Thursday, October 21, 2021

Giant ‘Mushballs’ May Hide Missing Ammonia on Neptune and Uranus

An advantage for planetary science is that insights from one planet can explain phenomena on another. We understand the greenhouse gas effect of Venus from our own experience on Earth, and Jupiter and Saturn share some characteristics.

But Jupiter also provides insight into other, distant systems such as Uranus and Neptune.

Now, a discovery by a spacecraft orbiting Jupiter may have solved a long-standing mystery about Uranus and Neptune — where has all the ammonia gone?

Scientists have long observed the absence of ammonia in the atmospheres of Uranus and Neptune, compared to the amounts seen on Jupiter and Saturn.

Many considered that fact strange because planet formation models suggested that all gas giants arose from the same “primary soup”, so their compositions must be similar.

Theories abound about where the ammonia went, but a closer inspection of Jupiter hints at a possible explanation.

Juno, a probe currently exploring the Jupiter system, observed that ammonia in the upper atmosphere forms “mushballs” by merging with water present in the atmosphere.

Like hail, these mushballs are more liquid than conventional hail, as ammonia is exposed to liquefied water even at extremely low temperatures, such as those found in Jupiter’s upper atmosphere.

These amalgamated mushroom balls may be larger than some of the more massive hailstones on Earth. They are also prone to falling rapidly through the atmosphere, dragging their constituent parts down from the upper reaches of the atmosphere.

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As they get closer to Jupiter’s center, the temperature rises, evaporating the ammonia and water, allowing them to climb back toward the observable upper reaches.


According to Tristan Guillot of CRNS Laboratoire Lagrange, the same process may occur on Neptune and Uranus, but the mashballs keep ammonia in the lower atmosphere longer, without the greater chance of it returning to observable altitudes.

At such a low altitude, ammonia would seem to be missing with current observational capabilities. The upper layers of the clouds obscure any ammonia readings, making it appear as if it has disappeared.

Seeing the disappeared ammonia would require a dedicated mission specifically to explore the lower atmospheres of the outer planets. Some missions have been postponed in the past, but none are currently operational.

As Dr. Guillot explains, understanding the outer planets in our own solar system will help us understand the atmospheres of exoplanets far beyond our own solar system. Perhaps it is time to send a dedicated probe to learn more about our most distant planetary neighbors.

This article was originally published by Universe Today. Read the original article.


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