Scientific Perspectives: Challenges for Human Cognition

Abstract

One of key issues in the study of Earth’s habitability concerns the origin and preservation of volatiles, which are related either to Earth accretion processes associated with some special giant impacts or to solidification of magma ocean and vertical tectonics, which were responsible for volatile transport and distribution.

2.1 Early Earth

One of key issues in the study of Earth’s habitability concerns the origin and preservation of volatiles, which are related either to Earth accretion processes associated with some special giant impacts or to solidification of magma ocean and vertical tectonics, which were responsible for volatile transport and distribution. Obviously, all these processes are associated with the evolution of early Earth (Armstrong et al. 2019), understanding of which requires following investigations in the future: ① Earth accretion processes (material types and acquisition of volatile substances); ② the number and pattern of giant impacts on the Earth and their influence on the preservation of different types of volatiles; ③ the connections between giant impact events and the present core-mantle boundary structure, and how did giant impacts cause the preservation of some primodial components in the lower mantle; ④ how did the core-mantle differentiation form Earth’s layered structure; ⑤ the scale of the latest magma ocean and its solidification, and the probability of the existence of a deep magma ocean; ⑥ precise dating of major geologic events; ⑦ primitive core constituents and the onset of the Earth magnetic field, etc.

One of significant difficulties in carrying out this research is the almost complete lack of geologic samples. Although ~4.4 Ga zircons and primordial materials as old as the Earth itself residing at the core-mantle boundary (brought to the surface by mantle plumes, Mundl-Petermeier et al. 2020) have recently been discovered, they are nevertheless indirect samples in the sense that they have been displaced from their genetic environment and therefore cannot provide a full range of information. Earth’s original appearance and constitution can only be inferred from these indirect ancient samples using isotopic and elemental geochemical methods. Meanwhile, high-temperature and high-pressure experiments, Earth dynamics calculation, comparative planetology, and planetary accretion dynamics should be conducted to simulate early Earth evolution (Fig. 2.1). Future Earth scientists must integrate these study methods and develop new research paradigms.

Fig. 2.1

Diagram illustrating showing deep-Earth water and carbon circulation patterns. During magma ocean

The Earth’s magnetic field is one of major factors in providing the necessary conditions for the formation and development of Earth’s habitability. Its formation is highly relevant to understandings of the Earth’s formation and evolution, deep interior dynamics and space environment. Earth’s magnetic field is generated by the ‘dynamo’ induced by motion in the liquid outer core. The ‘dynamo’ transforms some of the kinetic energy of fluid motion into electromagnetic energy by MHD (magnetohydrodynamics) and electromagnetic induction. Autogenesis and maintenance of Earth’s magnetic field involve planetary rotation, the presence of conductive fluids, and sufficient heat energy (Olson et al. 2018). The existence of a solid inner core, around which molten conductive fluids rotate, is also a likely contributing factor in generating a strong magnetic field. The key issues for research on Earth’s magnetic field include the mechanisms of formation and operation. Although important advances have been made in this field in recent years, the dynamic parameters required for the maintenance of the geomagnetic dynamo remain unclear.

During magma ocean crystallization in early Earth, volatiles (water and carbon dioxide) rose to the surface. At that time, there was over 100 atmospheric pressure of CO_2, several hundreds of atmospheric pressure of water vapor, and a little nitrogen in the atmosphere. A supercritical CO₂ layer between the primitive ocean and the atmosphere might be the key to the origin of life (Zhang et al. 2020). When plate tectonics operate on Earth, plate subduction and volcanic activity are the main cycling pathways of material circulation between Earth’s interior and exterior (Schmidt and Poli 2014; Dasgupta 2013; Hou et al. 2019; Zhang et al. 2020).

2.2 Effect of Deep Dynamic Processes on Earth’s Habitability

2.2.1 Materials Circulation and Deep Earth Structure

Materials and energy exchanges between Earth’s different spheres are essential to the ‘dynamic Earth’ and are also crucial processes in making our planet habitable and favorable for the birth of life. Spreading mid-ocean ridges, plate subduction and mantle plumes are the principal pathways for materials and energy exchanges between the solid Earth’s different spheres. These apparently independent dynamic processes are indeed mutually and closely related.

