| Ice
Dwarfs - Pluto and Charon |
Above: Pluto, showing the heart-shaped Tombaugh Regio of deposits with a high albedo (they reflect about 90% of incident sunlight and so appear bright). This region covers about one-quarter of Pluto's surface.
Above: a Pov-Ray simulation of an orange ice-dwarf like Pluto. Pluto belongs to a type of planet we could loosely call Ice Dwarfs, but there are really several types within such a broad class of planet.
Below: The Ice Mountains of a Pluto-like planet. A Sun-like star is visible in the Plutonian sky at a an equivalent distance of Sol from Pluto to illustrate the size of Sol as seen from Pluto.
Pluto,
an orange ice dwarf is aptly named after a mythical god of the
Underworld as it is far from the Sun. It is
so far from the Sun that the Sun appears as a bright star when Pluto
is furthest away (at aphelion) and the disc
of the Sun is just about discernible when Pluto is at its closest
(at perihelion). Light from the Sun is about one to
two thousand times as dim here, but since the eye detects the log of
light intensity the perceived light levels are
only about 3 times lower (about one-third) than on Earth and looking
at the Sun will still hurt the eyes.
Planet type: carbohydrate and water
(?) ices, cold desert planet. The largest known object in the Kuiper
Belt.
The Kuiper Belt is a ring or band of objects orbiting Sol beyond
Neptune from about 30 to 50 AU (astronomical
units). I consider Pluto to be a planet of the dwarf category
(rather than a 'dwarf planet' per se).
Equatorial Diameter: 2372 km (radius 1186 km).
Orbit:
Pluto orbits Sol in a very eccentric (elliptical) orbit ranging from
29.657 to 48.871 AU from Sol, with a year
lasting 247.68 Earth years and a day length of 6.387 Earth days.
Atmosphere: Very thin (about 1 pa or
one hundred thousandth of Earth's atmospheric pressure), consists of
nitrogen, methane and carbon monoxide, a visible hydrocarbon haze
extends about 130 km (80 miles) above
the planet's surface; it possible snows hydrocarbon ice. The
atmospheric gases are in equilibrium with surface
ices of nitrogen, methane and carbon monoxide. A cycle of ice
sublimation and ice snow probably maintains
this equilibrium.
Surface Temperature: 33 to 55 K (-240 to -218 degrees C).
Magnetic Field: no data.
Life:
none as yet discovered.
Some key tourist attractions:
The surface of Pluto is relatively crater-free, suggesting that it
is relatively young and geologically active. Large uncratered plains
occur, such as Sputnik Planum.
Color
- Pluto's surface varies in hue and brightness, but has an orange
color due to organic compounds such as tholins (polymers
formed by the action of the Sun's UV light on hydrocarbons such as
methane).
Ice
Mountains - a layer of water ice probably
exists beneath Pluto's surface layers of methane, nitrogen and
hydrocarbon ices. This is thought to account for the existence of
mountain ranges on Pluto, with the mountains consisting of a water
ice core (water ice is stronger than hydrocarbon ice). The tallest
mountain range on Pluto is 6.2 km (Tenzing Montes).
Ice Plains
- vast plains with a network of troughs containing darker material
(some of which forms hills) possibly consist of frozen 'mud', the
'mud' consisting of particles of methane ices.
Dune
Fields - large plains rippled with regular dune
systems thought to be made chiefly of grains of methane ice acting
like silica sand. this implies that winds on Pluto can cause
significant geological activity, at least during certain times of the
year. These transverse dunes are regularly spaced, with a wavelength
of 0.4 to 1 km and are thought to have been deposited by winds between
1 m/s and 10 m/s in speed. (Pluto's cold and thin atmosphere is
generally thought not to support higher wind speeds).
Nitrogen
Glaciers - evidence indicates that Pluto
possibly has glaciers of frozen nitrogen (and other ices such as CO
and methane). Water ice is too brittle at these low temperatures to
flow, behaving more like silica rock on Earth. Despite the low
temperatures frozen nitrogen may remain sufficiently soft and fluid
to slowly flow across Pluto's surface. Nitrogen freezes at about
-210 degrees C (63 K) and boils at about -196 degrees C (77 k).
