The Winds and Clouds of Neptune

Neptune, it seems, gets little attention. Dwarf planet Pluto, which occasionally crossed the orbital path of Neptune and is closer to the sun than the ice giant, has certainly been in the news frequently in recent days. Neptune itself seems to draw little interest. It is, however, an interesting planet in its own right.
The four outer planets (Jupiter, Saturn, Uranus and Neptune) all feature interesting clouds. Jupiter is known for its Great Red Spot, while Saturn’s Great White Spots stay in the metaphorical shadow of the planets majestic rings. Uranus also has spots, some dark, some light.
Neptune has bands. It also has two spots, the Great Dark Spot and Dark Spot 2.
An image of Neptune's cloud bands from 1989, from the Voyager 2 spacecraft. Image credit NASA/JPL.
An image of Neptune’s cloud bands from 1989, from the Voyager 2 spacecraft. Image credit NASA/JPL.
The atmospheres of Jupiter, Saturn and Uranus are all fairly stable. Neptune, on the other hand, has shown some changes since Voyager 2 imaged it in 1989.  This difference from the other outer planets makes it of special interest.
This brings us to today’s featured article, “Neptune’s zonal winds from near-IR Keck adaptive optics imaging in 2001” written by Shuleen Chau Martin, Imke de Pater, and Philip Marcus. Published in 2011 in Astrophysics and Space Science, the title can be a bit of a mouthful if you are not familiar with the terminology. Zonal winds are the east-west winds, and near-IR says that it was imaged in the near infrared, in this instance, with adaptive optics at the Keck Observatory in Mauna Kea in Hawai’i. The images were taken in 2001. More specifically, they were taken on August 20th and 21st, followed by more images on September 1st. Neptune was observed for four hours each night in August, and fifty minutes in September. The exposure time for each image was sixty seconds. The first night produced 58 images, the second 76, and the follow up observations in September gave 16 images.
So, what was done with all these images? The answer is that cloud features were tracked. By tracking the same cloud over time, one can derive the speed that cloud is travelling at, as well as any changes that might occur. The authors observed thin bands of bright clouds in the mid latitudes. Latitude is east-west, in line with the equator. There was a band of clouds at 27 degrees north, and above 60 degrees north was not visible due to the tilt of Neptune with respect to their vantage point on Earth. The brightest bands were between 22 and 48 degrees south. Near the equator, some clouds appeared and vanished again within minutes. Near 50 degrees south, the speed of the clouds were about the same as Neptune’s rotational rate (the speed at which Neptune turns on its axis). However, this was the only area where the speeds were the same.
Voyager 2 did not see any bands at the mid latitudes, and almost no clouds at 30 degrees south. The cloud pattern has clearly changed, unlike the nearly entirely stable atmosphere’s of Neptune’s counterparts.
An image of Neptune from Voyager 2, with the Great Dark Spot appearing prominently. Image credit NASA.
An image of Neptune from Voyager 2, with the Great Dark Spot appearing prominently. Image credit NASA.
Interestingly enough, some features at the same latitude do not go at the same speed. The authors tracked a total of nearly 200 cloud features and found that the rotational rates of these features, even in the same latitude, varies as much as five hours from each other, which translates to about 500 meters/second (1118 mph) in the case of Neptune. There were gaps of time in between some images taken, so it is possible that what was thought to be one cloud being tracked was an entirely different cloud, and data is inherently imperfect with margins of errors, but the impressive spread in speeds does appear to be quite real.
Also of note is that 17 of  the tracked southern mid latitude features seem to oscillate, or move back and forth in a regular pattern. This could be an illusion from the data, as there are multiple things that can affect the data, such as varying brightness in the clouds, or pixel-to-pixel differences in the camera. However, two facts support the oscillations being real; (1) it was 17 features, not just a few and (2) they correlate to the timing of tidal forces that would be placed on Neptune’s atmosphere due to Neptune’s largest moon, Triton. Tidal forces are caused by gravity, and the fact that gravity will pull more on the closer side of an object (such as Neptune) than the far side, as gravity drops off as distance between bodies increases. If you think Triton would not have an effect, think again; our moon is primarily responsible for the tides of the Earth’s oceans! While the data is inconclusive, the timing of the oscillations and of Triton is highly suspicious. As often is the case in science, more research needs to be done. The authors suggest looking at different phases of Triton (it has phases the same way our moon does) and seeing if this has any correlation to the behavior of Neptune’s clouds.
Often in science, learning one thing (like the variations of speed and position in Neptune’s clouds) lead to more questions (such as what is causing these variations). There’s always something more to investigate. With ever-present questions, Neptune will have some more attention coming its way.

 

Bibliography
Martin, Shuleen Chau, Imke de Pater, and Philip Marcus. “Neptune’s zonal winds from near-IR Keck adaptive optics imaging in August 2001.” Astrophysics and Space Science 337.1 (2012): 65-78.
The main article for today.
National Aeronautics and Space Administration. “Neptune: Overview.” Solar System Exploration. N.p., n.d. Web.
A NASA webpage about Neptune. This page, and the navigation tabs, were used to information about Neptune, and were also the source of the images used in the article.
US Department of Commerce, National Oceanic and Atmospheric Administration. “Tides and Water Levels: What Causes Tides?.” N.p., 28 March 2008. Web.
A webpage discussing the causes of ocean tides on Earth.
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