"All solar systems missions are subject to constraints designed to minimize the unintentional impacts of Mars and, when applicable, the contamination of icy satellites such as Europa, Enceladus, and Ganymede." [1], also see [2]

"The Huygens probe which landed on Titan was not sterilized as the chances of finding life were considered insignificant. From further investigations it is evident that chances for life on Titan are higher than initially thought. Although forward contamination in this case is still considered unlikely, it reinforces the need to ensure we protect extra-terrestrial chances of life." [3], also see the second page of [4]

[1] https://planetaryprotection.nasa.gov/requirements [2] https://saturn-archive.jpl.nasa.gov/faq/FAQHuygens/ [3] http://www.spacesafetymagazine.com/space-exploration/extrate... [4] http://www.esa.int/Our_Activities/Space_Science/Cassini-Huyg...