THE MARTIAN MOONS: PHOBOS AND DEIMOS
Mars, fourth planet from the Sun, is the outermost rocky planet of the inner solar system and marks the boundary between the terrestrial planets with solid surfaces and the giant planets beyond. Mars has two known moons: Phobos and Deimos. Phobos is roughly 22 km in diameter and orbits closer to Mars, with a semimajor axis of 9,377 km. Deimos is smaller—roughly 12 km in diameter—and orbits farther from Mars, with a semimajor axis of 23,460 km. The orbital periods of the two moons are very different, at 7.66 hours for Phobos and 30.35 hours for Deimos. Both moons are tidally locked, always presenting the same face toward Mars.
The orbits of both moons are unstable. Phobos’s orbit lies inside the areosynchronous radius (i.e., the distance at which a martian satellite’s orbital period is equal to one Mars day), and tidal forces are causing it to spiral in toward the planet on a time scale of 10-100 Ma. Deimos’s orbit lies outside the areosynchronous radius, and tidal forces are causing it to spiral away from Mars on a similar time scale. As will become clear later (see “Sterilization by Radiation on Phobos/Deimos Surfaces,” in Chapter 2), the time scale for orbital changes is significantly greater than that of relevance to the planetary protection issues being discussed in this report. Therefore, the effects of orbital changes can be ignored. A complete list of the physical characteristics of Phobos and Deimos is presented in Table 1.1.
No spacecraft mission has explored Phobos (Figure 1.1) or Deimos (Figure 1.2) as a primary objective, but several Mars-observing spacecraft have conducted remote, opportunistic observations of these bodies. In 1970, NASA’s Mariner 7 first took pictures of Phobos silhouetted against Mars and revealed its small size, irregular shape, and dark surface. In 1971, NASA’s Mariner 9 sent back the first images that were able to resolve surface features on Phobos and Deimos. Several other orbiting spacecraft have subsequently performed long-range observations, including the Viking 1 and 2 orbiters (NASA, 1970s and 1980s); the Phobos 2 mission (Soviet Union, 1988-1989); Mars Global Surveyor (NASA, 1997); the Mars Express mission (European Space Agency [ESA], since late 2003); NASA’s Mars Reconnaissance Orbiter (NASA, since 2008); Mars Atmosphere and Volatile Evolution (MAVEN) mission (NASA, since 2014); Mars Orbiter Mission (Indian Space Research Organization, since 2014); and most recently, Mars Odyssey (NASA, Phobos observations since 2018). Russia attempted to send a sample return mission to Phobos—called Phobos-Grunt—in 2011, but the spacecraft failed to escape Earth’s orbit and returned to Earth. While rovers and landers on the surface of Mars cannot get close to the moons, they have also provided some
TABLE 1.1 Phobos and Deimos Orbital Parameters
|Orbital semimajor axis||9,377 km||23,460 km|
|Orbital period||7.66 hr||30.35 hr|
|Orbital inclination, to Mars’s equator||1.093 degrees||0.93 degree|
|Size||26.06 × 22.80 × 18.28 km3||15.0 × 12.1 × 10.4 km3|
|Density||1860 ± 13 kg m−3||1490 ± 190 kg m−3|
|Gravity||5.7 × 10−3 ms−2||3 × 10−3 ms−2|
|Normal reflectance, 0.55 µm||0.071 ± 0.012||0.068 ± 0.007|
|Estimated surface temperature range (min-max)||~150-300 K||~160-270 K|
SOURCE: Table adapted from S.L. Murchie, P.C. Thomas, A.S. Rivkin, and N.L. Chabot, 2015, Phobos and Deimos, in Asteroids IV (P. Michel et al., eds.), University of Arizona, Tucson, doi:10.2458/azu_uapress_9780816532131-ch024.
Original source references: P.C. Thomas, 1993, Gravity, tides, and topography on small satellites and asteroids—Application to surface features of the martian satellites, Icarus 105:326; D.P. Simonelli, M. Wisz, A. Switala, D. Adinolfi, J. Veverka, P.C. Thomas, and P. Helfenstein, 1998, Photometric properties of Phobos surface materials from Viking images, Icarus 131:52-77; R.A. Jacobson, 2010, The orbits and masses of the martian satellites and the libration of Phobos, Astronomical Journal 139:668-679; K. Willner, X. Shi, and J. Oberst, 2014, Phobos’s shape and topography models, Planetary and Space Sciences 102:52-59.
Temperature ranges references: J. Lunine, G. Neugebauer, and B. Jakosky, 1982, Infrared observations of Phobos and Deimos from Viking, Journal of Geophysical Research 87:B12; R.O. Kuzmin, T.V. Shingareva, and E.V. Zabalueva, 2003, An engineering model for the Phobos surface, Solar System Research 37:266; R.O. Kuzmin and E.V. Zabalueva, 2003, The temperature regime of the surface layer of the Phobos regolith in the region of the potential Fobos-Grunt Space Station landing site, Solar System Research 37:480; D.K. Lynch, R.W. Russell, R.J. Rudy, S. Mazuk, C.C. Venturini, H.B. Hammel, M.V. Sykes, R.C. Puetter, and R.B. Perry, 2007, Infrared spectra of Deimos (1-13 µm) and Phobos (3-13 µm), Astronomical Journal 134:4; J.L. Bandfield, S. Piqueux, T.D. Glotch, K.A. Shirley, T.C. Duxbury, J.R. Hill, C.S. Edwards, J.J. Plaut, V.E. Hamilton, and P.R. Christensen, 2018, “Mars Odyssey THEMIS Observations of Phobos: New Spectral and Thermophysical Measurements,” Lunar and Planetary Science Conference, Abstract #2643.
disk-resolved images that show the moons’ surfaces and have been useful in refining their ephemerides. A full summary of spacecraft exploration of Phobos and Deimos through 2014 is provided in a paper by Duxbury et al.1
The origins of Phobos and Deimos are unknown. Given their similarities in albedo, spectral properties, and density with carbonaceous material and D-type main belt asteroids, Phobos and Deimos were originally proposed to be captured objects from the inner or outer solar system.2,3 However, the capture hypothesis is difficult to reconcile with the dynamics of Phobos’s and Deimos’s orbits. Formation from an accretion disk following a giant impact into early Mars or co-accretion in Mars orbit from Mars-like material can provide sufficient dissipation to damp the resulting debris disks down to the present orbital configuration and has been proposed as an alternative explanation for the moons’ origins.4,5,6,7,8 Estimates for the ages of the moons vary significantly depending on the formation model chosen.
