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.

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