Tag Archives: Charon

Dwarf planets

Charon has had an active youth

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Hi there! The scientific review Icarus releases an issue dedicated to New Horizons, which made a fly-by of the system of Pluto-Charon in July 2015. Now all the data have been transmitted to Earth. This is the opportunity for me to present you one of the new papers, entitled Charon tectonics, by Ross Beyer et al. This papers presents evidences of an active tectonic youth of Charon, and probably a former subsurface ocean, which is now frozen.

Charon facts

Charon has been discovered in 1978, as the first known satellite of Pluto. It actually appeared that Charon is massive enough, so that Pluto-Charon should be considered as a binary system, which orbits around a common barycenter. Moreover, the gravitational interactions (one call them tidal interactions) between these two bodies are so strong that they rotate synchronously with their mutual orbit. This means that they always show the same face to each other.

Charon seen by New Horizons. Credit: NASA

The recent flyby of the space mission New Horizons gave us several details on Charon, the paper I present here addresses some of them, more specifically linked to the features observed at the surface of Charon, which are linked to a past geophysical activity. Let us now speak a little about planetary tectonics.

Planetary tectonics

The tectonics is the process that controls the shaping of the surface of the Earth. It is responsible for the apparition of mountains, for earthquakes, for the continental drifts (plate tectonics). Tectonics does not appear only on Earth, this is why we can speak of planetary tectonics.

Tectonics results from the heating of a planetary body, and the loss of this heat. This heat is responsible for melting of some elements, differentiation of the planet, and thus activity. A spectacular example in the Solar System is the intense volcanic activity of Io. This satellite of Jupiter is intensively heated by the tidal interaction with its parent planet.
Another example is the geysers on the satellite of Saturn Enceladus.
Beside this observable activity, the observation of irregular features at the surface of a planetary body is an evidence of a past tectonic activity, which is actually ubiquitous in the Solar System. Just a few examples:

  • Plains have been detected at the surface of Mercury, which means that these are renewed terrains,
  • Our Earth has many volcanoes,
  • The highest known volcano in the Solar System is Olympus Mons, on Mars (see this post),
  • The surface of the satellite of Jupiter Europa presents many ridges,
  • The satellites of Uranus Ariel and Miranda present interesting features as well.

And now Charon!

A glossary of planetary features

Some of the definitions I present above have been borrowed from the official nomenclature of the International Astronomical Union. Such a nomenclature has been established to name the planetary features actually observed.

And the terms to know are:

  • Chasma: A deep, elongated, steep-sided depression. The plural is chasmata.
  • Scarp: An escarpment, i.e. a vertical feature which separates two zones of different elevation.
  • Macula: A dark spot, which may be irregular. The plural is maculae.
  • Planum: A plateau, or a high plain.
  • Elastic thickness: This is not a topographical feature. This is the thickness that would have the crust if it were fully elastic in showing the features actually observed. This quantity helps to characterize the crust from the observation of the surface.

The spacecraft New Horizons

The spacecraft New Horizons has been launched in January 2006 and has encountered the system of Pluto in July 2015, after a Jupiter flyby in February 2007. It is composed of 7 science instruments:

  • The ultraviolet imaging spectrometer Alice, dedicated to the study of the atmosphere of Pluto,
  • The imager Ralph, actually composed of 8 imagers, in different wavelengths. It is in charge of mapping the encountered bodies,
  • The Radio Science Experiment REX, which has measured the masses of Pluto and Charon, and probed their atmospheres,
  • The Long Range Reconnaissance Imager LORRI, which gave the first images of Pluto and its satellites by New Horizons,
  • SWAP, for Solar Wind Around Pluto, dedicated to the Solar wind,
  • PEPSSI, for Pluto Energetic Particle Spectrometer Science Investigation. It studied the interactions of the atmosphere of Pluto with the Solar wind,
  • and the Venetia Burney Student Dust Counter SDC, which studied the dust in the system of Pluto. This instrument was part of a New Horizons Education and Public Outreach project, it was designed and built by students. It was named after Venetia Burney, who proposed the name of Pluto after its discovery. She was 11 then. The 250-km-wide Burney Crater, on Pluto, is named after her.

The paper I present today use mainly LORRI and LEISA data, LEISA being an infrared detector of Ralph (not to be confused with LESIA, which is a planetary lab of Paris Observatory).