1 T.C. Duxbury, A. Zakharov, H. Hoffmann, and E.A. Guinness, 2014, Spacecraft exploration of Phobos and Deimos, Planetary and Space Science 102:9-17.
2 K. Pang, J. Pollack, J. Veverka, A. Lane, and J. Ajello, 1978, The composition of Phobos: Evidence for carbonaceous chondrite surface from spectral analysis, Science 199:64-66.
3 J.B. Pollack, J.A. Burns, and M.E. Tauber, 1979, Gas drag in primordial circumplanetary envelopes: A mechanism for satellite capture, Icarus 37:587-611.
4 S.F. Singer, 1966, “On the Origin of the Martian Satellites Phobos and Deimos,” pp. 317-321 in Moon and Planets (A. Dollfus, ed.), COSPAR Seventh International Space Science Symposium, COSPAR, Paris, France.
5 R.A. Craddock, 2011, Are Phobos and Deimos the result of a giant impact?, Icarus 211:1150-1161.
6 R.I. Citron, H. Genda, and S. Ida, 2015, Formation of Phobos and Deimos via a giant impact, Icarus 252:334-338.
7 A.J. Hesselbrock and D.A. Minton, 2017, An ongoing satellite-ring cycle of Mars and the origins of Phobos and Deimos, Nature Geoscience 10:266-269.
8 J.A. Burns, 1992, “Contradictory Clues as to the Origin of the Martian Moons,” pp. 1283-1302 in Mars (H.H. Kieffer et al., eds.), University of Arizona, Tucson.
Resolving the compositions of Phobos and Deimos will be a large step toward determining their origin. If the moons are captured bodies from the inner or outer solar system, they will probably resemble primitive meteorites or ordinary chondrites. If the moons formed via co-accretion or impact into differentiated Mars, they will probably resemble bulk Mars or differentiated basaltic martian crust.9 Unfortunately, observations of Phobos’s
9 S.L. Murchie, P.C. Thomas, A.S. Rivkin, and N.L. Chabot, 2015, “Phobos and Deimos,” in Asteroids IV (P. Michel et al., eds.), University of Arizona, Tucson, doi:10.2458/azu_uapress_9780816532131-ch024.
and Deimos’s composition to date have been ambiguous. Visible to near-infrared spectral data strongly suggest a chondritic composition,10,11,12 while thermal infrared data suggest that a small basaltic component may be present.13
The spectral features suggesting a chondritic composition support the idea that the moons are captured asteroids. However, these features can be explained by exogenic processes such as the implantation of hydrogen from the solar wind. A strong argument against the capture hypothesis is based on the fact that each moon’s orbit is almost circular and lies close to Mars’s equatorial plane. Thus, the moons’ current dynamical configurations would require the substantial dissipation of energy and angular momentum during the capture process. Explaining how this dissipation was achieved is difficult.
EARTH INVENTORY OF MARTIAN METEORITES
Important context for studies of Mars and its moons is provided by meteorites. Major impacts on Mars may deliver martian materials to Phobos and Deimos.14,15 This process also delivers fragments of Mars—that is, martian meteorites—to Earth. The inventory of martian meteorites on Earth consists of about 115 volcanic and plutonic rocks whose chemical and oxygen–isotopic compositions differ from those of other meteorites and suggest their origin from differentiated parent bodies.16 Sedimentary rocks that have been proven to exist on Mars—for example, from observations conducted by the Mars Global Surveyor spacecraft—are not among the martian meteorites identified so far. However, there may be additional complications in recognizing the martian origin of such samples, and also of other so far unknown materials from Mars, once they have been recovered on Earth.
Young igneous crystallization ages of 180 to 1,300 Ma for a large proportion of recognized martian meteorites point to derivation from a planet-size body, and martian atmosphere found trapped in impact-produced glass inclusions strongly advocate for their origin from Mars.17,18 The majority of martian meteorites comprise shergottite
10 S.L. Murchie and S. Erard, 1996, Spectral properties and heterogeneity of Phobos from measurements by Phobos 2, Icarus 123:63-86.
11 A.S. Rivkin, R.H. Brown, D.E. Trilling, and J.F. Bell, 2002, Near-Infrared Spectrophotometry of Phobos and Deimos, Icarus 156(1):64-75.
12 A.A. Fraeman, R.E. Arvidson, S.L. Murchie, A. Rivkin, T. Choo, J.-P. Bibring, B. Gondet, D. Humm, R.O. Kuzmin, N. Manaud, and E.V. Zabalueva, 2012, Analysis of disk-resolved OMEGA and CRISM spectral observations of Phobos and Deimos, Journal of Geophysical Research 117, doi:10.1029/2012JE004137.
13 T.D. Glotch, C.S. Edwards, M. Yesiltas, K.A. Shirley, D.S. McDougall, A.M. Kling, J.L. Bandfield, and C.D.K. Herd, 2018, MGS-TES spectra suggest a basaltic component in the regolith of Phobos, Journal of Geophysical Research: Planets 123(10):2467-2484.
14 L. Chappaz, H.J. Melosh, M. Vaquero, and K.C. Howell, 2012, “Transfer of Impact Ejecta Fragments Material from the Surface of Mars to Phobos and Deimos,” Conference Paper AAS 12-212, 22nd AAS/AIAA Space Flight Mechanics Meeting, Univelt Inc., San Diego, Calif.