Giant
Blades of Ice - jagged ridges of methane ice near
Pluto's equator several hundred feet tall. These are thought to form
when methane ice deposits are eroded by sublimation (evaporation
directly into methane gas) due to relatively warm periods in Pluto's
climate. Material can be seen that has flowed around hills and through
breeches in crater rims.
Impact craters - though not especially frequent do occur and "many appear to be substantially degraded or infilled, and some are highlighted by bright ice-rich deposits on their rims and/or floors" (Stern et al. 2015) such as those in the dark equatorial region called Cthulhu Regio (CR).
Features are generally named as follows:
Planum - large plain
Below - artistic renditions of Pluto's ice mountains produced by a computer simulation. Can you see Sol in the sky on one of these views?
Below: the frozen mud-planes of Pluto. Charon is visible in the sky. Charon is Pluto's largest moon and although much smaller than Earth's Moon it is also much closer to its parent planet and so appears about 6 or 7 times as large in Pluto's sky as the Moon does on Earth.
Above:
domes of ices (probably mainly methane or nitrogen ice) forming
polygonal hills about 100 m in height and more than 10 km in width.
This could be due to circulating convection cells within the ices
causing cells of warmer ice to slowly flow to the surface before
cooling and sinking below at the troughs around the margins.
Pluto is tiny as far as planets go. It is a dwarf planet, but in my classification a dwarf planet is not a distinct class from a planet. Rather, planets can be divided into dwarf planets, like Pluto, medium planets, like Earth, and giant planets, like Jupiter. In this scheme, a 'dwarf planet' is a subtype of 'planet'. Anything below 1000 km in radius I would not classify as a planet but rather as a planetoid. Let us compare Pluto, its largest moon Charon, The Earth and Earth's Moon. Also included for comparison is Eris, probably the most massive Kuiper Belt object and all the moons in the Solar system with a radius above 1000 km.
Values
listed are: radius, mass relative to the Earth and surface gravity
relative to the Earth
Pluto:
1186 km,
0.00218, 0.063g
Charon: 603.5 km,
0.000254, 0.028g
Earth:
63741
km, 1, 1g
Moon: 1737 km,
0.0123, 0.1654g
Eris:
1163
km, 0.0028, 0.084g
Triton: 1353
km, 0.00359, 0.0794g
Titan:
2576
km, 0.0225, 0.14g
IO:
1822
km, 0.015, 0.183g
Ganymede: 2634 km, 0.025, 0.146g
Callisto: 2410 km, 0.018,
0.126g
Europa: 1561 km, 0.008,
0.134g
In
our classification the Moon would be a planet of the dwarf type,
however, since it orbits the Earth which is much more massive the
Moon can be considered a secondary planet. Objects like Charon and
Ceres, a more-or-less spherical asteroid with a mean radius of 473
km would not be classes as a dwarf planet, but rather as a
planetoid, since it has enough gravity to form a roughly spherical
object. Objects with a radius of about 1000 km or greater usually
have a richer and more varied geology. The four largest moons of
Jupiter: IO, Ganymede, Callisto and Europa are secondary planets of
the dwarf type, along with Titan and Triton. This would give the
Solar System a total of 17 planets, of which 9 are dwarf planets.
Furthermore, 7 of the nine dwarf planets are secondary planets
(major moons), leaving ten primary planets in the Solar System. This
seems the most natural and convenient classification to me. Eris is
currently about three times as far from Sol as Pluto, though its
highly eccentric orbit means that it is sometimes closer (perihelion
of 37.9 AU, aphelion of 97.65 AU).
Above: Pluto itself as imaged by NASA's New Horizons space probe:
The bright heart-shaped region in the south is
Tombaugh Regio. To the left of Tombaugh Regio is the dark region:
Cthulhu Regio. The upper left lobe of Tombaugn Regio is Sputnik
Planitia (Sputnik Planum). The region of bright and dark streaks near
the equator on the right is Tartarus Dorsa - an area of large ridges.
