Wild Winds On Neptune

Neptune’s upper atmosphere contains some of the fastest winds in the solar system, reaching speeds upwards of 400 m/s (900 mph). How these blustering winds are sustained is a mystery to planetary scientists – Neptune is 30 times farther away from the Sun than Earth is, meaning the planet receives 900 times less flux than the Earth. Thus, it appears that incoming radiation alone can not power the extreme winds on Neptune.

Voyager 2 found that Neptune emits over twice as much energy as it would receive from solar insolation alone [1], so some internal force must drive the energetics within Neptune. In a recent paper [2], scientists observed bright cloud features on Neptune with the Keck telescope to infer the structure and dynamics of the planet’s upper atmosphere. The speed and location of these clouds are linked to the atmospheric temperature, composition, and convective motions. In turn, these properties provide clues about the formation history and evolution of Neptune and can help explain how Neptune is powered.

Neptune’s wind speeds are derived by tracking bright cloud features as they zip around the planet (Figure 1). By tracking the location of each cloud frame-by-frame, a velocity can be determined. This velocity is almost purely in the East-West direction. Due to Neptune’s fairly quick rotation rate (a day on Neptune is 16 hours), the Coriolis force fixes these features in latitude, similar to how it is difficult to raise or lower your arms if you hold them out while spinning in place very rapidly. A global wind speed profile, called the “zonal wind profile” is formed by combining the velocities of cloud features at different latitudes.

Figure 1 – GIFs of Neptune taken in two Keck filters: the H and Kp-bands. Each frame is scaled to have the same normalized brightness. Velocities are determined for prominent bright cloud features by tracking their locations frame-by-frame. More bright features are seen in the H-band than the Kp-band because the Kp-band is more sensitive to methane absorption. This means that the Kp-band can not probe as deep into Neptune’s atmosphere, implying that the ~100 m/s difference in Neptune’s zonal wind profile between filters is due to looking at different altitudes. Credit: Joshua Tollefson

Voyager 2 took the first measurements of Neptune’s zonal wind profile. Subsequent observations of Neptune with the Keck telescope have provided the latest zonal wind measurements. Significant wind speed differences of up to 100 m/s (225 mph) between zonal wind profiles taken in different filters and wavelengths have been observed at Neptune’s equator. These wind speed differences may be due to the planet changing over time, or due to observing in different spectral windows.

For the first time, scientists were able to rule out the former explanation. By observing Neptune in two filters, alternating back-and-forth between the two over a single night, they were able to construct a set of images in both filters that minimized cloud motions between frames in each filter. Since each filter probes distinct altitudes on Neptune, the researchers concluded that the 100 m/s differences they saw must be because the wind speeds change with depth.

These vertical differences in wind speeds provide many clues about the structure of the surrounding atmosphere and what drives its dynamics. Pressure gradients due to latitudinal temperature or compositional variations are necessary to drive winds. The magnitude of the vertical wind speed difference is mathematically linked to the strength of these variations. Clouds are most prominent at Neptune’s mid-latitudes, suggesting those regions are much colder and richer in condensible methane than the equator. Scientists assumed that this picture extended vertically throughout Neptune’s atmosphere. However, the observed 100 m/s vertical wind speed difference appears to contradict this finding.

In their recent paper, the authors argue that below the observed cloud deck, the above picture may be inverted. They conclude that if the equator is warmer than the mid-latitudes, consistent with mid-infrared observations of the planet, methane should be depleted by up four times as much at the mid-latitudes compared to the equator. This finding also has implications for how the atmosphere circulates and distributes energy. Methane-rich regimes are consistent with rising air, which adiabatically expands and cools into condensed cloud particulates. Conversely, methane-poor locations imply downwelling motions. Much like Hadley cells on Earth, this forms a global circulation model for Neptune’s atmosphere. But, this study argues that Neptune’s atmosphere is structurally complicated. It is uncertain how many cells there are, if there are circulation cells stacked on top of each other, and just how deep they go.

Ground-based observations and the Hubble Space Telescope have been the prevailing way of studying Neptune’s cloudy atmosphere ever since Voyager flew by in the late 1980s. Recently, Neptune has been a host of unusual stormy activity. A gigantic storm in Neptune’s normally dark and quiet equatorial region was discovered in the summer of 2017 with Keck [3], and a brand new dark spot, only the fifth ever seen, was spotted with the Hubble Space Telescope in September 2015 [4] (Figure 2).

Figure 2 — Dark vortices on Neptune lie at deeper levels of the atmosphere, below the bright methane ice clouds such as those seen in the Keck data. Size, shape, oscillations, and drifts of these vortices give clues to the dynamics of these deeper layers, but deciphering these clues requires knowledge of the zonal winds, and thus the wind shear conditions in the vortex environments. Left: NASA, JPL (https://photojournal.jpl.nasa.gov/catalog/PIA00052). Right: NASA, ESA, M.H. Wong, J. Tollefson https://hubblesite.org/news_release/news/2016-22/107-illustrations).

What forms these features, how deep they extend, and what they are made of is currently unknown. NASA has identified Uranus and Neptune as potential targets for a new flagship spacecraft mission in the upcoming decades. Another close up of Neptune would provide a high-resolution glimpse at these intriguing cloud systems and help settle these unanswered questions.

These findings are described in the article entitled Vertical wind shear in Neptune’s upper atmosphere explained with a modified thermal wind equation, recently published in the journal Icarus. This work was conducted by Joshua Tollefson, Imke de Pater, Philip S. Marcus, and Michael H. Wong from the University of California, Berkeley, Statia Luszcz-Cook from the American Museum of Natural History, Lawrence A. Sromovsky and Patrick M. Fry from the University of Wisconsin, Madison, and Leigh N. Fletcher from the University of Leicester.

References:

1. J.C. Pearl, B.J. Conrath., The albedo, effective temperature, and energy balance of Neptune, as determined from Voyager data. JGR Space Physics 96, S01. pp. 18921-18930 (1991). https://doi.org/10.1029/91JA01087
2. J. Tollefson, et al., Vertical wind shear in Neptune’s upper atmosphere explained with a modified thermal wind equation. Icarus 311, 1. pp. 317-339 (2018). https://linkinghub.elsevier.com/retrieve/pii/S001910351630210X
3. http://www.keckobservatory.org/new_storm_makes_surprise_appearance_on_neptune/
4. Wong et al., A new dark vortex on Neptune. Astrophysical Journal 155, 3. pp. 117-125 (2018). https://linkinghub.elsevier.com/retrieve/pii/S001910351630210X