Study Reveals MESSENGER Watched a Meteoroid Strike Mercury

Fri, 01/29/2021 - 09:39
Jeremy Rehm

NASA’s MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission to Mercury has been out of operation for nearly six years, but the data it collected keeps on giving, from revealing new insights about Venus’ atmosphere to providing a new way to measure the length of time neutrons can survive on their own.

Now, a recent study in Nature Communications shows the spacecraft can add one more feather to its cap (or, perhaps more aptly, winged feet): It very likely witnessed a large meteoroid impact on Mercury — the first ever observation of an impact on the surface of another planet. Before, meteoroid impacts had been observed by telescopes only on Earth and the Moon.

“It’s just incredible that MESSENGER could watch this happen,” said Jamie Jasinski, a space physicist at the Jet Propulsion Laboratory in Pasadena, California, and the lead author on the study. “This data plays a really important role in helping us understand how meteoroid impacts contribute material to Mercury’s exosphere.”

At two-fifths the size of Earth, Mercury has just a sliver of an atmosphere, called an exosphere, with a pressure that’s one-quadrillionth of that felt at sea level on Earth. The exosphere forms on Mercury’s Sun-facing side from material originally on the planet’s surface, including sodium and around a dozen other molecules. Scientists believe meteoroid impacts, in part, are responsible for putting such material into Mercury’s exosphere.

“Large meteoroid impacts can blast off an enormous amount of material from the surface, briefly exceeding the mass of Mercury’s entire exosphere,” Jasinski said.

The meteoroids come from the asteroid belt, more than 200 million miles away, where gravitational interactions between asteroids and either Jupiter or Mars send small space rocks spiraling into the inner solar system. Some of them should inevitably hit Mercury, throwing particles thousands of miles into its exosphere.

But such an impact had never been recorded — it was purely hypothetical.

Scientists hedged their bets on MESSENGER, which would orbit Mercury for four years. They expected the spacecraft to see two impacts per year during its mission. But 2 1/2 years after falling into orbit, MESSENGER hadn’t seen any.

“It just shows how rare it is to have the spacecraft at the right place and time to be able to measure something like this,” said Leonardo Regoli, a space physicist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland — where MESSENGER was built and operated — and study co-author.