What goes on beneath Jupiter’s clouds?

Read time: 3 minutes

Do you like watching the clouds? It’s a pleasing pastime. If you have enough imagination, the cloud
above your head sometimes resemble faces or animals.

Here on Earth, we always look at the clouds from below. But on other planets, we can only see
clouds from above. Even with a large telescope, it’s impossible to see what goes on beneath the
cloud deck.

Now, ALMA has succeeded in peering through the cloud deck of the giant planet Jupiter. Gases deep
in Jupiter’s atmosphere emit millimeter radiation. This radiation can pass upward through the clouds,
ready for ALMA to detect.

Clouds on Earth consist of tiny droplets of water, or ice crystals. On Jupiter, however, the upper cloud
layers consist of small crystals of frozen ammonia gas. Thanks to Jupiter’s fast rotation, the clouds are
stretched out into belts that encircle the planet.

In early January 2017, amateur astronomers discovered a new, white plume that appeared to erupt
in one of these cloud belts. Just a few days later, astronomers used ALMA to look beneath the new
plume, down to a depth of 80 kilometers below the cloud deck.

The new ALMA observations reveal that ‘cloud eruptions’ on Jupiter push ammonia gas upward.
Ammonia from deep within Jupiter’s atmosphere can be pushed up to well above the cloud deck.
When the gas freezes into ice crystals, a new white plume appears.

So thanks to ALMA, we now know what goes on beneath the swirling clouds of Jupiter. Just as well,
since there’s no one down there looking at the clouds from below!

By the way, ALMA could not have made this discovery all by itself. The astronomers combined their
measurements with other observations, made by ultraviolet and radio telescopes.

What?

Jupiter is the largest planet in the solar system. If it were hollow, it could contain well over a
thousand Earths. Unlike Earth, Jupiter doesn’t have a solid surface. Instead, the giant planet consists
mainly of the light gases hydrogen and helium, just like our Sun. High up in the atmosphere are cloud
layers of frozen ammonia crystals. Below the ammonia cloud deck is a layer of ammonium
hydrosulfide – a compound of hydrogen, nitrogen and sulfur. Even further down are cloud layers of
water droplets. Despite its giant size, Jupiter is spinning very fast: a day on Jupiter lasts less than ten
hours. Because of the fast rotation, the clouds in Jupiter’s upper atmosphere are stretched out in
dark belts and light zones. They can already be seen with a small amateur telescope.

Who?

The ALMA observations of Jupiter were carried out by Imke de Pater and her colleagues. Imke
studied astronomy in the Netherlands, but she is now a professor of planetary sciences at the
University of California at Berkeley. Imke worked together with a large group of scientists from the
USA, the United Kingdom, Australia, Japan, and Chile. They published their results in the
Astronomical Journal.

ALMA URL