Current Understanding of Early Earth and the Moon
More than 30 years of steady research and explosive technical advances have occurred since the Apollo program dramatically expanded understanding of the origin and evolution of the Moon and its broad significance to planetary science. A set of overarching hypotheses now prevail in lunar science that have broad applicability and implications for planetary science. The maturity of current understanding of the Earth-Moon system has enabled the formulation of tests that, with well-chosen new data, could strengthen, or overturn, some or all of the ruling hypotheses to provide better insight into the character of our solar system.
THE MOON SINCE APOLLO: MAJOR HYPOTHESES AND ENABLING FACTORS
Three hypotheses provide a context for understanding the origin and evolution of the Moon:
The giant impact hypothesis explains the origin of the Moon as being assembled from debris after the impact of a Mars-sized object with the early Earth. (An artist’s conception of a giant impact is shown in Figure 2.1.)
The lunar magma ocean hypothesis governs understanding of the formation of lunar rocks following lunar formation, and suggests that the outer portions of the Moon (several hundred kilometers in depth) were entirely molten. Differentiation of the vast magma body, a magma ocean, resulted in the formation of the earliest crust and mantle and produced the rocks observed today.
The terminal cataclysm (sometimes called the Late Heavy Bombardment) hypothesis concerns the timing of the impact flux in the 600 million years (Ma) after lunar formation. It proposes that the largest craters observed on the Moon, vast multi-ringed impact basins (e.g., see Figure 2.2), were formed in a brief pulse of impacts of large objects near 4 billion years (Ga) ago, well after impact-causing debris left over from solar system formation had died away (see Figure 2.3). The reality or not of an inner solar system cataclysm is important in understanding conditions on Earth at the time that life was first emerging. An alternate hypothesis is that the rate of impacts to the Moon and Earth declined with time and no cataclysm occurred (also in Figure 2.3).
In addition to these well-formulated hypotheses, there is an emerging recognition that the lunar plasma environment, tenuous atmosphere, regolith, and polar regions in permanent shade constitute a single system in dynamic flux that links the interior of the Moon with the space environment and the volatile history of the solar system.
Since Apollo, several key factors have enabled the formulation of these unifying hypotheses: (1) time for the scientific integration of the large body of disparate data, (2) improved and expanded analytical capability that
exploited the well-documented and curated collection of lunar samples and meteorites, (3) the increase in computational capability, (4) the recognition of meteorites from the Moon, and (5) remote sensing space missions.
The first factor, time, has enabled decades of intense scrutiny of lunar data and materials by a small but dedicated cadre of lunar scientists. This group has explored in great detail the available data and developed the currently known characteristics of the Moon, from its atmosphere to its core. A highly important revelation during this period of contemplation was the formulation of the giant impact hypothesis for lunar origin. The fundamental question of lunar origin persisted beyond Apollo, and prevailing hypotheses all suffered from one or more serious shortcomings. The giant impact hypothesis overcame these problems and meets all known constraints. The new insight was prompted less by working with new data, than by integrating the available data and engaging in
deep thinking about the conditions present in the early solar system. The period of contemplation featured many other lesser but still important insights, such as the recognition from detailed crater counts that lunar volcanism extended far later than the youngest dated basalt samples, perhaps being as young as 1 Ga, placing a significant constraint on the lunar thermal history.
A second factor enabling the formulation of the unifying hypotheses has been the exponential improvement in analytical capability that has had a revolutionary impact on the value of lunar samples collected by the Apollo program and Russian Luna missions (1959-1976). These samples have been conserved and very well documented by NASA’s curatorial facility, providing a significant body of data, some of which is still untapped. Analysis at spatial scales and analytical precision inconceivable in 1970—especially more recently developed isotopic systems such as Sm-Nd and Hf-W as well as analysis at the nanoscale by secondary-ion mass spectrometry, transmission electron microscopy, and other methods—has produced new insights in the formation of the Moon, from its core to the regolith. This new capability has enabled the “discovery” (or at least strong inference) about the presence of garnet in the lunar mantle; constraints on the processes attendant on a giant impact origin of the Moon, such as evaporative processes in a silicate vapor cloud; precise refinement of the chronological relationships among ancient lunar rocks; and the recognition of pervasive nanoscale processes involved in regolith evolution.
