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Mercury is the smallest planet in the solar system. It’s a ball of rock and metal just under 5,000 kilometers across (about 40% wider than our Moon), and the closest planet to the Sun. That means it’s hot, of course, with a surface temperature that reaches well over 400° C (750° F). Lead melts at 327°C, by the way.
It’s common to describe Mercury as airless, lacking an atmosphere, and that’s true, more or less… though more less than more. It doesn’t have an atmosphere per se, but it does have an extremely tenuous gas over its surface called an exosphere. It’s so dispersed that the atoms in it are collisionless, meaning they’re far enough apart that it’s pretty rare that two atoms will collide.
The source of this exosphere is twofold. One is the constant barrage of tiny micrometeoroids shed by what are called Jupiter family comets, ones that orbit the Sun close enough that Jupiter affects them strongly. This material spreads out around the inner solar system and can run into a planet. They burn up in Earth’s atmosphere so that protects us on the surface, but on Mercury they directly slam into the surface, sending molecules flying.
The other source is the solar wind: Subatomic particles from the Sun moving at very high velocity ping into Mercury’s surface. In both the micrometeoroid and solar wind cases, because they aren’t likely to hit another atom, this freed material (individual atoms and molecules) can actually fly off the planet into space. This means Mercury’s exosphere isn’t stable like Earth’s atmosphere; it’s at a kind of dynamic balance between being created and being lost.
And that’s why it’s so interesting that a team of planetary scientists have reported that, on three separate occasions, NASA’s MESSENGER spacecraft saw Mercury’s exosphere suddenly increase in brightness, sometimes by as much as 20 times!
MESSENGER orbited Mercury for over 4 years, mapping its surface in high resolution, examining its mineralogy, and probing its internal structure. It also had an ultraviolet camera that could detect the light emitted by elements such as calcium, magnesium, and sodium, which are major constituents of Mercury’s exosphere.
Not that this adds up to much! One paper looking at MESSENGER data showed that the amount of calcium in Mercury’s exosphere is roughly 20 kilograms. Total. Over the whole planet. The office I’m sitting in while I write this has more air than that, and it’s only the size of a small bedroom.
Still, this is enough to glow feebly in ultraviolet, and MESSENGER measured it quite often. The exosphere density increases and decreases over time, generally following Mercury’s seasons — the planet’s orbit is fairly elliptical and so sometimes it’s in more favorable positions to get hit by the solar wind. This causes the exosphere to increase in density twice per 88-Earth-day orbit over a few weeks. Even then, though, it’s barely there.
But, in August 2011, October 2012, and April 2013, the exosphere suddenly jumped in brightness, meaning there was a quick blast of atoms ejected off its surface and into its exosphere. These were definitely not from some increase in the solar wind; too much material was blown off and conditions weren’t conducive for a strong gust of Sun particles to create the increase.
The most likely culprit? Meteoroid impacts. The cometary material that helps create the exosphere is small, basically teeny grains of dust. But these impacts must have been much larger: Two of them were from impactors probably 10 centimeters across (about the size of a softball or grapefruit) and the biggest likely 16 cm (a little smaller than a volleyball), assuming they were rocky in composition. The material may have come from small asteroids or comets; it’s not possible to know.
The impact speeds aren’t known, but on average a Mercury impact would be at about 30 kilometers per second — over 100,000 km/hr! Because Mercury is close to the Sun, objects that get that close are accelerated by the Sun’s gravity to higher speeds; typical Earth impacting objects are “only” moving around 20 km/sec.
This gives the impactor a lot of kinetic energy (energy of motion) and upon impact that’s converted to light and heat — the impact flashes were probably at about 10,000° C — as well as blasting out surface material. Judging from the brightness of the exosphere glow, the planetary scientists estimate that the first impact generated 500 grams (about a pound) of sodium vapor and 1,500 grams of magnesium vapor. The second impact created 1,500 grams of sodium, and the third roughly 250 grams. That’s a significant fraction of the total mass of the entire exosphere.
The ratio of sodium to magnesium seen in the first flash is about the same as their ratio in Mercury’s rocks, so that fits with an impact scenario. And impacts of this size aren’t unheard of; they happen all the time in our atmosphere. Judging from the three seen by MESSENGER (and a possible fourth one as well in October 2011) the scientists estimate impacts like this happen on Mercury roughly once per day. There’s large uncertainty in that though, it could be as few as 80 up to 800 annually. MESSENGER misses most of them because the geometry and timing have to be just right for it to detect the increase.
They didn’t see any calcium, but that’s not too surprising. It exists on the surface as a molecule of calcium oxide (CaO), and upon impact a molecule would get blasted off the surface. If and when it hits sunlight the ultraviolet light breaks the molecule up into a single atom of calcium and one of oxygen, and it’s the calcium that MESSENGER could see. However, on average, it takes about 13 minutes for a molecule of CaO to get zapped, and by this time the impact plume would have dispersed enough that the light it emits would be too dim to spot.
Magnesium is similar, in that it sits in rocks as magnesium oxide (MgO), but it only takes a few seconds on average for it to get broken apart by sunlight. The plume is still relatively dense when this happens, so the glow from magnesium atoms can be seen by MESSENGER.
In 2025 the joint ESA/Jaxa mission BepiColombo will go into orbit around Mercury, only the third spacecraft to visit the tiny world, and it also has a nice UV camera (called Phebus) that will examine Mercury’s tenuous exosphere. Hopefully it will see more impacts and we can get a better handle on how often and how hard Mercury is pummeled by interplanetary debris.
Mercury is a surprising and interesting place. It’s almost entirely alien, so different from Earth, but like us it too suffers the slings and arrows of outrageous impacts. The more we learn about Mercury the more we can about Earth, too, so as weird as the solar system’s tiniest planet is, there’s a lot it can teach us.