An animated globe of Pluto viewed from the equator with the northern hemisphere tilted 15 degrees towards the viewer. The large bright heart-shaped feature is Tombaugh Regio.
The
Moons of Pluto
Pluto
has one large moon, Charon. Indeed, Charon is more than half of
Pluto's diameter, though much less massive, and both Pluto and
Charon orbit a common center of mass which is above Pluto's surface
(making this a binary system of two primary bodies, of which one is
a planet and one a planetoid; though both Pluto and Charon are also
considered dwarf planets). Charon orbits about 19 570 km from
Pluto's center or 17 540 km from the common center of mass (barycenter).
The Earth-Moon system is sometimes considered a double or binary
planet, however, the center of mass lies beneath the surface of the
Earth and so the Moon is a secondary planet in our classification.
Pluto has four other moons, which are tiny asteroids captured from
the Kuiper Belt. These are Styx, Nix, Kerberos and Hydra. Of these,
Hydra is the largest at 55 by 40 km across (and hence not even a
planetoid in our classification as it is not even roughly
spherical). These orbit around the central Pluto-Charon system.
Charon's surface is apparently dominated by water ice rather than by
methane or nitrogen ices and is less orange and more brownish in
color. One theory is that an impact with Pluto vaporised part of its
icy mantle which condensed into Charon. There is some evidence that
Charon may have cryogeysers. The surface of Charon is also
relatively crater-free, suggesting that it's surface is relatively
young and geologically active.
Is Pluto a Planet?
Scientists
disagree on whether Pluto is a planet or not. The IAU (International
Astronomical Union) decided to reclassify Pluto as a 'dwarf planet',
which is somehow not a 'planet'. Yes, Pluto is a dwarf planet as in
a planet that's small but it can still be considered a planet. Why?
Because science does not work by diktat: unions and 'official'
bodies can define their own definitions and classification systems
but they are not absolutely official as the Media have portrayed
them to be. Many scientists still regard Pluto as a planet and they
have the official capacity to do so. Geologists and interplanetary
scientists are among those who study planets, apart from
'astronomers' per se and they have just as much say in the
matter. The Media have been disingenuous in explicitly stating the
IAU definition as set in stone and official, as if in some legal
capacity: they are wrong! Science is not politics! In my honest
opinion, the IAU overstepped its authority.
Cronodon
agrees with many planetary scientists in defining planets on their
own merits, as objects in their own right, rather than on orbital
properties. Other scientists may agree or disagree. A planet can be
considered as an object with sufficient gravity to pull itself into
an approximate sphere. Large moons can be considered secondary
planets. Pluto is clearly sufficiently spherical and has interesting
geological features showing clear signs of current geological
activity. Cronodon considers Pluto to be a true planet of the dwarf
class.
There
is, however, a growing need to classify planets into sub-categories.
This should incorporate what is known of extrasolar planets, those
in other star systems, and not just focus on the Solar System. Such
classifications could include a descriptor for orbit as well as
descriptors for intrinsic geological properties. Physicists tend to
shy away from what they sometimes lazily refer to as 'stamp
collecting'. That is they tend to prefer to study the underlying
causes of phenomena rather than the phenomena themselves. This is an
unfortunate oversimplification. Analyzing phenomena often drives the
physics and if one adopts solely a bottom-up approach then one risks
missing emergent properties. Finally, equations may look nice, but
they are always nicer when considered alongside the phenomena they
model. Physicists really ought to take a leaf out of the biologist's
book and pay more attention to the diversity of phenomena in nature,
and in all fairness some do. Some may distinguish Astronomy from
astrophysics by claiming that Astronomy is really stamp collecting.
Stamp collecting is a valuable hobby, so I will have no more
dismissal of it! Scientists could perhaps apply a dichotomous scheme
of classification to planets.
Cronodon
agrees with Dr Allen Stern, who lead NASA's New Horizons mission to
explore the Pluto system, when he states that Pluto is a planet. The
question should be: what type of planet is it?
