Science

Great Red Spot Getting Taller as It Shrinks, Planetary Scientists Say

Planetary scientists have noticed that Jupiter’s most distinctive feature — the Great Red Spot (GRS) — has been getting smaller in area over time. Because of this, many expected to see the wind speeds inside the GRS increasing as the storm was shrinking. But surprisingly, this isn’t the case — the wind speeds aren’t changing; instead, the GRS is actually growing taller, and is deepening in color as well.

This enhanced-color image of the Great Red Spot was created by citizen scientists Gerald Eichstaedt and Sean Doran using data from the JunoCam imager on NASA’s Juno spacecraft. Image credit: NASA / SwRI / MSSS / Gerald Eichstaedt / Sean Doran.

This enhanced-color image of the Great Red Spot was created by citizen scientists Gerald Eichstaedt and Sean Doran using data from the JunoCam imager on NASA’s Juno spacecraft. Image credit: NASA / SwRI / MSSS / Gerald Eichstaedt / Sean Doran.

The GRS is a spinning, cyclone-like storm just south of Jupiter’s equator. Its reddish color is likely a product of chemicals being broken apart by solar UV light in the gas giant’s upper atmosphere.

Measuring in at 10,000 miles (16,000 km) in width, the storm is 1.3 times as wide as Earth.

The GRS has been monitored since 1830 and has possibly existed for more than 350 years. In modern times, the storm has appeared to be shrinking.

“Storms are dynamic, and that’s what we see with the GRS,” said Dr. Amy Simon, an expert in planetary atmospheres at NASA’s Goddard Space Flight Center.

“It’s constantly changing in size and shape, and its winds shift, as well.”

Dr. Simon and co-authors drew on the rich archive of historical observations and combined them with data from NASA spacecraft, starting with the two Voyager missions in 1979.

In particular, the researchers relied on a series of annual observations of Jupiter that they have been conducting with the NASA/ESA Hubble Space Telescope as part of the Outer Planets Atmospheres Legacy (OPAL) project.

They traced the evolution of the GRS, analyzing its size, shape, color and drift rate. They also looked at the storm’s internal wind speeds, when that information was available from spacecraft.

The findings indicate that the GRS recently started to drift westward faster than before.

The storm always stays at the same latitude, held there by jet streams to the north and south, but it circles the globe in the opposite direction relative to the planet’s eastward rotation.

Historically, it’s been assumed that this drift is more or less constant, but in recent observations, the scientists found the spot is zooming along much faster.

The study confirms that the GRS has been decreasing in length overall since 1878 and is big enough to accommodate just over one Earth at this point. But the historical record indicates the area of the spot grew temporarily in the 1920s.

“There is evidence in the archived observations that the GRS has grown and shrunk over time. However, the storm is quite small now, and it’s been a long time since it last grew,” said co-author Professor Reta Beebe, of New Mexico State University in Las Cruces.

Because the storm has been contracting, the team expected to find the already-powerful internal winds becoming even stronger. Instead of spinning faster, the storm appears to be forced to stretch up.

In the case of the GRS, the change in height is small relative to the area that the storm covers, but it’s still noticeable.

The storm’s color has been deepening, too, becoming intensely orange since 2014.

Scientists aren’t sure why that’s happening, but it’s possible that the chemicals which color the storm are being carried higher into the atmosphere as the spot stretches up.

At higher altitudes, the chemicals would be subjected to more UV radiation and would take on a deeper color.

The findings appear in the Astronomical Journal.

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Amy A. Simon et al. 2018. Historical and Contemporary Trends in the Size, Drift, and Color of Jupiter’s Great Red Spot. AJ 155, 151; doi: 10.3847/1538-3881/aaae01

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