Could we colonize the big cratered moon Callisto?
In this video we will deal with Callisto,
one of the main moons of the gaseous giant planet Jupiter,
and we will try to understand if its hypothetical future colonization is feasible or not.
Callisto, together with Io, Europa and Ganymede, is one of the medicean satellites,
and was discovered by Galileo Galilei on 13th January 1610, who reported his observations
in his work Sidereus Nuncius. These satellites are called "medicean" because the Italian
astronomer dedicated them to Cosimo II De Medici, his benefactor. Galileo observed
the medicean satellites with a very rudimentary telescope and with a very small field of view:
even today one wonders how he managed to discover them, given that in this very small field of view
they could hardly see all four at the same time.
In Greek mythology Callisto was a nymph or, according to other versions, the daughter of King
Lycaon or of one of his sons, Keteus or Nykteus. She was one of the handmaids of the goddess
Artemis (Diana for the ancient Romans), whom Zeus seduced because he was attracted by her beauty.
According to some versions, inspired by the homonymous comedy by Anfide,
an Athenian playwright and poet, Zeus transformed himself into Artemis in order to attract Callisto
and lie with her. As a result of this intercourse she became pregnant and, when she was discovered,
she was expelled and turned into a bear by Artemis or Hera. As a bear she then gave birth to Arcade,
the eponymous hero of the Arcadia region. In the last part of the myth,
just before being killed by an arrow of her son (who had mistaken her for a normal bear),
Zeus saved her life by placing her among the stars like the Bear Greater.
With a diameter of 4818 km, Callisto is the third largest moon in the Solar System after Ganymede
(diameter 5264 km) and Titan (diameter 5150 km), Saturn's main satellite. In practice, Callisto
is larger than our Moon, which has a diameter of about 3475 km, and slightly smaller than the
planet Mercury, whose diameter is 4879.4 km! The surface area of Callisto is seven times
smaller than the area of the earth's surface; the circumference of Callisto is equal to 15000km,
the terrestrial one to 40000 km. The fastest airliners on Earth travel at 850 km/h:
it would take two days to travel the entire circumference of the Earth, while on Callisto this
time would amount to just 17 hours! Would you like to try this experience? Write it in the comments!
Its average density is 1.85 g/cm3, corresponding to about half the lunar average density and three
times lower than the terrestrial average density. This density value suggests a composition of
approximately equal parts of rock material and water ice, with traces of volatile ices
such as ammonia. The percentage of mass of ice present is 49-55% while the rocky component is
not known exactly, although it is probably similar to the composition of ordinary L and
LL chondrites, which are characterized by a low content of metallic iron and a more
abundant presence of iron oxide than H chondrites. Callisto's surface has an albedo of about 0.2,
that is, it reflects 20% of the sunlight it receives.
The surface composition is believed to be substantially similar to that of the rest
of the satellite. Spectroscopic observations in the near infrared revealed the presence
of water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers.
Water ice appears to be ubiquitous on Callisto's surface, with a fraction of the total mass
of 25-50%. The high resolution analysis of the near infrared and ultraviolet spectra obtained
by the Galileo spacecraft revealed the presence of several non-frozen materials on the surface,
such as iron and magnesium hydrosilicates, carbon dioxide, sulfur dioxide, and possibly ammonia and
various organic compounds. The spectral data also indicate that the Callisto surface is
extremely heterogeneous on a small scale. Small bright areas formed by pure water ice are mixed
with areas formed by a mixture of rock and ice and large dark areas of non-frozen materials.
Callisto's surface is asymmetrical: the hemisphere facing the direction of the orbital motion is
darker than the other, contrary to what happens on the other Galilean satellites. The darker
hemisphere seems richer in sulfur dioxide than the other, in which carbon dioxide is more abundant,
an element that seems to be associated with many recently formed impact craters, such as the Lofn
crater. Generally the chemical composition of the surface, especially that of dark areas, appears
to be similar to that of type D asteroids, whose surfaces are covered with carbonaceous materials.
Callisto is the most heavily cratered natural satellite in the Solar System.
Unlike neighboring Ganymede, which displays varied terrain,
Callisto does not exhibits evidence of plate tectonic-like activity. Although these are two
relatively similar objects, Callisto seems to have had a simpler geological history.
In fact, the impact craters and rings concentric with them represent the only structures present on
Callisto; there are in fact no large mountains or other prominent features. This was probably
due to the nature of the surface: the sliding of ice in geological times has obliterated
the craters and the largest mountains. Impact craters and multi-ring structures, escarpments
and associated deposits are the only major features found on the surface. Playing hide
and seek at the bottom of never-illuminated craters could be one of the first hobbies of
future astronauts who will colonize Callisto. What do you think? Write it in the comments!
The surface of Callisto is dominated by two huge structures:
Valhalla (the largest) has a bright central region 600 km wide and concentric rings
that reach 3000 km in diameter; the second, Asgard, has an external diameter of 1400 km.
There are also chains, such as the Gipul Catena, a long series of impact craters in a straight line
on the surface. The origin of each of them is due to the impact on Callisto of objects captured by
Jovian gravity and then fragmented by the tidal forces of the planet.
