Watching the Sun Rip into Planets' Atmospheres

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Every second, the sun is emitting a wind of countless billions of charged particles, sending them screaming outwards, traveling at hundreds of kilometers per second. Watching what happens when this solar wind slams into planetary atmospheres is the mission of SPRINT-A, the first space telescope dedicated to observing the planets.

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Constructed by JAXA, the Japanese Aerospace eXploration Agency, SPRINT-A (full name, the Spectroscopic Planet Observatory for Recognition of Interaction of Atmosphere) is due to be launched later this year. From its vantage point in an elliptical orbit around Earth (not unlike the orbit occupied by the Chandra X-ray Observatory), it’ll turn it’s gaze towards our neighboring planets.

Seeing with ultraviolet eyes, SPRINT-A’s main objective will be to watch for high energy interactions in the upper atmospheres of the planets. Taking both images and spectra in the Extreme Ultraviolet (EUV) wavelength range, it should afford us our best view yet of the solar wind crashing into our neighboring worlds.

We already know that the solar wind gradually erodes the atmospheres of every planet in our solar system — even Earth, hidden under its protective magnetic field, isn’t safe from the assault.

But of the rocky inner planets in our solar system, Earth was the lucky one. Many planetary scientists believe that Mars, Earth, and Venus began life as very similar worlds. Certainly, they condensed from the same cloud of dusty, planet forming material, so they must have originally had similar compositions. Needless to say, they didn’t all turn out the same way.

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In no uncertain terms, the atmosphere of Venus is doomed. With charged solar wind particles grinding away at its atmosphere and no internally generated magnetic field to protect it, Venus is steadily losing billions of particles of its atmosphere into space every second.

Mars has no internally generated magnetic field either, and most of its atmosphere has already been lost to space. With so little atmosphere left to keep it warm, Mars is a cold, dry little planet.

Determining exactly how the atmospheres of these planets escaped into space is a rather important goal, especially if we want to ensure that Earth doesn’t one day share the same fate. It may also shed some light on the early days of the solar system, when the sun was much more active and the constant barrage of solar wind was even stronger than it is now!

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SPRINT-A’s second goal lies a bit further out. Around our solar system’s largest planet lies the most volcanically active object we know of. Where the planets I mentioned before don’t have much of a magnetic field to speak of, Jupiter has a field strong enough to more than make up for them. And deep within that magnetic field lies Io, Jupiter’s ill tempered, sulfurous moon.

Exactly how Io interacts with its surrounding environment is also a topic in need of more research. Jupiter is a harsh planet to get close to, and Io, close as it is, gets constantly squeezed and warped by the giant planet’s gravity. As the tortured little moon orbits Jupiter, complaining perpetually through endless volcanic eruptions, it leaves a trail of sulfur ions in its wake.

As these sulfur ions leave Io, they collide with some of the harshest radiation belts in the solar system, trapped by Jupiter’s magnetic field. These collisions are actually remarkably similar to those that happen when the solar wind hits the atmosphere of a planet.

Precisely what SPRINT-A discovers remains to be seen, but it’s scheduled to launch on Aug. 22. As it leaves the atmosphere, it’ll also be sitting atop a launch vehicle adorned with messages sent to JAXA by 5812 people (of whom 5360 sent messages in Japanese), printed as part of the craft’s decoration.

Image: Collage Showing SPRINT-A in orbit, together with images of Mars and Jupiter. Credits: JAXA/NASA