This mission did not permit a global high-resolution mapping of Charon, since New Horizons did not orbit in the Pluto system. So, the highest resolution images we dispose of are limited to one hemisphere, and the way to analyze them depends on the varying Solar insolation angle. A scarp, a mountain, a crater… will appear differently if enlightened from the zenith or from the horizon.

This paper

This paper represents the main surface features that can be seen, before discussing their origin.

The most striking features are:

  • Mordor Macula, which is a polar dark spot,
  • an equatorial belt of chasmata, which splits the hemisphere into two plains: Oz Terra (North), and Vulcan Planum (South),
  • Argo Chasma, which appears at the limb,
  • many craters.

Craters give us the chronology of the tectonics. Tectonic activity tends to melt the surface, renew it, and relax the crater basins, which should then be barely visible. The fact that many craters can be seen means a very old surface. This also means that the other features are even older, i.e. they were created some 4 Gyr ago.

Let us concentrate now on the equatorial belt. The two main features are Serenity Chasma, which is 40-50 km wide and over 200 km long, and Mandjet Chasma, which is 30 km wide and at least 450 km long. These two structures have a depth of typically 5-7 km.

These chasmata suggest an elastic thickness of 2.5 km. Moreover, the structures indicate that Charon experienced a radial extension, which could be due to the freezing of a global surface ocean. So, in its early ages, Charon has had a global subsurface ocean, which is now frozen.

Creating a subsurface ocean requires some heating. The system Pluto-Charon could originate from the destruction of a progenitor by an impact, which would have induced intense heating. Moreover, this heating has probably been assisted by the tidal heating of Charon by Pluto.

The discovery of these features gives us another signature of the early ages of the Solar System, and would surely contribute to the global understanding of the formation of planetary systems.

To know more

Links to the study, the authors, and the mission

That’s all for today! I hope you liked it. As usual, you are free to comment. You can also subscribe to the RSS feed, and follow me on Twitter.

Dwarf planets

Matter exchange in Pluto’s backyard

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Hi there! Today I will tell you about a recent study accepted for publication in The Monthly Notices of the Royal Astronomical Society. This study, by Rachel A. Smullen and Kaitlin M. Kratter, addresses The fate of debris in the Pluto-Charon system, and has been conducted at Steward Observatory, AZ (USA).

The Pluto-Charon system

Pluto has been discovered by Clyde Tombaugh in 1930 at Lowell Observatory in Flagstaff, Arizona. This was the first discovered object of the Kuiper Belt, and it has been considered as the 9th planet of the Solar System until 2006. Still in Flagstaff, its satellite Charon has been discovered in 1978. Later on, the next arrival of the New Horizons spacecraft motivated observing this system, and thanks to the Hubble Space Telescope, 4 other small moons were discovered: Nix and Hydra in 2005, and Styx and Kerberos in 2012. You can find below images of these 6 bodies taken by New Horizons, and some of their orbital and shape parameters.

Pluto (left) and Charon (right) seen by New Horizons. The white heart on Pluto’s surface is an ice-covered basin named Sputnik Planitia, while the dark spot on Charon’s north pole is named Mordor macula. Copyright: NASA
The small moons, seen by New Horizons. Copyright: NASA
Discovery Radius Distance
Pluto 1930 1187 km 0
Charon 1978 606 km 19571 km
Hydra 2005 23 km 64738 km
Nix 2005 18 km 48694 km
Kerberos 2012 5 km 57783 km
Styx 2012 5 km 42656 km

The pair Pluto-Charon is fascinating from a dynamical point of view, since they represent a case of double synchronous spin-orbit resonance. You know that the Moon is always showing the same face to the Earth, which is due to its synchronous rotation. This means that its orbital period around the Earth is exactly the same as its rotation period, this is a dynamical equilibrium which has been reached after tides had dissipated the rotational energy of the Moon. But the phenomenon goes further for Pluto-Charon, since not only Charon shows the same face to Pluto, but Pluto shows the same face to Charon! This is a consequence of the relative size of the two bodies, each of them being sufficiently large to affect the other one.
On the contrary, the small moons have a much more rapid rotation, which is less obvious to explain.

