Mercury appears to share its orbit with a huge ring of dust millions of kilometres thick, and scientists aren鈥檛 sure how it got there. None of the mechanisms researchers could think of to explain it create enough dust, and the region of space it passes through is so close to the sun that we would expect any dust there to be pulled in by the sun鈥檚 gravity almost immediately.
The dust ring was discovered in 2018 after researchers analysed data from the Solar Terrestrial Relations Observatory, which is composed of a pair of satellites that orbit the sun along a similar trajectory to Earth. Although Earth and Venus also share their orbits with clouds of dust, the discovery of such a cloud near Mercury was a major surprise.
鈥淵ou would not expect anything to be there, because things are not stable in that area 鈥� it鈥檚 a really violent region of the solar system,鈥� says聽 at the Catholic University of America in Washington DC. Objects in that area tend to quickly fall into the sun or be destroyed by its radiation. The objects could also face a close encounter with Mercury鈥檚 gravity that throws them either into the sun or towards the outer regions of the solar system.
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Pokorn媒 and his colleagues investigated a slate of possible explanations for the dust cloud鈥檚 existence. But the only scenario that could explain the dust鈥檚 presence was that debris was created within Mercury鈥檚 orbit. The simplest way to produce such debris is through impacts with Mercury聽that throw dust and rocks off the surface of the planet.
The researchers estimated that in total, the dust ring has a mass of less than 4 billion tonnes, which is the mass of an asteroid about 1570 metres across. That relatively low mass makes the ring extremely tenuous and hard to spot. 鈥淚f you were up close, you would not see [the ring] 鈥� we have something similar at Earth, and you cannot see it,鈥� says Pokorn媒.
Nevertheless, when they used data from聽the Messenger spacecraft to compare the mass of the ring to the estimated mass of ejected material from known large impacts on Mercury, the researchers couldn鈥檛 find a way to produce enough dust. While it is possible that there were thousands of tiny collisions, or that some of the large ones produced far more debris than expected, the researchers don鈥檛 know enough about the dust or Mercury to be sure.
鈥淭here are so many numbers you can use that you would spend the rest of your lifetime just trying to figure out exactly what happened, without more data,鈥� says Pokorn媒. 鈥淎t the moment I have no idea what could cause it.鈥� There are two major observatories 鈥� NASA鈥檚 Parker Solar Probe and the European Space Agency鈥檚 Solar Orbiter 鈥� on their way to observe the sun now, as well as the BepiColombo spacecraft heading towards Mercury. So, there may soon be enough information to start piecing the ring鈥檚 origins together.
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