The Great Red Spot on Jupiter is one of the most recognizable features in the Solar System: a massive hurricane-like oval-shaped storm in the atmosphere of the gas giant planet. It has been observed by telescopes and spacecraft for centuries. How a storm much larger than Earth could persist for so long has been mystery to astronomers, but something is changing: The Spot is shrinking. Recent images from the Hubble Space Telescope now show it at the smallest size ever seen.
This gradual shrinking has actually been known about since the 1930s, but the new Hubble images are the best illustration to date of how much the Spot has decreased in size over these past several decades. The earliest documented historical observations, from the 1800s (with other possible mentions from as far back as the 1600s), showed that the Spot was about 25,000 miles (41,000 kilometres) across the widest point of the oval. By the time the Voyager spacecraft flew past Jupiter in 1979 and 1980, measurements indicated the Spot had shrunk down to 14,500 miles (23,335 kilometres).
A great paragraph from the book “NASA’s Voyager Missions: Exploring the Outer Solar System and Beyond” (Ben Evans with David Harland, Springer-Praxis 2003) gives an eloquent overview of astronomers’ fascination with the Spot:
“Of all the storms and clouds in the atmosphere, none is more prominent or long-lasting than the famous Great Red Spot, a colossal hurricane, twice the width of Earth, which has unleashed its fury in the southern hemisphere since at least the 17th century. Voyager time-lapse movies of the spot revealed it to rotate counter-clockwise, although the clouds within it moved at various speeds. Those clouds nearest to its outermost edge were found to complete a full lap of the spot in 4-6 days, whilst material closer to the centre moved more slowly and erratically. The spot’s discovery is usually attributed to either Giovanni Cassini – the celebrated Franco-Italian astronomer – or English scientist Robert Hooke, more than 300 years ago. In 1664, Hooke wrote in the Philosophical Transactions of the Royal Society about his discovery of ‘ … a spot in the largest of the three observed belts of Jupiter and that, observing it from time to time, within two hours after, the said spot had moved east to west about half of the diameter … its diameter is one-tenth of Jupiter … ‘ Whether Hooke’s discovery and the spot of today are one and the same is unclear, but certainly Cassini made similar observations in 1665 and the object was seen intermittently until about 1713. After this, there was a lull for more than a century, until German astronomer Heinrich Schwabe saw it in 1831. Since then, although it has faded from view occasionally and changed dramatically in both size and colour – extending to 40,000 km (almost twice its current length) at one stage and displaying an astonishing brick-red hue at another – it has nevertheless remained Jupiter’s constant, faithful companion.”
(As readers here will know, Ben Evans is also senior writer for AmericaSpace).
But now the new observations from Hubble show the Spot to be at its smallest ever seen. According to Amy Simon of NASA’s Goddard Space Flight Center in Maryland, “Recent Hubble Space Telescope observations confirm that the spot is now just under 10,253 miles (16,500 kilometres) across, the smallest diameter we’ve ever measured.”
Other amateur observations since 2012 indicate that the shrinking rate may also be increasing, with the Spot getting smaller by just under 1,000 kilometres per year.
The Spot is often compared to hurricanes on Earth, although that may not be exactly the case. Also from the same book (2003):
“Today’s spot is distinctly oval, roughly 26,000 km along its east-west axis and 13,000 km along its north-south axis, and the counter-clockwise motion of its winds (some of which reach 400 km/h at the outermost edges) suggest that, far from being a Jupiter-sized version of a low-pressure Caribbean hurricane, it is actually a high-pressure region whose cloud tops are significantly colder and higher than surrounding areas.”
Of course, the big question is why the Spot is shrinking to begin with. Is it simply slowly dissipating as storms do on Earth, but at a much slower rate? Another observed phenomenon may provide a clue:
“In our new observations it is apparent that very small eddies are feeding into the storm,” said Simon. “We hypothesised that these may be responsible for the accelerated change by altering the internal dynamics of the Great Red Spot.”
If this shrinking continues, as seems likely, how much longer will the Spot last? Astronomers don’t know yet, but there will be another chance to see the Spot up close when the Juno spacecraft arrives at Jupiter on July 5, 2016. Juno will study in depth Jupiter’s atmosphere, magnetosphere, interior, and origins. It should be able take detailed measurements of the Spot as it examines “the global structure and motions of the planet’s atmosphere below the cloud tops for the first time, mapping variations in the atmosphere’s composition, temperature, clouds and patterns of movement down to unprecedented depths.”
Jupiter’s Great Red Spot is an amazing marvel of nature and an unprecedented opportunity to study alien storms in the atmospheres of gas giant planets, the likes of which are never experienced on Earth.
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