A possible anorthositic continent of early Mars and the role of planetary size for the inception of Earth like life Accepted Manuscript A possible anorthositic continent of early Mars and the role of[.]
Accepted Manuscript A possible anorthositic continent of early Mars and the role of planetary size for the inception of Earth-like life James M Dohm, Shigenori Maruyama, Motoyuki Kido, Victor R Baker PII: S1674-9871(16)30215-8 DOI: 10.1016/j.gsf.2016.12.003 Reference: GSF 521 To appear in: Geoscience Frontiers Received Date: March 2016 Revised Date: December 2016 Accepted Date: 11 December 2016 Please cite this article as: Dohm, J.M., Maruyama, S., Kido, M., Baker, V.R., A possible anorthositic continent of early Mars and the role of planetary size for the inception of Earth-like life, Geoscience Frontiers (2017), doi: 10.1016/j.gsf.2016.12.003 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT A possible anorthositic continent of early Mars and the role of planetary size for the inception of Earth-like life James M Dohma,*, Shigenori Maruyamab, Motoyuki Kidoc, Victor R Bakerd a 113-0033, Japan b Merugo-ku, Tokyo 152-8550, Japan c Aramaki, Aoba-ku, Sendai 980-0845, Japan RI PT SC The University Museum, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo M AN U Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1-IE-1 Ookayama, International Research Institute of Disaster Science, Tohoku University, 468-1, Aza Aoba, 10 d 11 * Corresponding author E-mail address: jmd@um.u-tokyo.ac.jp 12 Abstract EP TE D Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721-0092, USA The Moon has an anorthositic primordial continental crust Recently 14 anorthosite has also been discovered on the Martian surface Although the 15 occurrence of anorthosite is observed to be very limited in Earth’s extant 16 geological record, both lunar and Martian surface geology suggest that 17 anorthosite may have comprised a primordial continent on the early Earth during 18 the first 600 million years after its formation AC C 13 ACCEPTED MANUSCRIPT We hypothesized that differences in the presence of an anorthositic continent 20 on an Earth-like planet are due to planetary size Earth likely lost its primordial 21 anorthositic continent by tectonic erosion through subduction associated with a 22 kind of proto-plate tectonics (PPT) In contrast, Mars and the Moon, as much 23 smaller planetary bodies, did not lose much of their anorthositic continental crust 24 because mantle convection had weakened and/or largely stopped with time they 25 had appropriately cooled down Applying this same reasoning to a super-Earth 26 exoplanet suggests that, while a primordial anorthositic continent may briefly 27 form on its surface, such a continent will be likely transported into the deep 28 mantle due to intense mantle convection immediately following its formation TE D M AN U SC RI PT 19 The presence of a primordial continent on an Earth-like planet seems to be 30 essential to whether the planet will be habitable to Earth-like life The key role of 31 the primordial continent is to provide the necessary and sufficient nutrients for 32 the emergence and evolution of life With the appearance of a “trinity” consisting 33 of (1) an atmosphere, (2) an ocean, and (3) the primordial continental landmass, 34 material circulation can be maintained to enable a “Habitable Trinity” 35 environment that will permit the emergence of Earth-like life Thus, with little AC C EP 29 ACCEPTED MANUSCRIPT 36 likelihood of a persistent primordial continent, a super-Earth affords very little 37 chance for Earth-like life to emerge RI PT 38 Keywords: Anorthosite on Mars, Moon, Habitable trinity; Super-Earth; Plate 40 tectonics; Origin of life AC C EP TE D M AN U SC 39 ACCEPTED MANUSCRIPT 41 Introduction Rocky terrestrial planetary bodies such as the Earth, the Moon, and Mars 43 have continental crust exposed at their surfaces For example, the Earth 44 comprises 35-km-thick (average) granitic continental crust covering one-third of 45 its surface The bulk chemical composition of its continental crust is andesitic 46 (Taylor and McLennan, 1985), composed primarily of 75% SiO2 and 15% Al2O3 47 with the remaining 10% mainly being by Ca, Fe, and Mg Mafic rocks are thought 48 to be dominant in the lower continental crust M AN U SC RI PT 42 Granitic rocks on Earth are generally considered to be formed primarily as a 50 result of the accumulation of andesitic magma generated along the zone where 51 an oceanic plate is being subducted (Ringwood and Green, 1966), though an 52 alternative hypothesis is that arc magma (arc tholeiite) and its fractionation yield 53 andesite magma (Tatsumi et al., 1983; Kushiro, 1990) Granitic (andesitic) 54 magma is caused by partial melting of basaltic magma estimated to be about 55 20% Basaltic magma on Earth is caused by the following two processes: (1) 56 partial melt of mid-oceanic ridge basalt by about 20%, and (2) partial melt of the 57 mantle that contains hydrous minerals at a plate subduction zone, as 58 exemplified by plate tectonics in the Phanerozoic It is a key role of granitic rocks AC C EP TE D 49 ACCEPTED MANUSCRIPT on Earth to supply the nutrients that nurture life through material circulation, 60 involving a “trinity” of (1) a granitic landmass, (2) an ocean, and (3) an 61 atmosphere (Maruyama et al., 2013), which comprise what we term known as 62 “Habitable-Trinity conditions” (Dohm and Maruyama, 2015) In other words, 63 Earth’s life would not be possible without the presence of these granitic rocks 64 However, an important question is posed because extensive research on the 65 surface geology of Earth reveals that there are no Hadean