It rains regularly on Titan, Saturn’s largest moon. As with the earth, these rains are the result of evaporation of liquid from the surface, condensation in the sky, and falling back on the surface as precipitation. On earth this is known as the hydrological (or water) cycle, which is an indispensable part of our climate. In Titan’s case, all the same steps are present, but methane is exchanged, not water.
In recent years, scientists have found evidence of similar patterns in exoplanets, from molten metal to lava rain! This begs the question of how exotic the rain can be on alien worlds. Recently, a team of researchers at Havard University conducted a study that looked at how rain would differ in a variety of extrasolar planetary environments.
This research was supported by Kaitlyn Loftus, Ph.D. Student at the Harvard Department of Earth and Planetary Sciences. Her supervising professor (and co-author of the study) was Robin D. Wordsworth, who heads the research group on planetary climate and atmospheric evolution at Wordsworth at the Harvard School of Engineering and Applied Sciences (SEAS).
The rocks seen here along the coast of Lake Salda in Turkey were formed over time by microbes that trap minerals and sediments in the water. Photo credit: NASA / JPL-Caltech
Research on rainfall and records of past rainfall on Earth have taught scientists much about the dynamics of their climate. Unfortunately, this research with exoplanets is not yet possible, which prevents scientists from being able to further restrict their potential habitability. However, knowing these conditions on Earth has helped scientists predict the planetary climates of Mars and Titan.
For their study, Loftus and Wordsworth looked at how this can also be applied to exoplanets. As Loftus explained to Universe Today via email:
“A key component of habitability is climate (to test whether a planet can support liquid surface water). A major reason for the uncertainty in understanding the climate in different planetary environments (even with the modern earth’s current transition to higher levels of CO2) is the behavior of clouds. Precipitation is a key method for cloud “death”. So if we understand how precipitation works, we can limit the behavior of clouds and ultimately better predict the planet’s climate.
“Precipitation also helps control how much water remains in an atmosphere. Since water vapor is a very good greenhouse gas, this balancing of the amount of water in an atmosphere can also influence the climate … After all, precipitation is an essential part of the negative feedback mechanism for stabilizing the planet’s climate (carbonate-silicate cycle). this is the basis of the concept of the exoplanet “habitable zone”.
The exoplanet Kepler 62f would need a carbon dioxide-rich atmosphere for water to be in liquid form. Artist illustration: NASA Ames / JPL-Caltech / T. Pyle
That knowledge will be essential, Loftus added, as next-generation telescopes join the search for potentially habitable exoplanets. In the coming years, astronomers and astrobiologists will be able to conduct direct imaging studies of exoplanet atmospheres. Models that predict how clouds and water vapor will behave on these planets go a long way towards measuring their habitability.
While predicting the precipitation patterns of a distant exoplanet is very difficult, one component that is easy to understand is the behavior of individual raindrops. Given that every raindrop that falls from a cloud is determined by a combination of fluid dynamics, thermodynamics, and atmospheric conditions, studying them can reveal a lot about a planet’s climate.
Loftus and Prof. Wordsworth showed how three key properties can be calculated based on three key properties: their shape, their rate of fall, and the rate at which they vaporize. Said Loftus:
“Clouds and precipitation depend heavily on what happens on very small scales (cloud drops / raindrops ~ micrometer millimeters), medium-sized scales (clouds, kilometers up to 10 kilometers) and very large scales (water budgets on a planetary scale). . The accurate representation of all these measures in a single model is not possible with modern (or in the foreseeable future) computers. “
An artist illustration of the exoplanet HR8799e. ESO’s GRAVITY instrument on its Very Large Telescope Interferometer made the first direct optical observation of this planet and its atmosphere. Photo credit: ESO / L. Calçada
“We’re trying to use the simplest and best understood component of the water cycle – raindrops under a cloud – to narrow down what’s ‘important’ in all the complexity,” she added. A subjective term is certainly important, but in this case it is about tracking how much atmospheric water vapor actually becomes water on the surface – a key requirement for the existence of life as we know it.
From these three properties they could get a simple expression to explain the behavior of raindrops from more complicated equations. Ultimately, they found that (over a wide range of planetary conditions) only raindrops in a relatively narrow size range could reach the surface. As Loftus has indicated, her research could enable an improved representation of precipitation in complex climate models in the future:
“Right now, much of what we understand about how clouds and precipitation work in a larger climate system is driven by what we see (and have seen) on earth. However, this leaves a great deal of uncertainty as to how valid it is to apply such empiricisms to regimes in which many physical conditions are different.
“[S]o There are many big question marks surrounding non-modern geoscientific questions that depend on how clouds / precipitation behave. This work tries to slowly build up the ability to develop theoretically well-founded expectations for the behavior of clouds and precipitation outside the modern earth and ultimately to better limit these big question marks. “
NASA’s James Webb Telescope featured in this artist’s conception provides more information on previously discovered exoplanets. After 2020, many more next-generation space telescopes are expected to build on what they discover. Photo credit: NASA
This will be very useful when the James Webb Space Telescope launches on October 31, 2021. With its advanced suite of infrared instruments and spectrometers, the James Webb can study the atmospheres of lower-mass exoplanets that are closer to their exoplanets – that is, where potentially habitable rocky planets are most likely to live.
In this way, scientists can determine the chemical composition of the atmosphere on these planets, which can include water vapor and other tell-tale “biosignatures”. Other telescopes such as ESO’s Extrem Large Telescope (ELT), Giant Magellan Telescope (GMT), and Roman Grace Telescope Nancy Grace can perform similar direct imaging studies of exoplanets.
These tools will enable an unprecedented level of exoplanet characterization that exoplanet studies have shifted into in recent years. With over 4,000 confirmed exoplanets available for study, astronomers are no longer focused solely on finding promising candidates for study. At this point, the point is to find out which of these candidates meets the requirements for life!
The results were published in a recently published article entitled “The Physics of Falling Raindrops in Different Planetary Atmospheres,” which was published online and submitted for publication in the Journal of Geophysical Research: Planets.
Further reading: arXiv