When a spacecraft slips behind Jupiter, something counterintuitive occurs: when we lose the signal the measurement is then the loss of signal. When Juno passes out of sight of the Earth, its radio signal passes deep into the upper atmosphere of Jupiter and curves, decelerates, and finally goes dead creating a time piece of the largest planet of the solar system.
That emperor has now been made fined to a point of kilometers off a world which already towers more than anything around. Researchers (with high-precision radio tracking) estimate that Jupiter is a bit smaller and flatter than normal values which it inherits dating back to the 1970s, when Pioneer and Voyager gave only six radio-occultation snapshots. It is subtle in the sense that a slightly different lens prescription is subtle: the scene is almost the same, but the physics deduced using this physics is different.
In the revised analysis, the team used 24 measurements of a new orbital route that saw Juno repeatedly be on the opposite side of Jupiter as seen by the Earth. The procedure, radio occultation, involves the atmospheric refraction of radio wave by Jupiter to rebuild the vertical structure and, most importantly, the planetary limb. As signals pass through the ionosphere and deeper layers, changes in frequency encode conditions such as temperature and pressure, while the precise geometry pins down the planet’s outline. “Jupiter’s passing behind Jupiter provides an opportunity for new science objectives,” Scott J. Bolton said in a statement. “When the spacecraft passes behind the planet, its radio communication signal is blocked and bent by Jupiter’s atmosphere. This enables an accurate measurement of Jupiter’s size.”
The resultant dimensions constrict the fundamental shape of the planet at the level of 1-bar pressure which is a point of reference to the standard reference that is linked to the atmospheric pressure similar to the sea level of the earth. The research gives 66,842 km as the polar radius and 71,488 km as the equator radial which are in decrease of 12km and 4km respectively compared to long-serving values. Stated differently, taking into consideration other effects, Jupiter would emerge some 8 km narrower at the equator and 24 km flatter at the poles.
These are the same “additional effects” in which the engineering of the measurement is achieved to meet the meteorology of a giant planet. “The decades-old measurements do not account for the influence of Jupiter’s strong winds.” The fact that Jupiter already rotates very quickly, 9 h 55 min 29 s, already qualifies it as an oblate spheroid, with the equator being about 7% greater than the distance between the poles and the center. However, the effective centrifugal balance is slightly disturbed by the strong zonal winds on the planet, and the shape of the planet is slightly realigned by about ten kilometers particularly at the low latitudes. The work is able to obtain an order-of-magnitude reduction in uncertainty relative to previous standards by folding winds into the occultation geometry.
One sentence from the research team captures why a few kilometers matter on a planet nearly 140,000 kilometers across. “Shifting the radius by just a little lets our models of Jupiter’s interior fit both the gravity data and atmospheric measurements much better,” Eli Galanti said in a statement.
That better fitting does not come in silk. The interior of Jupiter cannot be directly imaged; it is deduced by balancing gravity, rotation, the structure of the atmosphere and a surface boundary which will have to be defined in a consistent way. A radius differs slightly and alters the interpretation of deep winds and the distribution of density gradients and the constraint of interior layers. The same revised “calibration planet” is also applied in cases when astronomers can scale observations of the distant gas giant whereby radii are usually deduced using the silhouettes and imperfect atmosphere assumptions- so Jovian newly improved dimensions are a quieter, albeit solid, basis.
