A storm that has been looming within the clouds of Neptune for two years is shrinking right before NASA’s Hubble Space Telescope’s very lens. A dark, tempestuous storm, which was once long enough to stretch from the United States to Portugal, is slowly shrinking and disappearing according to Hubble’s latest pictures.
NASA’s Voyager 2 spacecraft first discovered storms on the face of Neptune in the 1980s, as it flew past the enormous blue planet on its way to the interstellar medium. Now, Hubble has taken up the task of observing such features, this is because the telescope’s capabilities in blue light are great enough to track storms that continue to disappear and reappear. Hubble detected two dark storms that appeared in the mid-1990s, and have vanished from existence since. The latest storm, which was first observed in 2015, has been caught shrinking.
Similar to Jupiter’s Giant Red Spot (GRS), these storms behave in an anti-cyclonic manner. This means the storms flow in an opposite direction to the flow of its surrounding region. These types of storms bring vast amounts of hidden material, buried within icy atmosphere, to the visible face. Opportunities like this give astronomers a rare chance to study Neptune’s deep winds, which can’t be directly measured.
According to astronomers, the dark spot material may be hydrogen sulphide, giving off the extremely dreadful smell of rotten egg. “The particles themselves are still highly reflective; they are just slightly darker than the particles in the surrounding atmosphere,” explains Joshua Tollefson from the University of California at Berkeley, United States.
The fact that these storms disappear relatively quickly make them extremely different to Jupiter’s GRS, which has been visible for over 200 years. This is the first time a storm has actually been pictured during its decline. “We have no evidence of how these vortices are formed or how fast they rotate,” says Agustín Sánchez-Lavega from the University of the Basque Country, Spain. “It is most likely that they arise from an instability in the sheared eastward and westward winds.”
“It looks like we’re capturing the demise of this dark vortex, and it’s different from what well-known studies led us to expect,” says Michael H. Wong of the University of California at Berkeley. “Their dynamical simulations said that anticyclones under Neptune’s wind shear would probably drift toward the equator. We thought that once the vortex got too close to the equator, it would break up and perhaps create a spectacular outburst of cloud activity.”
The observed dark spot, which was first spotted midway between the planet’s equator and south pole, has apparently faded away rather than vanishing in an explosive fashion. This could be related to the surprising direction of its measured drift toward the south pole, as opposed to the drift northward towards the equator. Where Jupiter’s GRS is kept in shape by its numerous alternating win jets, recognisably seen as different coloured bands, Neptune’s storms have more freedom to move around. There are only three broad jets, a westward one at the equator and eastwards ones around the north and south poles, leaving the vortex with the right to roam between these jets.
“No facilities other than Hubble and Voyager have observed these vortices. For now, only Hubble can provide the data we need to understand how common or rare these fascinating neptunian weather systems may be,” says Wong.
Hubble gained the first images of the storm as part of the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project that captures global maps of each the Solar System’s Jovian planets each year. Hubble is also the only space telescope to observe an object in ultraviolet, allowing astronomers to learn more about an object from light that is invisible to our eyes.
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