From its discovery, to its importance around Jupiter, and more!
Join us as we explore Callisto, Jupiter's Moon.
9.
Discovery and Naming Of Callisto Callisto was discovered Jan. 7, 1610, by Italian
scientist Galileo Galilei along with Jupiter’s three other largest moons: Ganymede, Europa
and Io.
Callisto is named after one of Zeus's many lovers in Greek mythology.
Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated
with the goddess of the hunt, Artemis.
Who was also the goddess of the moon for the record.
The name was suggested by Simon Marius soon after Callisto's discovery.
Marius attributed the suggestion to Johannes Kepler.
However, the names of the Galilean satellites fell into disfavor for a considerable time,
and were not revived in common use until the mid-20th century.
In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral
designation, a system introduced by Galileo, as Jupiter IV or as "the fourth satellite
of Jupiter".
Now though it's known as Callisto by most texts, including ones you'll see in school
in hear about when moons like these are discovered.
The desire to keep things simple while also rooting much naming in mythology has been
desired by astronomers in earlier decades.
8.
Orbit and Rotation Callisto is the outermost of the four Galilean
moons of Jupiter.
It orbits at a distance of approximately 1,170,000 miles (26.3 times the radius of Jupiter itself).
This is significantly larger than the orbital radius of the next-closest Galilean satellite,
Ganymede.
As a result of this relatively distant orbit, Callisto does not participate in the mean-motion
resonance—in which the three inner Galilean satellites are locked—and probably never
has.
Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with
its orbit.
The length of Callisto's day, simultaneously its orbital period, is about 16.7 Earth days.
Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity
and inclination changing quasi-periodically due to solar and planetary gravitational perturbations
on a timescale of centuries.
These orbital variations cause the axial tilt (the angle between rotational and orbital
axes) to vary between 0.4 and 1.6°.
The dynamical isolation of Callisto means that it has never been appreciably tidally
heated, which has important consequences for its internal structure and evolution.
Its distance from Jupiter also means that the charged-particle flux from Jupiter's magnetosphere
at its surface is relatively low—about 300 times lower than, for example, that at Europa.
Hence, unlike the other Galilean moons, charged-particle irradiation has had a relatively minor effect
on Callisto's surface.
The radiation level at Callisto's surface is equivalent to a dose of about 0.01 rem
per day, which is over ten times higher than Earth's average background radiation.
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7.
Exploration The moons of Jupiter have been something of
a fascination for many astronomers and scientists.
So when the Earth had the ability to look at the moons via satellites and probes they
almost literally jumped at the chance.
To the extent that Callisto has been visited many times of the last several decades.
The Pioneer 10 and Pioneer 11 Jupiter encounters in the early 1970s contributed little new
information about Callisto in comparison with what was already known from Earth-based observations
ironically enough.
The real breakthrough happened later with the Voyager 1 and Voyager 2 flybys in 1979.
They imaged more than half of the Callistoan surface with a resolution of 1–2 km, and
precisely measured its temperature, mass and shape.
A second round of exploration lasted from 1994 to 2003, when the Galileo spacecraft
had eight close encounters with Callisto, the last flyby during the C30 orbit in 2001
came as close as 138 km to the surface.
The Galileo orbiter completed the global imaging of the surface and delivered a number of pictures
with a resolution as high as 15 meters of selected areas of Callisto.
In 2000, the Cassini spacecraft en route to Saturn acquired high-quality infrared spectra
of the Galilean satellites including Callisto.
In February–March 2007, the New Horizons probe on its way to Pluto obtained new images
and spectra of Callisto as well.
There were other plans to send probes to Jupiter as well.
Formerly proposed for a launch in 2020, the Europa Jupiter System Mission (EJSM) was a
joint NASA/ESA proposal for exploration of Jupiter's moons.
In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the
Titan Saturn System Mission.
At the time ESA's contribution still faced funding competition from other ESA projects
and as such the mission never came to be.
