(ORDO NEWS) — Most clouds on Earth are made of water, but off our planet they have many chemical varieties.
For example, the upper part of the atmosphere of Jupiter is covered with yellow clouds, consisting of ammonia and ammonium hydrosulfide.
And on worlds outside our solar system, there are clouds made of silicates, a family of rock-forming minerals that make up over 90% of the earth’s crust.
But scientists have so far been unable to capture the conditions under which these clouds form from fine dust grains.
A new study reveals the temperature range at which silicate clouds can form and be visible in the upper atmosphere of a distant planet.
This conclusion was drawn from NASA’s Spitzer Space Telescope observations of brown dwarfs – celestial bodies that lie between planets and stars – but it fits into a more general understanding of how planetary atmospheres work.
“Understanding the atmospheres of brown dwarfs and planets where silicate clouds can form can also help us understand what we will see in the atmosphere of a planet that is closer in size and temperature to Earth,” said Stanimir Metchev, professor of exoplanet studies at Western University in London, Ontario, and study co-author.
The steps to create any type of cloud are the same. First, heat the key ingredient until it turns into steam. Under the right conditions, this ingredient can be a variety of substances, including water, ammonia, salt, or sulfur.
Place it in a trap, cool it down enough to condense, and voila, clouds! Of course, rock evaporates at a much higher temperature than water, so silicate clouds are only visible on hot worlds, such as the brown dwarfs used for this study and some planets outside our solar system.
Although they form like stars, brown dwarfs are not massive enough to trigger nuclear fusion, the process that makes stars glow.
The atmospheres of many brown dwarfs are almost indistinguishable from those of gas-dominated planets such as Jupiter, so they can be used as a proxy for these planets.
Prior to this study, Spitzer data had already indicated the presence of silicate clouds in the atmospheres of several brown dwarfs.
This work was carried out during the first six years of the Spitzer mission (launched in 2003), with three cryo-cooled instruments operating on the telescope. However, in many cases the evidence for silicate clouds on brown dwarfs observed by Spitzer was too weak.
As part of the latest study, astronomers collected more than 100 such minor detections and grouped them by the temperature of the brown dwarf.
They all fell within the predicted temperature range at which silicate clouds should form: from about 1,000° C to 1,700° C. Although the individual sightings are minor, together they show the definitive signature of silicate clouds.
In atmospheres hotter than the upper end of the range defined in the study, the silicates remain as vapor. Below the minimum limit, clouds turn into rain or fall lower into the atmosphere where the temperature is higher.
In fact, researchers believe silicate clouds exist deep in Jupiter’s atmosphere, where temperatures are much warmer than at the top, thanks to atmospheric pressure.
Silicate clouds cannot rise higher because silicates solidify at lower temperatures and do not persist as clouds.
If the upper atmosphere were thousands of degrees hotter, clouds of ammonia and ammonium hydrosulfide would evaporate, and silicate clouds could rise to the top.
Scientists are finding more and more diverse planetary environments in our galaxy. For example, they found planets with one side constantly facing their star while the other side is constantly in shadow – planets where clouds of different compositions can be seen depending on the side observed.
To understand these worlds, astronomers first need to understand the general mechanisms that shape them.
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