Oceanic lithosphere forms at mid-ocean ridges through mantle partial melting, representing one of principal sources of mantle cooling. Thermal fluid-rock reaction at in these areas not only alters raw materials supplied to ‘subduction factory’, but also affects sea water composition and submarine life processes. Plate subduction zones are vital sites for materials and energy exchange, fluid–solid couplage and Earth’s sphere interactions. Subduction acts an efficient circulation pathway in linking deep interior processes and surface processes, governing both the interior and exterior evolutionary processes of the globe. Dehydration of subducting slab and upward transfer of water trigger melting of the overlying mantle wedge and the formation of island arc or continental arc magmatic rocks, providing materials for the reworking of continental crust. In this sense, the subduction zone is considered as graveyard of subducting oceanic crust and birth site of new continental crust.

A great amount of slab materials is transported to the deep mantle by subduction, creating the heterogeneity of the mantle in terms of lithology, elements, isotopes, and volatiles (H₂O, CO₂, etc.). Mantle heterogeneity would in turn induce mantle convection and magmatism, exerting significant influence on the formation and evolution of the oceanic and continental lithosphere, enrichment of mineral resources in the shallow crust and changes to habitable environment on the surface. The detention of subducted plates at the core-mantle boundary may have resulted in the seismically detected Large low-shear velocity provinces (LLSVP) and ultra-low velocity zones (ULVZ) (McNamara 2019; White 2015).

The total quantity of all the volatiles transported in subduction zones exceeds considerably those of respective components on the surface. Elements with variable valency such as C, H, O, S, etc. are responsible for the variation of redox state of earth’s interior and have an enormous influence on the property of the Earth’s interior and geodynamic processes. Subducted slab and their derivatives of dehydration and melting are present at different depths of the mantle. Time-evolution of these components and their interactions with ambient mantle ultimately generated distinct types of mantle end-members (EM1, EM2, HIMU, FOZO, etc.).

In order to understand materials and energy exchange between the different Earth spheres and to elucidate their roles in the formation and evolution of habitable Earth, investigations from the Earth system viewpoints need to be carried out on the structure and origin of Earth’s different spheres, compositions and origin of diverse mantle end members, and the dynamic processes governing materials exchange and energy circulation in subduction zones. It is also important to quantitatively estimate transfer fluxes of key elements during major geological events. Finally, these new acquired data will be integrated to evaluate the effects of deep-Earth processes on changes in Earth’s habitability.

2.2.2 New Physical Chemistry of the Deep Lower Mantle and Deep Earth Engine

From the deep crust to the middle of the lower mantle (about 1800 km), minerals and rocks go through several facies changes, but the physical and chemical principles defined under normal temperature and pressure do not vary. However, in the lower part of the lower mantle (below 1800 km), familiar physical and chemical rules appear to change, and the properties and chemical behavior of the elements alter beyond recognition. For example, the properties of iron become similar to those of magnesium and the properties of hydrogen to those of lithium. When both ends of a single system exist under fundamentally different physical and chemical conditions, potentially extreme variations can occur, which may be evidence of an as yet unknown dynamic engine for Earth’s evolution (Mao et al. 2017). For instance, the current mainstream view is that the lower mantle (660 ~ 2900 km and 24 ~ 135 GPa) consists of bridgmanite and ferropericlase. Although it accounts for 55% of Earth’s total volume, it is thought to be in simple composition. Whereas, breakthroughs in X-ray spectroscopy and synchrotronic radiation analysis have revealed Fe-paired magnetic rotation of mantle minerals and the decomposition of bridgmanite under super-high pressure, as well as iron peroxide synthesis which releases hydrogen and remains vast amounts of oxygen when hematite comes into contact with water (Hu et al. 2017, 2020). This suggests that iron peroxide is a considerable component of the core-mantle boundary (Liu et al. 2017). These new understandings demonstrate that HPHT conditions in the deep lower mantle can have a profound effect on mineral density, wave velocity, melting temperatures, conductivity, magnetic properties, rheological intensity, and other physical properties, as well as on the controlling dynamics of the mantle, the heterogeneity of lateral structures and components, and the evolution of deep Earth. These new clues and discoveries appear to confirm that the super-deep mantle (>1800 km) and the shallow mantle are profoundly different (Fig. 2.2), which foreshadows another revolution in HPHT chemistry. Study of these questions will guide us in the construction of new theories and concepts.