But, it's still possible that they could go back to them depending on funding.
Especially with the Mars missions coming up.
6.
Surface Of The Moon Callisto’s rocky, icy surface is the oldest
and most heavily cratered in our solar system.
The surface is about 4 billion years old and it’s been pummeled, likely by comets and
asteroids.
Because the impact craters are still visible, scientists think the moon has little geologic
activity—there are no active volcanoes or tectonic shifting to erode the craters.
Callisto looks like it’s sprinkled with bright white dots that scientists think are
the peaks of the craters capped with water ice.
Callisto's surface has an albedo of about 20%.
Its surface composition is thought to be broadly similar to its composition as a whole.
Near-infrared spectroscopy has 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 seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%.
The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft
and from the ground has revealed various non-ice materials: magnesium- and iron-bearing hydrated
silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds.
Spectral data indicate that Callisto's surface is extremely heterogeneous at the small scale.
Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture
and extended dark areas made of a non-ice material.
The Callistoan surface is asymmetric: the leading hemisphere is darker than the trailing
one.
This is different from other Galilean satellites, where the reverse is true.
The trailing hemisphere of Callisto appears to be enriched in carbon dioxide, whereas
the leading hemisphere has more sulfur dioxide.
5.
Atmosphere Callisto has a very tenuous atmosphere composed
of carbon dioxide.
It was detected by the Galileo Near Infrared Mapping Spectrometer (NIMS) from its absorption
feature near the wavelength 4.2 micrometers.
The surface pressure is estimated to be 7.5 picobar (0.75 µPa) and particle density 4
× 108 cm−3.
Because such a thin atmosphere would be lost in only about 4 days (see atmospheric escape),
it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice
from Callisto's icy crust, which would be compatible with the sublimation–degradation
hypothesis for the formation of the surface knobs.
Callisto's ionosphere was first detected during Galileo flybys; its high electron density
of 7–17 × 104 cm−3 cannot be explained by the photoionization of the atmospheric
carbon dioxide alone.
Hence, it is suspected that the atmosphere of Callisto is actually dominated by molecular
oxygen (in amounts 10–100 times greater than CO2).
However, oxygen has not yet been directly detected in the atmosphere of Callisto.
Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible
concentration in the atmosphere, based on lack of detection, which is still compatible
with the ionospheric measurements.
At the same time, HST was able to detect condensed oxygen trapped on the surface of Callisto.
Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis
of 2001 Hubble Space Telescope data.
Spectral images taken on 15 and 24 December 2001 were re-examined, revealing a faint signal
of scattered light that indicates a hydrogen corona.
The observed brightness from the scattered sunlight in Callisto's hydrogen corona is
approximately two times larger when the leading hemisphere is observed.
This asymmetry may originate from a different hydrogen abundance in both leading and trailing
hemispheres.
However, this hemispheric difference in Callisto's hydrogen corona brightness is likely to originate
from the extinction of the signal in the Earth's geocorona, which is greater when the trailing
hemisphere is observed.
4.
Pop Culture Jupiter and its moons are popular subjects
for science fiction writers and Callisto is no exception.
It appears in several books and in the TV show “Cowboy Bebop,” Callisto has been
terraformed and is home to mostly men.
In the 1930s, writer Harl Vincent had Earth and Callisto at war in his novel “Callisto
at War.”
Isaac Asimov’s 1940 novel "The Callistan Menace” depicts Callisto as having an atmosphere
of carbon dioxide and oxygen with lakes and vegetation.
It’s also a deathtrap crawling with giant caterpillars.
Callisto gets a mention in Robert A. Heinlein's 1950 novel “Farmer in the Sky.”
The book is mostly about Jupiter’s moon Ganymede, but it discusses an atmosphere being
created so colonists can live on Callisto.
Philip K. Dick, author of “Blade Runner” and “The Minority Report” wrote a short
story in 1955 called “The Mold of Yancy” about colonists living in a totalitarian society
on Callisto.