Pluto
and Charon are almost Perfect Spheres
In
terms of sphericity, measurements suggest that the difference
between the equatorial and polar diameters of Pluto are less than 12
km different, so the polar diameter is at least 99% of the
equatorial diameter. Generally, planets with liquid interiors or
extensive gaseous envelopes extend along their equator due to the
centrifugal force of the planet's rotation. Tidal bulges due to the
gravitational pull of other nearby bodies can also deform a planet.
In Pluto's case no oblateness (polar flattening) has been detected.
Similarly, Charon has no detectable oblateness, placing a maximum
limit of a 1% difference in its polar and equatorial radii.
Pluto
is tidally locked with Charon: both bodies present the same
face to one-another (their rate of spin on their own axes is equal
to the average orbital period of the two bodies about their common
center of mass or barycenter which is outside the surface of
Pluto). The orbits of the two bodies about the barycenter are
circular. Furthermore, the equators and orbital plane are coplanar.
This situation could have arisen in two probable ways: either two
bodies collided, forming Pluto and Charon from the impact debris in
circular orbits, or Charon was captured by Pluto, perhaps after a
grazing impact, in which case the orbit of Charon may have been
elliptical or eccentric to begin with. Two such bodies orbiting in
close proximity exert gravitational pulls on one-another, resulting
in tidal bulges on each body: the bulges move as the bodies
spin, but because of friction within the deforming bodies, the
bulges lag behind the gravitational pull. This results in tugs
acting on the two bodies to slow their spins and transfer
angular momentum to their orbits. The total angular momentum
must be conserved in an isolated system, so as angular momentum is
transferred, the spins slow as the orbits speed up, until the two
become equal and the bodies become tidally locked. The orbits also
become circular as the tidal forces resist eccentric orbits in which
resistive tidal forces increase sharply when the bodies are closer
together: circular orbits offer less resistance.
The fact that very little or no polar flattening / equatorial extension has occurred in Pluto suggests that Pluto remained warm, soft and deformable during the early evolution of the Pluto-Charon binary system either during or after the tidal locking. It may also suggest that Pluto was not spinning very fast to begin with.
A Closer Look at Pluto's Geological Features
Here we will focus on a few selected regions of Pluto's surface. For
further details see Moore et al.
2016.
Sputnik Planum (now opfficially Sputnik Planitia) is an uncratered plain, filling a basin, within the Tombaugh Regio and is about 1000 km wide and spans an area of some 870 000 km2 centered at about 20oN, 175oE and is possibly a modified impact crater or some cryovolcanic outpouring and is 3 to 4 km below the surrounding highlands. The high albedo imparts a light color compared to surrounding terrain and is probably due to ices of nitrogen, methane and carbon monoxide. The lack of impact craters suggests it is a relatively young surface, at less than about 10 million years old (10 Mya).
This plain is formed largely of polygonal domes or hills, up to 50 m high in the center, surrounded by narrow troughs about 100 m deep. These are thought to be convection cells. On Earth, water ice is a highly viscous fluid (a solid fluid) flowing slowly, as in glaciers and ice sheets. On Pluto, water ice is so cold that it becomes rigid and brittle like rock. However, the frozen atmosphere of Pluto covers much of the surface in ices of nitrogen, methane and carbon monoxide. Heat from the warmer interior may cause these softer ices to slowly rise to the surface in convection cells, before the ice cools at the margins and sinks back below. This solid-state convection could account for the ice domes. The low surface gravity of Pluto would facilitate their elevation. Other ice hills on Pluto, which typically adorn smoothy plains and may be up to 50 km across, are possibly caused by the deformation of ices. Nitrogen ice is particularly soft at Plutonian temperatures.
The tops of these polygonal cells contain periodic ridges or ripples, aligned in one direction, which are thought to be ice dunes. These dunes are not thought to be made chiefly of silica sand as on Earth (though some could be present) but of grains of water ice. Despite the fact that atmospheric pressure at the surface of Pluto is only 1 Pa (Pa = pascal) - a mere one hundred thousandth of the atmospheric pressure on Earth - wind speeds could reach 10 m/s and calculations suggest this is sufficient to cause eolian (aeolian, i.e. generated by the wind) features such as dunes.