Fig. 2.2

Entirely different physical and chemical properties of the super-deep (>1800 km) and shallow mantle (amended by Mao and Mao 2020). The differences may relate to Earth’s interior working mechanisms, and might also be the causes of important geologic events on the surface

Earth’s evolutionary history began around 4.6 billion years ago (Ga). At 2.4–2.0 Ga a major oxidization event occurred and the diversity of life on Earth instantly began to increase exponentially. At about 550 Ma, the oxygen in the atmosphere abruptly increased again, coinciding with the advent of vertebrate lifeforms. This much is known, but what remains mysterious is the question of where the oxygen came from that drove these oxidization events. The conventional view has been that CO₂ on the surface was transformed into oxygen by cyanobacterial photosynthesis. This hypothesis matches the facts, as far as they are known, but there is no convincing geological evidence to support the hypothesis. More recent HPHT calculations and experiments have confirmed that, when subduction slabs subside at the core-mantle boundary, the water drawn down by subduction contacts with iron. Following a series of intermediary reactions that produce ferric oxide and FeH₂, the final product of this process is iron peroxide. A single subsiding tectonic plate could transport hundreds of millions of tons of water to the core-mantle boundary every year. Over billions of years, oxygen-rich blocks several thousands of meters thick probably accumulated at the bottom of the lower mantle (Mao and Mao 2020). Destabilization of these oxygen-rich layers could have released the oxygen into the atmosphere and generated the great oxidization event. The formation principle of oxygen-rich layers at the core-mantle boundary is that oxygen remains trapped there following release of hydrogen from the water drawn down by subduction, which suggests that the Earth’s interior is a vast hydrogen-producing machine. It is already known and understood that hydrogen molecules tend to release across crystal lattices, but the precise patterns and processes of hydrogen volatilization need to be more closely studied in the future. First, the possibility should be explored of hydrogen combining with carbon to form abiogenetic hydrocarbons, or performing as a catalyst in the genesis of organic matter in the shallow crust. Second, the potential trapping of naturally occurring hydrogen in clay mineral layers should be considered. Third, the possibility of hydrogen recombining with oxygen to form water during geological uplifting should be examined. Fourth, we should determine how hydrogen could control the migration of volatile substances from the interior of the Earth in combination with nitrogen, sulfur, phosphorus, and halogens. These avenues of study will open significant new perspectives for the geosciences.

At present, our knowledge of the super-deep Earth is based mainly on high-temperature and high pressure (HPHT) measurements and experiments in geophysics and geochemistry. Previous studies have focused on synthesis of individual minerals under high pressure to measure their basic physical properties and thereby interpret deep seismic structures. Future research will focus on developing in-situ HPHT testing technologies, measuring deep rock mineral properties, exploring physical and chemical behavior, and controlling geophysical and geochemical phenomena in high pressure environments. This will cast light on the critical role of the super-deep regions in Earth’s evolution from core to surface and from past to future, and will promote the re-evaluation of fundamental theories of a 4D Earth system.

2.3 Influence of Important Events on Earth Habitability

2.3.1 Deep Earth Processes and Constant Temperature Mechanisms of Earth’s Climate

Since 4Ga, surface temperatures have been maintained in the narrow range that supports the existence of liquid water—an essential factor in the continuous evolution of life. By comparison, climate-controlling factors (for example, solar radiation, land-sea distribution, and CO₂ gas emissions from the mantle) have varied immensely. Deep tectonic movements have also developed from the vertical motion of the early Earth into a complex system of coexisting and simultaneous vertical and horizontal movements. The mechanism that has maintained stable surface temperatures over this immense period of time, while other environmental factors have fundamentally changed, is clearly one of the keys for deepening our understanding of the formation and development of Earth habitability.

The traditional view is that long-term surface temperature stability is related to a negative feedback mechanism formed by continental weathering, a process which has been called “continental weathering geological air conditioning”. When the climate becomes warmer, the rate of atmospheric CO₂ adsorbed by silicate weathering increases, which leads to declining atmospheric CO₂ content, which in turn inhibits further temperature rises, and vice versa. Nevertheless, in the course of geological evolution, continental surfaces have been covered by sediments and granite that have undergone complete weathering cycles, with severe reduction in the efficiency of CO₂ adsorption by the weathered surface, weakening the capacity of the sediments for climate adjustment. The ‘air conditioning’ hypothesis is therefore not confirmed by the geological record. It is also difficult to explain why this mechanism should only have emerged on Earth but not on Venus or Mars, both of which are also within the Sun’s habitable zone.