15 K.R. Ramsley and J.W. Head III, 2013, Mars impact ejecta in the regolith of Phobos: Bulk concentration and distribution, Planetary and Space Science 87:115-129.
16 H.Y. McSween, Jr., 1998, Martian meteorites, Reviews in Mineralogy 36:6.1-6.53.
17 D.D. Bogard and D.H. Garrison, 1995, 39Ar-40Ar are of the Ibitira eucrites and constraints on the time of pyroxene equilibration, Geochimica et Cosmochimica Acta 59:4317-4322.
18 A.H. Treiman, J.D. Gleason, and D.D. Bogard, 2000, The SNC meteorites are from Mars, Planetary and Space Science 48:1213-1230.
(>80 percent of all known martian meteorites), nakhlite (~10 percent), and chassignite (~3 percent) groups. Based on texture and mineralogy, shergottites are subdivided into basaltic-aphantic rocks with subequal proportions of plagioclase and pyroxene; olivine-phyric-aphanitic basalts with phenocrysts of olivine; and poikilitic-coarser grained rocks with oikocrysts of pyroxene enclosing olivine. Nakhlites are olivine clinopyroxenites, and chassignites are olivine-rich rocks called dunites. Additional Mars meteorite types include monomict orthopyroxenite breccia, Allan Hills (ALH) 84001,19 and polymict regolith breccia Northwest Africa (NWA) 7034 and its pairs.20,21,22 These breccias sample ancient martian crust, with an age of about 4.1 Ga for ALH 84001.23 Zircons in NWA 7034 date to 4.428 Ga, with evidence of U-Pb disturbance at ~1.5-1.7 Ga.24 Two recently recognized meteorites, NWA 8159 and NWA 7635, expand shergottite types, sampling a discrete igneous unit from the early Amazonian (~2.3-2.4 Ga).25,26 Despite identification of sedimentary rocks on Mars through satellite- and rover-derived morphological observations, there are no such rocks represented within current meteorite collections.
All martian meteorites on Earth were ejected from Mars by hypervelocity impact, originating within a near-surface “spall” zone of inverted pressure gradient, caused by interference between shock waves and rarefactions near the free surface.27 This spall zone comprises accelerated solid rock and has been studied both numerically and analytically.28,29,30,31 Ejection ages indicate that the martian meteorites were delivered to Earth by less than eight discrete impact events between 0.7 and 20 Ma ago.32 Attempts have been made to identify meteorite source craters using spectral matching.33,34 However, such efforts have been hampered by dust that obscures primarily the youngest igneous terrains such as Tharsis.35 The bias of martian meteorites toward young igneous rocks has been investigated through computer simulation by Head et al.36 Their results show that the size of the ejected fragments is affected by target strength; weaker materials, like sedimentary rocks, require larger, and therefore rarer, impact events. This observation may account for the paucity of breccias in the current collection and the absence of sedimentary martian meteorites. There may be added complications in recognizing the martian origin of these samples once they have been recovered on Earth. Hypervelocity impact into coherent targets, such as
19 D.W. Mittlefehldt, 1994, ALH 84001: A cumulate orthopyroxenite member of the martian meteorite clan, Meteoritics 29:214-221.
20 C. Agee, N.V. Wilson, F.M. McCubbin, K. Ziegler, V.J. Polyak, Z.D. Sharp, Y. Asmerom, M.H. Nunn, R. Shaheen, M.H. Thiemens, A. Steele, et al., 2013, Unique meteorite from early Amazonian Mars: Water-rich basaltic breccia Northwest Africa 7034, Science 339:780-785.
21 M. Humayun, A. Nemchin, B. Zanda, R.H. Hewins, M. Grange, A. Kennedy, J.P. Lorand, C. Gopel, C. Fieni, S. Pont, and D. Deldicque, 2013, Origin and age of the earliest martian crust from meteorite NWA 7533, Nature 503:513-516.
22 A. Wittman, R.L. Korotev, B.L. Joliff, A.J. Irving, D.E. Moser, I. Barker, and D. Rumble III, 2015, Petrography and composition of martian regolith breccia meteorite Northwest Africa 7475, Meteoritics and Planetary Science 50:326-352.
23 J.J. Bellucci, A.A. Nemchin, M.J. Whitehouse, J.F. Snape, P. Bland, and G.K. Benedix, 2015, The Pb isotopic evolution of the martian mantle constrained by intial Pb loss in martian meteorites, Journal of Geophysical Research: Planets 120:2224-2240.
24 M. Humayun, A. Nemchin, B. Zanda, R.H. Hewins, M. Grange, A. Kennedy, J.P. Lorand, C. Gopel, C. Fieni, S. Pont, and D. Deldicque, 2013, Origin and age of the earliest martian crust from meteorite NWA 7533, Nature 503:513-516.
25 C.D.K. Herd, E.L. Walton, C.B. Agee, N. Muttik, K. Zeigler, C.K. Shearer, A.S. Bell, et al., 2017, The Northwest Africa 8159 martian meteorite: Expanding the martian sample suite to the early Amazonian, Geochimica et Cosmochimica Acta 218:1-26.
26 T.J. Lapen, M. Righter, R. Andreasen, A.J. Irving, A.M. Stakoski, B.L. Beard, K. Nishiizumi, A.J.T. Jull, and M.W. Caffee, 2017, Two billion years of magmatism recorded from a single Mars meteorite ejection site, Science Advances 3:6.
27 H.J. Melosh, 1985, Ejection of rock fragments from planetary bodies, Geology 13:144-148.
28 P.H. Warren, 1994, Lunar and martian meteorite delivery services, Icarus 111:338-363.
29 H.J. Melosh, 1995, Cratering dynamics and the delivery of meteorites to the Earth, Meteoritics 30:545-546.
30 J.N. Head, H.J. Melosh, and B.A. Ivanov, 2002, Martian meteorite launch: High-speed ejecta from small craters, Science 298:1753-1756.
