New NASA study explains why some planets have clouds of sand

Clouds on Earth are made of water, but on other planets, they consist of chemicals like ammonia and ammonium hydrosulfide. Extraterrestrial clouds can also be made of silicates, the family of rock-forming minerals that make up over 90 percent of Earth’s crust.

How do these clouds of small dust grains form?

The temperature range at which silicate clouds may form

A new study by NASA may hold the answers, according to a press release by the space agency published on Thursday. The research deciphers the temperature range at which silicate clouds can form high in a distant planet’s atmosphere. The researchers drew their conclusions using data that NASA’s now-retired Spitzer Space Telescope collected on brown dwarfs, which are celestial objects that fall in between planets and stars.

“Understanding the atmospheres of brown dwarfs and planets where silicate clouds can form can also help us understand what we would see in the atmosphere of a planet that’s closer in size and temperature to Earth,” said Stanimir Metchev, a professor of exoplanet studies at Western University in London, Ontario, and co-author of the study.

Clouds — no matter what they're made of — develop in the same basic way.  The ingredient heats up until it becomes a vapor. Then it's trapped, cooled until it condenses, and there you go: a cloud! Chemicals like water, salt, ammonia, and sulfur can form clouds. So can silicate, but that only happens on extremely hot worlds (such as brown dwarfs) because rock vaporizes at an extremely high temperature. 

Gathering over 100 marginal detections

To truly understand cloud formation on brows dwarfs, astronomers gathered more than 100 marginal detections and grouped them by temperature. They noticed that all of them fell 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). 

“We had to dig through the Spitzer data to find these brown dwarfs where there was some indication of silicate 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 once we had the right data to analyze.”

The researchers concluded that the temperature needed to be just right for clouds to form. Atmospheres hotter than the top end of the range identified in the study saw silicates become a vapor. Meanwhile, temperature below the bottom end, produced clouds that turned into rain or sank lower in the atmosphere.

The study is published in the Monthly Notices of the Royal Astronomical Society.

Abstract

We present a uniform analysis of all mid-infrared R ≈ 90 spectra of field M5–T9 dwarfs obtained with the Spitzer Infrared Spectrograph (IRS). The sample contains 113 spectra out of which 12 belong to late-M dwarfs, 69 to L dwarfs, and 32 to T dwarfs. Sixty-eight of these spectra are presented for the first time. We measure strengths of the main absorption bands in the IRS spectra, namely H₂O at 6.25 μmμm⁠, CH₄ at 7.65 μmμm⁠, NH₃ at 10.5 μmμm⁠, and silicates over 8–11 μmμm⁠. Water absorption is present in all spectra and strengthens with spectral type. The onset of methane and ammonia occurs at the L8 and T2.5 types, respectively, although ammonia can be detectable as early as T1.5. Silicate absorption sets in at spectral type L2, is on average the strongest in L4–L6 dwarfs, and disappears past L8. However, silicate absorption can also be absent from the spectra at any L subtype. We find a positive correlation between the silicate absorption strength and the excess (deviation from median) near-infrared colour at a given L subtype, which supports the idea that variations of silicate cloud thickness produce the observed colour scatter in L dwarfs. We also find that variable L3–L7 dwarfs are twice more likely to have above-average silicate absorption than non-variables. The ensemble of results solidifies the evidence for silicate condensate clouds in the atmospheres of L dwarfs, and for the first time observationally establishes their emergence and sedimentation between effective temperatures of ≈2000 and ≈1300 K, respectively.