Unique deep Earth processes may be the key to understanding these questions. Perhaps large igneous provinces, rifting and expansion, and volcanoes in subduction zones provided a continuous supply of fresh basalt to the surface, which could have ‘fueled’ the process, since basalt weathering has extremely high CO₂ adsorption efficiency (Dessert et al. 2003), roughly one hundred times that of granite weathering. Exhausted volcanoes could also have continued to pump CO₂ from deep Earth to the surface, helping to keep the ‘geological air conditioning’ system working. Study of the synergetic evolution of deep Earth events (e.g., plate tectonics initiation, continental crust growth, supercontinent cycle) together with continental weathering, atmospheric CO₂ content, and climatic environment would be an effective way to verify the ‘continental weathering geological air conditioning’ theory. Research should concentrate on the reconstruction of deep Earth carbon cycle processes, mantle exhaustion history, and continental weathering history. Numerical simulation of deep crust-mantle circulation can be conducted by extracting chemical footprint information from weathered continental deposits and remnants of mantle exhaustion, which may offer significant clues for a new approach to resolve this issue.

2.3.2 Why Does Plate Tectonics Occur Only on Earth and How Does It Affect the Habitable Environment?

Plate tectonics is a unique feature of the Earth, which is quite different from the other terrestrial planets and is a principal factor in the development of Earth habitability. The role and function of tectonic activity in the construction of the habitable environment are key questions in the study of the habitable Earth. Study of the emergence and evolution of plate tectonics should be placed in this context.

The timing of the beginning of plate tectonics is a contentious issue. Previous studies primarily examined clues found in ancient rocks to identify the beginnings of subduction. Whereas, new perspectives and methods are required to establish a definitive timeline. In fact, plate tectonics occurred as a result of the transition from pipe-like structures created by vertical movements into more complex structures associated with horizontal movements. This transition should be the focus of future study, particularly to determine the driving force of subduction zones and the reasons for variations in thermal system through time. This will be helpful in accurately determining the initiation, characteristics, and evolution of plate tectonics in early Earth.

Specific questions include: What are the sources of driving force for the global-scale plate tectonics? What are the mechanisms? When and how did plate tectonics initiate? How are plate tectonics maintained? How do subduction slabs interact with the mantle? What is the influence of tectonic activity on the surface environment?

2.3.3 Influence of Significant Geological Events on the Environment and the Evolution of Life

Major geological events have occurred throughout Earth’s history. They have directly affected the development of Earth habitability and have determined the origins and radiation of the different biomes, as well as global ecological crises and mass extinctions. For example, two major oxidation events occurred in the Early and Late Proterozoic. The first led to a rapid flourishing of eucaryotes and the second to the emergence of the first complex lifeforms. Rapid melting and breakup of the lithosphere in the Neoproterozoic were accompanied by volcanic activity on a vast scale which triggered a fast transformation from extreme ice room conditions (snowball Earth) to an extreme greenhouse climate. Large-scale continental magma eruptions in the Phanerozoic resulted in multiple mass extinction events. A huge meteorite impact may have caused the well-known ‘dinosaur extinction’ at the end of the Cretaceous. Thus, uncovering the full effects of these crucial geological events on biological and environmental evolution is clearly essential for understanding the development of Earth habitability (Fig. 2.3).

Fig. 2.3

Relationship of vital geological events to the Earth’s evolving temperature cycle (drawn by Tand, Chen and Gong, according to Condie 2010; Grotzinger and Jordan 2010; Tang and Li 2016)

2.4 Earth’s Atmosphere and Climate Change

2.4.1 Oceanic Heat Absorption and Carbon Sequestration, and Their Role in Climate Change

Since the Industrial Revolution, the cumulative carbon emissions due to usage of fossil fuels have reached about 450 billion tons (Friedlingstein et al. 2020), and led to a significant rise in atmospheric concentrations of CO₂. Over 90% of the heat added to the climate system (Cheng et al. 2019) and about 30% of the CO₂ emissions due to human activities (Pörtner et al. 2019) have been absorbed by the oceans. Therefore, the oceans play a crucial role in mitigating climate change. The oceans’ capacities for heat absorption and carbon sequestration are determined by material and energy exchanges at the air-sea interface, between the upper and deep layers of the oceans, and at the fluid–solid interface on the sea floor (Fig. 2.4). The oceanic dynamic processes including large-scale ocean circulation, mesoscale/sub-mesoscale eddies, and small-scale turbulence, and multi-scale air-sea interactions are the key processes for oceanic materials and energy cycle and storage. However, what is the upper limit of ocean’s capacities for heat absorption and carbon sequestration in response to climate change, and where are the tipping points of climate change? These are major fundamental scientific questions to understanding the oceans’ role in climate change. To address this question, it is needed to understand the role of mesoscale and smaller-scale processes, which contain the most part of ocean’s energy, in heat absorption and carbon sequestration; accurately quantify the heat and carbon exchanges at the air-sea interface; and clarify the contributions of ocean’s biological and physical carbon pump to absorbing atmospheric CO₂.

