Solar system bodies for which NASA has considered planetary protection precautions include:
- Earth’s Moon
- Europa, Ganymede, and Callisto (moons of Jupiter)
- Titan and Enceladus (moons of Saturn)
- Comets and Asteroids
Planetary protection controls on the forward contamination are intended to preserve the planetary record of natural processes on these bodies by preventing human-caused microbial introductions that could interfere with future astrobiological investigations.
For Category I, typically missions to Mercury and those of heliophysical interest are considered to be of no direct interest for understanding the process of chemical evolution and do not require planetary protection controls. However, reassessment of the planetary protection concerns for Mercury are underway due to the recent discovery of water ice and organics by the MESSENGER mission. Further, for heliophysical missions, assignment to Category II may be warranted if planetary bodies of concern may be impacted. Missions to Venus and Earth’s moon are typically assigned as Category II missions. Though some scientists have speculated that microbial life might be able to survive in the atmosphere of Venus, the consensus is that the environment of Venus is too harsh to support life as we know it, and planetary protection is currently not a major concern for that planet.
While some of the moons of Jupiter and Saturn are considered good sites for astrobiological investigation, the consensus is that the environments of Jupiter and Saturn themselves are not suited to supporting life as we know it, and planetary protection is currently not a major concern for these gas giants. Likewise, planetary protection is not a major concern for the other outer planets Uranus, Neptune and Pluto and many of the satellites of the outer planets. This same assessment also applies to comets and asteroids.
The most stringent requirements are those for Mars, Jupiter’s moons Europa, Ganymede and Callisto, and the Saturn’s moon of Titan and Enceladus due to the great potential for studying the origin, evolution, and distribution of life on these bodies. The preceding links provide more information regarding the specific planetary protection concerns. For Mars, the requirements are based upon orbital lifetime constraints for Category III missions, and numerical bioburden constraints for the Category IV missions. For the Jovian and Saturn moons, the primary planetary protection focus is on the probability of contamination of a liquid water body.