A second Great Spot has been discovered on Jupiter by University of
Leicester astronomers, rivalling the scale of the planet’s famous Great
Red Spot and created by the powerful energies exerted by the great
planet’s polar aurorae.
Dubbed the ‘Great Cold Spot’, it has been observed as a localised dark
spot, up to 24,000 km in longitude and 12,000 km in latitude, in the gas
giant’s thin high-altitude thermosphere, that is around 200K (Kelvin)
cooler than the surrounding atmosphere, which can range in temperature
between 700K (426ºC) and 1000K (726ºC).
The results are published in Geophysical Research Letters.
Dr Tom Stallard, Associate Professor in Planetary Astronomy and lead
author of the study, said: “This is the first time any weather feature
in Jupiter's upper atmosphere has been observed away from the planet's
“The Great Cold Spot is much more volatile than the slowly changing
Great Red Spot, changing dramatically in shape and size over only a few
days and weeks, but it has re-appeared for as long as we have data to
search for it, for over 15 years. That suggests that it continually
reforms itself, and as a result it might be as old as the aurorae that
form it - perhaps many thousands of years old.”
The Great Cold Spot is thought to be caused by the effects of the
magnetic field of the planet, with the massive planet’s spectacular
polar aurorae driving energy into the atmosphere in the form of heat
flowing around the planet.
This creates a region of cooling in the thermosphere, the boundary layer
between the underlying atmosphere and the vacuum of space. Although we
can't be sure what drives this weather feature, a sustained cooling is
very likely to drive a vortex similar to the Great Red Spot.
The astronomers used the CRIRES instrument on the Very Large Telescope (VLT)
to observe spectral emissions of H3+, an ion of hydrogen present in
large amounts in Jupiter’s atmosphere, which allowed the scientists to
map the mean temperature and density of the planet’s atmosphere. They
then used images of H3+ emission from Jupiter’s ionosphere taken by
NASA’s InfraRed Telescope Facility between 1995-2000 to compare.
Through combining images taken over a period of time, including over
13,000 images taken over more than 40 nights by the InfraRed Telescope
Facility, the astronomers revealed the presence of the Great Cold Spot
as an area of darkness amongst the hot environment of Jupiter’s upper
Dr Stallard, who is funded by the Science and Technology Facilities
Council, added: “What is surprising at Jupiter is that, unlike weather
systems on Earth, the Great Cold Spot has been observed at the same
place across 15 years. That makes it more comparable to weather systems
in Jupiter's lower atmosphere, like the Great Red Spot.
“Observations and modelling of Earth's upper atmosphere have shown that,
on the short term, there may be changes in the temperature and density
of the upper atmosphere.
two main differences are firstly that Earth's aurora sees dramatic
changes caused by activity from the Sun, whereas Jupiter's aurora are
dominated by gases from the volcanic moon Io, which are relatively slow
and steady, and secondly that the atmospheric flows generated by Earth's
aurora can drive heat quickly across the whole planet, making the upper
atmosphere ring like a bell, while Jupiter's fast spin traps this energy
nearer the poles.”
Dr Stallard added: “The detection of the Great Cold Spot was a real
surprise to us, but there are indications that other features might also
exist in Jupiter's upper atmosphere. Our next step will be to look for
other features in the upper atmosphere, as well as investigating the
Great Cold Spot itself in more detail.
“The Juno spacecraft is currently in orbit around Jupiter and the
observations of Jupiter's aurora and upper atmosphere by the JIRAM
instrument that have been released so far already provide a wealth of
new information about the planet. When combined with our ongoing
campaign of observations using telescopes on Earth, we hope to gain a
much better understanding of this weather system in the next few years.”