'Washboard terrain' is an upland terrain found to the north and northwest of Sputnik Planum and consists of sets of parallel ridges separated by troughs. The ridges are about 1 km apart (crest-to-crest) and oriented NE-SW. They are superficial, consisting of material deposited over an existing terrain or material eroded from a surface covering to reveal the underlying terrain. These parallel ridges are thought to be ice dunes, consisting of ice particles especially lighter methane ice. Sublimation of ices can help raise overlying particles of ice into the air, as already explained, and the action of wind could result in dune formation.
On the western margin of Sputnik Planum are mountain ranges in which the mountains consist of randomly oriented angular blocks up to 40 km in diameter and 5 km high forming ranges hundreds of km in length. The slope and surface texture of these blocks reveals that they formed from the breakup of a pre-existing surface. Spectroscopy suggests these are blocks chiefly of water-ice that perhaps floated up through nitrogen and other ices by buoyancy.
At the eastern side of Tombaugh Regio there are high-albedo pitted uplands bordered by lower albedo bladed terrain. These pitted uplands contain pits, up to 25 km or more in diameter, intersected by long straight troughs several km deep. These perhaps formed by sublimation of ices carrying away the upper layers, and/or by melting deep below causing them to sink. Such sublimation pits perhaps form when overlying ice acts as a lens to focus the weak rays of the Sun to melt the more volatile ices (such as nitrogen and carbon monoxide ices). Modeling suggests this greenhouse effect can cause overlying particles of ice to become airborne.
The bladed terrain consists of ridges, hundreds of meters high, spaced 5 to 10 km apart (crest-to-crest) and aligned approximately N-S and separated by V-shaped valleys, some of which intersect in a Y-pattern.These ridges perhaps formed by the deposition of volatiles. We envisage a cycle of sublimation of volatile nitrogen, methane and carbon monoxide ices followed by their cooling and deposition back on to the surface. Such a cycle, driven by unequal heating of Pluto's surface could be coupled to winds on Pluto.
Along the boundary between the Sputnik Planum and the pitted uplands there are troughs, up to 6 km wide descending a slope of 2-3 degrees over about 50 km with a smooth channel floors. Did ice flows, that is rivers of ice, carve these troughs? Did the troughs form like the pits by sublimation? Fluted channels form a characteristic type of upland terrain, with fluted and dendritic (branching in a treelike manner) channels found on plateaus and in mountains. Such channels were likely carved by glacial rivers of softer nitrogen ice, perhaps sliding faster due to basal melting or sublimation. These channels frequently end abruptly in depressions and on crater floors, but there is no evidence of deposition suggesting they carried volatile material that has long since sublimated.
This cycle of sublimation and deposition of ices is expected to result in a gradual loss of these ices to outer space, especially given Pluto's low surface gravity. This suggests that they may be periodically replaced by melting/sublimation of ices deep beneath Pluto's crust and expelled onto the surface through cryovolcanic vents. Large mounds with central depressions have been found on Pluto, for example Wright Mons and Piccard Mons in the Norgay Montes at the SW border of Tombaugh Regio. Material around the depressions is layered, suggesting periodic deposition. An active cryovolcano on Pluto would be definitive proof of cryovolcanism but has yet to be seen. (Note: I once entered a competition with New Horizons to speculate what features might exist on Pluto and my reply, which was published, suggested cryovolcanoes to replenish the atmosphere. However, I doubt that any discovered cryovolcano will get named after myself as promised).
Pluto also shows signs of tectonic activity. There are several regions of Pluto's surface that are cut by long rifts up to 4 km deep and hundreds of km in length. Two such regions are highlighted by the dashed boxes below.
