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When something is as incredibly difficult to get to as Mercury, it is extremely tricky to study.
Which is one of the reasons why in all of human history, there have only ever been 2 missions to
Mercury, with just one more on the way. Mariner 10 in 1974, MESSENGER in 2011 and BepiColombo due to
arrive in 2025. Of these prior two missions, only MESSENGER went into orbit around Mercury, and so
it is the only mission to ever give us close up shots of Mercury’s surface. And crazily enough,
some of the formations we’ve seen on the surface are still unsolved mysteries even more than a
decade on, while other formations give us hints at the raw primal power of the early solar system.
I’m Alex McColgan, and you’re watching Astrum. Join with me today as we examine the imagery
taken by the MESSENGER probe, focusing on some of the geological formations on this rarely visited
planet, to try to decipher what they imply about the planet’s evolution and history.
I hope by the end of the episode to have earned your like and subscription.
When you look at the surface of Mercury, there are a few features that immediately jump out
at you. First, its colour. Mercury’s colour is not actually monochrome,
and is smattered with speckled greys, creams and beiges, with lighter sections and lines.
These darker sections are believed to indicate high levels of graphite,
the same material used in pencils! And the lighter sections? Well, we’ll get onto them later.
Beyond that, you most likely noticed the craters. Much like the moon, Mercury is
covered with craters, as ancient pieces of space debris crashed down on the unprotected planet
with a roughly even distribution. These offer fascinating insights into the planet’s violent
history. You can get a sense for how old a crater likely is by how sharp its crater rims are. Sharp
and crisp rims are likely a lot more recent than the older, rougher rims that have had more time
to erode down due to the natural processes happening on the planet. Sometimes asteroids
strike within the same place as older collisions, creating overlapping craters of differing ages,
such as the craters here. But it is in the difference between these older and younger craters
that we get our first fascinating clue about the surface of Mercury. It is an active, flowing
place. Although there is no real atmosphere to produce the weathering we would imagine,
evidently things on the planet’s surface do not remain static on an astronomical time scale.
In fact, here’s an interesting phenomenon to illustrate what I mean.
As you look over the surface of Mercury, you might notice certain ridges that crisscross
certain parts of the planet. Here are some very prominent examples towards Mercury’s north.
When I first saw these strange ridges, it made me think of crumpled paper.
Is this something similar? These ridges are hundreds of kilometres across and can be found
all over the planet. So, what has caused them? Well, scientists believe that in Mercury’s past,
the planet was much warmer. This may seem weird considering Mercury is so close to the Sun,
why would it have cooled? However, these lines prove this is the case.
As the core of the planet cooled, it contracted. And much like a grape shrivelling down to become a
creased raisin, the surface crust of Mercury seems to have buckled and bunched up in line with this
cooling process. This is a fascinating phenomenon that distinguishes Mercury from our Moon,
as this cannot be found there to anywhere near the same degree. But Mercury’s surface is not
just crumpling. It is also smoothing out. In this crater, there is evidence of slumping
having taken place. While about 90 degrees of the crater wall has retained its shape, the remaining
270 degrees has slipped further into the crater bottom, breaking away from the rest of the rim
under the force of its own weight. Scientists are not entirely sure why this happened to only some
of the crater and not all of it. Is the soil particularly hard in that bottom right corner?
Was it something to do with the angle the impactor struck at? We don’t really know. And that’s one of
the things that is so intriguing about Merury – there is still so much more to discover.
Here's another interesting phenomenon. In my videos about the Moon, I mentioned crater rays,
or ray systems. These spidery lines that radiate out from certain craters
are a prominent part of Mercury’s surface as well, with some stretching over 400km across.
Their lighter colour is a sign that the material kicked up from under Mercury’s surface is
less graphite-rich, or at least is a different chemical composition to the Sun-exposed surface.
But did you know that for a while, scientists could not account for how these spidery lines
were formed? When they tested different weights, consistencies of terrain, and speeds of impact,
they were unable to recreate these patterns in lab conditions. Whatever they tried, the material
they kicked up would always return back down in a consistent circle, not thin, spider web lines.
Scientists wracked their brains for years, but now it seems that this mystey has been solved.
