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Study Reveals MESSENGER Watched a Meteoroid Strike Mercury
Mercury鈥檚 surface, as captured here in 2013 by the APL-operated MESSENGER spacecraft, is pockmarked with craters that have formed from millions of years of meteoroid impacts. MESSENGER very likely witnessed one of the most recent of these crater-forming collisions in 2013.
Credit: NASA/秘密直播 APL/Carnegie Institution of Washington
狈础厂础鈥檚 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鈥 atmosphere to providing a new way to measure the length of time neutrons can survive on their own.
Now, a 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 鈥 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.
鈥淚t鈥檚 just incredible that MESSENGER could watch this happen,鈥 said Jamie Jasinski, a space physicist at the Jet Propulsion Laboratory in Pasadena, California, and the lead author on the study. 鈥淭his data plays a really important role in helping us understand how meteoroid impacts contribute material to Mercury鈥檚 exosphere.鈥
At two-fifths the size of Earth, Mercury has just a sliver of an atmosphere, called an exosphere, with a pressure that鈥檚 one-quadrillionth of that felt at sea level on Earth. The exosphere forms on Mercury鈥檚 Sun-facing side from material originally on the planet鈥檚 surface, including sodium and around a dozen other molecules. Scientists believe meteoroid impacts, in part, are responsible for putting such material into Mercury鈥檚 exosphere.
鈥淟arge meteoroid impacts can blast off an enormous amount of material from the surface, briefly exceeding the mass of Mercury鈥檚 entire exosphere,鈥 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 鈥 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鈥檛 seen any.
鈥淚t just shows how rare it is to have the spacecraft at the right place and time to be able to measure something like this,鈥 said Leonardo Regoli, a space physicist at the 秘密直播 Applied Physics Laboratory in Laurel, Maryland 鈥 where MESSENGER was built and operated 鈥 and study co-author.
Artist鈥檚 illustration depicting how MESSENGER observed the first meteoroid impact on another planet鈥檚 surface. Particles (neutral atoms) ejected by the meteoroid skyrocketed over 3,000 miles above Mercury鈥檚 surface, outside the bow shock of Mercury鈥檚 magnetosphere. There, photons of light turned the neutral particles into charged particles (ions), which one of MESSENGER鈥檚 instruments could detect.
Credit: modified from Jacek Zmarz
On December 21, 2013, as MESSENGER glided over Mercury鈥檚 sunward side, one of its instruments 鈥 the Fast Imaging Plasma Spectrometer (FIPS) 鈥 saw something strange: an unusually large number of sodium and silicon ions blowing in the Sun鈥檚 solar wind, a powerful gale that spews charged gases from the Sun. Oddly, these particles were traveling in a tight beam, nearly all in the same direction, and at the same speed.
When Regoli, Jasinski and other team members looked over the data years later, those details were like a signpost. They indicated the particles were likely 鈥測oung,鈥 having only recently floated into the solar wind. But where did they come from?
Using the particles鈥 speed and direction, the researchers rewound the clock, tracking the particles鈥 motion back to their source. They found the particles clustered in a dense plume, one that had erupted from Mercury鈥檚 surface and extended nearly 3,300 miles into space.
The researchers considered various possible causes for the plume, but a meteoroid impact made the most sense. They estimate the meteoroid was likely just a little over three feet long 鈥 relatively small, but large enough that models suggest it would create a plume with a height and density closely matching what FIPS detected.
鈥淭his was a special observation, and really cool to see the story come together,鈥 Regoli said.
The team plans to leverage an instrument similar to FIPS on the European Space Agency鈥檚 BepiColombo mission, which launched for Mercury in 2018 and will approach the planet in late 2025, to make analogous observations and look for more meteoroid impacts during its year orbiting Mercury. Regoli noted that researchers will need to hone their models before using BepiColombo to make new observations, but the opportunity to see another Mercurian impact would be invaluable.