These are thought to be fault lines as a result of tectonics in which sections of the water-ice crust are moving. In particular these fault lines indicate extension or stretching of the crust, causing it to fracture. These are extensional fault lines. An example is the Virgil Fossa which is the arc connecting to the crater in the blower box above. These features suggest that Pluto's lithosphere is a thick layer of water ice. The lithosphere (lit. 'rocky sphere') of a planet is the outer rigid part, including the crust and any rigid part of the outer mantle. Their presence also suggests that Pluto has or is expanding slightly.
One possible explanation is that Pluto has a subsurface ocean, the outer layers of which froze into standard water ice type I, expanding as it did so, since ice I is less dense than liquid water. If the depth of the ocean is greater than about 260 km and the whole of it froze then the pressure should be sufficient to generate enough type II ice to cause Pluto to shrink rather than expand. Type II ice is a high pressure form of ice that is denser than liquid water. Other indications support the notion of a subsurface water ocean and both basic and more sophisticated mathematical models predict that the ocean is likely to be only partly frozen.
Dark (low albedo) reddish areas called maculae (sing. macula =
'spot') and a more extensive dark area called Cthulhu Regio (named after
the octopus-like god of Lovecraftian myth) contain a coating of material
that is poorly reflective. This is possibly a deposit of tholins
(or an outer layer of tholins formed in situ) which is a mixture
of organic molecules formed when UV light or electrical discharge acts
on a mixture of gases or when UV light acts on a mixture of ices; the
gases or ices in the mixture including water, methane and ammonia and
other simply molecules. Such a mixture is thought to be essential for
organic life to evolve and represents prebiotic chemistry.
Above: Krun and Balrog Maculae.
Could There be Life on Ice Dwarfs?
The surface of Pluto is too bitterly cold to support life dependent on complex chemical reactions without some other source of heat. However,
When I was a young child, I imagined creatures living on Pluto. I depicted them as humanoids with two horns and insect-like mouthparts with a proboscis that could extend and enzymically digest the ices to obtain nourishment. I also imagined they would be covered in insulating hairs and because of their horns I called them Minatories. They had enormous strength and so were put to use for labor by space-faring races. I imagine they might dwell in sub-surface caverns away from the full harshness of the cold outside. Networks of ancient cryovolcanic vents deep beneath Pluto's surface could contain an atmosphere, insulated by many kilometers of overlying ices. The ocean itself could theoretically contain aquatic lifeforms, but terrestrial forms could dwell in deep caverns. Such a dark world could have bioluminescent lifeforms, if sufficient energy was available for light production or such organisms might live in a world of total darkness.
In order to flourish life needs two things: a source of minerals and a source of energy. Life utilizes the most abundant minerals and elements in the universe, and the ices of Pluto and its probably silica interior and salty oceans likely contain ample nutrients. To assimilate these nutrients life requires a source of energy that can do useful work like building bodies to overcome the natural tendency of ordered structures to decay due to entropy. We think of Pluto as a cold, somewhat dark and lifeless world, so where could useful energy come from?
Pluto is some 40 times further from the Sun than the Earth and since the intensity of sunlight falls off with the square of distance the sunlight reaching Pluto is only one 1600th (402 = 1600) or 0.0006 % of the intensity of sunlight reaching Earth. However, surprisingly this is still sufficient for photosynthesis and we have to take into account the fact that Pluto's very thin atmosphere screens out a negligible amount of light. Indeed, the compensation point for photosynthesis for shade plants on Earth is only one ten-thousandth of the intensity of direct sunlight. The compensation point is the amount of light needed so that the plant breaks even and makes as much organic material as it respires. Some bacteria can utilize even dimmer light. However, the surface of Pluto is simply too cold for photosynthetic lifeforms.
One solution would be to use waveguides to conduct the light down to the dark warmer subsurface parts. Some living organisms are known to utilize waveguides to conduct light, essentially organic fiber optics. However, the thickness of the ice on Pluto is of the order of 100 to 200 km thick so a lot of growth of such structures would be needed before they can access the light above. Organisms could live within the ice where it contains some melting from warmer subsurface waters welling upwards but the icy crust of Pluto is so cold that it is largely dry and rigid and the temperatures are prohibitively low.