Fig. 2.4

Heat absorption and carbon sequestration processes in the ocean and their role in climate change. Ocean dynamic processes and biogeochemical processes fundamentally affect the capacity of the oceans to absorb surplus heat and human carbon emissions. Heat absorption and carbon sequestration processes in the ocean can in turn significantly alter the marine environment and exert important influences on Earth’s climate system through cross-sphere interactions

The continuous absorption of heat and carbon by the ocean changes the marine environment significantly, exerts enormous and even disastrous effects on Earth’s climate system through cross-sphere coupling, and would significantly affect the Earth’s habitability in a visible future by human beings (Fig. 2.4). However, it remains unknown when and how the continuous oceanic change would shape Earth’s climate in context of global warming, and how to better predict ocean’s feedback to global climate change in the future. The answers to these questions are key to understanding the habitability of future Earth. In the future, it is needed to carry out systematic research in terms of: the mechanisms through which the continuous heat and carbon absorption affects ocean’s dynamics and thermal structures; the processes that determine the changes of ocean heat content, polar ice and snow, and sea level; the mechanisms through which the air-ice-ocean interactions affect global and regional climate change; and improved prediction of the frequency and intensity of future extreme weather and climate events; and a better understanding of the ocean’s feedbacks to global climate change and underlying mechanisms.

2.4.2 Climate Change in Extremely Hot Geological Periods

As the largest active carbon reservoir on the Earth’s surface, the ocean is the most crucial buffer against climate change in Earth’s surface systems. It’s fundamental in maintaining a habitable climate and a tenable ecologic environment for human life. In the geological past, the Earth has experienced extreme climate and environment change events on a vastly greater scale than those that have occurred during human history. During those periods, the atmospheric CO₂ concentration has been much higher in the past than it is now. Climate change mechanisms in hyperthermal periods of geological past and the corresponding response of oceanic ecological systems have critical reference implications for comprehending the requirements for human survival in the current conditions of accelerating environmental change driven by global warming. However, the detailed evolutionary processes and forcing mechanisms of paleoceanography, paleoclimate, and palaeontology in periods of extremely rapid warming in geological history such as, the Cretaceous anoxic events, the Paleocene-Eocene Thermal Maximum (PETM), the Eocene and Middle Miocene Climatic Optimum, remain unclear. The opening and closure of major ocean gateways may also have played a significant role in past climatic and ecological change, but are not well understood. The following questions must be urgently addressed: What are the thresholds of abrupt changes in global climate, environment and ecosystem in response to tectonic activity and rapid emission of greenhouse gases? How did the marine ecosystem and climatic condition rapidly restore after the disaster? Are the current conditions of carbon emissions and the present rate of global temperature increase sufficient to drive a transition to a different state of the oceans’ ecological system?

2.5 Interactions Between the Oceans and the Earth’s Interior

2.5.1 Lithosphere Structure and Composition of Oceanic Plates

The composition and structure of the oceanic lithosphere are the keys to unlocking the driving forces of plate tectonics. In 1957, the “Moho” project—an ambitious plan to penetrate the Mohorovicic discontinuity in the sub-oceanic crust—was set out by the eminent American scholar of physical oceanology, Walter Munk, and the pioneer of plate tectonics theory, Harry Hess, amongst others (Hess and Ladd 1966). Their intention was to observe and study the lithospheric structure and composition of the sub-ocean plates at first hand by drilling through to the Earth’s mantle. At the time, this plan was thought to rival the Apollo program in scale and importance. Whereas, the primary objective of the Apollo program—to send men to the moon and return them safely to Earth—was achieved half a century ago, together with the collection of large quantities of geological samples from the lunar surface, representing a giant leap in our knowledge and understanding of the moon. Nevertheless, although deep-sea drilling has been conducted throughout fifty years, the Moho discontinuity remains untouched. In recent years, an increasing number of studies have confirmed that the structure and composition of the oceanic lithosphere are very complex and quite different from classical models (Wilson et al. 2006; Gillis et al. 2014; Sutherland et al. 2017). To reach new understandings, multi-disciplinary cooperation is required, involving marine geology, geophysics, high pressure and high temperature experimental simulation, and computer science. Major questions include: How does the oceanic lithosphere thicken from mid-ocean ridges to subduction zones? What is the composition of the thickened lithospheric mantle? What are the alteration processes of the oceanic lithosphere and their controlling factors? How influential is the alteration of the oceanic crust on deep-water circulation and fluid–solid interactions?

