Sample return missions to Phobos are the subject of future exploration plans. Given the proximity of Phobos to Mars, Mars’ potential to have supported life, and the possibility of material transfer from Mars to Phobos, careful consideration of planetary protection is required. If life exists, or ever existed, on Mars, there is a possibility that material carrying organisms could be present on Phobos and be collected by a sample return mission such as the Japanese Martian Moons eXplorer (MMX). Here we describe laboratory experiments, theoretical modelling and statistical analysis undertaken to quantify whether the likelihood of a sample from Phobos material containing unsterilized material transferred from Mars is less than 10−6, the threshold to transition between restricted and unrestricted sample return classification for planetary protection. We have created heat, impact and radiation sterilization models based on the Phobos environment, and through statistical analyses investigated the level of sterilization expected for martian material transferred to Phobos. These analyses indicate that radiation is the major sterilization factor, sterilizing the Phobos surface over timescales of millions of years. The specific events of most relevance in the Phobos sample return context are the ‘young’ cratering events on Mars that result in Zunil-sized craters, which can emplace a large mass of martian material on Phobos, in a short period of time, thus inhibiting the effects of radiation sterilization. Major unknowns that cannot yet be constrained accurately enough are found to drive the results – the most critical being the determination of exact crater ages to statistical certainty, and the initial biological loading on Mars prior to transfer. We find that, when taking a conservative perspective and assuming the best-case scenario for organism survival, for a 100 g sample of the Phobos regolith to be below the planetary protection requirement for unrestricted sample return, the initial biological loading on Mars must be <8.2 × 103cfu kg-1. For the planned MMX mission, a ∼10 g sample to be obtained from a 25–30 mm diameter core as planned would require an initial martian biological loading to be <1.6 × 104cfu kg-1, in order to remain compliant with the planetary protection threshold
Bibliographical noteFunding Information:
This work was funded by the European Space Agency under contract number 4000112742/14/NL/HB (“Sterilization limits for sample return planetary protection”). We note appreciation to J. Melosh and S. Werner regarding impact and crater discussions throughout the study, and R. Moeller and T. Berger regarding radiation results discussions. We would also like to thank the COSPAR PP Working Group for Phobos, who provided significant input to the overall interpretation of the results from this study, and G. Kminek in particular.
This work was funded by the European Space Agency under contract number 4000112742/14/NL/HB (?Sterilization limits for sample return planetary protection?). We note appreciation to J. Melosh and S. Werner regarding impact and crater discussions throughout the study, and R. Moeller and T. Berger regarding radiation results discussions. We would also like to thank the COSPAR PP Working Group for Phobos, who provided significant input to the overall interpretation of the results from this study, and G. Kminek in particular.
© 2019 The Committee on Space Research (COSPAR)
- Planetary protection
- Sample return