The improvement in analytical technology also included the patient application of ground-based astronomy. High-performance ground-based telescopic remote sensing, especially infrared spectroscopy, shows that the diversity of the lunar crust revealed at millimeter scale in the samples exists at the kilometer scale, and that rock types unknown in the sample collection exist far from the Apollo landing sites. Ground-based radar has revealed new insights into the nature of the lunar regolith and placed tight constraints on the nature of the permanently shaded portions of the Moon. Astronomical observations also enabled the detection of a tenuous atmosphere of sodium and potassium to supplement the constituents discerned by Apollo surface experiments.
Regarding the third factor, the increase in computational capability, a major beneficiary of this new capability has been the giant impact hypothesis. The ability to apply successful computational tests of this hypothesis using two- and three-dimensional fluid dynamics codes graduated what might have been simply an interesting and competitive notion to the status of a ruling paradigm. Another beneficiary of this new capability is the cataclysm hypothesis according to which models of the time evolution of the large bodies in the outer solar system suggest that planet migration could have instigated the cataclysm. Other fields have benefited as well, such as the reanalysis of Apollo seismic data using modern and computationally intensive techniques and analysis and the integration of remote sensing and new spacecraft geophysical data.
With respect to the fourth factor, the recognition of meteorites from the Moon, many of these meteorites have characteristics which suggest that they originate far from the Apollo zone, being extremely poor in incompatible elements—known collectively by the acronym KREEP (for enrichment of some of the first recognized incompatibles: potassium [K], rare-earth elements [REE], and phosphorus [P])—that are characteristic of the Apollo samples. These meteorites raise questions about the notion that the Moon can be understood solely in terms of the samples from Apollo, since the meteorites exhibit subtle differences that may ultimately challenge one or more of the prevailing hypotheses and lend additional impetus to the need for new lunar samples.
The fifth key factor enabling the formulation of the unifying hypotheses is the three post-Apollo missions that returned global lunar remote sensing data. The pioneer was Galileo, which on its way to Jupiter flew by the Moon twice, in 1990 and 1992; its multispectral image data first drove home the compositional distinctiveness of the vast South Pole-Aitken Basin. Next, in 1994, was the Clementine polar lunar orbiter, a joint NASA/Department of Defense technology demonstration mission run in part by lunar enthusiasts who allowed aspects of its sensor payload to be tailored to lunar issues. That 2-month mission resulted in the first unified global remote sensing data set; revealed the large-scale topography of the Moon, including the extent and depth of the South Pole-Aitken Basin; allowed near-global estimates of crustal thickness; provided the first global evaluation of selected elemental and mineral abundances; and brought the importance of the lunar poles into sharp focus with innovative, if controversial, measurements of polar radar properties aimed at the detection of water ice. The third and arguably most scientifically significant mission was the NASA Discovery mission, Lunar Prospector, in 1998. This mission initiated a possible paradigm shift, as it showed the unanticipated extent to which the Moon is asymmetric in composition, in particular for the heat-producing elements. The profound asymmetry, well out of the scope of the
lunar magma ocean hypothesis as originally formulated, is prompting a rush of new research attempting to accommodate this fundamental observation into current understanding. Lunar Prospector also identified a concentration of hydrogen at both poles, renewing the discussion of possible volatile deposits.
The more than 30 years of study since Apollo, the technical advances, and the space missions bring us well prepared, at this dawn of a new era of return to the Moon with robots and humans, to test current, durable models and to pose new questions not previously considered. With time, some of these hypotheses have attained the status of “paradigms.” Because of the tendency to view a paradigm as accepted truth, it is important to mount challenges constantly, and the new era of lunar exploration provides the opportunity to test each paradigm to varying degrees. Among the highest priorities for scientific understanding will be investigations that either challenge or expand the ruling paradigms.
TESTING THE PARADIGMS
Perhaps the hypothesis least likely to be significantly altered in the near future is the giant impact hypothesis. Tests of this model rest on new samples and on developing a much more detailed understanding of the composition of both Earth and the Moon. Clearly, such a quest will be significantly aided by the receipt of lunar samples from diverse locations and by a more thorough and detailed robotic assessment of the compositional character and structure of the Moon. Despite the directed quest for “genesis rocks,” there are no samples as old as the Moon, and the first 100 Ma of lunar history remains obscure. Improvement of the knowledge of the Moon’s composition relative to the composition of Earth is also ultimately limited by uncertainties in the composition of the accessible and well-sampled Earth. Anomalies in several isotope systems, including Hf, Nd, and Ba, have recently challenged faith in the chondritic composition of the Earth-Moon system, leading to proposals of unsampled domains on Earth (the core-mantle boundary) and on the Moon. At present, straightforward compelling tests do not exist; however, illumination of the giant impact hypothesis may be possible with new sampling, more abundant data, better analytical techniques, or conceptual breakthroughs.
