The transfer of unsterilized material from Mars to Phobos: Laboratory tests, modelling and statistical evaluation.

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.

Three-dimensional load-displacement curves due to forces on the cervical spine.

Load-displacement curves were measured for six types of pure force loading of the cervical spine specimens obtained from fresh human cadavers. A new measuring and mounting technique was developed that yielded data for all of the functional spinal units for each specimen tested. All five of the coupled, as well as the main, load-displacement curves were studied. For anterior and posterior shear loadings, the main resulting motions were translation in that direction (1.6 +/- 0.3 and 1.9 +/- 0.3 mm), and the major coupled motions were flexion and extension (3.6 degrees +/- 1.2 degrees and 6.3 degrees +/- 1.2 degrees). The main motions with right and left lateral shear loadings were translations laterally (1.4 +/- 0.3 and 1.6 +/- 0.3 mm), and the major coupled motions were axial rotations (1.5 degrees +/- 0.6 degrees and 2.3 degrees +/- 0.6 degrees) and not lateral bending. For compression and distraction loadings, the main motions were translations in that direction (0.7 +/- 0.3 and 1.1 +/- 0.3 mm), and the major coupled motions were flexion and extension (2.0 degrees +/- 1.0 degrees and 2.8 degrees +/- 1.0 degrees) and lateral bending (1.4 degrees +/- 0.3 degrees and 1.9 degrees +/- 0.3 degrees). The neutral zones for anterior and posterior shear forces were 1.6 +/- 0.2 mm of translation and 5.8 degrees +/- 1.3 degrees of rotation, for lateral shear force 1.4 +/- 0.3 mm and 2.0 degrees +/- 0.5 degrees, and for compression/distraction 0.6 +/- 0.1 mm and 2.8 degrees +/- 0.9 degrees.