Mercury is Bombarded by Ancient Comet Dust that Impacts Its Atmosphere

Mercury is being pelted regularly by bits of dust from an ancient comet. Scientists have taken a closer look at the planet and have found that this bombardment has a discernible effect in its tenuous atmosphere.

On Earth, we can see the effects of cometary dust on a regular basis. This dust takes the form of "shooting stars" that burn up in Earth's atmosphere. At certain times of the year, the numbers increase manifold and create a natural fireworks display known as a meteor shower. This is caused by the Earth passing through a stream of dust particles left behind by certain comets.

One of the most well-known meteor showers, called the August Perseids, originates from comet Swift-Tuttle, which was last seen in 1992 and won't be back in the inner solar system for another century. However, Earth isn't the only planet to encounter cometary dust.

Mercury is typically thought of as airless, but we've known since the time of the Apollo moon landings that Mercury and other airless bodies, like the moon, are surrounded by clouds of atomic particles either launched from the surface or brought in by the solar wind. Though tenuous by comparison to the dense atmospheres of the Earth or Mars, the observational record has revealed these "surface boundary exospheres" to be complex and dynamic entities, fascinating to study in their own right.

The researchers found some strange readings when it came to emissions seen right after Mercury passes through its closest point of its orbit to the sun. In this case, the researchers believe that the evolution of the dust stream over thousands of years somehow shifted the stream away from the comet Encke's present orbit.

But what's causing the shift? To find out, the researchers simulated the evolution of the Encke stream. The researchers found that there was a subtle interaction between the dust grains and sunlight called Poynting-Robertson drag. Over time, this creates an extra, tiny force on the grains which can add up over time.

The findings have set an interesting precedent on the importance of the different dust populations in exosphere production. The researchers already knew that impacts were important in producing exospheres, but now they know the relative importance of comet streams over zodiacal dust.

The findings are published in the journal Geophysical Research Letters.

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