The crust of the satellite has been assigned an age of about 4.5 billion years,
thus dating back almost to the formation of the solar system, while the concentric ring
structures have an age between 1 and 4 billion years, depending on the sources considered.
Its average distance from Jupiter is about 1882700 km, equal to 5 times the mean distance
between Moon and Earth, and this makes it the outermost of the Galilean satellites.
Its rotation is synchronous: this means that Callisto takes the same time to complete a
rotation on itself and an orbit around Jupiter, specifically it takes 16.7 days. In other words:
on Callisto a day lasts as long as a year! It follows that one face of Callisto is always
facing Jupiter, while the other always faces the other side, as is the case with our Moon.
Callisto travels an almost circular orbit and very close to the equatorial plane of Jupiter,
with eccentricity and orbital inclination that undergo variations on a time scale of centuries
due to solar and planetary gravitational perturbations. The eccentricity varies from
0.0072 degrees to 0.0076 degrees, while the orbital inclination is around 0.192 degrees.
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Callisto's dynamic isolation implies that it was never significantly affected by
gravitational tides, and this affected its evolution and internal structure.
Given its distance from Jupiter, the flow of charged particles belonging to the Jupiter
magnetosphere reaching its surface is quite low and has had negligible effects on it,
unlike what happened on the other Galilean satellites.
The level of radiation on Callisto's surface is equivalent to a dose of about 0.01 rem per day,
seven times less than the level of radiation that the Earth receives. Here are some reference
values of the radiation levels for comparison: - 0.14 rem / day: normal dose at sea level,
coming from cosmic ragigs and natural radioactivity of rocks;
- 50 rem / day: typical dose assumed for a 2.5-year Martian expedition, outside the Earth's
magnetosphere. This level of radiation increases the risk of developing cancer by 1% each year,
more or less the same as induced by smoking. - 75 - 200 rem in 30 days: the human body cannot
repair cellular damage caused by radiation quickly enough; nausea, fatigue, hair loss, malformations
in children, cancer in later life can occur. - 500 rem: fatal dose.
Up to now there has never been a serious medical emergency in space,
partly for the short distances from Earth, partly for the periods spent in space largely
not exceeding a few months. Conversely, common ailments from space (constipation, nausea,
headache, bone pain, etc.) have always been on the agenda and managed without major problems.
In the case of long-duration missions, such as a trip to Callisto,
common medical risks will be minimized by careful screening and vaccinations before flights.
But the exposure to radiation, much greater than that which could be experienced on a trip to Mars,
and to the reduction of gravity, as well as unexpected accidents, will constitute an
unknown not yet well defined. According to the old protocols, a small mission of 4 - 6 people should
include a doctor. However, in a larger mission of about 20 people, the presence of a paramedic
or nurse is also desirable, which would also make surgical interventions possible. If you were to
be chosen to colonize Callisto, which activities would you apply for? Write it in the comments!
The idea of including a medical and surgical robot managed by a crew member trained with help from
Earth is being studied. But here too there is a problem, namely the delay with which the signals
travel: due to the speed of light, which is very large but not unlimited, communications between
the ship's medical officer and the Earth would be interspersed with long waits. How would you behave
in the event of an unforeseen event during a major surgery, whilw you aren’t able to communicate with
Earth in real time? Write it in the comments! In the hypothesis that a solution to these
problems is found, where could a colony be created on Callisto? Considering that the basic
idea is to try to exploit as much as possible the materials present in situ to build habitats
and structures for a human crew, one possibility would seem to be to build the base inside craters
rich in water ice, such as for example the aforementioned Valhalla. In this way,
in addition to building materials for the colony, the crew could have an equally important resource:
water, which could be obtained by melting the ice. The frozen water present at moon
poles could also be used to produce oxygen for the crew. In practice, the water molecule would
be split into hydrogen and oxygen: hydrogen would be used as a propellant, oxygen for respiration.
In any case, the colony should be built on the face of Callisto from which Jupiter is visible,
to allow an in-depth study of the gas giant and its surface details. In any case,
before sending a human crew, it would be necessary to send habitat and supplies first:
the colony would be built thanks to completely autonomous machinery and governed by advanced
artificial intelligence systems. Once the base is built, other problems
would arise. The first is energy supply: due to Callisto's great distance from the Sun,
roughly equal to 5 astronomical units (i.e. five times the average distance
Terr - Sun), it would be extremely difficult to use solar panels: an alternative could be the
construction of a nuclear fusion power plant or the use of radioisotope thermoelectric generators,
already in use on some space probes, such as New Horizons or the Curiosity rover.
The second major problem is waste management and hygiene within the settlers' habitat.
Unlike what happens on the International Space Station,
where waste is stowed and burned upon re-entering the atmosphere in cargo modules,
in a closed habitat on Callisto, contamination by waste materials could soon become a huge problem.
As if all this were not enough, it will be necessary to guarantee a
certain comfort to the crew: in addition to the psychological aspect of being so far from Earth,
the astronauts will also need to ensure proper daily physical exercise
to avoid muscle atrophy: due to the very low acceleration of Callisto gravity,
it is very likely that astronauts will be prone to loss of muscle and bone mass.
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