2.5.2 Driving Forces of Oceanic Plate Movements

Plate tectonic theory is the basis of solid geoscience (Fig. 2.5). However, the nature of the forces that drive plate movements are still unclear and are hotly disputed (Zheng and Zhao 2020). Current mainstream opinions relate the driving forces of plate movement to mantle convection, mantle plume upwelling, slab pull, and the ‘magma engine’ hypothesis. Whereas, the rate of mantle convection is much slower than the movement of the oceanic plates, so mantle convection cannot be the principal driving force for tectonic activity (Anderson 1998). Super-mantle plume upwelling could trigger continental disintegration but could not have driven the gradual and continuous sea floor spreading and oceanic basin formation that followed the breakup of the supercontinents. Subduction slab dragging could certainly influence local plate movements but could not be responsible for large-scale ocean-floor spreading, which is not related to subduction zones, or provide sufficient force for oceanic plate re-orientation or initiation of subduction (Sun 2019; Stern and Gerya 2018; Arcay et al. 2020). The ‘magma engine’ hypothesis proposes that heat from the Earth’s interior is the main energy source for plate movement. Newly formed oceanic crust around mid-ocean ridges is light and thin, while old oceanic crust is heavy and thick. This combination causes tectonic plates to be inclined relative to the asthenosphere, generating sliding forces that cause continuous mid-ocean ridge spreading driven by sinking of old crust and the formation of more new oceanic crust by upwelling of magma. The sinking of old, heavy crust in subduction zones therefore provides the power to drive this vast ‘engine’ (Sun 2019). However, this hypothesis has never been verified. Questions to be resolved include: Where does the energy driving plate movements come from? Why do plate movements occur only on Earth of all the terrestrial planets? What role does fluid–solid interaction led by sea water play in plate movements? How can we verify the ‘magma engine’ driving force hypothesis for plate movements?

Fig. 2.5

Sketch of the driving forces of plate movement. Verification defects occur in the driving force models of slab dragging, mantle convection and plumes. The rate of mantle convection is slow. In addition, the flow direction of the mantle at mid-ocean ridges is opposite to the direction of plate movement. Plate movement without subduction cannot be caused by subduction slab dragging. Continuous plate movement cannot be driven by mantle plumes. The magma engine hypothesis, although promising, requires further verification

2.5.3 Deep-Water Circulation and Sea Level Fluctuation

At present, about 60% of the world’s population reside in coastal areas, i.e., within 100 km of the sea, so that sea level fluctuations have a huge influence on the human habitable environment. Traditional studies of sea level fluctuations have focused on ice cap melting, glacio-isostatic adjustment, variations in sea water density, changes in inland water storage conditions, vertical displacement caused by movements of the crust, and the overall influence of sea water motion. The amount of water in the Earth’s interior exceeds that held in the surface oceans (Langmuir and Broecker 2012), with plate subduction and magmatic activity controlling water circulation in the deep Earth (Cai et al. 2018; Galvez et al. 2016; Parai and Mukhopadhyay 2018). With respect to Earth’s evolution, sea level fluctuation is defined by the total amount of sea water and the average age of the oceanic crust, with the former being primarily controlled by water exchange between the Earth’s interior and the surface driven by plate subduction and hydrothermal fluid processes in the mantle, with the latter controlled by super-continent convergence and breakup and super-ocean opening and closing. These geological processes have also controlled ice cap formation and dissolution in the past. For example, during the event known as the Permian Extinction, volcanic activity in large igneous provinces, combined with plate subduction, released vast quantities of greenhouse gases in a concentrated discharge over a relatively short period of time, causing rapidly increasing air temperature and creating a greenhouse climate with no polar ice caps. The result was the extinction of almost 80% species on Earth (Fig. 2.6). Key issues in this area include: What is the relationship between the exchange flux of water in the Earth’s interior and sea water on the surface? How does the total volume of surface water vary? Will Earth’s oceans dry up like those on Mars and when might that happen? How do “greenhouse” and “ice house” conditions on Earth flip from one state to the other? How do large-scale geological events affect the formation and melting of the ice caps?