In 2018, a scientist called Tapan Sabuwala was scouring the internet and discovered that a group
of students had managed to recreate the spidery line pattern of crater rays. Sabuwala was excited,
but also confused. Why were these students able to manage what other scientists had not?
Interestingly, he realised this was an example of scientists being too neat. Before performing
their tests in lab conditions, Sabuwala and other researchers had always prepped the experiment by
smoothing out the sand their test asteroid was impacting into. The students had not done this
step, leaving their test surface rough. This made sense in hindsight, as it more closely mimics the
rough terrain on the surface of an alien planet. And as it turns out, this was the entire key
to how these rays were formed. Crater rays do not care about the speed of impact, the angle
or the composition of the crust. They only care about the surface shape and how rough it is.
Knowing that this is how these lines are formed,
it really opens your eyes to the scope of some of the impacts that have struck Mercury in its
past. Remember I mentioned how some of these ray systems stretch over 400km? That was just
the smaller ones. Just look at the ray system originating from the crater known as Hokusai.
These rays must have been created from an incredible impact, as their lines stretch
almost entirely around Mercury’s surface – which, by the way, has a circumference of 15,000km.
And although not quite as large, the ray system of the crater Debussy covers over 1000km.
While the Moon also has ray systems, they usually smaller than these. In fact, one of the main
visual distinguishing aspects of Mercury are these giant ray systems. These show us that one of the
formative processes that explain Mercury’s unique surface is incredibly powerful bombardments.
Seeing as the Sun is so nearby, objects caught in this intense gravitational pull would
crash into Mercury with far more force than Mercury could produce with its own gravity.
Mercury’s gravitational pull is so weak compared to the Sun
that Mercury cannot normally capture objects as moons – they get pulled past, instead.
This is one of the reasons why visiting Mercury is so difficult for spacecraft. But that’s not
to say that Mercury can’t stop such an object reaching the Sun, it just has to bodyblock it.
Yet, this explanation cannot explain this last formation. Take a look at
one of the most fascinating craters on Mercury – Apollodorus, and its surrounding Pantheon Fossae.
At first glance, you might think that there is nothing to unusual about the crater Apollodorus.
Yes, those fractures running out from the centre are a little unusual.
There are a few features that are odd here. To begin with, the fractures,
and their surrounding radial fractures bear a strange resemblence to fractured glass.
Glass fractures in this way due to its hard but brittle qualities.
As you have seen, most other craters we have seen on Mercury do not follow this pattern.
The crust of Mercury does not tend to fracture, but instead sprays in ray systems
or just leaves perfectly round craters. This is in keeping with a looser material make-up.
Sand does not fracture when hit. We do not see this fracturing anywhere else on the planet
either, so something unusual is clearly happening here. Was the surface of Mercury particularly cold
and hard when this impact occurred, thus making it more brittle? Mercury’s nights can get as cold
as -180 degrees C. But if that was so, why has this not happened in other places? About half the
impacts should be hitting Mercury’s night side, at least. When we take a closer look at Apollodorus,
the crater you might assume is the cause of the fractures, we notice something even stranger.
Apollodorus is not quite the epicentre. While it’s pretty close, doesn’t actually line up.
This might suggest that Apollodorus and the spidery fractures of Pantheon Fossae are actually
unrelated. Whatever caused this phenomenon may have happened, only to later be hit by
an asteroid near to - but not on - its epicentre. But if that’s so, what caused Pantheon Fossae? The
intriguing thing is that we don’t know. Evidently, something created this fractured glass shape,
but left no crater. Might that imply this is the result of not something hitting it from above, but
pushing its way up from beneath? Perhaps this is the result of immense volcanic activity, suddenly
pressing up and cracking the crust? That’s just my guess. There are still many mysteries to be found
on Mercury’s surface, and many other fascinating insights to be gleaned as science advances.
I hope to do more videos on Mercury’s surface in the future, as there’s too much to delve
into in just one video. For now, at least know that Mercury’s surface is an evolving place,
moving and developing, albeit over a timescale of millions of years.
Science has advanced in other, remarkable ways too.
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Thanks for watching! If you liked this video, check out my other MESSENGER videos in this
playlist here, where I go into detail about what makes Mercury such a hard planet to visit.
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