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MESSENGER was a triumph of a mission.
It was designed to last one year in orbit around Mercury, but it ended up lasting four,
from 2011 to 2015.
During those four years, it took over 200,000 images of the planet, it scanned the surface
for various minerals, and investigated its bizarre magnetic field.
Its findings have been critical to understanding Mercury, because the only other spacecraft
to even get close to the planet was Mariner 10 back in 1975.
Without MESSENGER, we wouldn’t even know what the entirety of the planet looks like.
I’m Alex McColgan, and you’re watching Astrum, join me today as we investigate the
science collected by the MESSENGER probe, the things that surprised NASA scientists,
and look at some of the most impressive features on Mercury’s surface.
One of the fascinating things about Mercury’s orbit and rotation is that while it isn’t
tidally locked to the Sun, it is locked in a rare 3:2 spin-orbit resonance, meaning it
rotates 3 times for every 2 orbits around the Sun.
This means that its sidereal day, or the time it takes to do a full rotation on its axis,
takes 59 days.
By pure coincidence, this is almost exactly half its synodic period in respect to Earth,
which is 116 days, so between conjunctions of Earth and Mercury, Mercury rotates on its
axis exactly twice.
Historically, there was a big problem with that.
Because of this coincidence, we believed that Mercury was tidally locked to the Sun for
the longest time.
You see, Mercury orbits closely around the Sun, meaning it was always tricky for astronomers
to get a good look at it for most of its year.
When it finally got in a good viewing angle from our perspective, we’d have a look at
the face of the planet.
118 days later, we’d have another look during this prime observation alignment, and see
the same face again.
So, to them, it showed that Mercury was tidally locked to the Sun.
What astronomers didn’t realise is that Mercury had rotated exactly twice on its axis
during this time.
It wasn’t until radar observations of the planet that we found out it does rotate slightly
faster than it orbits.
Mariner 10 had a similar problem.
While it did give us great images of the planet, its orbital period was almost exactly twice
that of Mercury’s, meaning that even though Mariner 10 did 3 flybys of Mercury, the same
side of the planet was always sunlit every time it passed by.
In order to map the full surface in detail, we needed an orbiter, something that could
follow Mercury as it went through its day night cycle while it travelled around the
Sun.
As my previous video showed, this was harder than it sounds.
However, in 2011, MESSENGER successfully overcame those issues, and entered Mercury’s orbit.
On board MESSENGER were a host of scientific instruments, including a magnetometer to map
out Mercury’s magnetic field.
Unlike Venus and Mars, Mercury has a significant magnetic field originating from its core.
Like Earth’s, it is likely generated by a dynamo effect in its molten core.
Our fast rotation and tidal stretching from our Moon keeps our core molten, but Mercury
doesn’t have a moon, or a fast rotation.
What it does have, however, is an eccentric orbit, more so than any other planet.
Gravitational strength increases and decreases as it gets closer and further away from the
Sun, so the tidal forces pull and squeeze on the planet, the friction of which keeps
Mercury’s core hot and the dynamo going.
Unlike Earth’s, it is offset from the centre by about 20% of the planet’s radius, and
we don’t really know why.
Its magnetic field is only about 1% as strong as Earth’s, but this still has an impact
on deflecting a lot of the solar wind around the planet.
However, due to it being closer to the Sun, the solar wind pressure is a lot greater here
than it is around Earth.
Add a weak magnetic field to the mix, and the magnetosphere around Mercury is compressed
closely to the planet’s surface.
Earth’s, on the other hand, extends many times the diameter of the planet away from
the surface.
Interestingly, these factors make the magnetosphere of Mercury highly dynamic.
What does this entail?
Well, for one, reconnection events are 100 times more common around Mercury than around
Earth.
Reconnection events occur when magnetic field lines snap together as the charged solar wind
pushes against a planet’s magnetosphere.
When this happens, it allows a few of these charged particles to break into the planet’s
magnetosphere, entering a region of plasma in the planet’s magnetotail.
The flows you see in this simulation in the plasma region are from reconnection events.
Another feature of the magnetosphere that MESSENGER detected was energetic bursts of
electrons, producing hundreds of thousands of electron volts of energy.
As MESSENGER orbited Mercury, it picked up thousands of these events, and mysteriously,
they were mainly localised in the northern hemisphere, and were compressed towards the
planet along the sun facing side.
This is still an ongoing field of study; however, scientists believe these electrons have been
accelerated through breakdowns in the magnetotail, and they follow the direction of the magnetic
field around from the south pole to the north.
MESSENGER also hosted a wide array of spectrometers.
Spectrometers are important for detecting the composition of mineral deposits on the
surface without having to actually take a sample.
Spectrometers can also be used to detect the particles in the atmosphere.
Now, Mercury doesn’t have an atmosphere per se, rather an exosphere, or an extremely
tenuous atmosphere.
It is so thin that the particles within it don’t interact with each other.
But what MESSENGER found out about this exosphere’s relationship to the surface really surprised
scientists.
Mercury is covered with volatile substances, it isn’t just a fried, rocky planet.
