Two or three ways to live on Mercury
"Welcome to Kandinsky Station! The hour is 06:00 AM Mercury Standard Time, and the local
temperature is a balmy 93°C! We hope you had a pleasant journey to the surface via
the Spindle, Mercury's shining tether to space! We also hope that you have acclimated to the
local gravity. If you are traveling from a low-g point of origin, we recommend you consult
the data provided in your virtual pamphlet. If you feel any symptoms of 'gravity sickness'
please seek immediate medical help. Our final destination is Terminator City,
where you will be transported in extreme comfort to Mercury's equator and treated to scenic
landscape views amid perpetual twilight!” Someday, announcements like this could be
a real thing. While the thought of living, working, or visiting a planet like Mercury
might sound like something out of science fiction (or completely insane), the idea is
grounded in science fact. And with the right strategies and technologies for living in
a hostile environment, humans could make a life for themselves on Mercury.
Don't think it's possible? Well, let's try to find out together!
Colonizing other worlds: a dream that inspires the human imagination for over a century,
but until a few years ago was relegated mostly to science fiction films and novels. For some
time now, however, even science seems to have begun to deal with it, and thanks to advances
in space technology, long-term missions to other planets of the Solar System now seems
increasingly within reach. But what are the most likely destinations?
If you ask around for an opinion, the most popular answers will talk about the Moon,
of course ... and then Mars. Even Venus, perhaps. No one, but no one will think of proposing
Mercury, the planet that in the collective imagination is considered the most inhospitable
and least attractive of all! Who would ever want to go and live in a place
like Mercury? Nobody, of course! If you are thinking of
moving somewhere else in the Solar System, discard Mercury, they will tell you. Sure,
it's a rocky planet like Earth, and you could walk on it...but its proximity to the Sun
makes it decidedly inhospitable. However, you'll be surprised to learn how
many good and beautiful qualities the torrid planet hides. Starting with its being officially
named the "closest planet to Earth". Yes, you got it right!
Despite what common sense suggests, Mercury, not Venus, would be the closest planet to
Earth: the relative position of the orbits is always the one taught in school texts,
but the distance between the planets changes when measured in respect to time.
In practice, the orbits of Venus and Earth are separated from each other by a distance
of 0.28 astronomical units, while that of Mercury is 0.61 AU away from Earth's orbit.
There seems to be no doubt: from a geometric and absolute point of view, Venus is the planet
that is closest to Earth! However, if the calculation is made over a
fairly long time, we find that Mercury spends more time in the vicinity of Earth than Venus
does, so that the average distance between Mercury and Earth, in the long run, is 1,04
AU, while the one between Venus and Earth is 1.14 AU!
It is a little bit what happens to us in everyday life... Between an uncle who comes to visit
us once a year and then he goes back to his house 100 km away, and the neighbor who lives
for the whole year 100 meters from our house, according to you who is closer to us?
You have to take this as a curiosity... it is only a different way to consider the measures,
and not a revolutionary discovery as most of the media reported it!
But let's go further. If we want to be able to give a judgment on the habitability of
Mercury it is necessary to accumulate much more data.
After Pluto was downgraded to dwarf planet status in 2006, Mercury has once again become
the smallest of the planets in the solar system. With a diameter of 4879 km, less than half
that of Earth, it is even smaller than the moons Ganymede and Titan.
Mercury also orbits at an average distance from the Sun of 58 million kilometers, but
its orbit is the most eccentric among the planetary orbits, so that at perihelion it
approaches the Sun up to a distance of 46 million kilometers, while at aphelion it can
move away up to 70 million kilometers. But the planet's oddities in terms of celestial
dynamics don't end here. Do you remember how our Moon rotates on itself with the same period
with which it rotates around the Earth? This is defined as a 1:1 tidal synchronization.
Well, the very slow rotation of Mercury, which lasts well 58.65 days, is instead synchronized
with the period of revolution around the Sun, which is 88 days, according to the ratio of
2:3. And this, too, is a way of standing out... In practice, Mercury makes two revolutions
for every three rotations, and this means that a possible Mercurian inhabitant would
have to wait 176 days before being able to see the Sun return to the sky in the same
position. The very special relationship between the
period of rotation and the period of revolution, combined with the high eccentricity of the
orbit, would produce strange effects for a possible inhabitant of Mercury placed on the
surface of the planet. Within a Mercurian day, it would see the sun rising in the east
and advancing very slowly in the sky (about three times slower than the sidereal motion
of the stars) increasing more and more the apparent diameter; near the meridian, the
Sun would slow down even more until it would cease completely; then the Sun would produce
a double reversal of motion, to resume its path towards the western horizon, which would
reach 88 days later, returning to its original size.
Other observers, who were at points at a different longitude, would see different but equally
bizarre motions.
The proximity to the Sun, even in perspective, has always made very difficult the observation
and study of this small planet, which in this way has always remained on the margins of
general attention, and in part also of the scientific one.
For example, there are no photos of Mercury taken by the Hubble Space Telescope, which
has never pointed its photographic instrumentation in its direction (and not even in that of
Venus). And only two probes have visited Mercury: it may be the closest of the planets, but
it is also terribly difficult to reach!
How come? Well... The problem is that to reach Mercury means to jump headlong into the Sun's
gravitational well, which would attract the spacecraft until it would reach a speed so
high to prevent its insertion in Mercury's orbit.