What other energy sources may be available? Deep within Pluto where there are still liquid waters there could well be volcanic vents. Pluto may contain a warm core, heated by radioactive decay and the heat of the planet's formation. The heat could be used to directly drive biochemical reactions. Some bacteria on Earth are also able to photosynthesize using infrared light. Perhaps our Plutonian might have skin capable of utilizing volcanic heat in such a way. Some materials, such as silk, will also generate electrical currents in response to heat which could be coupled to a respiratory chain to drive energy providing chemical reactions within an organism's cells or tissues.
Another potential source of energy is piezoelectric current. Piezoelectric materials generate electrical currents when mechanically distorted, converting mechanical energy into electrical energy. This could be used to harness wind or oceanic currents either to generate heat or to directly power respiration by providing a source of electrons. In the subterranean tunnels and caverns of Pluto there may well be winds driving the gases sublimated from ices or driven off bodies of liquid, particularly as nitrogen is especially volatile.
Of course, the fact that life could potentially survive deep beneath the surface of ice dwarfs like Pluto does not mean that life will necessarily get going or evolve into very complex ecosystems. Science currently has insufficient information on how life gets started to address the first obstacle. Suffice it to say that the necessary chemical building blocks are available. Even if life begins, then if total energy availability is low then a relatively sparse ecosystem of largely single-celled lifeforms could result with no complex multicellular forms. Even on Earth, bacteria were the dominant lifeform for most of prehistory, though they evolved considerable diversity and immense molecular diversity and ecosystem complexity. However, we should bare in mind that the Universe is possibly infinite and ice dwarfs abundant and in the fullness of time every possibility may be realized somewhere sometime.
Charon
Smaller than Pluto, Charon is a more inert and more frozen world and
most of its surface is estimated to be about 4-billion years old and the
northern hemisphere is known to be particularly cratered. For a movable
virtual globe of Charon visit: https://solarsystem.nasa.gov/moons/pluto-moons/charon/in-depth/.
Above: a 3D model of Charon made by simply wrapping a map of Charon's surface (supplied by NASA/Cassini-Huygens) around a sphere. Note the large crater Dorothy and the large fissure on the equator. What this model does not show is the relief: there is some 20 km between the top of the tallest peak and the bottom of the deepest depression which gives Charon a noticeably rough outline on this scale. This high relief is maintained due to the rock-like rigidity of water ice that covers most of the surface and the low gravitational field (g = 0.288 ms-2 or 0.029 of that on the surface of the Earth).
Unlike Pluto, due to its weak gravity Charon lost its atmosphere ages ago and so is not covered by frozen gases and hence its surface geology is not dominated by cycles of sublimation and condensation as is Pluto's. Instead it is dominated by rock-like water ice and is mostly a very old surface with little sign of recent resurfacing. It seems to be largely inert.
The equatorial fissures are a series of ridges and canyons or chasmata, including Serenity Chasma which is about 50 km wide and 5 km deep. These are probably tectonic in origin and reflect stretching and fracture of the icy crust, followed by freezing of deeper layers. This suggests that Charon expanded slightly and suggests that a subsurface ocean froze, forming low density ice I, the form of ice familiar to us which has a lower density than liquid water, causing the water to expand upon freezing. This suggests that there is likely to be less of the other forms of high-density water ice that form under high pressure which indicates a relatively shallow subsurface ocean compared to Charon's weak gravity. Thus, it is quite possible that the subsurface ocean is completely frozen or that a slushy mixture of ice and salty brine remain and perhaps a shallow subsurface ocean of brine. However, no significant internal heat source is expected so Charon is perhaps totally frozen.
