Ice shell covers Titan

Since 2004, the Cassini spacecraft has been returning daily information on the Saturn system.  Cassini is currently on its second extended mission called the Cassini Solstice Mission, completing in May 2017 at the end of Saturn’s 13 year seasonal period, and thus allowing for the first time the study of a complete seasonal cycle of Saturn.
The extended mission is also focussing on Saturn largest moon, Titan.

View of the mountainous surface of Titan as seen by the Huygens probe during its descent, 14 January 2005. (Credit: ESA/NASA/JPL/University of Arizona )

Titan’s mountainous surface is covered in relatively pristine ice, suggesting that some sort of active geology – maybe even global tectonics similar to the Earth’s plate tectonics – constantly resurface the planet with fresh ice and created the mountains in the process.
The study the topology of Titan’s surface, astronomers use the radar capabilities of Cassini. Synthetic aperture radar (SAR), which is capable of penetrating Titan’s opaque atmosphere to observe the surface, is used for radar imaging, and radar altimetry, which is capable of measuring the distance from the spacecraft to the surface, is used the map the surface topology of Titan.

(a) Gravity field map and (b) elevation map of Titan. (Credit: Hemingway et al. 2013, Nature)

(a) Gravity field map and (b) elevation map of Titan. (Credit: Hemingway et al. 2013, Nature)

The combined results from these radar measurements have revealed that Titan has a rigid and weathered ice shell, rather than a geologically active, low-rigidity ice shell. This means that Titan is covered in thick rigid ice rather than a thin flexible ice shell as previously thought. These new results are published in this week’s edition of Nature. Hemingway et al. (2013) found negative gravity anomalies on the surface of Titan. Gravity is expected to be higher over a mountain than at ground level due to the increase in mass of the mountain, however gravity measurements of Titan suggest the opposite is happening.  In the image above the gravity field is negative (blue in Figure (a)) in regions of high evelvation (red in Figure (b)).  They suggest that these results are due to crustal thickness in regions of the ice shell where the topography is low, similar to icebergs on Earth. Through modelling of the surface, they were able to show that a rigid ice shell, over 40 km thick, and several meters of surface erosion are needed to replicate these results.
If these results are confirmed, this would suggest the plate tectonics on Titan would not recycle the ice shell.

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[Catarina Ubach & Sarah Maddison]

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