The lunar magma ocean hypothesis, formulated almost immediately after the receipt of the Apollo 11 samples, was a vital conceptual breakthrough in understanding the early formation of planetary crusts. The concept has provided a new framework for understanding the early Earth, Mars, Mercury, and differentiated asteroids such as Vesta. It explains the observed ancient plagioclase-rich lunar rocks and their relationship to the source area of basalts that cover portions of the Moon through differentiation of a global magma body, the lunar magma ocean (see Figure 2.4). However, the hypothesis is now known to be inadequate to capture everything that has been learned about the early Moon. As Newtonian physics is to Einsteinian physics, the lunar magma ocean hypothesis as formulated in 1970 and refined in the following decades is not wrong, but incomplete; its shortcomings lie not in failures, but in scope. In its pure form, the lunar magma ocean hypothesis is fundamentally a one-dimensional
crystallization sequence, and a one-dimensional model was all that was required at the close of the Apollo program, when little data existed that placed ancient rocks into a global, three-dimensional spatial context.
Global remote sensing data from the post-Apollo missions forced rethinking of the simple lunar magma ocean hypothesis, principally through their documenting of the distribution and heterogeneity of crustal components. A major enigma is the dramatic asymmetric distribution of thorium, a significant heat-producing element. According to the lunar magma ocean hypothesis, as the globe-encircling magma body cooled and minerals precipitated, a lunar mantle formed from dense olivine and pyroxene, and a crust accumulated composed of buoyant plagioclase. In the waning stages of crystallization, elements that are not accommodated easily into the structures of these major minerals became enriched in the residual melt. This process led to a layer sandwiched between the aluminous crust and the iron- and magnesium-rich mantle that is strongly enriched in incompatible elements, known collectively as KREEP. One of these incompatible elements, thorium, is relatively abundant and was readily detected by the Lunar Prospector’s gamma-ray spectrometer (see Figure 2.5).
Apollo measurements from equatorial orbit hinted at a hemispherical asymmetry in Th; Lunar Prospector showed that the thorium (and the rare-earth elements Sm and Nd) and hence KREEP is distributed strikingly
asymmetrically and is strongly concentrated on the lunar nearside, with profound implications for lunar thermal evolution. While the lunar magma ocean hypothesis predicted a global zone of incompatibles concentrated at the base of the crust, Lunar Prospector showed that not even the immense farside South Pole-Aitken Basin exposed such a subcrustal enriched layer comparable to that seen in the area sampled by Apollo. High concentrations of Th (and by inference KREEP) are only found on the nearside. The seismic discontinuity provisionally detected at ~500 km depth by Apollo has often been suggested as the base of the original planetwide magma ocean. However, this feature (whose existence is in some doubt) has only been inferred beneath a limited area of the Moon, owing to the restricted geographic span of the Apollo seismic network. An alternative explanation is that it represents the regional depth of melting for the nearside mare basalts and thus might also be consistent with major lateral crustal heterogeneity. If the magma ocean originally was well mixed and essentially one-dimensional, at some epoch its character inflated to three dimensions and the Th (and KREEP) underwent massive lateral migration.
This recent recognition has spurred ongoing efforts to understand the asymmetry within the context of the lunar magma ocean model. Petrologic models suggest that throughout the crystallization of the magma ocean, precipitating minerals became increasingly dense as they accommodated more iron relative to magnesium, with the dense iron-titanium oxide ilmenite being among the last to crystallize (though prior to the crystallization of the last KREEP-rich residuum), perched atop the mantle. This sequence leads to a lunar mantle that was gravitationally unstable, and energetic overturn is enabled. The wide variation in Ti observed in lunar mare basalts in samples and in remote sensing data indicates a highly heterogeneous mantle with respect to Ti and has been cited as evidence of redistribution of magma ocean components. There have been recent attempts to produce models that exploit this gravitational instability to result in large-scale lateral motion. At this early stage, the efforts to model mantle overturn require sensitive and unsatisfying adjustment of viscosity in order to enable this type of global overturn while also preventing breakup into small convection cells; thus, at present this fundamental problem is far from solved, but it is certain that the magma ocean model as it existed prior to Lunar Prospector will require substantial revision and enhancement to take into account the new constraints.
