Juno helping us learn about Jupiter


Jupiter and Saturn are now readily visible in our evening sky just after dark. Look to the southeast for bright Jupiter. Over to the right in the south, Saturn shines a bit more dimly.

Check out Jupiter as quickly and as often as possible, fellow and sister stargazers. With whatever optical aid you can muster, now is the time to go out and check out the solar system’s most massive planet.

Since 2016, the robotic spacecraft Juno has been orbiting Jupiter. As a result, our understanding of Jupiter and its extraordinarily complex meteorology has increased exponentially.

However, observing Jupiter from your backyard on Earth can still teach you a lot.

Binoculars show its four brightest satellites, the Galilean moons, lined up around the planet. Even a small telescope reveals its weird cloud bands stretched around it like horizontal zebra stripes.

Jupiter’s stunning visibility rises partly from its enormity. At 89,000 miles wide, Jupiter could comfortably contain almost 1,500 Earths. Some surface details are relatively easy targets, even at half a billion miles away.

Jupiter is essentially a ball of liquid hydrogen and helium covered with clouds divided into alternating bright “zones” and darker brown and yellow “belts.”

Also, notice that the planet is not truly spherical. It bulges out distinctly along its equator in the direction of the cloud bands.

Jupiter may be the planet with the largest diameter, but it has the shortest day, which, oddly, makes it the most exciting planet to observe with a telescope.

Tiny Earth rotates once every 24 hours. Jupiter spins on its axis once every 10 hours. At 40 degrees latitude, we central Ohioans rotate at about 680 miles per hour.

Jupiter has 12 times the diameter of Earth. At 40 degrees latitude, its outer region zips around at 27,500 miles per hour.

Spin a giant, liquid planet around that fast, and it will bulge at the sides.

At Jupiter’s equator, the planet takes an average of 9 hours and 50 minutes to rotate once. Near Jupiter’s poles, it takes 9 hours and 55 minutes. The planet’s weird “differential” rotation is one factor that stretches its clouds into zebra stripes extending around Jupiter.

Add rising and falling heat, and you have a recipe for atmospheric chaos.

Meteorologists have a difficult time predicting Earth’s weather because of the many variables involved. However, Earth is a calm and straightforward place compared to turbulent Jupiter.

Larger telescopes show details within the clouds — hook-shaped festoons and light and dark ovals that are giant storm systems the size of our planet.

You also might see the pale orange-red oval in the planet’s southern hemisphere. That so-called Great “Red” Spot (GRS, for short) is the most significant storm of all. Currently, you could fit 1.6 Earths inside the Red Spot.

Those readers with long memories will recall that decades back, astronomers used to estimate the size of the GRS at three times Earth’s diameter. Over the years, that estimate has shrunk to 2.5, then 2.0, and now 1.6.

The Juno spacecraft has detected chunks of reddish material flaking away from the main storm.

The conclusion is obvious and startling. The GRS is shrinking and may soon disappear entirely.

Why startling? The GRS has been visible to Earth’s telescopes since they got large enough in the 17th century to see it. Some astronomers estimate that Jupiter’s red cyclone has graced the planet for 50,000 years. That it could disappear in our lifetimes is, well, humbling.

The color of the Red Spot changes slowly over time. Over the years, I’ve seen it fluctuate from white to pink to vivid orange-red.

The exact causes of the changes are still unknown, but the reason is probably less astronomical and more meteorological.

As the Red Spot churns around like a gigantic typhoon, it dredges up material from deep inside Jupiter’s atmosphere. The sun’s energy or electromagnetic energy generated by Jupiter (or both) creates colorful chemical changes.

We now understand why the GRS has lasted so long, thanks to the Juno orbiter. It exists at the boundary between a belt and a zone. The complex interaction at that interface causes two jet streams traveling in opposite directions.

The two fast-moving streams stir up the storm. Jovian meteorologists compare the jet streams to two fast-moving conveyor belts spinning in opposite directions. The GRS is like a ball caught between the conveyor belts.