Lin Carter created a series of eight books in the 1970s called the Callisto series.
The books featured a soldier who is teleported to Callisto where he finds an ancient human
civilization.
In the book Callisto is like Earth, but some sort of illusion makes it seem uninhabitable
to outsiders.
In Kim Stanely Robinson’s book “2312,” settlers have colonized Callisto’s giant
moon-spanning impact crater Valhalla, starting from its innermost ring.
The author envisions windows lining the inward-facing parts of the rings.
As the colony grows and residents seek to maintain their space and independence, they
settle in the different rings, creating separate colonies in each one.
It's rather interesting that so many authors across many decade have made Callisto a focal
point of their stories in one form or another.
It may have been a cascade effect of sorts if you look at the time they were written,
or it could just be that they really thought that Callisto would be a good setting.
Hard to say.
3.
Waters Underneath?
Once thought to be a dead, inactive rocky body, data gathered by the Galileo spacecraft
in the 1990s indicate Callisto may have a salty ocean beneath its icy surface.
It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly
conducting sphere; that is, the field cannot penetrate inside Callisto, suggesting a layer
of highly conductive fluid within it with a thickness of at least 10 km.
Failing an ocean, the icy lithosphere may be somewhat thicker, up to about 300 km.
More recent research reveals that this potential ocean may be located deeper beneath the surface
than previously thought, or may not exist at all.
If an ocean is present, it’s possible the ocean is interacting with rock on Callisto,
creating a potential habitat for life.
But just as important, if there is water of any kind on this moon, it's one that warrants
further investigation because it's not the only moon orbiting Jupiter that has an underground
ocean.
Yet we know that the vast majority of Jupiter's moons don't have water or fluid of any kind
on them or under them.
Which would beg the question of why these particular moons are so special and why they
and not others have liquid.
Further research is obviously still required.
But the potential is definitely undeniable.
2.
Potential For Life?
As noted by NASA and others, there is a chance that Callisto has life within it.
Or more specifically, within its waters that could be under the surface.
But let's look at the facts for a moment and truly determine if this is true.
Many hypothesize that there are waters deep within the surface of the moon known as Callisto,
assuming this to be true, could life truly live there?
The answer is yes...if certain conditions are met.
We know just via Earth that life can live in the waters of a world (or in this case
a moon), but whether it actually will be there is a bit up for debate.
The biggest problem for us is that the supposed ocean where the creatures live is many miles
down.
Even if we landed a rover on the moon, it wouldn't be able to reach the ocean based
on the depth we believe it to be at.
And even if we did, there's no guarantee we'd get a sample that would have life in it.
But it could.
It might just be simple creatures, multi-celled organisms, or something of the like.
We won't know until we dig deep and find it.
1.
Potential Colonization?
Given the potential for water, a thin atmosphere that could be worked to our favor, and a rocky
surface, Callisto has gotten many people's attention in regards to colonization.
In 2003 NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE)
regarding the future human exploration of the outer Solar System.
The target chosen to consider in detail was Callisto.
The study proposed a possible surface base on Callisto that would produce rocket propellant
for further exploration of the Solar System.
Advantages of a base on Callisto include low radiation (due to its distance from Jupiter)
and geological stability.
Such a base could facilitate remote exploration of Europa, or be an ideal location for a Jovian
system waystation servicing spacecraft heading farther into the outer Solar System, using
a gravity assist from a close flyby of Jupiter after departing Callisto.
In December 2003, NASA reported that a manned mission to Callisto might be possible in the
2040s.
Which by that time, humanity is aiming to have a colony on Mars in some capacity.
And that could help facilitate a mission to Callisto even more.
Thanks for watching everyone!
What did you think of this look at the moon known as Callisto?
Do you understand now why so many astronomers and scientists want to look at it and explore
it more?
Do you think we'll find something truly life-changing there one day?
Let us know in the comments below, be sure to subscribe and I'll see you next time on
the channel!