Other explanations for the past expansion of Charon have been suggested based on mathematical models of the objects evolution. For example, the model by Malamud et al. (2017) factors in various processes that can generate internal heat within Charon's core, such as radioisotope decay, tidal heating and hydration of rock. Sorting and compaction of material under gravity is also a potential source of heat. The model assumes that Charon is made-up initially of porous material which undergoes an initial period of shrinkage as the material compacts, followed by internal melting as the core temperature rises which releases further heat by serpentization of the rocky core. Serpentine is a form of hydrated aluminosilicate and a major mineral component of rock. Hydration of aluminosilicates to form serpentine is the chemical reaction of serpentization which releases heat. The remaining free liquid water then freezes in the outer mantle, causing expansion of Charon. In the latter stages further compaction by gravity causes shrinkage and finally a brief period of expansion is predicted as rising core temperatures reverse the serpentization by driving off water of hydration from rocks in the central core, this released water then expands upon freezing. In short such models can predict the formation of tectonic rifts due to shrinkage/expansion without the freezing of a subsurface ocean as such. This model thus incorporates some kind of activity throughout Charon's existence in a gradual evolution rather than assuming it to be totally inert for the past 4 billion years.
Above: models such as that by Malamud et al. (2017) predict a layered structure for Charon as shown in this cutaway model. The central core consists largely of anhydrous silicate rock (brown), surrounded by an outer core of hydrated serpentine rocks (green) and a mantle of ice, the lowermost part of which froze slowly to form crystalline ice. The outermost crust consists of amorphous ice, or glassy ice that formed by rapid freezing.
Above: The dark reddish northern cap, called Mordor Macula (macula literally means 'spot' and refers to dark spots on Charon and Pluto). This region is rich in organic molecules and is thought to be formed by the action of UV light on frozen methane, forming tholins. The source of this methane is debated and it has been suggested that it may have come from Pluto's atmosphere or from past cryovolcanic activity.
Above: the Nasreddin crater at 25.5oN, 51.4oW.
Above: Equatorial fissures scar Charon. This main set of chasmata (troughs) running E-W includes Serenity Chasma. The large Dorothy crater on the southern edge of the North cap is visible. The northern region between the chasmata and the northern cap is called Oz Terra.
Above: Pluto and Charon seen side-on. Pluto is about twice the radius of Charon, at a mean radius of 1188 km compared to Charon's 606 km and the center-to-center distance is about 16.5 times the radius of Pluto at 19 640 km. Pluto is about 8.2 times as massive as Charon, so the barycenter (center of mass) of the system is about 2400 km from the center of Pluto. Both objects orbit about this barycenter as can be seen in this external link. This is in contrast to the Earth-Moon system in which the barycenter is much closer to the Earth's center, due to its greater mass, and beneath the surface of the Earth. Thus Pluto and Charon is often considered a binary planetary system or double planet, with both members planets of the dwarf class.
Charon and Pluto are both tidally locked to one-another, meaning that over the ages the gravitational drag, due to tidal friction, that each exerts upon the other has synchronized their rotations, such that both objects rotate once on their own axes every 6.4 Earth days and thus the same face of Pluto always faces the same face of Charon. Again, in contrast the Moon is tidally locked to the Earth, always keeping the same face towards her, but the Moon has not yet been able to break the rotation of the Earth enough to lock its rotation, hence the Moon is tidally locked to the Earth but the Earth is NOT tidally locked to the Moon.
Kepler's third law states that the square of the period of orbit of one body about another is proportional to the cube of the radius of the orbit. The constant of proportionality depends on the mass of the objects involved and hence the mass of the Pluto-Charon binary can be readily estimated. Given a knowledge of the diameters and compositions of the two objects their individual masses can then be obtained. This is just one method by which the mass of a planet or satellite can be estimated. Alternatively by comparing the orbits of the Earth-Moon and Pluto-Charon system the mass of Pluto can be obtained given the mass of the Earth.
Credit: Image maps of Pluto and Charon courtesy of NASA / Cassini-Huygens.
References / Further Reading
Stern, S.A., Bagenal, F., Ennico, K., et al. 2015. The Pluto system: Initial results from its exploration by New Horizons. Science 350(6258): aad1815. DOI: 10.1126/science.aad1815
Telfer, M.W., Parteli, E.J.R., Radebaugh, J., et al. 2018. Dunes
on Pluto. Science 360(6392): 992-997.
DOI: 10.1126/science.aao2975
Article
updated:
26 July 2015
30 June 2020
11 Dec 2022
27 Dec 2022
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