Most clouds on Earth are made of water, but beyond our planet they exist in many chemical varieties. The top of Jupiter’s atmosphere, for example, is covered in yellow clouds of ammonia and ammonium hydrosulfide. And on worlds outside our solar system, there are clouds made up of silicates, the family of rock-forming minerals that make up over 90% of Earth’s crust. But the researchers were unable to observe the conditions under which these clouds of small dust grains form.
A new study appearing in Monthly Notices of the Royal Astronomical Society provides some insight: The research reveals the temperature range at which silica clouds can form and be visible at the top of a distant planet’s atmosphere. The finding came from observations by NASA’s retired Spitzer Space Telescope of brown dwarfs—celestial bodies that fall between planets and stars—but fits into a more general understanding of how planetary atmospheres work.
“Understanding the atmospheres of brown dwarfs and planets where silicic clouds can form can also help us understand what we would see in the atmosphere of a planet that is closer in size and temperature to Earth,” said Stanimir Metchev, professor of studies of exoplanets at Western University in London, Ontario, and co-author of the study.
Nebular chemistry
The steps to create any type of cloud are the same. First, heat the base ingredient until it becomes steamy. Under the right conditions, this ingredient could be any number of things, including water, ammonia, salt, or sulfur. Trap it, cool it enough to condense, and voilà—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 big enough to begin fusion, the process that makes stars shine. Many brown dwarfs have atmospheres almost indistinguishable from those of gas-dominated planets such as Jupiter, so they can be used as proxies for these planets.
Silica clouds may be visible in brown dwarf atmospheres, but only when the brown dwarf is cooler than about 3,100 degrees Fahrenheit (about 1,700 degrees Celsius) and hotter than 1,900 F (1,000 C). Too hot and the clouds are evaporating. very cold and turn to rain or sink lower in the atmosphere. Credit: NASA/JPL-Caltech
Prior to this study, data from Spitzer already suggested the presence of siliceous clouds in a handful of brown dwarf atmospheres. (NASA’s James Webb Space Telescope will be able to confirm these types of clouds on distant worlds.) This work was done during the first six years of the Spitzer mission (which launched in 2003), when the telescope operated with three cryogenic refrigerated instruments. In many cases, however, the evidence of siliceous clouds in brown dwarfs observed by Spitzer was too weak to stand on its own.
For this latest survey, the astronomers collected more than 100 of these edge detections and grouped them based on the temperature of the brown dwarf. All of these fall within the predicted temperature range for where silicate clouds should form: between about 1,900 degrees Fahrenheit (about 1,000 degrees Celsius) and 3,100 F (1,700 C). While the individual detections are marginal, together they reveal a definitive feature of siliceous clouds.
“We had to search the Spitzer data to find these brown dwarfs where there was some evidence of silicic clouds, and we really didn’t know what we would find,” said Genaro Suárez, a postdoctoral researcher at Western University and lead author of the new study. “We were very surprised at how strong the conclusion was when we had the right data to analyze.”
In atmospheres warmer than the upper end of the range identified in the study, the silicates remain a vapor. Below the low end, the clouds will turn to rain or sink lower in the atmosphere, where the temperature is higher.
In fact, researchers believe that silica clouds exist deep in Jupiter’s atmosphere, where the temperature is much higher than at the top due to atmospheric pressure. Silicate clouds cannot rise higher, because at lower temperatures the silicates will solidify and not remain in cloud form. If the top of the atmosphere were thousands of degrees warmer, the planet’s ammonia and ammonium hydrosulfite clouds would evaporate, and silicate clouds could potentially rise to the top.
Scientists are finding an increasingly diverse menagerie of planetary environments in our galaxy. For example, they have found planets with one side permanently facing their star and the other permanently in shadow—a planet where clouds of different composition may be visible depending on the side viewed. To understand these worlds, astronomers must first understand the common mechanisms that shape them.
Scientists improve weather forecasts for brown dwarfs More information: Genaro Suárez et al, Ultracool dwarfs observed with the Spitzer infrared spectrograph. II. Occurrence and precipitation of silicate clouds in L dwarfs and analysis of the complete M5-T9 field dwarf spectroscopic sample, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac1205 Provided by Jet Propulsion Laboratory
Reference: NASA helps decipher how some distant planets have sand clouds (2022, July 7) retrieved July 7, 2022 by
This document is subject to copyright. Except for any fair dealing for purposes of private study or research, no part may be reproduced without written permission. Content is provided for informational purposes only.
