The collision of the deep impact probe with the nucleus of comet tempel 1 in 2005 revealed the nucleus as a rubble pile held together by

Self Gravity

Explanation:

Tempel 1 is a periodic Jupiter-family comet discovered in 1867 by Wilhelm Tempel which is where it got its name from. It takes 5.5 years for it to complete an orbit of the Sun.

After the probe of the collision which took place in 2005, it was discovered that the comet was held together only by self-gravity rather than by structural bonds between the cometary particles themselves.