31 N. Artemieva and B. Ivanov, 2004, Launch of martian meteorites in oblique impacts, Icarus 171:84-101.
32 L.E. Nyquist, D.D. Borg, C.-Y. Shih, D. Greshake, D. Stöffler, and O. Eugster, 2001, Ages and geologic histories of martian meteorites, Space Science Review 96:105-164.
33 A. Ody, F. Poulet, C. Quantin, J.P. Bibring, J.L. Bishop, and M.D. Dyar, 2015, Candidate source regions of martian meteorites as identified by OMEGA/MEx, Icarus 258:366-383.
34 V.E. Hamilton and P.R. Christensen, 2003, High spectral and spatial resolution analyses of martian meteorite-like compositions on the surface of Mars, Meteoritics and Planetary Science 38:76.
35 N.P. Lang, L.L. Tornabene, H.Y. McSween, Jr., and P.R. Christensen, 2009, Tharsis-sourced relatively dust-free lavas and their possible relationship to martian meteorites, Journal of Volcanology and Geothermal Research 185:103-115.
36 J.N. Head, H.J. Melosh, and B.A. Ivanov, 2002, Martian meteorite launch: High-speed ejecta from small craters, Science 298:1753-1756.
shergottite-nakhlite-chassignite source terrains, may eject decimeter-size rocks, leaving their trace as craters as small as 3 km diameter.37
This same process of impact spallation may also eject martian materials into Phobos- and Deimos-crossing trajectories,38,39 necessitating further assessment of the amount of martian material on these moons, under the auspices of planetary protection (this study). Ejecta arriving directly to Phobos from Mars (referred to as primary ejecta), intersects the surface at ~2-3 km/s. Due to the moon’s small size, and therefore low escape velocity (~4-10 m/s), a large amount of secondary ejecta (~95-99 percent) that is temporarily inserted into martian orbit may subsequently reaccrete on the moon. The reaccretion interval for secondary ejecta to Phobos ranges from several days to hundreds of years.40 Based on these models, the amount of martian material in the regolith of Phobos was computed to be ~75 ppm in the last 10 Ma and ~250 ppm delivered during the last 3.5 Ga. This material is primarily within 0.4-1.0 m of the surface, with 10 to 60 times less in terms of bulk concentration in deeper, and therefore older (>500 Ma), regolith units. The process of delivery of Mars’s material to its moons has been revisited by the Japan Aerospace Exploration Agency (JAXA), and a synopsis of their results can be found in Chapter 2.
During impact ejection, the rock fragments—some destined to become meteorites— are shock-metamorphosed. Shock effects in martian meteorites are recorded as petrographically observable features in constituent minerals, including mechanical deformation and transformation. Transformation of plagioclase to a diaplectic glass called maskelynite is sensitive to composition (Ca-content) and shock pressure, which has been calibrated by shock-recovery experiments.41 All martian meteorites record shock effects, and their study can be used to estimate shock pressure and post-shock temperature.42 The shock-induced temperature increase is governed by the pressure-volume work achieved by the shock wave, which may be estimated using the linear relation of shock wave and particle velocity across specific pressure intervals, as described in a 2005 paper by Fritz et al.43 Studies of shock effects in martian meteorites show that they have experienced a range of shock conditions, from weakly shocked nakhlites (5-10 GPa) to more strongly shocked shergottites (20-55 GPa). These pressure estimates are based on mineral deformation in olivine and pyroxene, including, but not limited to, planar fractures, undulose extinction, planar deformation features and mechanical twinning (pyroxene only), and complete to partial transformation of plagioclase to maskelynite. Calculated post-shock temperature increase (∆T) ranges from 10 ± 20 K in nakhlites, to 50 ± 5 K at the lower end of shock in shergottites (20 GPa; Yamato-980459), to 800 ± 200 K at the upper limit (55 GPa; ALH 77005).44 Low post-shock temperatures are supported by study of ALH 84001 magnetization, demonstrating that this meteorite did not realize temperatures greater than 313.15 K since its formation.45 These shock conditions apply to those experienced by the bulk rock; however, during shock, shear zones may develop and open spaces (e.g., cracks, fractures, vesicles) collapse, forming hot spots within the rock and generating small volumes of shock-produced melt. The localized temperature conditions within shock melt may be in excess of 1500-2000 K; however, these represent small volumes of melt (<1 to ~3 percent) that are heterogeneously distributed throughout the sample.46
38 L. Chappaz, H.J. Melosh, M. Vaquero, and K.C. Howell, 2012, “Transfer of Impact Ejecta Fragments Material from the Surface of Mars to Phobos and Deimos,” 22nd AAS/AIAA Space Flight Mechanics Meeting. AAS Conference paper 12-212.
39 K.R. Ramsley and J.W. Head III, 2013, Mars impact ejecta in the regolith of Phobos: Bulk concentration and distribution, Planetary and Space Science 87:115-129.
41 D. Stöffler, C. Meyer, J. Fritz, G. Horneck, R. Möller, C. Cockell, S. Ott, J.P. de Vera, U. Hornemann, and N.A. Artemieva, 2006, Impact Experiments in Support of “Lithopanspermia”: The route from Mars to Earth, Abstract 1551, 37th Lunar and Planetary Science Conference, https://www.lpi.usra.edu/meetings/lpsc2006/pdf/1551.pdf.
42 See, for example, J. Fritz, N. Artemieva, and A. Greshake, 2005, Ejection of martian meteorites, Meteoritic and Planetary Science 40:1393-1411, and references therein.
43 J. Fritz, N. Artemieva, and A. Greshake, 2005, Ejection of martian meteorites, Meteoritic and Planetary Science 40:1393-1411.
