Planetary Protection Classification of Sample Return Missions from the Martian Moons (2019) / Chapter Skim
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2 Overview and Assessment of the SterLim and JAXA Studies
2 Overview and Assessment of the SterLim and JAXA Studies
Pages 21-34
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From page 21... ...
Truscott, SterLim, 2017, "Sterilisation Limits for Sample Return Planetary Protection Measures ‘SterLim': Presentation to the Committee on the Review of Planetary Protection Requirements for Sample Return from hobos and Deimos," http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_183902.pdf; referred to here as the P "SterLim report." 2 M Patel, D
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From page 22... ...
Fujita, 2018, "JAXA's Planetary Protection Activities for Sample Return from Martian Moons," TR 2018-13-02, presentation to the committee, http://sites.nationalacademies.org/SSB/CurrentProjects/SSB_181917, used with permission. Each phase represents a specific model developed by the SterLim and JAXA teams or certain assumptions made.
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From page 23... ...
reported that PLFA analysis indicated 2.0 × 106 to 1.0 × 107 cell equivalents per gram. Thus, the cell density of the Atacama Desert is likely to be greater than what was used by the SterLim and JAXA teams.4,5 Meanwhile, even if the dryness of Atacama is similar to Mars, it is probably true that the Atacama Desert is an energetically favorable environment for life compared with Mars.
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From page 24... ...
No other impact events on Mars were considered, although the contamination risk from unrecognized young craters was also investigated. From the dimensions of these five craters, possible combinations of impactor sizes, impact velocities, and impact angles were identified using a Monte Carlo approach and semi-empiric scaling laws.
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From page 25... ...
This discrepancy is attributed to the different approaches followed by the two teams. The SterLim team relied on the work from Melosh and coworkers,10 who used semi-empirical impact crater models.11 This approach inherently includes most impact angles, but depends on the distribution of velocity of ejected fragment as a function of the mass of the ejected fragments, which is uncertain, particularly when including the impact angle.12 The JAXA team, in contrast, used the results from numerical simulations specifically performed for its study.
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From page 26... ...
Committee Assessment The numerical simulation performed by the JAXA team suggests that the ejecta accelerated sufficiently to reach the martian moons experience significant heating that enables sterilization.28 Additionally, the JAXA team reviewed investigations of martian meteorites, some of which show significant shock heating and others showing no signs of shock heating. (See "Earth Inventory of Martian Meteorites," in Chapter 1.)
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From page 27... ...
This effect reduces the aerodynamic deceleration and brings into question any subsequent analyses making use of this lower limit on the size of particles capable of escaping the martian atmosphere. However, it is clear, as the JAXA team concluded, that aerodynamic heating of ejecta during passage through the martian atmosphere does not cause any significant sterilization.
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From page 28... ...
. The JAXA team performed Monte Carlo simulations to obtain the distribution of collision velocity of Mars rocks onto Phobos and Deimos.
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From page 29... ...
Is Accessible to a Phobos (and Deimos) Sample Return Mission," SterLim-Ph2-TAS-TN21, June 9, http:// sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_190403.pdf; referred to as "SterLim TN-21." 40 See Equation 6-1 in Fujita et al., 2018, "Assessment of Microbial Contamination Probability for Sample Return from Martian Moons." 41 See Thales Alenia Space, 2018, "Sterilisation Limits for Sample Return Planetary Protection Measures: TN 19 Description of the S terilisation Statistical Model," SterLim-Ph2-TAS-TN19, June 9, http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ ssb_190404.pdf, and SterLim TN-21.
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From page 30... ...
, the JAXA team estimated the amount of martian ejecta with a velocity that could reach the moons by SPH simulations, by changing the impactor's angle and velocity toward Mars.43 Based on this SPH modeling, the JAXA team estimated the impact velocity and angle of the Mars ejecta onto Phobos and Deimos by the Monte Carlo method. Since the size distribution of Mars ejecta colliding with the moons is unknown, the number of martian rocks colliding at Phobos and Deimos was estimated from the total mass of the ejecta while assuming that the martian rocks were all 10 cm diameter spheres.
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From page 31... ...
Sangen, 2013, Degree of impactor fragmentation under collision with a regolith surface -- Laboratory impact experiments of rock projectiles, Meteoritics and Planetary Science 49:69-79.
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The Radiation Environment on the Martian Moons A critical issue upon which support or not for the presence of preserved life on Phobos and Deimos transferred from Mars is radiation -- mainly, ionizing forms. If a radiation-resistant organism similar to D
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From page 33... ...
:294-320. 68 See Table 7.2 in Fujita et al., 2018, "Assessment of Microbial Contamination Probability for Sample Return from Martian Moons." 69 M
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Although the effect was not considered by either the SterLim or the JAXA team, it could significantly enhance the rate at which any biological materials present are exposed and degraded by radiation.
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