A few thousand miles below the cloud tops, Jupiter’s hydrogen is so cold and pressurized by Jupiter’s intense gravity that the planet becomes a giant, unending sea of liquid hydrogen. Below that, the hydrogen acts much like a metal, although it remains liquid.

Because it is in metallic form, hydrogen can do what most metals do — conduct electricity. All that metallic hydrogen spinning and sloshing around gives Jupiter the mother of all planetary magnetic fields.

Orbiting the whole swirling, seething mass is a thin, nearly invisible ring system and at least 80 moons.

Astronerds like me often blather about how a spacecraft could never land on Jupiter because it has no solid surface.

But the situation is much weirder than that. Let’s dive into Jupiter’s atmosphere in our mind’s eye.

At first, we would marvel at how clear the atmosphere is. It is 90 percent hydrogen and 10 percent helium. Mere traces of ammonia, methane, and sulfur do not block the view.

It wouldn’t be long before our spacecraft would be crushed like an empty soft-drink can by the planet’s enormous atmospheric pressure, even in the upper regions of its atmosphere.

But let’s imagine that we had somehow built a spacecraft that could handle the burden. We would notice that the atmosphere got denser and denser as we descended, like a thick fog on bone-crushing steroids.

The gas would slowly change into a liquid as we got further down. We would note that Jupiter doesn’t even have a liquid surface. The change from a gas to a solid would be subtle and imperceptible.

The atmosphere would get denser and denser until you were immersed in liquid. You’d never notice the transition.

Below that layer of liquid metallic hydrogen, the composition of Jupiter’s core remained a mystery until the Juno spacecraft produced a detailed analysis of the planet’s magnetic field.

Before Juno, astronomers divided into two camps about the center of Jupiter.

Some astronomers hypothesized that Jupiter’s core was composed primarily of heavy elements, the tiny hunks of ice, rock, and metal that inhabited the early solar system four billion years ago. Those heavy chunks were attracted to each other to form larger clumps called planetesimals.

Those planetesimals then collided to form Jupiter’s core, which was perhaps 14 or so times the mass of Earth. The heavy core swept up lighter gases, mostly hydrogen and helium, to form the planet we see today.

The second camp of astronomers argues that Jupiter has no core at all. Just after the sun formed, Jupiter was an amorphous cloud of gas and dust a bit denser than the gas and dust surrounding it.

As Jupiter’s cloud cooled and condensed, regions within it were slightly denser than the rest of the cloud. The densest region gathered gas and dust from the rest of the cloud, swirling the cloud into the ball of liquid we see today.

If Juno’s data are correct, then both theories are wrong. Jupiter has a core, all right, but it is far less dense than the core posited in the first theory.

The core turns out to be a fuzzy, diluted sphere extending up to about half the planet’s diameter.

As usual, the findings create more questions. Did Jupiter ever have a dense core? If so, did it slowly wear away? Did a collision with another planetesimal shatter the core?

Such questions are not trivial. They strike at the heart of how gas giants like Jupiter form in the first place.

Juno’s 53-day, elongated orbit carries it around Jupiter’s north and south poles. Juno imaged cyclonic storms wider than the continental United States centered at both poles. Surrounding those central cyclones are rings of smaller storms, each larger than Texas. Initially, Juno spotted five storms circling the south pole and eight around the north pole.

The central storms seem relatively stable, a function of Jupiter’s rotation around the poles and its magnetic field.

The same cannot be said of the surrounding cyclones. In November 2019, Juno imaged a sixth southern-polar storm that had joined the other five circling the central cyclone.

Juno’s accomplishments seem to make our Earth-based observations irrelevant. However, sharp-eyed amateurs have observed pieces of the Great Red Spot breaking off from the primary swirl as the GRB shrinks.

Besides, I still remember with spine-tingling intensity the first time I saw Jupiter’s four Galilean moons in my father’s plastic-lens opera glasses.

So get out there and show a child (and yourself) the splendor that is Jupiter. A lifetime of joy awaits.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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