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It’s Raining on the Sun

By David Reneke

It’s raining on the Sun, and the asteroid Vesta may reshape our knowledge of planet formation.

Just like Earth, the Sun has spells of bad weather, with high winds and showers of rain. However, rain on the Sun is made of electrically charged gas (plasma) and falls at around 200,000 km/h from the outer solar atmosphere to the Sun’s corona, or surface. The thousands of droplets that make up a “coronal rain” shower are as big as Ireland.

Now a team of solar physicists from Trinity College Dublin have explained this intriguing phenomenon, with imagery that shows a type of “waterfall” in the Sun’s atmosphere.

Coronal rain was discovered almost 40 years ago, and solar physicists are now able to study it in great detail thanks to state-of-the-art satellites like NASA’s Solar Dynamics Observatory and several ground-based observatories. The scientists see regular and massive shifts in the solar “climate” but have until now been unable to understand the physics of coronal rain.

It turns out that the process through which hot rain forms on the Sun is surprisingly similar to how rain happens on Earth. If the conditions in the solar atmosphere are just right, then clouds of hot, dense plasma can naturally cool, condense and eventually fall back to the solar surface as droplets of coronal rain.

In another parallel with terrestrial weather, the material that makes up the hot rain clouds reaches the corona through a rapid evaporation process caused by solar flares, the most powerful explosions in the solar system.

Using images from the Swedish Solar Telescope based in the Canary Islands, the Irish team suggests a model of “catastrophic cooling”, where an exceptionally rapid fall in temperature causes material to change from rarefied coronal gas to “raindrops”.

Asteroid May Reshape Knowledge of Planet Formation

Researchers at the Earth and Planetary Science Laboratory (EPFL) now have a better understanding of the asteroid Vesta and its internal structure, with new data from the Dawn spacecraft questioning contemporary models of rocky planet formation, including the Earth.

With a whopping 500-km diameter, Vesta is one of the largest-known planet embryos formed at the same time as the solar system. Dawn spent a year in Vesta’s orbit between Mars and Jupiter, and the data gained revealed that the asteroid’s crust is almost three times thicker than expected. The study not only has implications for the structure of this asteroid located, it also challenges the fundamental planetary formation model in which the original cloud of matter coalesced together and crystallised to form planets.

At EPFL’s laboratory, Harold Clenet examined the composition of the rocks scattered across Vesta’s surface. “What is striking is the absence of a particular mineral, olivine, on the asteroid’s surface,” he said. Olivine is a main component of planetary mantles and should have been found in large quantities on the surface of Vesta since a double meteorite impact excavated its southern pole to a depth of 80 km, catapulting large amounts of minerals to the surface.

The two impacts were so powerful that more than 5% of the Earth’s meteorites come from Vesta, yet these cataclysms were not strong enough to pierce through the crust and reach the asteroid’s mantle. The meteorites originating from Vesta and found on Earth confirm this since they generally lack olivine or contain only minute amounts compared with the amount observed in planetary mantles.

Furthermore, Dawn did not find olivine in the vicinity of the two impact craters, which means that the crust of the asteroid is not 30 km thick, as models suggest, but almost three times that figure. These discoveries challenge models that describe the formation of Vesta, and consequently the formation of rocky planets in the solar system, including planet Earth.

Vesta is the only known asteroid that has an Earth-like structure, with a core, mantle and crust, making it an incredible laboratory for testing hypotheses and theories, but it’s clear that a more complex model of planet formation therefore has to be considered.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com