Of the above-named hypotheses, the most vulnerable, because of the existence of well-defined tests aimed at its overturn, is the terminal cataclysm hypothesis. The validity of this hypothesis is the subject of vigorous current debate, and the possible existence of the hypothesized pulse of impacts of large objects has both profound scientific consequences and compelling tests. The consequences derive from the implications of such a pulse for the origin and development of life very early in Earth history and from its implications for the early evolution of the solar system. Astrobiologists have coined the term “impact frustration” to describe the possible effect on terrestrial life of impact by objects hundreds of kilometers in diameter. Models suggest that an impact on Earth comparable with the impact that formed the lunar Imbrium basin would vaporize the oceans and sterilize the top of the crust to depths of hundreds of meters. The hyperthermophilia of Archaea and bacteria near the base of the tree of life suggest a genetic bottleneck only survived by hardy organisms resistant to high temperature. Large objects did strike the Moon, as demonstrated by the many documented impact basins, and therefore a proportionate number also struck Earth. If these impact events occurred over the period 4.5 billion to 3.85 billion years ago, tens to hundreds of millions of years would separate the impacts, enabling the cooling and recovery of existing life and the population of refuges against the next impact. However, a narrow pulse of dozens of such impacts in a few million years would cause greater heating and leave little time for recovery and repopulation of refuges between events. This leads to the suggestion that any surface life that persists today must have either originated after the cataclysm (i.e., after about ~3.85 Ga, which appears to be the time of the last of the lunar basins) or survived deep in the crust.
What are the possible implications? This late date of heavy bombardment in the Earth-Moon system would allow half the time believed necessary to produce the extreme biochemical complexity observed in the Archaea and bacteria. It would also allow only half the time likely necessary for the organisms that produced the first generally accepted microfossils at 3.5 Ga. This implies that the origin of terrestrial life and its development to the level of biochemically complex and sophisticated microorganisms occurred at an exceedingly fast rate, over only a few hundred millions years at most. If the intensity of late bombardment was heavy enough to sterilize the surface of Earth at ~3.9 Ga, then it is possible that life emerged more than once on Earth, but that all present life is derived from the last developing life, not the first life on Earth. On the other hand, if the observed lunar impact basins
formed over a much longer period (with no peaked cataclysm occurring), the development of life was permitted to occur at a more leisurely pace, and planetary recovery from each impact event was sustained by hundreds of millions of years of relative quiet.
Models to explain the apparent lunar cataclysm also have long-reaching implications for our understanding of solar system dynamics. For instance, one such model postulates that exchange of angular momentum between the giant planets and an early solar system teeming with smaller objects causes the orbits of the large planets to migrate. As a consequence, the giant planets pass through highly disruptive resonances that ought to scatter small objects formed in the outer solar system both outward and inward. The excited orbits of the Kuiper Belt objects are testimony to a scattering event or events that may also have sprayed the inner solar system with icy objects large and small long after planetary accretion had waned and background impact fluxes had drastically dropped. These planet migration and scattering calculations solve profound problems with the terminal cataclysm, that is, where were the objects stored, and what precipitated their bombardment of the inner solar system? They also suggest that Mars too would have experienced a cataclysm, removing that planet as a safe refuge for life to later populate Earth by way of meteorite exchange between planets.
Work on early Earth is beginning to intersect work on the early Moon and to provide critical constraints on both bodies. In particular, the implications of early bombardment and the terminal cataclysm hypothesis must also accommodate new results from the study of early Earth which strongly suggest that relatively clement conditions existed on Earth as early as 4.4 Ga to 4.2 Ga, raising the possibility that life might have emerged within 200 Ma of the formation of Earth (see Figure 2.6).
Zircons (which are incredibly difficult to alter) with ages as great as 4.4 Ga have been found in the ancient terrestrial Archaean metasediments (metamorphosed but recognizably sedimentary rocks). Because most terrestrial zircons are formed in granitic rocks, these ancient zircons imply the presence of continent-building activity at the time of zircon formation, rather than the presence of a simple basaltic crust. The mildly elevated values of δ18O in these zircons support the proposal that, during this period, chemical weathering and erosion had occurred, both of which require liquid water to alter the protoliths (original rocks) of subsequently formed granitic rocks. If the high impact flux suggested by lunar chronology could have caused the oceans to vaporize repeatedly, there may be an inconsistency in the lunar and terrestrial observations. More investigations of the lunar record and terrestrial history are indicated.
