Given the task definition and input, reply with output. In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].

It is a mystery that has baffled the world's top scientists - why are there 'blood streaks' on one of Saturn's moons? 'It's clearly painted on the surface in some way that we do not as yet understand,' says Paul Schenk of the Lunar and Planetary Institute, who presented the results of the latest investigation at the American Geophysical Union's annual meeting. 'We basically have a little mystery.' Cassini first spotted these mysterious red lines, which stretch 1,000 of miles across, scratched across the surface of Saturn's icy moon Tethys. The unexplained arc-shaped 'blood trails' are among the most unusual colour features on Saturn's moons to be revealed by the probe.The Cassini probe spotted mysterious red lines on Tethys, several hundred miles long and only a few miles wideScientists thought they may be the result of some type of deformation on the moon caused by a strange orbitBut simulations ruled out this possibility, and now astronomers say fractures unseen by Cassini may be the cause 
 Question:This may be because its proximity to Saturn causes more tidal warming, and that warming kept _ partially molten longer.

Tethys