The planet Mercury occupies an odd place in the solar system in more ways than one.
Unlike Uranus and Neptune, Mercury is one of the original planets long known in ancient
times stretching into prehistory. It’s visible to the naked eye, bright, but not easy to see. It’s
often hiding behind trees near the horizon. This was noted most prominently in the distant past,
in that it never gets very far above the horizon and it moves very quickly. This is because it’s
orbiting the sun very rapidly, leading the Greeks and Romans to equate it with Hermes, or Mercurius,
the fast footed messenger god of the greco-roman world. But it was known to many others worldwide,
astronomers in China and the Middle East and many others watched and recorded it. Mercury can be
seen in the evening or the morning, and by 350 BC it was known that both were the same object.
We know today that Mercury is the closest planet to the sun, which puts it into the
strange status of being sort of tidally locked to the sun, but not in the usual sense of an
exoplanet only showing one face to its star. Rather mercury is locked to its orbital period,
leading to a 3:2 resonance, with a very long day during a somewhat short year. Mercury is
a terrestrial world, rocky, and it’s the smallest planet if you don’t count Pluto
but it’s actually significantly smaller than some of the solar system’s moons
like Ganymede and Titan, but not actually in mass. And that’s where this gets weird.
That mass means it’s not like any other planet in the solar system and that’s where the mysteries
of Mercury begin. Mercury’s composition is strange. It’s 70 percent metallic and only
30 percent silicate rock, which is just about the exact opposite of the rest of the terrestrial
planets. It’s a largely metal world and this is weird. In short, in some ways Mercury looks
more like a planetary core than it does a normal planet which has led to a hypothesis that, well,
that’s exactly what it is. It’s possible that during the early solar system,
Mercury was significantly larger, about 2.5 times what it is now with a composition
analogous to the other terrestrial planets. It might have started out in an orbit something
closer to Mars than what it is today, potentially placing it at one time in Earth’s neighborhood.
And this is where the first oddity lies. So for all that material to have been stripped off
leaving mostly just a core, Mercury would have had to collide with another sizable object early on,
or recent work suggests several of them in such a way as to glancingly pare it down rather than
outright destroy it. This would have thrown a massive amount of material into the solar system
that’s been very tenuously linked to two rare meteorite types, the enstatites and the aubrites
that may have resulted somehow from that collision or collisions, but the evidence is tenuous.
Enstatites are odd in their composition, which is significantly different from normal run of
the mill chondrite stony iron meteorites. Their composition suggests an origin near
the inside of the protoplanetary disk of the solar system, somewhere around or within the
current orbit of Mercury. There have been several hypotheses as to what the parent
body of these meteorites is, but one option is Mercury because they are similarly metal rich,
but the other is the asteroid 16 psyche, itself anomalously metal rich, which we have a NASA
mission en route to investigate this object. It won’t be long before we will know more.
Aubrites are even stranger. These rocks are igneous, meaning volcanic,
and they often very closely resemble rocks from the moon, but not enough to suggest
that its their origin. They often feature chunks of other meteorites embedded in them,
suggesting that the parent body was undergoing violent collisions. Mercury ticks both of
those boxes, it’s somewhat similar to the moon, and it’s covered in impact craters.
Whatever resulted in Mercury being like it is however is weird because the same sort of
thing happened to Earth. The leading model for the formation of the moon is a blow between an
object roughly the size of Mars named Theia with proto-earth which flung off the material
that formed the moon. This situation may have been unique in the solar system as
far as moon formation, but it definitely required a glancing blow of proto-earth,
something Mercury also seems to have required to strip its outer layers. Protoplanetary impacts
are well attested in the solar system. This is also noted with objects like
Uranus’ moon Miranda which appears to have been completely shattered at some point,
presumably in another collision, and then reformed out of the pieces.
This represents an awful lot of collisions in the solar system and may suggest that such things are
very common early in the history of many star systems such as our own, bolstered by hints in
infrared of recent collisions in young exoplanet systems. This may in turn mean that the situation
that formed the earth moon system may not be as uncommon as it might seem, even involving
glancing blows, which would go some way in eliminating a solution to the Fermi Paradox,
the rare moon hypothesis where the presence of a moon like ‘earth’s is required for life.
This hypothesis has been taking hits already however in a different regard, in that the
main reason it was invoked was the thinking that the most likely place for life to have
arisen on earth are tidal pools. No tides, no calm pools for the chemistry to occur in. That said,
Earth’s tides would not go away if we plucked the moon out of the system, rather we would
then have solely the solar tides created by the sun, which are weaker, but they exist.
Recent work in abiogenesis however is pointing towards volcanic hot springs
having been the environment, which volcanism and liquid warm water in the universe are
not likely to be rare. Another outstanding mystery of Mercury is where exactly all of
this happened. There’s uncertainty that Mercury has always been in its current
orbit. Hints from the Messenger spacecraft to Mercury actually showed that it has a
very high ratio of potassium in its makeup when compared to Thorium. The thing with
potassium is that it tends to evaporate in a hot environment, whereas Thorium does not.
