Mercury, the solar system's closest planet
to the Sun. Everything I'll show you today will be an
actual picture or video image of Mercury from the MESSENGER probe. We discuss Mercury’s orbit and rotation,
its physical characteristics, its surface conditions and the magnetic field and magnetosphere
of the planet. I’m Alex McColgan, and you’re watching
Astrum. Stick with me on this video and you will learn
almost everything you could want to know about this tiny yet fascinating planet. Now when you think about the physical characteristics
of Mercury, I'm sure you imagine it being the closest planet to the Sun, but also that
it’s this giant rock floating in space. You wouldn’t be too far wrong with that,
but it is much more interesting than what you may first think. For example, when I look at Mercury, I do
think of our Moon. But Mercury actually is visually more appealing
than our Moon. Look at it in its true colour. The first thing that I notice is that it actually
does have a colour. It's not just different shades of grey. And what else? Well did you know, for example, that Mercury
consists of approximately 70% metallic and 30% silicate materials. It’s actually more metallic than rocky. Because of this, Mercury's density is the
second highest in the solar system at 5.427 grams per centimetre cubed, only slightly
less than the planet with the greatest density - that of Earth at 5.515 grams per centimetre
cubed. If Mercury happened to be the same size as
Earth, that would mean it would have pretty much the same gravitational pull as its surface. But being the size that it is, its surface
gravity is only 3.7 metres per second squared. If you were to compare its gravity to Earth,
it would look something like this. This means the surface gravity of Mercury
is only slightly less than what it is on Mars, and considering that Mars is a much bigger
planet, that just says something about the density of Mercury. Before we leave the subject of Mercury’s
size, I want to show you one last comparison - that of Ganymede and Titan against Mercury. Now Ganymede is the solar system's biggest
moon and also the biggest moon of Jupiter, while Titan is Saturn's biggest moon and the
second biggest moon in the solar system. These two giant moons are bigger than Mercury
as you can see here, but their masses are far less. If you look closely at Mercury's surface,
you'll see its appearance is similar to that of our Moon. It shows extensive mare like plains and heavy
cratering indicating that it has been geologically inactive for billions of years. But it obviously was geologically active at
one point, because one of the distinctive features of Mercury's surface is the presence
of many narrow ridges extending out to several hundred kilometres in length. It's believed that these were formed as Mercury's
core and mantle cooled and contracted over time when the crust had already solidified. And one of the most distinctive things you'll
notice about Mercury is this huge crater on its surface called Caloris Basin, with a diameter
of 1,550 kilometres. The impact that created Caloris Basin was
so powerful it caused lava eruptions and left a concentric ring over two kilometres tall
surrounding the impact crater. At the antipode of Caloris Basin is a large
region of unusual hilly terrain known as the weird terrain. If you compare this region to the rest of
Mercury, you can see why it would have this name. So, what's it like on the surface of Mercury? Well, to start with, the surface temperature
is hugely different all over. It can range from minus 173 degrees Celsius
to over 400 degrees Celsius. It never rises above minus 93 degrees on the
poles though, because there's no atmosphere retaining the heat. This means that there's quite a big difference
between the equator and the poles, but this variation is also due to its orbit and rotation
which we'll get back to later. The subsolar point reaches about 400 degrees
while on the dark side of the planet the temperatures are, on average, minus 163 degrees Celsius. Because Mercury is too small and hot for its
gravity to retain any significant atmosphere over long periods of time, it's not able to
retain any of the heat it gets from being so close to the Sun, which is why the dark
side of the planet is so much colder than the side facing the Sun. Mercury, however, does have an exosphere,
which is like an extremely thin atmospheric like volume surrounding the planet. Molecules in an exosphere are gravitationally
bound to a planet but the density is so low that it can't behave like a gas because the
molecules don't collide with each other. In this picture you can the MESSENGER probe’s
view of Mercury’s exosphere. When solar wind hits the planet, it rips off
certain atoms out of the exosphere, and what's left is this trail of atoms going into space. We call this the planets tail, and every planet
has this to a certain extent. Earth even does have an exosphere, but it
starts at 600 kilometres above the surface. It's really the point where space and the
atmosphere meet. Now, in the case of Mercury, this exosphere
is not at all stable. Atoms are continuously lost and replenished
from a variety of sources. NASA has been able to confirm that craters
at the North Pole of Mercury contain water ice. Mercury also has something which Mars lacks,
an actual magnetosphere, or a magnetic field all around the planet. It is only about 1.1 percent as strong as
Earth’s, but it's still strong enough to deflect a lot of the solar wind around the
planet. Now we're going to get to one of the things
which I find the most interesting about Mercury. Its orbit and its rotation. Mercury has the most eccentric orbits of all
the planets with its distance from the Sun ranging from 46 million kilometres to 70 million
kilometres. Now, this is something a bit hard to imagine,
but bear with me. Mercury takes about 88 Earth days to complete
an orbit around the Sun. It also has a 3-2 spin orbit resonance of
the planet’s rotation around its axis. This means it spins three times around its
axis for every two times that it orbits around the Sun. So, although it takes about 59 Earth days
for Mercury to rotate on its axis once, which is what we call a sidereal day, this 3:2 orbital
resonance means that if you were actually standing on Mercury, it would appear that
one day - from sunrise to sunrise, or what is called a solar day - is two Mercurian years. Standing on Mercury, that would look something
like this. You would see the Sun rise relatively fast,
and then as it approaches midday, it slows down and even starts going backwards before
continuing on again to sunset. As you can see, that took a whole year, which
means a night-time on Mercury also takes a year. The Sun starts going backwards in the sky
because approximately four Earth days before perihelion, the speed in which Mercury travels
along its orbit equals the speed in which it is rotating. At this point, the Sun's apparent motion stays
stationary. At perihelion itself, Mercury’s orbital
speed exceeds its rotational speed, so to a person actually standing on Mercury, the
Sun appears to move backwards. Four days after perihelion, the sun's normal
motion resumes. You can see this even clearer from a top-down
perspective of Mercury. Twice a day on one of its poles, the Sun seems
to pause and then continue on again. Something else to note about Mercury's orbit
is that its inclined by seven degrees to the plane of Earth's orbit. As a result of this, we can only see Mercury
transit in front of the Sun when it's directly between us on Earth and the Sun itself. And because its orbit is inclined by seven
degrees, this only happens about once every 7 Earth years. The last thing we’ll discuss about the rotation
of Mercury is that its axial tilt is almost zero, with the best measured value as low
0.027 degrees. This is even smaller than that of Jupiter
which has been measured at 3.1 degrees. And finally, do you want to see Earth from
Mercury? Well here we are, just a couple of pixels
across. This photo was taken from the MESSENGER probe
several years ago, and barring new-borns, every single one of us was in this picture. Well, thank you so much for watching this
video. I hope you learned something today about Mercury,
and that it seems more interesting to you now than when you started this video. If you do want to learn more about our solar
system, I’m planning on remastering all of the old planet videos I’ve done in the
past to bring them up to the quality of my channel currently. Find them in this playlist here. The original videos in this series had been
part of the flagship playlist of my channel for a long time, however, the audio quality
hasn’t aged that well, so it wasn’t a good reflection on my channel anymore. Hopefully now people will be able to enjoy
them for years to come! All the best and see you next time.
