was a multi-volume encyclopedia from the 1920s I’ve never been able to identify. All I have is the memory of looking through its musty volumes and realizing that Pluto was not listed in it, as the publication date was a few years earlier than Clyde Tombaugh’s epic search for the world.

I do remember thinking that without Pluto, the Solar System only had eight planets, and musing in my teenage boy way about how odd this incomplete view of the Solar System was. Little did I know how much more was in store! As to that eighth planet, Neptune was a puzzler not only to the encyclopedia but to science fiction writers of the Gernsback era. Thus James Morgan Walsh’s “The Vanguard to Neptune,” published in Wonder Stories Quarterly in the Spring, 1932 issue. In the cover by Frank R. Paul, that’s Neptune hanging in the sky, looking for all the world like a terrestrial planet, here seen from Triton. The explorers assume the blue areas are ice until they cross to the planet.

Image: Frank R. Paul’s cover illustration for J. M. Walsh’s “The Vanguard to Neptune.” Walsh (1897-1952) was an interesting figure in his own right for those of us who spent a career living off the printed word. Settling in the UK, the Australian novelist would pen an astounding 94 novels across a wide range of genres and under a variety of pseudonyms. It was possible to do that kind of thing in the pulp era.

Spurring these recollections are images of Neptune revealed in a new study on the planet’s cloud cover and its relation to the solar cycle. They’re so stunning that I wanted to reproduce them here, thinking about how our knowledge of the Solar System has advanced since my first acquaintance with the planet in that encyclopedia as no more than a speck of light amongst countless others. There’s also a bit of the Voyager 2 thrill as the craft approached Neptune back in 1989 deep in the summer night here. To see new worlds open before us. Astonishing.

I suppose one day we’ll get so completely accustomed to imaging exoplanets that such thrills will seem commonplace, or maybe not, given their sheer diversity. But the images below still work for me, the first set from Hubble.

Image: This sequence of Hubble Space Telescope images chronicles the waxing and waning of the amount of cloud cover on Neptune. This nearly-30-year-long set of observations shows that the number of clouds grows increasingly following a peak in the solar cycle – where the Sun’s level of activity rhythmically rises and falls over an 11-year period. The Sun’s level of ultraviolet radiation is plotted in the vertical axis. The 11-year cycle is plotted along the bottom from 1994 to 2022. The Hubble observations along the top, clearly show a correlation between cloud abundance and solar peak of activity. The chemical changes are caused by photochemistry, which happens high in Neptune’s upper atmosphere and takes time to form clouds. Credit: NASA, ESA, LASP, Erandi Chavez (UC Berkeley), Imke de Pater (UC Berkeley).

I don’t mean to neglect the import of the paper that features these observations, which comes from astronomers at UC-Berkeley, or their conclusions, which use the numerous changes in the patterning of Neptune’s clouds to point to the connection with the flip in the Sun’s magnetic field every eleven years. It’s intriguing to learn that when the Sun emits more intense ultraviolet light, and in particular the strong hydrogen Lyman-alpha emission, there is increasing cloud cover on Neptune fully two years later.

Imke de Pater (UC-Berkeley) is senior author on the study:

“These remarkable data give us the strongest evidence yet that Neptune’s cloud cover correlates with the Sun’s cycle. Our findings support the theory that the Sun’s UV rays, when strong enough, may be triggering a photochemical reaction that produces Neptune’s clouds.”

We see 2.5 cycles of cloud activity on Neptune recorded over a 29-year period in observations not only from Hubble but Keck Observatory and Lick Observatory, in which it also becomes clear that there is a relationship between the number of clouds and the planet’s observed brightness. Below is the Keck imagery.

Image: A dramatic change in Neptune’s appearance was observed in late 2019 and has persisted through June 2023. As shown by this compilation of images at 1.63 µm (microns) obtained with the NIRC2 and adaptive optics system on the Keck II Telescope, Neptune had numerous cloud features organized in latitudinal bands from before 2002 through late 2019. Afterwards, clouds appeared almost absent except near the south pole. The images are displayed using a Asinh function which, like a log-scale display, decreases the contrast between the features; if displayed on a linear scale, only the brightest features would be visible. Credit: Imke de Pater, Erandi Chavez, Erin Redwing (UC Berkeley)/W. M. Keck Observatory.

This is tricky analysis, because as the paper points out, clouds not related to photochemical reactions, as for example those produced by storms rising up from the deep atmosphere, would complicate correlations with the solar cycle. More recent imagery from the summer of this year has begun to show more clouds in the northern latitudes and at high altitude, which de Pater says reflects the observed increase in the solar ultraviolet flux in the past two years. It’s chastening to realize that even with 30 years of high resolution data covering almost three solar cycles, we have still covered only 20 percent of Neptune’s orbit. Oh for an ice-giant orbiter to depict up close the chaotic actions on a planet whose winds are the strongest known in the Solar System.

The paper is Chavez et al., “Evolution of Neptune at near-infrared wavelengths from 1994 through 2022,” Icarus Vol. 404 (1 November 2023), 115667 (abstract).