As on the Moon, terrestrial zircons are the end product of extensive igneous processing, and they host some of the important incompatible elements of the KREEP association (of potassium, rare-earth elements, and phosphorus found in lunar basalts). Zircons generally require an evolved host rock such as granite, and on Earth the presence of large-scale processes giving rise to granite and mountain belts account for most, though not all, zircons. In addition to hosting a range of incompatible elements, zircons are extremely durable and are resistant to melting, abrasion, and weathering. It is possible that these minerals are the end result of differentiation of basaltic magma, but their sheer abundance in Archaean sedimentary rocks on Earth suggests that most were derived from rocks of evolved, possibly granitic, composition. Radiometric dating of these detrital zircons shows that some were formed before 4.0 Ga, with several terrestrial ages near 4.4 Ga. Elevated oxygen isotope ratios in the most ancient zircons appear to require that the protoliths were altered by liquid water as long ago as 4.3 Ga. Although work on these zircons has only begun, they already indicate the presence of buoyant, probably granitic components in the crust shortly after the formation of Earth (while the lunar magma ocean completed crystallizing many of its primary rocks), and the zircon host rock was profoundly affected by the presence of liquid water. Further study of ancient zircons and rocks from Earth may resolve these differences. Relatively few zircons have been studied from the Moon, and further analysis will elucidate differences in magmatic environment. It is also possible that zircons from otherwise-unsampled domains of early Earth will be located on the Moon, delivered as meteorites, if sufficient quantities of regolith are searched.
The terminal cataclysm hypothesis can be definitively tested by measuring the ages of large impact basins that are far from the Apollo sampled zone. The basis for the hypothesis, a cluster of radiometric dates near 3.8 Ga, may all be related to the vast Imbrium basin that dominates all portions of the Moon visited by Apollo. Thus, sample-derived radiometric dates of basins far from Imbrium, and most especially the largest and oldest basin of them all, South Pole-Aitken Basin, will rigorously test the terminal cataclysm hypothesis.
THE LUNAR ATMOSPHERE
The lunar atmosphere links the Sun and solar system volatiles through the lunar surface. The Sun provides one input, the solar wind, to the lunar environment, some of which is entrained in the lunar plasma environment, some of which is implanted into grains of the lunar surface, and some of which serves as a critical loss mechanism for the lunar neutral atmosphere. Hydrogen and helium are the dominant species that saturate the outermost surfaces of lunar soil grains, but the solar wind provides a wide diversity of other components, particularly carbon and nitrogen. Many other sources can provide volatile inputs to the lunar environment: comets, asteroidal meteorites, interplanetary dust particles, the transit of interstellar giant molecular clouds, Earth itself (ions from its atmosphere are continuously shed down the magnetotail or episodically removed as impacts eject atmosphere into space), and even the Moon’s interior (which may occasionally still outgas at high rates, as suggested by some young geologic features). These sources provide volatile species that may then be transported across the lunar surface until either lost or sequestered in lunar sinks. This atmosphere is extremely tenuous, having a total mass of a few tons. Research shows that the lunar atmosphere is generally resilient and can restore its original state even a few weeks or months after a disturbance, but its present state is relatively fragile against extended large-scale lunar activities.
The lunar atmosphere may actually be dominated by dust, although its properties are not well known. The mass of suspended dust may be larger than the mass of the atmosphere. Observations by the Surveyor, Apollo, and Clementine missions showed the presence of high-altitude dust, presumably lofted by electrostatic levitation.
Permanently shaded regions of the lunar poles may constitute one of the important sinks for volatiles. Lunar Prospector showed excesses of hydrogen at the 50 km to 100 km scale near the poles, although radar detects no widespread thick deposits of volatiles (unlike at the poles of Mercury). Despite the fact that these cold, shadowed surfaces are expected to act as lunar cold traps, their effectiveness in trapping volatiles for long periods is unknown. Regardless of total efficiency, the lunar polar soil may contain a wealth of scientific information on solar system volatiles.
Significant gaps remain in the understanding of the lunar atmosphere system, especially the three-dimensional flows and nature of constituents through the atmosphere, the detailed behavior of dust and its relationship to the vapor component, and the role and state of the polar cold traps. It is safe to say that the dynamic system exists, but there is no predictive understanding of its behavior.
The more than 30 years since the Apollo program have enabled lunar and other scientists to make great strides in the understanding of the origin and evolution of the Moon, Earth, and other planets using the Moon as a lens. But this era has also revealed troubling defects in that lens owing to the lack of key data in time, space, and physical characteristics. Building on the foundation of the Apollo program and over 30 years of contemplation and technical progress, a well-formulated new era of lunar exploration will enable key tests of hypotheses major and minor that may revolutionize the understanding of the origin and evolution of the Moon, Earth, and the planets.