Hubble unlocks the secret behind their colors

Carl Sagan and André Brahic is unfortunately no longer among us to comment on the discoveries made on giant planets of the solar system, but their colleagues continue to explore these worlds. If the gases that are Jupiter and Saturn is much better known to us now after the missions Juno and Cassini, the Frozen – Discovered by Herschel (Uranus) and Le Verrier (Neptune) – still holds many mysteries.

Only the Voyager 2 probe approached Uranus and Neptune and discovered in the latter case a meteorology much more active than expected for a planet so far away Sun (therefore receives much less sound energy than Saturn), like it possesses a spot, black this time, suggesting a huge storm planetary anticyclone.

NASA’s Hubble Space Telescope has taken time-lapse images of a large dark storm on the vanishing Neptune. A recent Hubble program called Inheritance from the atmosphere of the outer planet, 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 get a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles were then to appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Automatically translate”. Select “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 Earth or with Hubble in orbit. We see a new illustration of this with an article published in Journal of Geophysical Research: Planetsbut also in free access on arXivshowing the work of an international team led by patrick irwinprofessor i physical planetary at Oxford University.

This work is based on analyzes of archival data covering several years, first collected with Hubble, which was obtained with its spectro-imager (STIS) and ranges fromultraviolet infrared, as well as the many images that Hubble has taken of Uranus and Neptune with the mythical Wide angle camera 3 (WFC3). Data collected with the telescope Twin north andInfrared telescopic system (IRTF) off Nasa also proved valuable.

These observations have provided scientific models for radiation transfer atmospheres of the two giants (and of these atmospheres themselves) largely consist 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 mists and various turbulent atmospheres

Initially, planetary scientists were just looking to develop a model that would help them understand clouds and mists in the atmosphere of the ice giants. But as Mike Wong, an astronomer at the University of California at Berkeley and a member of the team behind the discovery of Uranus and Neptune’s colors, 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 that also takes into account the observations of light reflected sun 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 nebula (remember that a aerosol is a suspension of fine droplets or particles in a gasfor example in the form of smoke or fog) at different altitudes in the atmosphere of each planet. That intermediate 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 in their models have taken into account Rayleigh and Mie light scattering effectsnamed after physicists involved in explaining these effects more than a century ago.

Jet currents only affect Neptune and Uranus on the surface

Article by Laurent Sacco published on 23/05/2013

That jet streams on Neptune and Uranus would be limited to their surface, according to a group of planetary scientists. This is the conclusion reached by the researchers who indirectly studied the gravitational field generated by the two gas giants. The method they used should be able to be applied to Jupiter in a few years.

The conquest of space has enabled the birth of a comparative planetology, using the tools of internal and external geophysics, tested on Earth, to try to understand geology and climatology from other planets Solar system. In this way, it is possible to learn more about our planet itself.

You can not vary soil mass, its composition or its temperature, as one would do in the laboratory to better understand a physical system. But’universe is responsible for performing these experiments with the various stars of the solar system. We can thus transpose the models of climate on earth at March and Venusto see if they make it possible to predict, via simulations, the meteorology of the planets.

Scientists like Yohai Kaspifrom Department of Environmental Science from Weizmann Institute of Science (Israel), is also trying to understand the climate of gas giants like Jupiter and Saturn. The Geophysicist has just been co-published, in Naturean article in which he provides a new assessment of the thickness of the layers where jet streams (Where jet streamsin English) about Neptune and Uranus.

Winds blow at 1,000 km / h on Neptune and Uranus

Since the observations of the mission Voyager 2we know these win violent are found on the surface of the two planets and that they even blow there by one velocity over 1,000 km / h. A kind of cyclones whose size exceeds that of the Earth also occurs there.

It was also a surprise too astrophysicistswho did not expect to see so much activity on icy planets with low energy from the light of Sun, as they are located at distances of several billion kilometers from it. Though planet uranus has a strongly inclined axis of rotation so that it is almost parallel to the planet’s orbital plane and that one of its poles is almost opposite to the Sun in winterwe see that the atmospheric phenomena on Uranus and Neptune are very much alike.

To decipher what is happening in atmospheres of these giants it is necessary to specify the models for their internal structure. We can use the laws of physics to build them, but it is necessary to limit them by observations and measurements. It is possible to use, for example magnetic field where is gravitational field planets for this purpose. This is how the measurements are gravimetry of the mission Grill gave us information about the interior of Moon.

A method of exploring Jupiter and exoplanets

It turns out, however, that the gravitational field affects the properties of the fluid flow on a rotating planet. On Earth, this is a fact well known to external geophysicists. So velocity field winds show that they enclose areas of low and high pressure. Via the estimates of these pressures, we can go back to the planet’s inhomogeneous gravitational field and finally to the characteristics of the distribution of fabric in the inner layers. This is exactly what the scientists realized, based on data on the jet streams from 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 the scientists, to limit the inner structure of the largest planet in the Solar System. We should be able to do the same in the future warm Jupiters.

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