The spacecraft would have to find a way to slow down, but this would mean carrying huge
amounts of fuel. The only other way is to slow down by making use of one or more gravitational
assist with Venus and with the Earth itself; with changes of path that, however, inevitably,
lead to greatly lengthen the duration of the journey.
This explains the lack of attention to the planet shown by the various space agencies.
The first probe to visit Mercury was the NASA Mariner-10 in the first half of the seventies,
which took only 134 days to reach Mercury. The second was MESSENGER with the insertion
in orbit that occurred in 2011, after 7 years of travel.
The huge disparity in the flight times of the two missions depends on the fact that
Mariner 10 had only flown over the planet, without trying to orbit around it. It was
attracted by the gravitational field of the Sun and without ever slowing down it had a
very fast encounter with Mercury. Messanger's goal was to orbit the planet, which he managed
to do only after an endless wandering between Venus and Earth in an attempt to dissipate
energy and speed. But now let's ideally try to descend on the
surface to see what would really wait for us if we really decide to consider Mercury
a possible destination. We would find first of all that the force
of gravity is a little more than a third of Earth's and virtually identical to that on
Mars, this means that a person who here "weighs" 80 kilograms, on Mercury would weigh only
30. According to astronomers, Mercury would be
made mostly of iron. Almost three-quarters of its diameter would be occupied by a large
ferrous core (3600 km compared to a total diameter of almost 4,900). This core would
occupy 40% of the volume of the planet and would constitute even 70% of its mass. In
comparison, the Earth's core is only 17% of the volume of our planet.
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One of the hypotheses of scientists to explain a core of these proportions, larger than that
of any other known celestial body, is that the planet was originally larger and that
a collision with another large object has torn away its outermost part. Since Mercury
has a strong magnetic field, the iron of its core must also be partly in a liquid state.
The rest of the planet would be composed of a mantle and a crust made of materials similar
to those of which is made the Earth's crust, mainly based on silicon.
On Mercury, however, there is no tectonic activity and the crust does not move. Therefore,
seen from outside the planet resembles our Moon: it has a gray color and preserves the
craters produced by the impact of meteorites, without reshaping its surface.
The atmosphere of the planet, composed of hydrogen, helium, sodium, potassium, and calcium,
is extremely rarefied and therefore does not contain a sufficient amount of gas to reduce
the heat and light that receives from the Sun, also its slow rotation exposes for long
periods to direct sunlight the different areas of the surface. This results in the highest
temperature range of all planets in the Solar System.
Without the moderating effects of an atmosphere or oceans like the Earth, the intense solar
radiation (the sun appears about 2½ times larger in Mercury's sky) results in surface
temperatures, reaching an extreme of 427°C at the local noon time.
The nights on Mercury are long due to its slow rotation. Since there is no atmosphere
to trap heat, it radiates from the surface directly into space at night and the surface
gets very cold, about -180°C). Such temperature extremes do not occur on planets with atmospheres
due to the insulating effects of the atmospheres that moderate temperature extremes and maintain
a warmer temperature at night. Having said that, I think you are beginning
to wonder, "but then, why would we even think of going down to such a planet? To die there
soon after?" And here's the kicker!
Only a few years ago it was known that on the surface there may also be frozen water.
The probe Messenger has found it in areas that always remain in the shadows of some
craters in the polar regions of the planet. Observations suggest ice deposits at least
several meters thick around the planet's poles, and modeling of radar data suggests that these
deposits are composed of 95% pure water ice. While some deposits closer to the pole are
exposed at the surface, most deposits appear to be covered by a layer of hydrocarbon-like
materials up to 30 centimeters thick. In these regions, a human settlement could
be constructed using a process called "paraterraforming" which can be described as a pressurized enclosure
placed within a crater to create a self-contained atmosphere. Over time, the ecology within
this dome could be modified to meet human needs.
In the case of Mercury, this would include pumping in a breathable atmosphere and then
melting the ice to create water vapor and natural irrigation. Eventually, the region
within the dome would become a livable habitat, complete with water cycle and carbon cycle...
Or one could build a base underground by exploiting some of the many lava tubes whose existence,
a bit like on the Moon, is taken for granted by our geologists.
It is hypothesized in fact that just below the surface, on Mercury the environmental
temperature can even settle on values definitely compatible with human life.
Or, and here we come to explain the situation described at the beginning of this video,
we could exploit the most prominent feature of the entire planet, the one that made it
famous in many science fiction stories: the very slow terminator line!
On Earth, the line that separates day from night moves at a very high speed: at the equator,
it reaches as much as 1670 km per hour... and it is obviously impossible to follow its
movement on foot: only the fastest planes and artificial satellites can keep up with
the speed at which sunrise and sunset cross seas and continents.
But on Mercury everything is different. As we have seen, the solar day is very long,
176 Earth days, and the planet has a circumference that is a little more than a third of the
Earth's: 15,331 km. As a result, the terminator moves at a maximum speed of only 3.6 kilometers
per hour! Even an elderly person could walk at that
speed, thus managing to keep the Sun always close to the horizon!
From this, to imagine that one day motorized and self-propelled bases could be built astride
the Mercury terminator, able to keep the temperature on acceptable levels, well... the step is
short.
Yes, the imagination flies, but obviously, a colony on Mercury would still be a huge
challenge, both economically and technologically. Right now, however, humans don't have the
faintest idea how to create a sustainable human colony in space. Neither on Mercury
nor anywhere else.
We are in the process of doing so, it's true. On the Moon, and then on Mars, but we still
have very fuzzy ideas. Mercury, though the most consistently close
planet to Earth, will have to wait its turn.