When you look at these radios with Earth and the moon, Mars and also Venus these
all seem to be the right ratios for their overall temperatures. But not for Mercury,
it looks like an object that formed further from the sun,
and that the huge collision or collisions may have happened in that original position,
and its current position close to the sun is the result of getting disturbed orbitally by
the hits. But something even stranger has been advanced, though it's very much in dispute.
It’s the idea that maybe Mercury IS Theia and the debris stripped from it is now contained
in the moon and earth. This scenario would have required something on the
order of around 20 Mars sized objects in the early solar system colliding with and
enlarging earth and Venus. Mars here is just one of those surviving by chance,
and Mercury being a core remnant, that potentially is or was Theia.
Proving or disproving that will take an enormous amount of measurement,
which is set to to happen with the european space agency’s Bepi-Columbo mission which is
enroute to Mercury to arrive in December 2025. That would be a game changer in
astrobiology if we get any further hints of that scenario however, if indeed it turns out some
aspect of life on earth is related, albeit peripherally, to a collision with Mercury.
Another question on Bepi Columbo’s agenda is searching for water on Mercury. There
are tantalizing hints that Mercury shares a position like that of the moon,
where there are ice bearing craters that never see sunlight that could potentially provide a
water source if anyone ever tries to found a Mercury colony. That’s not talked about much
this early in the game of space exploration, but if a science outpost or something like
that is ever created there it would have a known water local supply, and colonizing it would also
benefit from experience in lunar colonization, which is talked about and also involves craterst,
because Mercury doesn’t seem to be that different of an environment, at least in some respects.
But there will be some differences, Mercury can be very hot. But also, Because of how dense it is,
Mercury’s gravity is closer to Mars than the moon and we already know about one situation
that appears to be unique in the solar system with Mercury. Situated around a handful of
Mercury’s craters are weird depressions, or hollows, that seem to be caused by something
beneath evaporating. This appears to be recent activity, and no one knows why, other than some
volatile appears to be coming up inside mercury from deep down and sublimating out at some level.
These were first seen by messenger, and Bepicolombo comes a decade later,
so scientists are hoping to look for any changes that might have happened in the
interim. The moon doesn’t do any of this, so it would mean that there is active geology of
some sort going on at Mercury driven by an unknown process that differs from the moon.
Another way that Mercury is weird is in that it’s very dark for a solar system object. Much
darker than the moon, and no one knows why. If it were in the sky replacing the moon,
it would only be about 2/3rds as bright. It might simply be that the extreme heat Mercury is subject
to darkens its surface materials in comparison to the moon. Or it could be something really odd like
graphite that had some reason for being there that other bodies of the solar system did not.
Mercury also somehow still maintains a magnetic field, despite appearing,
from what we know, physically too small to have one. It’s very weak, but it’s there,
yet the larger Mars does not have this. This may suggest that somehow Mercury’s core is
still molten, at least partially. There is more than circumstantial evidence for
this in that Mercury from tidal data seems to have a liquid layer below the surface,
and certain geologic cracks and features suggest its shrinking due it still being in
a cooling process. Why the core has remained molten however is still completely unknown.
And lastly is Mercury’s very tenuous atmosphere. It’s extremely thin, but was actually a debated
topic before any spacecraft visited Mercury. As far as atmospheres go, it’s unique in the solar
system. That very heat that should have evaporated Mercury’s potassium, is definitely doing so now
with a variety of materials getting evaporated and along with components from the solar wind
making up the atmosphere. But the atmosphere is also being stripped off by the sun, so Mercury
actually sports a tail not unlike a comet as this process of evaporation and stripping continues.
Mercury is often an overlooked world. It’s very difficult to get to and explore, and
for many years it was thought to be a relatively uninteresting moon-like world. As is usually the
case with the solar system, once we learn more, the more interesting objects get and if it can
be shown that Mercury is indeed Theia, then it becomes one of the most important and formative
objects in the solar system and without it, we may not have ever been here. Without Mercury, it may
be that Earth might well have ended up a smaller Mars like world, unable to support complex life.
Thanks for listening! I am futurist and science fiction author John Michael Godier currently
fessing up to an astronomical shame. A black mark if you will. So in nearly 40 years of
amateur astronomy and hundreds upon hundreds of visual observations of all the major planets,
I have only ever caught Mercury in a telescope twice. Once as a teenager painstakingly going
through the solar system to check off all the major planets after much searching for a clear
view, and the other serendipitously in the Mojave Desert at a star party. I blame the
copious amounts of trees always in my way and be sure to check out my books at your
favorite online book retailer and subscribe to my channels for regular, in-depth explorations into
the interesting, weird and unknown aspects of this amazing universe in which we live.