Fig. 2.6

Sketch of deep-water circulation and sea level fluctuation. Sea water enters the oceanic lithosphere by seafloor seepage and is carried into Earth’s interior by plate subduction. It is then released to the surface by magmatic and hydrothermal activities, completing the deep-water circulation cycle. On a geological scale, this process controls sea level fluctuations

2.6 Evolution of the Oceans and the Origins of Life

2.6.1 Extreme Oceanic Life Processes and Origins of Life

We know that life originated on Earth no later than 3.8 Ga, but the mechanisms that created the conditions for life remain highly controversial. In 1953, Harold Urey and Stanley Miller synthesized amino acids from reductive gases (ammonia and methane, etc.) using electrical discharges. Therefore, intense lightning storms in the strongly reductive primordial atmosphere have been considered as the key factor in creating the chemical precursors of life on Earth. Nevertheless, subsequent studies have shown that the atmosphere of early Earth was not as reductive as originally thought, casting doubt on the mechanism suggested by the Urey-Miller experiment. The current mainstream view is that localized extreme environment (deep-sea hydrothermal fluids and cold springs, etc.) were probably the original sites of life (Fig. 2.7). For instance, alkaline thermal fluids associated with serpentinization at low temperature generate large amounts of methane, which could support chemosynthetic ecosystems and have been the hotspot for research of the origins of life. The study of lifeforms in extreme deep-sea environment could assist us in understanding the origins of life and the coherent evolution of organisms and the environment. Scientific issues to focus on in the future include: key chemical and physical processes in the origin of life; deep-sea serpentinization and transition mechanism from inorganic to organic; the coupling mode and mechanism between the origin of life and major geological processes; hypothesis of origin and evolution of photosynthetic organisms in deep-sea hydrothermal fluids; the deep-sea origin hypothesis of eukaryotic life.

Fig. 2.7

Life origin and evolution hypothesis

2.6.2 Evolution of Marine Life and Its Adaptability to the Environment

The oceanic environment is complex and variable, so marine organisms have evolved a wide range of adaptation mechanisms. Such organisms can survive independently in hydrothermal fluids, cold springs, abysses and other extreme environment, and have also evolved a high level of biological diversity. Studies of deep marine life indicate that physical and chemical factors, as well as geological processes, shape the environmental constraints on marine life (Heuer et al. 2020). However, the evolutionary processes and mechanisms under environmental change are still unclear. Important issues awaiting clarification and interpretation include: the limits and controlling factors of life in extreme marine environment; epigenetic and genetic mechanisms of adaptive evolution during the migration of deep-sea organisms between habitats; the driving processes and mechanisms of diversity differentiation in marine organisms from different environment; variation, response strategies and ecological function changes of marine organisms in the context of global change.

2.6.3 Role of Marine Life in Earth Evolution

Marine life has had a significant effect on Earth’s evolution by its influence and participation in substance circulation and energy flow. Organic matter from the ocean surface was gradually degraded by marine organisms and was deposited in water bodies and sediments, entering into material circulation in the lithosphere and Earth’s interior through diagenesis (Fig. 2.8), controlling the formation of hydrocarbon resources and marine gas hydrates. Against the global background of climatic variation, the community structures and ecological functions of marine organisms have changed markedly, and their roles in generating feedback phenomena in climate change are now receiving unprecedented attention (Cavicchioli et al. 2019). Crucial questions that call for breakthroughs in this field include: What are the associations between deep-sea biological activity and specific geological structures and events? What are the contributions and ecological effects of nutrient transportation by marine organisms to surrounding ecological systems? How are materials transformation and energy flow driven by marine organisms and what are their roles in regulating Earth’s climate system (for example, the mechanisms of methane leakage, their scope, and their influence on global warming; the contribution of biogenetic sulfur to cloud formation and negative greenhouse effects; etc.)?

