A giant impact four billion years ago could have caused Mercury to lose most of its mantle to deep space, potentially explaining one of the planet’s key mysteries.
Mercury is unusual compared with other rocky planets in the solar system, as its iron core accounts for more than 80 per cent of its radius. Earth’s core, by comparison, makes up just 50 percent of the planet’s radius. This makes it very dense, with just a thin mantle around 500 kilometres thick above the core.
Astronomers have long wondered why this is the case, with no clear answer. One idea is that in Mercury’s first 100 million years, a large object thousands of kilometres across hit the planet, vaporising its surface and launching debris into space. But the problem with this idea is that we would expect this material to eventually fall back on Mercury.
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Now Christopher Spalding at Yale University and Fred Adams at the University of Michigan have proposed a solution to this issue. They say that since the solar wind, which is made up of charged particles streaming out from the sun, would have been 10 to 100 times stronger than it is today, it could have blown this debris away and caused Mercury to lose a large fraction of its mantle.
“Mercury’s got this huge mystery that we’ve known about for decades,” says Spalding. “However, back then, we know the solar wind must have been stronger. If you blast off material from Mercury in the environment of this young solar wind, [it can] be removed from Mercury’s orbit in a million years.”
Depending on the strength of the solar wind at the time, this cloud of debris may have then fallen into the sun. However, if the solar wind was strong enough, the debris – estimated to be grains of centimetre-sized material – could have been pushed out into the solar system. “There may be some remnants of Mercury left on Venus and potentially Earth,” says Spalding.
The finding may also have implications for exoplanets, suggesting planets closer to their stars can reach even higher densities than Mercury if more of their mantle is stripped away. “I would expect the maximum density to go up as you get closer to the star,” says Spalding.
The Planetary Science Journal
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