An amateur astronomer used an old technique to study Jupiter — and found something strange

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Researchers and amateur stargazers have joined forces to challenge a longstanding assumption that Jupiter's distinctive swirling clouds are made of frozen ammonia — a quite fundamental discovery about the gas giant we believed we already knew well.

Hill discovered unexpected information about the gas giant's atmosphere, contrary to what earlier models predicted.

Jupiter's atmosphere is mainly made up of hydrogen and helium, along with small amounts of ammonia, methane, water vapor, and other gases. These other gases form clouds at different altitudes, which reflect sunlight and create Jupiter's distinctive appearance. Since ammonia is known to be present in Jupiter's atmosphere and is expected to form clouds at the lowest pressure of any other gas, researchers have generally assumed that the planet's prominent upper clouds are made of ammonia ice.

Astronomers will generally default to a simple model unless there's a strong indication that it's not accurate," Irwin said. "Given that we can see ammonia gas in Jupiter's atmosphere [...], it was simply assumed that its most visible clouds were probably made up of ammonia ice.

Although relatively well established, this technique had not seen much use recently.

The technique is called band-depth analysis and is used to estimate the concentration of a particular gas based on the amount of light absorbed at specific wavelengths unique to that gas – in this case, methane and ammonia.

Using the absorption bands of methane (619 nm) and ammonia (647 nm), which are both characteristic features visible in Jupiter's spectrum, Hill was able to calculate the concentration of these gases near Jupiter's cloud tops. The absorption of methane at 619 nm serves as a firm reference point due to its well-known abundance and the fact that it can be used to determine atmospheric pressure levels. By comparing this to ammonia's absorption at 647 nm, Hill was able to calculate and chart the distribution of ammonia across the clouds of Jupiter with a high level of precision.

We have a reliable understanding of how evenly methane is distributed in the atmosphere and we have a reasonable estimate of how much of it is present," Irwin explained. "As a result, we can use the variation in reflectivity between images taken in these two absorption bands to calculate both the pressure of the clouds and the relative amount of ammonia.

The team found that the reflected light was coming from cloud layers where atmospheric pressure was too high and temperatures were too warm for ammonia to condense. "[The observations] show very clearly that the main layer of reflection [...] is much deeper than the predicted condensation level of ammonia at 0.7 bar, actually occurring much deeper at 2-3 bar," said Irwin.

The only conclusion was that ammonia ice couldn't be the main part of Jupiter's clouds. Models indicate that the clouds are probably made of ammonium hydrosulfide and possibly also smog caused by chemical reactions in the atmosphere, as the cloud color doesn't match pure ice.

However, we can't confirm this specific composition for sure," said Irwin. "Some scientists also propose that the clouds could be a rare mix of water and ammonia.

What it shows, he continued, is that there is a lot of complex chemical reactions happening in the atmosphere of Jupiter. "It appears that in most areas, ammonia is broken down and destroyed more quickly than it can be carried up, upward," Irwin said. "So ice clouds made of pure ammonia are actually quite rare and are only found in small areas of very fast and intense mixing and movement."

Mission. This is major because it not only verifies these remarkable discoveries, but it also makes it easier and more convenient to do observations of Jupiter—and other planets like Saturn.

Hill in his original paper that was published in the journal Earth and Science one year ago.

Although a groundbreaking discovery, the researchers acknowledge that there is still work to be done. Specifically, the present findings rely on the assumption that the ammonia profile is "vertical," a condition that scientists often take for granted.

It's actually more accurate to think of this varying depending on the height below the point where ammonia condensation occurs, but it's not easy to pinpoint with our existing data," Irwin said. "We need to compare the results from the VLT/MUSE, Juno, and VLA more closely. The data should be consistent with each other, but we'll have to work through some of these differences to get a better understanding of how ammonia behaves at different altitudes in Jupiter's atmosphere.

This work shows how contributions from both professional and amateur astronomers broaden our understanding significantly. Even observations that initially appear "simple" can offer valuable insights and greatly expand our knowledge of the universe.