Jupiter’s Great Red Spot – the largest and longest-persisting storm in the solar system – may provide the energy required to heat the planet’s upper atmosphere to the unusually high values observed. So says an international team of astronomers, who believe they have found that the atmosphere above the storm is hundreds of degrees hotter than anywhere else on the planet.
Thanks to our relative proximity to the Sun, the Earth is efficiently warmed by solar energy. Indeed, both the surface and the atmosphere of our planet is heated by sunlight, up to altitudes as high as 400 km, where the International Space Station orbits. On the other hand, Jupiter is more than five times more distant from the Sun, and yet its upper atmosphere has average temperatures that are comparable to those of Earth. Indeed, the temperatures of the upper atmospheres of all of the solar system’s giant planets are much hotter than would be expected if the Sun were their only heat source. This anomaly was first noticed nearly 40 years ago, and has since been dubbed the giant-planet “energy crisis”. But an actual energy source, beyond sunlight, has evaded scientists.
Hot at the poles
For Jupiter specifically, it is known that the planet’s powerful aurorae at the poles impart nearly 200 GW per hemisphere into the atmosphere, which explains the temperatures measured in those areas. However, for the low to mid-latitudes of Jupiter, no such source has been identified. This is despite the fact that the temperature measured there is nearly 800 K, which is 600 K warmer than it should be if the planet was merely heated by the Sun.
Now though, thanks to new observations of the Jovian atmosphere above the Great Red Spot (GRS), James O’Donoghue and Luke Moore at Boston University in the US, together with Tom Stallard and Henrik Melin at the University of Leicester in the UK, suggest that it is the storm driving the planet’s atmospheric heating. After ruling out solar heating from above, the team “designed observations to map the heat distribution over the entire planet in search for any temperature anomalies that might yield clues as to where the energy is coming from”, says O’Donoghue.
Stormy coincidence or clue?
In December 2012, the team observed Jupiter for nine hours using the SpeX spectrometer on the NASA Infrared Telescope Facility. A Jovian day itself is rather short – a measly 9 hours and 56 minutes – thanks to the planet’s quick spin. The planet-wide infrared emissions that the team observed showed that much-higher-than-expected temperatures exist at high altitudes. “We could see almost immediately that our maximum temperatures at high altitudes were above the Great Red Spot far below,” says O’Donoghue, adding that the team immediately wondered if this was a “weird coincidence or a major clue”?
The GRS storm has been raging for more than three centuries – while its size and colour has fluctuated over time, it is so big it could swallow Venus, and has winds that take six days to complete one spin. The atmosphere above the storm is on average 1600 K hotter than anywhere else on the planet, including the auroral region. Until their recent observations, there was no evidence that linked this huge powerhouse to the heated upper atmosphere, according to the researchers.
“Energy transfer to the upper atmosphere from below has been simulated for planetary atmospheres, but not yet backed up by observations,” O’Donoghue says. “The extremely high temperatures observed above the storm appear to be the ‘smoking gun’ of this energy transfer, indicating that planet-wide heating is a plausible explanation for the ‘energy crisis’,” he adds. The researchers concluded that, if sufficient heating does not come from above (i.e. via the Sun) and there is no heating in situ via magnetospheric interactions (like with the aurorae), then by a process of elimination, it must come from below.
As to the mechanism via which this energy is imparted, O’Donoghue and colleagues deduce that the storm produces acoustic waves – which are vertically propagated from the lower atmosphere – that deposit their energy, via viscous dissipation, into the upper atmosphere. Acoustic waves are routinely produced above thunderstorms and this kind of effect has been observed on Earth above the Andes mountains. Indeed, this method has been previously suggested for Jupiter, with models suggesting that hundreds of degrees of heating could be imparted this way. But this is the first direct observation of a localized source of heating, which clearly links the upper and lower atmosphere, according to the researchers.
“This fantastic result, showing how the upper atmosphere is heated from below, was produced directly from Leicester’s 2012 observing campaign, which was designed to try and answer why Jupiter’s upper atmosphere is so hot,” says Stallard. He adds that the Juno mission, which reached the planet earlier this month, “will be measuring the aurora and its sources, and we expected the auroral energy to flow from the pole to the equator. Instead, we find the equator appears to be heated from plumes of energy coming from Jupiter’s vast equatorial storms.”
The research is published in Nature.