Daniel MEGE







students & postdocs


hot BasaltAlteration VM PlutoTriton GSOgaden VolcOgaden DS DykeProp Termites cooling frozen





Pluto, Triton, the KBOs and the New Horizonsmission



New Horizons

The New Horizons mission (NASA), designed and managed by Southwest Research Institute (Boulder, CO) and Johns Hopkins' Applied Physics Laboratory (Laurel, Maryland) was launched by Lockheed Martin's Atlas V 551 le 19 janvier 2006 for a flyby of Pluto (on July 14, 2015) and other Kuiper Belt Objects.

The instruments onboard New Horizons have been mainly designed to study the surface and atmospheres of Pluto, its moons Charon, Nix, and Hydra, and the other KBOs. See the seminar on New Horizons and the KBOs given at ESA in Noordwijk in June 2006 as a collaborator of the New Horizons Scence Team.

A workshop supported by CNES was held in Nantes in January 2007 to summarize the state of the art in Pluto and KBO science and identify French research perspectives in this field.

Pluto : still a planet (though dwarfified) !

In August 2006, in spite of campaigning aiming at keeping Pluto a planet, vigorously supported by Alan Stern, Principal Investigatot of the New Horizons mission, resolutions B5 et B6 of the International Astronomical Union (IAU) deprived Pluto of its planetary status, and classified it in a new class of object, the plutoids, from the newly defined dwarf planets category of objects. When the resolution was voted, Alan was with one of her daughters for the start of the new college year; no doubt that should he have inverted his priorities Pluto would still be a planet.

yes but it did not work finally :(



Cryovolcanism of Triton and Pluton

In 2009 a prospective study will start on the tectonics and cryovolcanism of Pluto in relation with Triton, a likely KBO captured by Neptune.

How do we know that there is cryovolcanism on Triton?

Geysers are observed at surface; They are probably due to the gravitational attraction of Neptune. Despite the low resolution of the available images (from Voyager 2, 1989), landforms of apparently volcanic origin are observed (see Kargel, Earth, Moon and Planets 67, 101-113, 1995). Something must be happening at depth.

How do we know that there has been cryovolcanism on Pluto?

First, we don't know. Then, thre must have been some internal dynamics early in the history of Pluto (and Charon?), surface albedo is highly contrasted, so why not cryovolcanism? Let us dream a bit before New Horizons shows us reality.

Both Triton and Pluto have a rocky core and an icy outer layer. Triton was captured by Neptune, and the Pluto-Charon binary system originates from a collision between a proto-Pluto and another objects. Triton, observed by Voyager 2 in 1989, shows an amazing variety of cryovolcanic landforms not more than several tens of m.y. old. These include volcanic edifices and associated lava flows, calderas, diapirs, lava lakes, rilles, pit craters, and probable ice dykes, in addition to active geysers. This activity probably originates from tidal forces exerted by Neptune, and gradually increases as Triton gets closer to Neptune with time.
In July 2015, the New Horizons spacecraft from NASA/SwRI/APL will observe the surface of Pluto and Charon in panchromatic mode at a nominal resolution up to 70 m/pixel (LORRI), and in colour mode (RALPH, blue, red, near-IR, CH4 absorption IR band) up to 670 m/pixel. Cryovolcanic activity is expected be observed on Pluto as well, and would result from tidal interactions with Charon early in their history, when Pluto and Charon were rotating faster and closer to each other. This contribution explores the implications of Triton's cryovolcanism for its internal structure, and some implications the observation of cryovolcanic landforms on Pluto would have for its internal dynamics in the past.

The interior of Triton and Pluto is assumed to be differentiated into a rocky core overlain by a thick H2O ice crust, whose thickness is determined from the total radius and mass of the satellite. A tidal energy dissipation model for Triton and Pluto is calculated using a model earlier developed for Titan. Assuming a typical cometary composition for the ice phase, a few percent of contaminants such as ammonia and/or methanol are considered. The model predicts that the presence of these anti-freezing contaminants permits the persistence of a molten zone at the bottom of the ice layer on Pluto early in its history, and during the history of Triton until the present days. As this liquid zone crystallizes as a function of time, the concentration of contaminants increases, resulting in a progressive decrease of the liquid density. At some point during the evolution, the density becomes lower than the density of the overlying ice. This leads to the rupture of the icy crust above the liquid reservoir and the rise and emplacement of molten ice dykes whose vertical extent is calculated from the fracture toughness of low temperature ice Ih. If the cryomagma supply rate allows, dykes propagate vertically to their level of neutral buoyancy (LNB), whose depth is calculated from the balance between the density of the molten ice and the density profile of the surrounding ice layer. Dyke thickness and lateral extent can be calculated for a range of cryomagma flow rates. Dykes attaining the LNB can lengthen as long as they are fed from the reservoir. LNB depth is calculated as a function of time and tidal dissipation energy decrease. A number of dykes propagating at the LNB are likely to reach the surface and erupt at some point, giving birth to a series of cryovolcanic morphologies examples of which are observed on Triton, providing a constraint on the minimum age of Plutonian landscape modification by cryovolcanic processes.

This ongoing work is carried out with Gabriel Tobie, also from University of Nantes. A poster has been presented at the 6th International Dyke conference on this topic. It discusses cryovolcanic landforms on Triton, and LNB depth s a function of thermal history of Pluto and Triton assuming a crust whose composition is inferred from cometary composition (N2, CO, CO2, CH4, NH3...).