Thursday, October 31, 2013

Mountains on Titan

Titan is by far the largest moon in orbit around Saturn and the 2nd largest moon in the Solar System. It has a diameter of 5,152 km, making it nearly 1.5 times the size of Earth's Moon. Titan has a thick atmosphere and opaque haze layers obscure its entire surface. From inside out, the bulk of Titan is believed to be comprised of a partially differentiated interior of rock and ice, a high pressure ice layer (consisting of ice III, V, and VI), a subsurface ocean of liquid water and an outer ice I shell. Ice III, V, and VI are high pressure phases of ice which do not occur naturally on Earth. Ice I is basically normal ice and all naturally occurring ice on Earth is ice I.

Figure 1: Saturn’s fourth-largest moon, Dione, can be seen through the haze of the planet’s largest moon, Titan, in this view of the two posing before the planet and its rings from an image taken by the Cassini spacecraft. Credit: NASA/JPL-Caltech/Space Science Institute.

Figure 2: Artist’s concept showing a possible scenario for the internal structure of Titan, as suggested by data from the Cassini spacecraft.

A thermal model of the interior of Titan developed by Mitri et al. (2010) show that the long term cooling of Titan can cause a global volume contraction of ~0.01. As Titan cools, the base of its subsurface ocean would freeze onto the top of its high pressure ice layer while the top of its subsurface ocean would freeze onto the underside of its outer ice I shell. Because high pressure ice is a lot denser than liquid water (~10 to 30 percent denser) and ice I is only marginally less dense than liquid water (<10 percent less dense), the gradual freezing of Titan's subsurface ocean into high pressure ice and ice I would cause an overall reduction in the volume of Titan.

It seems that the presence or absence of a high pressure ice layer in the interior of an object can determine whether or not it will undergo global contraction or expansion during cooling. For example, the interior of Jupiter's moon Europa is comprised of a rocky interior, an overlying subsurface ocean and an outer ice I shell. Unlike Titan, Europa does not have a high pressure ice layer. Since its outer ice I shell has a lower density than the underlying subsurface liquid water ocean (i.e. water is less dense than ice), the long term cooling of Europa will cause the outer ice I shell to thicken and result in overall global volume expansion.

Figure 3: A model of the topography produced by the contractional deformation of Titan's icy lithosphere. (Mitri et al., 2010)

Figure 4: Cassini radar imagery showing three elongated radar bright features that may be fold ridges formed from the contractional deformation of Titan's icy lithosphere. A topographic profile across one of the ridges (black rectangle) show that it has a height of 1,930 m. (Mitri et al., 2010)

The global volume contraction of Titan leads to contractional deformation of Titan's icy lithosphere, producing fold features (i.e. mountains). These fold features can reach topographic heights of up to several kilometres, especially so if Titan underwent more rapid cooling in the early Solar System and thereby experienced more contraction. The radar instrument on the Cassini spacecraft has imaged mountainous topography on Titan that is consistent with fold features produced by the contractional deformation of Titan's icy lithosphere. Perhaps, such a contractional deformation process may have formed most of Titan's mountains.

Mitri et al., “Mountains on Titan: Modelling and Observations”, Journal of Geophysical Research: Planets, Volume 115, Issue E10, October 2010