Microwave measurements provide a window into the atmospheric processes of this strange seventh planet. Uranus doesn't look like much when compared to Jupiter's irregular, turbulent surface and Saturn's beautiful lines and unique rings. The featureless pale blue orb in the upper left image captured by Voyager 2 as it passed by the ice giant in 1986 didn't quite impress the public. Fortunately, there was no shortage of information about the rings and moons of Uranian to grab everyone's attention.
However, it turns out that being “boring” is only up to a point. Uranus' thick layer of high-altitude methane clouds can be seen in a visible-light photograph taken by Voyager 2. Researchers can explore deeper into objects by photographing at longer wavelengths, and have discovered that Uranus is more complex than it might seem at first glance.
Alex Akins of Caltech and his colleagues used the Very Large Array, consisting of 0,7 radio dishes spread over tens of kilometers across the high plains of New Mexico, to scan Uranus at wavelengths ranging from 5 cm to 28 cm. They also discovered a clearly visible cyclone churning around the planet's north pole.
There are polar cyclones on Earth. They have also been seen on every planet with an atmosphere in the solar system, including Jupiter, Venus, and Venus. The significance of the new observation stems from the unique orbital geometry of Uranus. Unlike most other planets whose spin axes are at least somewhat in line with their orbital axes, Uranus appears to be tilted with its poles alternately pointing directly at the Sun.
As a result, Uranus defies a basic rule of local climate on Earth, which states that the poles are colder than the equator because they receive less direct sunlight. It is unclear how this distinction affects or should affect atmospheric circulation.
Uranus's ability to be observed by astronomers is also affected by its orbital configuration. No spacecraft other than Voyager 2 has visited the planet, and terrestrial observatories can only see the side illuminated by the Sun. Thus, each pole of Uranus spends 84 years completely out of sight during its 42 Earth-year orbit. The last Uranian equinox occurred in 2007, and the north pole is visible from Earth for the first time since 1965 in the current northern hemisphere spring.
It's also the first opportunity to get a proper look at both poles since the south pole disappeared in the late 1990s and early 2000s.
These decades saw significant advances in microwave astronomy, particularly the Very Large Array, which received a significant upgrade in 2012. Thus, the north polar region is more deeply depicted in recent photographs of Uranus; one of them is shown above right.
It is nothing new to see a sizable bright patch to the right of the image, which corresponds to latitudes roughly north of 45°. It has long been known that the yet unexplained interplay of pressure, temperature, and chemical composition causes both poles of Uranus to shine brighter than its equator at long wavelengths.
A new feature is that the bright spot directly at the pole is surrounded by a slightly darker collar. The researchers conclude that these features are due to the temperature-pressure pattern of the core of the polar cyclone, similar to the eye of a hurricane on land.
The dynamics of the uranium atmosphere are still largely unexplored. But watching the cyclone as northern spring turns into summer may reveal important new insights into how a polar atmosphere that has spent 42 years in darkness works. Akins and his colleagues have already noticed signs that the cyclone may have strengthened during its time on the Sun, though it's too early to say for sure.
Source: Physics Today – Johanna L. Miller
📩 20/06/2023 17:15