How Deep is the Great Red Spot? (Planetary Science)

Two studies just published in Science, both based on data obtained with the Juno probe, offer – through complementary methods – estimates on the depth of Jupiter’s iconic vortex. One of the two managed to “push” up to 500 km using gravity measurements. We interviewed the first author, Marzia Parisi, a researcher from Rome today in California, at NASA’s Jet Propulsion Laboratory

The Great Red Spot of Jupiter – the iconic vortex that has shaken the planet’s atmosphere for centuries – is the largest storm in the entire solar system: it has an extension of over 16 thousand km, well over the diameter of the Earth. But how deep is it? Two studies published today in Science , conducted using data collected – with two complementary techniques – through NASA’s Juno probe , orbiting Jupiter since 2016, have now managed to provide a reliable estimate. And one of the two is led by an Italian researcher, Marzia Parisi .

Born and raised in Rome – “which I miss a lot”, she confesses to Media Inaf – Parisi earned her doctorate at Sapienza University with Professor Luciano Iess, principal investigator  of Juno’s KaT radioscience tool, before joining NASA – at Jet Propulsion Laboratory, in California, where it is located today – thanks to the participation, since the time of the doctorate, in missions of ESA and of NASA itself. His passions are languages ​​and reading, as well as science, of course, and if you are wondering, well no: he is not related to the recent Nobel Prize winner Giorgio Parisi, “But I am very happy to share his surname”. Its result – complementary to that of the team of colleague Scott Bolton, who used the microwave data collected with the Mwr instrument to estimate the depth of the Great Red Spot – was obtained with gravity measurements, thus exploiting the same Juno probe. as a sort of “sensor” sensitive to the fluctuations in the gravitational field of the planet caused by the storm.

Parisi, how can gravity be measured through a space probe which is Juno? Do you observe the way its orbit changes?

«From Earth we record the Doppler signal (therefore a frequency variation) of radio signals in the X band and Ka band – the latter supplied by the Italian Space Agency and produced by Thales Alenia Space. The Doppler signal is directly related to fluctuations in the probe’s velocity along the line of sight – that is, the line that connects the probe to the antenna on Earth. These variations are, in turn, linked to the gravity of the planet, through the equations of state ».

Artist’s impression of Juno in orbit around Jupiter. Credits: Nasa / Jpl-Caltech

What accuracy can you achieve? I read of a noise of just 5-10 micrometers per second: it seems incredible …

«The uncertainties on the radial velocity are just of the order of 10 micrometers per second. Consider that the relative speed of the probe is of the order of tens of km per second. So these accuracies are quite phenomenal, and are partly due to the quality of the Italian instrument ».

What kind of gravitational effect does the Great Red Spot exert on Juno? Does it attract more or less than the other areas of the Jovian atmosphere?

“It’s hard to say whether the Great Red Spot attracts more or less when the spacecraft flies over it, for geometry reasons. What I can say is that we see an excess of mass near its surface, and a mass defect at depth (hundreds of km below the Great Red Spot). This is due to the strong winds and the conditions of the atmosphere surrounding the vortex, in fact the pressure and density tend to increase with depth ».

The Great Red Spot in detail, taken by Juno during the 11 July 2017 flyover. Credits: Nasa / Swri / Msss / Gerald Eichstädt / Seán Doran

What are the pros and cons of your Great Red Spot depth measurement method versus Scott Bolton and colleagues’ microwave-based one?

«I would say that the two techniques cannot be compared, as they measure fundamentally different and complementary things. Theirs is a direct measurement of the ” brightness temperature ” on 6 channels, one of which can see up to a depth of about 350 km. In contrast, we with gravity can go much deeper with our measurements – up to 500 km but even beyond ».

Eventually you get that the Great Red Spot is between 200 and 500 km deep. It’s a lot? Is it little?

“It depends on the point of view. Reading various articles in the past I got the impression that most scientists expected the Great Red Spot to be very thin (we are talking about tens of km and not hundreds), so – if you take this point of view – 500 km represents a very deep vortex. On the other hand, if one compares the thickness with the radius of Jupiter (70 thousand km), we are clearly talking about a shallow phenomenon limited to the upper area of ​​the atmosphere “.

But how did you get the idea? And did he expect it, to be able to obtain information about the Great Red Spot using gravity measurements?  

‘The idea was born during brain storming sessions with my postdoctoral supervisor in Israel, Professor Yohai Kaspi. And yes, I have always had faith in the fact that we would be able to isolate the signal of the Great Red Spot, even if it is very elusive ».


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