45 B.P. Weiss, J.L. Kirschvink, F.J. Baudenbacher, H. Vali, N.T. Peters, F.A. MacDonald, and J.P. Wikswo, 2000, A lower temperature transfer of ALH 84001 from Mars to Earth, Science 290:791-795.
46 E.L. Walton and C.S.J. Shaw, 2009, Understanding the textures and origin of shock melt pockets in martian meteorite from petrographic studies, comparisons with terrestrial mantle xenoliths, and experimental studies, Meteoritics and Planetary Science 44:55-76.
There is considerable debate about the presence, origin, and meaning of organic material in martian meteorites. Chains of tiny magnetite crystals associated with carbonate globules found in ALH 84001 have been interpreted as evidence for possible ancient biological activity on Mars.47 However, the magnetite may also have formed by inorganic processes—for example, thermal decomposition of carbonates during shock heating.48 Also, organic compounds reported from ALH 84001 and EETA 79001 appear to be of terrestrial and not martian origin.49 More recently, kerogen-like organic matter present in the 2011 fall, Tissint, as well as methane released from six martian meteorites, have been taken as hints of biological activity.50,51 Nakhlites, the least shock-metamorphosed martian meteorites, contain various alteration assemblages—for example, clay minerals, sulfates, and halite—attesting to interaction between martian crustal fluids and the parent igneous rocks.52 In strongly shocked shergottites, geochemical signatures of martian alteration such as D- and Cl-enrichment are found preferentially in quenched shock melt.53 Despite these detailed studies in search of evidence for biological activity, there has been no unambiguous evidence for early life found in martian meteorites. This, however, does not generally preclude the possibility of sampling martian material that may contain signs of biological activity.
Chapter 3 returns to the discussion of martian meteorites, because they will prove to be an important factor in determining whether or not samples from the martian moons are designated restricted or unrestricted Earth return.
FUTURE MISSIONS TO THE MARTIAN MOONS: THE MMX MISSION
The preceding two sections have demonstrated that Phobos and Deimos are high-priority targets for a future dedicated spacecraft mission, especially one that could return samples for detailed study in terrestrial laboratories.54 Resolving the questions of the moons’ origins will advance researchers’ understanding of how planetary systems form. Studying material from Phobos and Deimos will also provide information about primordial material transport in the earliest period of solar system history. If the moons are captured bodies originating from the outer solar system, they would provide important clues about material transport across the snow line marking the frontier between the inner and outer solar system.
The Martian Moons Exploration (MMX) mission is a robotic spacecraft mission under development by JAXA for launch in September 2024. MMX will be a 5-year sample return mission with the following mission profile:
- September 2024—Launch
- August 2025—Arrive at Mars
- 2026—Observation of Phobos for landing site selection
- 2026 or 2027—Proximity phase: landing on Phobos for sampling
- August 2028—Depart from Mars
- July 2029—Arrive at Earth
47 D.S. McKay, J.E.K. Gibson, K.L. Thomas-Keprta, H. Vali, C.S. Romanek, S.J. Clemett, X.D.F. Chiller, C.R. Maechling, and R.N. Zare, 1996, Search for past life on Mars: Possible relic biogenic activity in martian meteorite ALH 84001, Science 273:924-930.
48 A.J. Brearley, 2003, Magnetite in ALH 84001: An origin by shock-induced thermal decomposition of iron carbonate, Meteoritics and Planetary Science 38:849-870.
49 A.J.T. Jull, C. Courtney, D.A. Jeffrey, and J.W. Beck, 1998, Isotopic evidence for a terrestrial source of organic compounds found in martian meteorites Allan Hills 84001 and Elephant Moraine 79001, Science 279:366-369.
50 Y. Lin, A. El Goresy, S. Hu, J. Zhang, P. Gillet, Y. Xu, J. Hao, et al., 2014, NanoSIMS analysis of organic carbon from the Tissint martian meteorite: Evidence for the past existence of subsurface organic-bearing ﬂuids on Mars, Meteoritics and Planetary Science 49:2201-2218.
51 N.J.F. Blamey, J. Parnell, S. McMahon, D.F. Mark, T. Tomkinson, M. Lee, J. Shivak, M.R.M. Izawa, N.R. Banerjee, and R.L. Flemming, 2015, Evidence for methane in martian meteorites, Nature Communication, doi:10.1038/ncomms8399.
52 J.C. Bridges, D.C. Catling, J.M. Saxton, T.D. Swindle, I.C. Lyon, and M.M. Grady, 2001, Alteration assemblages in martian meteorites: Implications for surface-near processes, Space Science Reviews 96:365-392.
53 C.R. Kuchka, C.D.K. Herd, E.L. Walton, Y. Guan, and Y. Liu, 2017, Martian low-temperature alteration materials in shock-melt pockets in Tissint: Constraints on their preservation in shergottite meteorites, Geochimica et Cosmochimica Acta 210:228-246.
54 See, for example, S.L. Murchie, D.T. Britt, and C.M. Pieters, 2014, The value of Phobos sample return, Planetary and Space Sciences 102:176-182.
