There are things that we learn in childhood that accompany us throughout our lives and one of them is to recite the Solar System at once, which has its disadvantages: for those of us who are already old, mentioning Pluto (which It is no longer a planet) either make mistakes when estimating distances interplanetary. Another classic misconception is the size of Jupiter. Data from the Juno mission published in Nature Astronomy They change the shape and size of the colossus of the Solar System.
Jupiter is flatter and smaller than we thought. We knew that Jupiter was the largest planet in the Solar System, a gaseous colossus whose mass exceeded that of the rest of the planets combined, which gave it the power to be almost the conductor of the orchestra (with the permission of the Sun) as long as its gravity had a lot of weight. Its large magnetic shield protects its moons from solar radiation, it has iconic clouds and storms in astronomy and its Great Red Spot It exceeds the Earth in size.
But there is something wrong with its shape and size.
The Context. The missions Voyager and Pioneerdating back to the 1970s, established figures that today we read in science books: that Jupiter has an equatorial radius of 71,492 kilometers and a polar radius of 66,854 kilometers. With this model, the planet was assimilated as a sphere flattened at the poles (oblate spheroid). These dimensions were calculated with just six indirect measurements with profiles of radio occultation.
The discovery. Because what Juno has seen shows that the equatorial radius is approximately 8 kilometers smaller and the polar radius is about 24 kilometers smaller than previous missions said. Qualitatively, Jupiter is flatter. The first thing that comes to mind is: How important are eight kilometers on a planet 140,000 kilometers wide? Well scientifically, it has it. In fact, it’s the difference between whether the laws of physics fit or not.
Why is it important. Well, because although the difference is comparatively minor, the fact that it is smaller and has a flatter shape has thermodynamic implications. Thus, it suggests a colder atmosphere enriched with heavy elements that better suit what the Galileo probe measured in 1995. Additionally, having accurate geometry is essential to understanding what’s inside and interpreting the gravity data provided by Juno, so we can accurately map how its mass is distributed inside and how hydrogen behaves under extreme pressures.
On the other hand, knowing Jupiter better is getting closer to the recipe of how the Earth was formed and going beyond: facilitating the understanding of thousands of other exoplanets giants that we are discovering in the stars.
Radio occultation operation diagram. MPRennie Wikipedia
Juno’s look. Both Pioneer and Voyager and Juno use radio occultation, that is, they use the same physical principle. The radio occultation technique consists of measuring how a planet’s atmosphere bends and slows down the radio signals of a probe when it is hidden behind it. By analyzing the delay and deviation of these waves from the Earth, the scientific team can precisely calculate the density and pressure and therefore the exact shape of the planet.
Of course, from a technological point of view there has been half a century of evolution and it is noticeable in terms of quality due to its multiband operation, precision and repetition. Thus, the probes of the 70s mainly used one radio band while Juno uses two, which allows, among other things, to eliminate noise. Likewise, the original ones were passing missions in front of the planned June orbit, that is, we have gone from having six points to an almost complete map. And finally, ground-based tracking systems are night and day when it comes to measuring changes in frequency and signal arrival time.
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Cover | NASA Hubble Space Telescope
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