The system of Pluto has been visited in 2015 by the spacecraft New Horizons, which gave us invaluable data and the nice images I show you today.

The Plutinos

The orbit of this system around the Sun is interested as well. Not only it has a significant inclination (17.16° wrt ecliptic), but it is also in a 3:2 mean-motion resonance (MMR) with Neptune. This means that Pluto makes exactly two revolutions around the Sun while Neptune makes three. Moreover, this is a pretty stable dynamical zone. This is probably why Pluto and its satellites are not the only bodies in this zone. Beside the Pluto system, the first Plutino has been discovered in 1993 at the Mauna Kea Observatory, HI.
The following figure gives a repartition of the known Trans-Neptunian Objects with respect to their semimajor axis, the Plutinos represent a peak at 39 astronomical units.

Distribution of the Kuiper-Belt Objects, plotted from the data of the Minor Planet Center, consulted on January 28th 2017. We see the Plutinos as an accumulation of objects close to 39 AU, which corresponds to the 3:2 MMR with Neptune. The second peak, close to 44 AU, does not correspond to a known resonance. Copyright: The Planetary Mechanics Blog.

Formation of planetary debris disks

The last thing I would like to tell you before presenting the study itself is: how to make a debris disk around a pretty massive body? It is thought to come from an impact. An impactor impacts the target, is destroyed into very small parts, which coalesce into rings, before eventually reaccreting and / or being ejected. The most famous debris disk in the Solar System in the system of the Saturnian rings, but there are actually rings about the four giant planets of the Solar System, and the Centaurs (asteroids between the orbits of Jupiter and Neptune) Chariklo and possibly Chiron.
It is thought that the Moon is the consequence of such a process, i.e. there has been a debris disk around the Earth. And it is also thought that Charon has been created the same way.

This paper

This study aims at understanding the fate of the debris disk which has created Charon. Once enough debris accreted to create Charon, or a proto-Charon, debris remained, and have been ejected. There are at least two ways to model a disk: either you consider it as a gas, i.e. some fluid, or you see it as a cloud of many particles, which interact. These interactions are close encounters and collisions, with translate into viscosity if you model the disk as a gas.

A numerical study

In this study, the authors chose to model the debris disk as a cloud of particles, which is probably the only way to model the path of ejecta. They made several simulations involved 27060 test particles, over 27.3 kyr, i.e. 1.5 million orbits of Pluto and Charon about their common barycenter. Such a study requires high performance soft- and hardware. Their code was based on the integrator Mercury, which is a commonly used N-body code modeling the motion of N body which interact gravitationally and may collide. The test particles are massless, so they have no gravitational action, but they are under the action of Pluto and its 5 satellites. In some of the tests, a migration of Pluto, which is predicted by models of formation of the Solar System, has also been considered.
The hardware is the Super-computer El-Gato (Extremely LarGe Advanced TechnOlogy), based at the University of Arizona, and partly funded by the National Science Foundation.

Once the simulations have run, the authors got the results. And the results are… drum roll please…

Making craters on Charon

The New Horizons images show that Charon is craterized. In all of their simulations, the authors have collisions between Charon and the debris disk. They show that the impact rate is higher if Charon formed on a wide and eccentric orbit. Moreover, they have fewer impacts if secular migration of Pluto is considered.
An issue is: what could be the signature of such an impact now? We know from its synchronous rotation and from the ridges at its surface that Charon has been hot. Hot enough would mean that part of its surface could have been renewed, and then the older impacts would have no signature anymore. Moreover, it would be interesting, but I doubt the information is present in the New Horizons data, to map the impact on the whole surface of Charon. If Charon was synchronous during the most intense episode of impacts, then we would expect a hemispheric repartition of the craters.

Making Plutinos

The simulations show that the most probable destination of the ejected debris is the 3:2 MMR with Neptune. This means that the observed Plutinos could originate from the impact which created Charon. This would mean that the Plutinos are a collisional family, which could be test from their composition. It should be similar to Pluto’s.

And the small moons?

The simulations do not manage to form the small moons. So, the question of their origin is still open.

Some links…

And that’s it for today! New Horizons is en-route to the asteroid 2014 MU69, which would be the first object visited by a spacecraft which had been launched before its discovery. It should reach it either on December 31th, 2018, or January 1st 2019.
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