TABLE 1.2 Instruments to Be Carried by the MMX Mission
(Telescopic Nadir Imager for Geomorphology)
|Geological features||Field of view (FOV): 1.1 × 0.82 degrees Spatial resolution: ~40 cm at 20 km altitude|
(Optical Radiometer Composed of Chromatic Imagers)
|Geological features Hydrated minerals Space weathering||FOV: 66 × 53 degrees Wavelength: 390, 480, 550, 650, 700, 860, 950 nm Spatial resolution: 20 m at 20 km altitude|
(Macroscopique Observatoire pour la Minéralogie, l’Eau, le Glaces et l’Activité)
|Hydrated minerals Water molecules Organic materials||FOV: 6 degrees Wavelength: 0.9-3.6 μm Spatial resolution: <20 m at 20 km altitude|
(Light Detection and Ranging)
|Topographic features||Ranging distance: 100 m-100 km Ranging resolution: 0.5 m|
(Mars Moon Exploration with Gamma Rays and Neutrons)
|Major element composition||Gamma-ray energy: 0.4-8 MeV Energy resolution: <5.1 keV (FWHM) at 1454 keV Neutron energy: thermal, epithermal, and fast (0.01-7 MeV)|
(Mass Spectrum Analyzer)
|Space ion environment Possible ice inside Phobos||Ion energy: 10-30 keV/q Energy resolution: ΔE/E ~ 20 percent Ion mass: 1-60 amu Mass resolution: M/ΔM ~ 100|
NOTE: TENGOO, MacrOmega (Macroscopic Observatory for Mineralogy, Water, Ice and Activity; contributed by the French Space Agency, CNES), and MEGANE (contributed by NASA) will play extremely important roles in the landing site selection on Phobos. In particular, TENGOO’s high-resolution imaging ability will be crucial to landing safely. FWHM, full width at half maximum.
MMX has three scientific objectives.55 They are, in priority order:
- To understand the origin of the martian moons. Are Phobos and Deimos captured primordial asteroids or leftover accreted debris from a significant impact in Mars’s history?
- To make progress in understanding planetary system formation and primordial material transport of material between the inner and outer portions of the early solar system.
- To understand processes in cismartian space and to investigate how it might have changed in response to the evolution of the surface and atmosphere of Mars throughout the history of the solar system.
The science goals of MMX are to be addressed with a comprehensive suite of instruments (Table 1.2) and two sampling systems, JAXA’s C-Sampler (a coring device) and NASA’s P-Sampler (a pneumatic device).
The P-Sampler, one of NASA’s major contributions to the MMX mission, is a pneumatic sample collection device mounted on one of the footpads of the spacecraft’s landing legs (Figure 1.3). The main characteristics of its operation are as follows:
- Samples only the top 1 cm of Phobos’s regolith.
- Uses gas pressure to agitate surface material and blow it to the Sample Canister.
55 H. Miyamoto, “Japanese Mission of the Two Moons of Mars with Sample Return from Phobos,” presentation to Mars Program Assessment Group, March 17, 2016, https://mepag.jpl.nasa.gov/meetings/2016-03/17_Miyamoto.pdf.
- Once a sample has been collected, a robotic arm, mounted on the underside of the spacecraft moves the Sample Canister to the Sample Return Capsule located on the side of the spacecraft.
The C-Sampler is mounted on the underside of the MMX spacecraft (Figure 1.4); it consists of three separate coring bits designed to retrieve samples from two different locations. The third bit is a spare. The general characteristics of the C-Sampler are as follows:
- Each core tube has an inside diameter of 2.5 cm and is 6 cm long.
- 10 g of material is gathered for each corer at a depth of greater than 2 cm.
- The corer is located on the end of a robotic arm attached to the underside of the spacecraft.
- Three-dimensional imaging is used to determine the best location for sampling.
- Once a core sample is collected, the robotic arm transfers the sample tube to the Earth-return capsule mounted on the side of the spacecraft.
Sample Return Capsule
The Sample Return Capsule is located on the side of the MMX spacecraft. Its design is based on that used for JAXA’s Hayabusa 1 and 2 asteroid sample return missions. Following the completion of sample acquisition and remote-sensing studies of the martian moons, the MMX spacecraft, with Sample Return Capsule still attached, uses its ion propulsion system to return to Earth. Just prior to entry into Earth’s atmosphere, the Sample Return Capsule detaches from MMX. The MMX spacecraft burns up in Earth’s atmosphere. But, the Sample Return Capsule, protected by its heatshield and slowed by a parachute, drifts down to a soft landing in the Woomera Prohibited Area, in South Australia.56 The specific characteristics of the Sample Return Capsule are as follows:
- Capsule diameter—60 cm (Hayabura 2, 40 cm)
- Capsule mass—38 kg (Hayabusa 2, 16.5 kg)
- Separation mechanism mass—7 kg
- Total subsystem mass—45 kg
- Payload volume—15 × 15 × 15 cm3
- Total sample mass—0.01-0.03 kg
- Thermal protection system—carbon phenolic
PLANETARY PROTECTION AND COSPAR POLICY
Planetary protection policies have a twofold goal, as follows:57
- The control of forward contamination in the form of viable microbial life from Earth; and
- The control of back contamination by extraterrestrial materials collected and returned to the Earth-Moon system by spacecraft missions.
The rationales for these goals are also twofold:58
- To preserve the integrity of Earth’s biosphere; and
- To protect the biological and environmental integrity of other solar system bodies for future science missions, especially those relating to the origins of life and prebiotic chemical evolution.
The 1967 United Nations (UN) Outer Space Treaty (OST),59 to which most spacefaring nations are signatory, includes the following language as part of Article IX: “States Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination, and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose.” In addition, Article VI of the same treaty specifies the following: “States Parties to the Treaty shall bear international responsibility for national activities in outer space, including the Moon and other celestial bodies, whether such activities are carried on by governmental agencies or by non-governmental entities.”
Technical aspects of planetary protection policies are developed by individual space agencies and coordinated through the Committee on Space Research (COSPAR), part of the International Council of Science (ICSU). International planetary protection consensus guidelines are developed through a harmonization process conducted by
56 For additional information about the Woomera Prohibited Area, see, for example, http://www.defence.gov.au/woomera/about.htm.
57 See, for example, National Academies of Sciences, Engineering and Medicine, Review and Assessment of Planetary Protection Policy Development Processes, The National Academies Press, Washington, DC, 2018, p. 9.
59 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies, opened for signature January 27, 1967, 18 U.S.T. 2410, 610 U.N.T.S. 25.
COSPAR’s Panel on Planetary Protection (PPP). The UN Committee on Peaceful Uses of Outer Space has accepted COSPAR’s planetary protection policy as a guide for compliance with the OST.
