ebb and flow

The tides of Jupiter can help scientists understand the history of the Solar System

Jupiter's moons inflict a gravitational tug on their host planet.

Jupiter isn’t much like Earth — which makes Caltech grad student Benjamin Idini’s research into its tides all the more interesting. There are no waves to ride for would-be Jovian surfers.

“If you were to try and dive into Jupiter, you would never stop,” Idini tells Inverse. “You would just keep falling in until the pressure is so big that it would probably crush you.”

In a study published in The Planetary Science Journal, Idini and colleagues took a look at the effects Jupiter’s 79 moons might have on our solar system’s largest planet, and compared it to the effects we see between the Earth and Moon. As a gas giant, Jupiter lacks the solid core of rocky planets like Earth and Venus, and is made of liquid metallic hydrogen under extreme pressure.

The study used data gathered from the Juno mission in hunting down the gravitational effects of the moon, and found that one had the most profound effect: Io.

What’s new — The Juno spacecraft has been studying Jupiter since 2016, orbiting the planet once every 53 days. In one set of data, scientists discovered something unusual: a small gravitational perturbation within Jupiter.

Further analysis yielded a different kind of tide that runs through the entire planet itself. Rather than the well-known effect of hydrostatic tides, which is at play on Earth, the team discovered an additional effect known as dynamical tides. Dynamical tides are when the gravitational influence of the moon causes oscillations in the whole interior of the gaseous planet.

Only one thing in Jupiter’s vicinity could cause these tides: Idini and his colleagues looked to the moons for a culprit. Jupiter has 79 moon, but only four of them are of substantial size:

  • Io — a hellish volcanic world constantly spewing lava into space due to Jupiter’s tug on it
  • Europa — a moon slightly smaller than our own, which is believed to have an ocean underneath its ice crust
  • Ganymede — the largest moon of our solar system, larger even than the planet Mercury
  • Callisto — the second-largest moon of Jupiter, slightly smaller than Mercury

Io is the third-largest and innermost moon of these natural satellites, often called the Galilean moons. While it may not be as massive as Ganymede or Callisto, its proximity to Jupiter ended up making it the culprit behind the dynamical tides.

The rest of the moons also have a gravitational effect on Jupiter, but it is much smaller and harder to detect.

The Juno spacecraft has been orbiting Jupiter for five years, giving scientists a better picture of the Solar System’s gas giant.


Here’s the background — Compared to the effect on Earth’s oceans, the effect of Jupiter’s moon is not that much different, except that it takes place over the whole planet.

“The main difference is that on Earth, we have the movement of the oceans over solid Earth, or the ocean floor,” Idini says. “Jupiter, we don't have an ocean floor or anything like that because Jupiter is almost entirely fluid.”

This means that Jupiter’s interior acts much different than Earth’s — for instance, some Juno data indicates that there may not be defined boundaries between the core and upper layers, with the core permeating upward.

Why it matters — The recent finding will help scientists investigate the interior of the Solar System’s gas giant planet.

By probing at the recent data set, scientists can find out how deep Jupiter’s interior is.

“The study we just published, this is the beginning of a line of research,” Idini says. “We can use that information to say something about the core of Jupiter.”

By going back in time with Jupiter, scientists also get an idea of the early Solar System and how it formed. And using that knowledge, they can then apply it to other planetary systems that they discover in the quest to search for exoplanets orbiting around foreign stars.

What’s next — The Juno mission is scheduled to wrap up in July 2021, but the team behind the recent study are hopeful that it will continue to feed them new information on Jupiter.

“Every time Juno passes by, we get a better picture of Jupiter,” Idini says. “People should stay tuned if they want to find out the interior of Jupiter.”

Based on the recent study, the researchers can create models of Jupiter with a tiny solid core or with a diluted core and find out which one corresponds to the same tidal effect observed by Juno.

By gathering more information on Jupiter, scientists are able to reconstruct the history of the planet and how it evolved over time.

“This is what the planet looks like looks today, but you can go back in the history of the Solar System from the measurements we have today together with our knowledge of physics and how matter behaves and try to reproduce the early stages of Jupiter,” Idini says. “What did Jupiter look like 4.5 billion years ago?”

Abstract: The Juno orbiter has continued to collect data on Jupiter's gravity field with unprecedented precision since 2016, recently reporting a nonhydrostatic component in the tidal response of the planet. At the mid-mission perijove 17, Juno registered a Love number k2 = 0.565 ± 0.006 that is −4% ± 1% (1σ) from the theoretical hydrostatic . Here we assess whether the aforementioned departure of tides from hydrostatic equilibrium represents the neglected gravitational contribution of dynamical tides. We employ perturbation theory and simple tidal models to calculate a fractional dynamical correction Δk2 to the well-known hydrostatic k2. Exploiting the analytical simplicity of a toy uniform-density model, we show how the Coriolis acceleration motivates the negative sign in the Δk2 observed by Juno. By simplifying Jupiter's interior into a coreless, fully convective, and chemically homogeneous body, we calculate Δk2 in a model following an n = 1 polytrope equation of state. Our numerical results for the n = 1 polytrope qualitatively follow the behavior of the uniform-density model, mostly because the main component of the tidal flow is similar in each case. Our results indicate that the gravitational effect of the Io-induced dynamical tide leads to Δk2 = − 4% ± 1%, in agreement with the nonhydrostatic component reported by Juno. Consequently, our results suggest that Juno obtained the first unambiguous detection of the gravitational effect of dynamical tides in a gas giant planet. These results facilitate a future interpretation of Juno tidal gravity data with the purpose of elucidating the existence of a dilute core in Jupiter.

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