It seems to be covered in potassium, magnesium, sulphur, sodium and chlorine, at a higher
level than any other terrestrial planet, and much higher than on our Moon.
The fact that its volatile ratios have more in common with Mars than with Earth and Venus
have completed disproved a lot of solar system formation theories that existed before MESSENGER
arrived at Mercury.
These volatiles are blasted by radiation from the Sun, more so at the equator than near
the poles, which may explain why on the surface, some substances, like potassium, are more
abundant in the northern hemisphere than around the equator.
It is much hotter on Mercury around its equator than at the poles, so the potassium there
would have been heated enough that much of it has been lost from the surface to the exosphere.
Now, the exosphere contains a lot of the particles you would also find on the surface, like sodium,
potassium, and the others I mentioned.
This exosphere is not at all stable.
Solar wind picks up and carries away a lot of charged particles, and solar light pressure
also pushes a lot of the neutral particles away.
Were it not for the processes that replenish the exosphere, Mercury would lose it all to
space over a relatively short time frame.
While most substances certainly do come from the planet’s surface, it also contains other
elements, like hydrogen and helium, which cannot be found there.
So where did they come from?
Well, as you may know, the Sun is made predominately of hydrogen and helium, and interestingly,
the solar wind carries these particles to Mercury.
Some of the solar wind actually gets caught up in the exosphere and stays for a while.
As far as we know, this is the only major source of hydrogen and helium in the exosphere.
The particles that do get stripped away have been spotted by MESSENGER too.
In these images, we see calcium, an unknown process of which means it’s much more prevalent
in the exosphere during the planet’s dawn than dusk, and magnesium streaming away from
the night side of the planet.
In fact, Mercury’s tail has been known about for a while.
In these images, sodium ions are lit up as they stream away from the planet, making Mercury
look like a comet.
Incredibly, if you were to look up into the night sky on Mercury, you would actually see
a faint yellow glow, reminiscent of city lights on Earth.
This tail is seasonal.
The eccentric orbit of Mercury means that its distance to the Sun varies throughout
its year, and as it orbits its orbital speed also changes.
So, the time of greatest sodium emission is actually when Mercury is at its middle distance
from the Sun.
There was one other curious substance found in Mercury’s exosphere that scientists really
weren’t expecting.
Water vapour.
This could come from cometary tails as they pass by, or it could come from ice deposits
MESSENGER detected around the planet’s poles.
Surprisingly, water ice can exist on this scorched planet, but only at the bottom of
permanently shadowed craters, forever protected from directly interacting with the Sun’s
light rays.
The Earth-based Arecibo Radio Telescope had already detected highly reflective regions
around the poles, and as images from MESSENGER came in, these regions matched up with regions
of permanent shadow at the bottom of large craters.
Estimates put the amount of water ice found on Mercury at a quadrillion kgs.
This isn’t huge by Earth standards, but it would be a significant boost to any future
colony there to have that much water accessible.
There were some other surprising features found on Mercury’s surface too.
Hollows were found dispersed all over.
This is a feature unique to Mercury.
While we aren’t completely sure what causes them, they may be volatile substances sublimating,
and they are unique to Mercury simply due to the proximity of Mercury to the Sun.
They seem to be an active geological process, apparently some of the youngest features on
the planet, and they are certainly not the result of meteor impacts.
Other young features include evidence of volcanic deposits.
Look how this crater appears to be completely filled in by this volcanic flow.
Mercury certainly isn’t volcanically active today, but billions of years ago, it may have
hosted numerous volcanoes across the surface.
We see evidence of shield and compound volcanoes that were active in the past, with at least
9 vents spotted in Mercury’s most famous surface feature, Caloris Basin.
Caloris Basin is a huge impact crater, one of the largest in the solar system, at 1,600km
in diameter.
A 100km wide impactor likely caused this crater, creating a global event that would have changed
the very nature of Mercury at the time.
The scars of it remain, there is a 2 km tall mountain range surrounding the rim, and radial
troughs coming away from the centre.
Further away from the centre, these troughs turn into concentric rings.
The reason for this is not known.
At the antipode of the impact, or the opposite side of the planet, is found a region of weird
terrain, likely formed when the shockwaves from the collision converged.
Here, the terrain is hilly and lineated, unlike much of the Mercurian surface.
The last thing I’ll discuss today are these long scarps, evidence that Mercury is cooling.
That may be a surprise to you considering it’s getting blasted by the heat of the
Sun, however overall, Mercury still loses more heat than it gains but from the Sun.
These scarps show that Mercury is contracting, and from MESSENGER’s data, Mercury has contracted
by over 14km in diameter since its formation, a lot more than was expected.
All these findings have thrilled scientists, yet even though we would barely know anything
about Mercury were it not for MESSENGER, somehow this mission isn’t that well known among
the general public.
Perhaps ESA’s BepiColombo mission, already on its way to Mercury right now, will better
capture the public’s imagination when it arrives in 2025.
In any case, personally, I really loved the MESSENGER mission and simply because it brought
Mercury to life in my eyes.
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Thanks for watching!
Want to find out why Mercury is such a difficult planet to get a probe around?
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