COSPAR’s deliberations occur regularly, with participants reporting new scientific findings with policy implications (e.g., water being more abundant at a particular target than was previously recognized) and raising questions regarding specific concerns (e.g., new activities in space exploration that could affect policy compliance). The PPP develops recommendations that the COSPAR Bureau and Council may adopt for inclusion into the official COSPAR planetary protection policy.60 Through this process, the COSPAR planetary protection policy has evolved steadily and incrementally over the years since it was initially created. Space agencies such as NASA, the European Space Agency (ESA), and JAXA formulate and implement planetary protection policies and procedures that are consistent with the COSPAR planetary protection policy.
NASA and ESA maintain their respective planetary protection policies, administer associated procedures to ensure compliance with them, and oversee compliance with formal implementation requirements that are assigned to each mission. Agency policies are informed by the most current scientific information available about the target bodies and about life on Earth.
Planetary protection policies are not static but evolve over time based on the increasing knowledge and understanding of both planetary environments and the physical and chemical limits of terrestrial life. Conclusions and recommendations generated by internal and external advisory groups chartered by space agencies such as NASA and ESA are weighted and assessed in an iterative manner by COSPAR’s PPP. Consensus policy recommendations developed by the PPP are then forwarded for discussion and ultimate approval by COSPAR’s Bureau and Council prior to becoming official COSPAR policy. The development of the concept of Special Regions on Mars is a good example of how planetary protection policies are developed and evolve as new information becomes available.61
COSPAR planetary protection policy sets requirements for each spacecraft mission and target body depending on the type of encounter it will have (e.g., flyby, orbiter, or lander) and the nature of its destination (e.g., a planet, a moon, a comet, or an asteroid). If the target body has the potential to provide clues about the origins and evolution of life or prebiotic chemical evolution, spacecraft going there are required to meet a higher level of cleanliness, and some operating restrictions will be imposed. Spacecraft going to target bodies with the potential to support Earth life undergo stringent cleaning and bioload-reduction processes, up to and including subjecting the entire spacecraft to a dry heat microbial reduction (heating to ~385 K for 30 hours) or equivalent process. Such missions may also be subject to operating restrictions.
The fundamental challenge for those drafting planetary protection policies and their implementations is to craft requirements that are conservative. In practice, conservatism means that when assessing the risks posed by forward or back contamination, unknown or poorly known factors are overestimated if potentially harmful or underestimated if potentially beneficial. However, requirements should not be so conservative as to preclude the design or operation of a spacecraft mission designed to explore a planetary body of scientific interest and planetary protection concern.
The MMX mission presents an interesting test case of balancing conservatism against practicality. Phobos and Deimos are not in themselves objects of interest to studies of the origins of life and prebiotic chemical evolution. Therefore, spacecraft missions to these bodies present little to no chance that any onboard biological contamination from Earth will compromise future scientific investigations. Such missions would be subject to only the most minimal of requirements (e.g., documentation as to where the spacecraft went and what it did) relating to the control of forward contamination.
Returning samples from Phobos and Deimos is a more difficult question. Extraterrestrial samples returned to Earth by spacecraft missions are subject to more- or less-rigorous inflight and post-return containment restrictions depending on whether the body from which they are collected is designated “restricted” or “unrestricted” Earth return, respectively. The latter designation is reserved for spacecraft missions returning materials from
60 G. Kminek, C. Conley, V. Hipkin, and H. Yano, 2017, COSPAR Planetary Protection Policy, Space Research Today 200:12-25.
61 See, for example, National Academies of Sciences, Engineering and Medicine, Review and Assessment of Planetary Protection Policy Development Processes, The National Academies Press, Washington, D.C., 2018, pp. 101-105.
extraterrestrial bodies whose environmental conditions are consistent with the maintenance of life.62,63 However, as of today, no categorization has taken place for the martian moons. Phobos and Diemos are a special case not because of what they are but because of where they are located.
The current categorization of planetary protection target bodies for Category V (i.e., all sample return) missions is as follows:
- Restricted Earth return—Mars, Europa, Enceladus, and other to-be-determined (TBD) bodies64
- Unrestricted Earth return—Venus, Moon, and other TBD bodies
The close proximity of Phobos and Deimos to Mars greatly complicates the planetary protection calculations because major impacts on Mars can scatter martian material throughout cismartian space. Some of the ejected martian material will end up on Phobos and Deimos. A sample return from the martian moons could be effectively a Mars sample return mission, and such missions are classified as restricted Earth return.
Samples classified as restricted Earth return are subject to stringent pre- and post-flight requirements. Current COSPAR policy mandates, in part, the following:65
For all other Category V missions, in a subcategory defined as “restricted Earth return,” the highest degree of concern is expressed by the absolute prohibition of destructive impact upon return, the need for containment throughout the return phase of all returned hardware which directly contacted the target body or unsterilized material from the body, and the need for containment of any unsterilized sample collected and returned to Earth. Post-mission, there is a need to conduct timely analyses of any unsterilized sample collected and returned to Earth, under strict containment, and using the most sensitive techniques. If any sign of the existence of a nonterrestrial replicating entity is found, the returned sample must remain contained unless treated by an effective sterilizing procedure. Category V concerns are reflected in requirements that encompass those of Category IV plus a continuing monitoring of project activities, studies and research (i.e., in sterilization procedures and containment techniques).
Implementing the preceding requirements and more is complicated, time consuming, and expensive. The MMX mission builds heavily on the heritage of JAXA’s successful Hayabusa 1 and ongoing Hayabusa 2 asteroid sample return missions. The asteroids visited by the Hayabusa missions were both categorized as unrestricted Earth return. The associated contamination avoidance and containment requirements were minimal to none, other than protecting the samples from being contaminated by Earth. Making MMX compliant with restricted Earth return requirements would mean costly redesign of the spacecraft so that samples were strictly contained. It would also have to be designed to break the chain of contact between the moon sampled and any uncontained portions of the spacecraft actually returned to Earth. In addition, a receiving facility would need to be constructed that is capable of both protecting Earth from the samples and the samples from Earth.
