Carl Sagan and André Brahic are unfortunately no longer with us to comment on the discoveries made on the giant planets of the Solar System but their colleagues continue to explore these worlds. If the gasses that are Jupiter and Saturn are much better known to us now after the missions Juno and Cassini, the frozen ones – discovered by Herschel (Uranus) and Le Verrier (Neptune) – still hold many mysteries.
Only the Voyager 2 probe approached Uranus and Neptune, discovering in the case of the latter a meteorology much more active than expected for a planet so far from Sun (therefore receiving much less sound energy than Saturn), possessing like it a spot, black this time, indicative of an enormous storm planetary anti-cyclonic.
NASA’s Hubble Space Telescope has captured time-lapse images of a large dark storm on the vanishing Neptune. A recent Hubble program called Outer Planets Atmosphere Legacy, or OPAL, provides annual global maps of our gas giant planets, allowing planetary scientists to see changes in formations such as Neptune’s dark storms. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © NASA Goddard Space Flight Center, Katrina Jackson
planetary scientists have therefore been content since the 1980s to continue their study of Uranus and Neptune using telescopes on the ground or with Hubble in orbit. We see a new illustration of this with an article published in the Journal of Geophysical Research: Planetsbut also in free access on arXivshowcasing the work of an international team led by patrick irwinProfessor of physical planetary at Oxford University.
This work is based on analyzes of archival data covering several years first collected with Hubble, which were obtained with its spectro-imager (STIS) and ranging fromultraviolet infrared, as well as the many images that Hubble has taken of Uranus and Neptune with the mythical Wide Field Camera 3 (WFC3). Data collected with the telescope Gemini North and theInfrared Telescope Facility (IRTF) of the Nasa also proved valuable.
These observations have fed scholarly models of radiative transfer in atmospheres of the two giants (and of these atmospheres themselves) largely made up of ice and which now allow astronomers better understand why with masses and very similar rays, and equally similar compositions and structures, Uranus and Neptune still have different colors.
Layers of aerosol hazes and different turbulent atmospheres
Initially, planetary scientists were just looking to develop a model that would help understand the clouds and the mists in the atmospheres of ice giants. But, as Mike Wong, an astronomer at the University of California at Berkeley and member of the team behind the discovery of the colors of Uranus and Neptune, specifies in a press release, “ explaining the color difference between Uranus and Neptune was an unexpected bonus! “.
Irwin explains that ” it is the first model to simultaneously take into account the observations of the light reflected solar wavelengths ultraviolet to near infrared. It is also the first to explain the visible color difference between Uranus and Neptune. “.
As explained in more detail in the diagrams below, the researchers’ new model involves three layers of aerosol haze (recall that a aerosol is a suspension of fine droplets or particles in a gasfor example in the form of smoke or fog) at different heights in the atmospheres of each planet. The middle layer haze particles, just above the level of condensation methane, turns out to be thicker on Uranus than on Neptune and this is what affects the visible color of the two planets.
As in the case of the explanation of the blue color of the sky and the white color of the clouds on Earth, planetary scientists have taken into account in their models the Rayleigh and Mie light scattering effectsnamed after physicists involved in explaining these effects more than a century ago.
Jet streams only affect Neptune and Uranus on the surface
Article of Laurent Sacco published on 05/23/2013
The jet streams on Neptune and Uranus would be confined to their surface, according to a group of planetary scientists. This is the conclusion reached by the researchers, indirectly studying the gravitational field generated by the two gas giants. The method they used should be applicable to Jupiter in a few years.
The conquest of space has allowed the birth of a comparative planetology, where the tools of internal and external geophysics, tested on Earth, are used to try to understand the geology and the climatology from other planets Solar system. In this way it is possible to learn more about our planet itself.
You cannot vary the earth mass, its composition or its temperature as one would do in the laboratory to better understand a physical system. But’universe is responsible for carrying out these experiments with the various stars of the solar system. We can thus transpose the models of the climate on earth at March and Venusto see if they make it possible to predict, via simulations, the meteorology of the planets.
Researchers like Yohai Kaspifrom Department of Environmental Sciences from Weizmann Institute of Science (Israel), are also trying to understand the climate of gas giants such as Jupiter and Saturn. The geophysicist has just co-published, in Naturean article in which he delivers a new evaluation of the thickness of the layers where jet streams (Where jet streamsin English) on Neptune and Uranus.
Winds blowing at 1,000 km/h on Neptune and Uranus
Since the observations of the mission Voyager 2we know that these winds violent ones exist on the surface of the two planets, and that they even blow there at a speed above 1,000 km/h. Kinds of cyclones whose sizes exceed that of the Earth also occur there.
It was also a surprise for the astrophysicistswho did not expect to see so much activity on icy planets with low energy from the light of the Sun, since they are located at distances of several billion kilometers from it. Although the planet uranus has a strongly tilted axis of rotation, so that it is almost parallel to the orbital plane of the planet, and that one of its poles is almost opposite the Sun in winterwe see that the atmospheric phenomena on Uranus and Neptune are very similar.
To decipher what is happening in the atmospheres of these giants, it is necessary to specify the models of their internal structure. We can use the laws of physics to build them, but it is necessary to constrain them by observations and measurements. For example, it is possible to use the magnetic field where the gravitational field planets for this purpose. This is how the measurements by gravimetry of the mission Grill gave us information about the interior of the Moon.
A method to explore Jupiter and exoplanets
However, it turns out that the gravitational field affects the characteristics of the flow of fluids on a rotating planet. On Earth, this is a fact well known to external geophysicists. So the velocity field winds show that they wrap around areas of low and high pressure. Via the estimates of these pressures, we can go back to the inhomogeneous gravitational field of the planet and finally to the characteristics of the distribution of matter in the inner layers. This is precisely what the researchers realized, based on data concerning the jet streams of Neptune and Uranus.
Planetologists conclude that jet streams should not extend to a depth of more than 1,000 km. Remember that the radii of Neptune and Uranus are approximately 24,600 and 25,400 km respectively.
When the missions Juno and Juice will arrive near Jupiter, the data they will collect should make it possible to use the same method as that of the researchers, in order to constrain the internal structure of the largest planet in the Solar System. We should, in the future, be able to do the same with hot Jupiters.
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