1. Meltzer, M. When Biospheres Collide: A History of NASA's Planetary Protection Programs (NASA SP-4234, 2011).
2. Fairén, A. G. & Schulze-Makuch, D. Nature Geosci. 6, 510–511 (2013).
3. National Research Council of the National Academies. Assessment of Planetary Protection Requirements for Mars Sample Return Missions (National Academies Press, 2009).
4. Ammann, W. et al. Mars Sample Return backward contamination – Strategic advice and requirements (European Science Foundation, 2012).
5. Kelley, D. S., Baross, J. A. & Delaney, J. R. Annu. Rev. Earth Planet. Sci. 30, 385–491 (2002).