rocks with an age 66 range between 4.0 and 4.56 Ga Therefore, it is unknown if there was any 67 primitive granitic continental crust or another type of continent to supply nutrients 68 during Hadean age when the first life may have been initiated on the Earth TE D M AN U SC RI PT 59 A possible answer to this enigmatic question was provided by the research on 70 the Moon that began with the Apollo Program of the 1960s Apollo-based 71 investigations revealed that continental crust of the lunar surface consists of 72 anorthosite (e.g., Wood et al., 1970; Jolliff et al., 2000) This anorthosite is 73 generally viewed to have been the upper part of a consolidated magma ocean 74 (Warren, 1985) Another important rock material exposed at the lunar surface is 75 KREEP basalt, so named because its composition includes potassium (K), rare 76 earth elements (REE) and phosphorous (P) (Hubbard et al., 1971; Warren and AC C EP 69 ACCEPTED MANUSCRIPT Wasson, 1979) This material has been more recently been referred to as 78 Procellarum KREEP terrain due to its concentration in Oceanus Procellarum and 79 Mare Imbrium (Jolliff et al., 2000) Interestingly, both these types of lunar rock 80 materials, which cover much of the surface of the Moon, are enriched in the 81 nutrients necessary for the emergence of Earth-like life Considering the 82 formation process of the Earth and the Moon, both rocky planetary bodies 83 should have followed the same evolutionary trends after their formation, 84 although their speeds of planetary evolution must have been quite different 85 mainly due to their size differences Because of its rapid early cooling, the Moon 86 has been a mostly dead planetary body, but it is also for this reason that the 87 lunar surface may be very similar to what was a very early stage in Earth’s 88 evolution EP TE D M AN U SC RI PT 77 Recently, rapid advances have been made in understanding of Martian 90 geology, including the documentation of anorthosite on the surface of Mars 91 (Carter and Poulet, 2013) This finding is consistent with a view of Earth-like 92 planetary evolution that will be further discussed below Moreover, the presence 93 of anorthosite is of great significance in regard to the emergence of Earth-like life 94 So, this paper will focus on the meaning of the discovery of Martian anorthosite AC C 89 ACCEPTED MANUSCRIPT 95 and the implications of this discovery for inferences concerning the existence of 96 Earth-like life Generalized geology of Mars 99 2.1 Crustal dichotomy SC 98 RI PT 97 Mars’ radius (~3390 km) is about half that of the Earth, and it has nearly 1/8 of 101 Earth’s mass (Fig 1A) Though a lack of seismic information means that its 102 interior structure and composition are not well known as Earth’s, Mars is, 103 nevertheless, inferred to be a differentiated planetary body It has a core 104 estimated to be nearly the same radius as Earth's solid inner core (~1200 km); a 105 thinner mantle (~2100 km), about ~ 2/3 that of Earth (~2100 km); and a thicker 106 crust, nearly double or more than that of Earth, reaching more than 60 km thick 107 in parts of the southern cratered highlands Oceanic-type crust has been 108 interpreted to be roughly representative of the northern plains, while continental 109 crust has been hypothesized to underlie the southern highlands (Baker et al., 110 2007; Maruyama et al., 2008) (Figs 1b and 2) AC C EP TE D M AN U 100 111 One of the most remarkable topographic features of Mars is the extremely 112 ancient hemispheric dichotomy that separates the southern, cratered highlands ACCEPTED MANUSCRIPT from the northern lowlands (Fig 1B) Multiple hypotheses have been put forth to 114 explain the origin of the hemispheric dichotomy boundary and the northern 115 lowlands These include subcrustal erosion resulting from mantle convection 116 (Wise et al., 1979), excavation by one or more large impacts (Wilhelms and 117 Squyres, 1984; Frey et al., 2002; Andrews-Hanna et al., 2009; Golabek et al., 118 2011), and overturn of unstable cumulates in an initial magma ocean (Hess and 119 Parmentier, 2001) Another interpretation, consistent with the geological 120 evidence, hypothesizes the landscape of the northern lowlands to be the result 121 of sea-floor spreading, that the southern highlands is a kind of a supercontinent, 122 and that the hemispheric dichotomy is a remnant of trench-related activity, all 123 associated with an ancient phase of lithospheric mobilism analogous to Earth’s 124 plate tectonics (Baker et al., 2007; Maruyama et al., 2008) This proposal holds 125 that Tharsis formed in a tectonic setting of double-sided subduction, with plate 126 convergence initiating a hydrous plume which controlled the activity in the 127 Tharsis and surroundings (Fig 3; Baker et al., 2007; Maruyama et al., 2008) In 128 other words, early Mars developed a large ocean basin in the area of its northern 129 plains and a larger supercontinent in its southern highlands At about 4.0 Ga, the 130 late phases of the lithospheric mobilism, involving subduction, perhaps as a kid AC C EP TE D M AN U SC RI PT 113 ...AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT A possible anorthositic continent of early Mars and the role of planetary size for the inception of Earth-like life James... 302 a rationale for a hypothesized lunar magma ocean composed of primordial 303 crustal and mantle materials similar to those of the Earth On the other hand, 304 geophysical arguments indicate... geology of Mars 99 2.1 Crustal dichotomy SC 98 RI PT 97 Mars? ?? radius (~3390 km) is about half that of the Earth, and it has nearly 1/8 of 101 Earth’s mass (Fig 1A) Though a lack of seismic information