The COSPAR requirements outlined above do not specify particular sterilization or containment protocols. Other groups have looked at the specifics and have recommended that protecting Earth from the samples requires that the receiving facility operate at a standard equivalent to a biosafety level (BSL)-4 biological containment
62 The status of sample return missions from small solar system bodies, for example, is determined by a test consisting of six questions, as follows. Does the preponderance of scientific evidence indicate (1) That there was never liquid water in or on the target body? (2) That metabolically useful energy sources were never present? (3) That there was never sufficient organic matter (or CO2 or carbonates and an appropriate source of reducing equivalents) in or on the target body to support life? (4) That subsequent to the disappearance of liquid water, the target body has been subjected to extreme temperatures (i.e., 433 K)? (5) That there is or was sufficient radiation for biological sterilization of terrestrial life forms? (6) That there has been a natural influx to Earth—for example, via meteorites, of material equivalent to a sample returned from the target body? Returning six “no” or “uncertain” answers requires that the sample return mission be designated restricted Earth.
63 G. Kminek, C. Conley, V. Hipkin, and H. Yano, 2017, COSPAR Planetary Protection Policy, Space Research Today 200:12-25.
64 With TBD indicating that additional analysis is required.
65 G. Kminek, C. Conley, V. Hipkin, and H. Yano, 2017, COSPAR Planetary Protection Policy, Space Research Today 200:14-15.
laboratory.66 Similarly, others have suggested a containment criterion of one in a million for the release of any particle ≥10 nm.67
The categorization of Phobos and Deimos is the subject of the present report. In order to proceed with the mission as currently planned and remain consistent with current planetary protection practice, JAXA needs to demonstrate that the probability of a single unsterilized particle from Mars, ≥10 nm in diameter, in an uncontained sample returned from Phobos or Deimos is less than 10−6. If the probability is greater than 10−6, then JAXA faces three alternatives: redesign MMX to be consistent with restricted Earth return requirements, cancel the program, or change the requirements.
STUDIES IN SUPPORT OF PLANETARY PROTECTION CLASSIFICATION OF MARTIAN MOONS SAMPLE RETURN
As mentioned in the preceding section, the planetary protection categorization for a sample return mission specifically from the martian moons is still to be decided. With the MMX mission being planned, it became necessary for relevant space agencies to develop a planetary protection policy for the martian moons.
As already mentioned, large meteorite impacts on Mars are expected to eject material from the planet’s surface, and some of this ejecta will ultimately be deposited on the martian moons. Sample return missions to the Phobos and Deimos therefore represent opportunities to collect pristine minerals and, potentially, molecular evidence of life transferred from the surface of Mars. Therefore, the potential for martian moons sample return missions to collect unsterilized martian material needs to be investigated.
The present report reviews the results of two such studies, one sponsored by ESA and the other by JAXA.
In 2014, ESA tasked Manish Patel and his team at the Open University to conduct “feasibility studies and tests to determine the sterilization limits for sample return planetary protection measures.” The final objective of that study was to evaluate the probability that unsterilized martian material could be naturally transferred to Phobos, and whether that material would be accessible to a Phobos and, by extension, a Deimos sample return mission. The Open University team produced several reports dealing with the various aspects of the material transfer from Mars to the martian moons due to a crater forming impact. The team and the report it produced will henceforth be referred to as, respectively, “the SterLim team” and “the SterLim report.”68
Additionally, JAXA tasked a multi-institutional review team, led by Kazuhisa Fujita,69 to assess the microbial contamination probability for sample return from the martian moons. The purpose of the study was to clarify the potential physical processes that can bring about microbial contamination on the surface of martian moons, to obtain a quantitative estimate of the density of microorganisms still surviving in the regolith of the martian moons through several sterilization processes, and to assess microbial contamination probability of samples collected on the surface of the martian moons for future sample return missions from the martian moons. The aforementioned study was presented to this committee and will be referred to as “the JAXA report” in the following sections.70
The results of both the SterLim and the JAXA studies were used to help assess the level of planetary protection measures that need to be implemented for a future sample return mission to Phobos and Deimos to mitigate the risk of release of nonterrestrial life into Earth’s environment upon delivery.
66 National Research Council, 2002, The Quarantine and Certification of Martian Samples, The National Academies Press, Washington, D.C.
67 These criteria—10 nm and 10−6—were recommended in an ESF study—Mars Sample Return Backward Contamination-Strategic Advice and Requirements, Report from the ESF-ESSC Study Group on MSR Planetary Protection Requirements, European Science Foundation, Strasbourg, France, 2011—and have subsequently been endorsed by ESA’s Planetary Protection Working Group. These criteria have not yet been officially adopted by COSPAR or NASA.
68 From the SterLim consortium, the authors of the reports. The consortium included the Open University, Public Health England, Thales Alenia Space, Kallisto Consultancy, and Fluid Gravity Engineering.
69 The JAXA review team included experts from the Institute of Space and Astronautical Science, the Chiba Institute of Technology, the Tokyo Institute of Technology, and the University of Tokyo and JAXA scientists.
70 K. Fujita, K. Kurosawa, H. Genda, R. Hyodo, T. Mikouchi, S. Matsuyama, and the Phobos/Deimos Microbial Assessment Team, 2018, “Assessment of Microbial Contamination Probability for Sample Return from Martian Moons,” GNG-2018003, http://sites.nationalacademies.org/SSB/CurrentProjects/SSB_181917.
In addition to the reports commissioned above, ESA and JAXA, with NASA also participating (see the Preface), also requested an independent review of these reports. As mentioned in the Preface, the present report is the result of this assessment by a joint European, Japanese, and American team of experts. The outcome and final recommendations of this review process are detailed in the following chapters.