Fig. 2.8

Organic matter decomposition and transformation driven by microbes in deep-sea water bodies and sediments. After hydrolysis and fermentation, organic matter precipitates and begins to generate methane. Most of the methane is consumed by anaerobic oxidization before being released into water bodies

2.7 Multi-sphere and Multi-scale Interaction Processes and Physical Mechanisms of the Sun and Earth in a Deep-Space Environment

2.7.1 Sun and Earth Interaction

Solar activity can vastly alter the energy and state of Earth’s atmosphere and ionosphere and can induce spatial weather variations, leading to changes in the lower atmosphere and even on Earth’s surface. Solar activity is therefore a non-negligible driving element for global change and for terrestrial environmental catastrophes (Bothmer and Daglis 2007; Kamide and Chian 2007; Zhang et al. 2012). Research has established that multi-scale space weather effects are closely related to Earth habitability and have a significant nonlinear influence on aspects of the evolution, transmission, and integration of Earth’s multi-sphere systems. A shortage of empirical observations and numerical simulations means that many important questions relating to mutual sphere boundaries and dynamic structural boundaries are still to be resolved on both macro- and microscopic scales. With respect to perception of the multi-sphere activity of the sun and the Earth and their influence on terrestrial geological activity, essential researches will involve: determining the mechanisms by which meteorological systems, landform variations, and lithospheric changes stimulate atmospheric fluctuations; assessing the influence of crustal activity on atmospheric ionization and electrostatic fields; and investigation of electromagnetic connections and chemical diffusion processes between the lithosphere, atmosphere, and ionosphere.

2.7.2 Geological Activity and the Formation Mechanisms of Icy Celestial Bodies

Across the solar system, clear traces of a variety of geologic processes have been identified on the surfaces of icy celestial bodies, for example, impact meteorite craters, ice volcanoes, faults, mountain ridges, and chaotic terrain areas. From our current state of knowledge, we can identify the geological processes that created many of these features. However, the geneses of other features are still unknown. For instance, normal extensional faults are fairly common on ice satellites, such as Jupiter’s moon, Europa, but reverse compression faults are rare. There are large areas of chaotic terrains on the surface of Europa, but their geneses are entirely unknown at present. How do these surface structures form? What are the deep structures of the ice crust? What are the patterns and extent of internal convection beneath the ice crust? Do both superficial and deep ice circulation occur? What is the nature of the boundary between the lower surface of the ice crust and the salt-water ocean that is believed to exist beneath the frozen surface of Europa? Does subduction occur in the ice crust? What about plate tectonics?

With respect to other icy celestial bodies, essential questions are as follows: What are the geneses of ice volcanoes? What are the internal structures of these bodies? How many impact craters are there and how are they distributed? What were the origins of the meteoroids? Could internal structural features be produced by viscouse relaxation? Are there active processes and features, such as liquid methane lakes, on the surface of Titan, Saturn’s largest moon? What is the composition of Pluto’s surface mountains and how were they formed?

2.7.3 Study and Evaluation of Small Extraterrestrial Celestial Bodies, and Prediction and Prevention of Earth Impacts

The possibility of a small celestial body impacting the Earth presents a significant threat to human existence. In 1908, a relatively small meteor (30–50 m in diameter) exploded in the sky over Tungus, Russia, destroying over 2000 km² of Siberian forest. In 2013, another small object (only about 18 m in diameter) impacted the Chelyabinsk Area, Russia, (Brown et al. 2013) injuring around 1500 people and damaging over 3000 buildings. Nevertheless, actual meteorite impacts (meteoroids that strike the Earth’s surface) are comparatively rare, since ablation, ariel explosions, and disintegration generally destroy small celestial bodies as they traverse the atmosphere. The atmosphere therefore represents Earth’s primary defense against ground impacts. Studies of small extraterrestrial celestial bodies must determine the following: the occurrence rate of gamma-ray bursts, calculated independently of existing models based on historical observation; the evolutionary laws of celestial bodies in the solar system and the flux of small meteoroid impacts on the Earth; the filtration effect of Earth’s atmosphere on small celestial bodies and the historical incidence of meteoroid events; the nature of the two-way material exchange processes between Earth and interstellar space; the likelihood of meteorites of earth origin being found on the moon and Mars; and, of course, the available mechanisms for defending the Earth against small celestial body impacts.

2.8 Detection of Habitable Extrasolar Planets

What information from planetary observations and measurements is useful in determining whether a planet is habitable or not? Based on understandings gained from observation of the planets in our own solar system, four factors need to be considered to answer this question. Firstly, the planet must be neither too small nor too large. Planets that are too small will be unable to maintain stable atmospheres or magnetic fields, while planets that are too large are likely to be gaseous, like Jupiter and Saturn in our solar system. One rough measure is that planets within 0.1–5 times the size of Earth have at least the potential to be habitable. Secondly, habitable planets must be located in stellar zones falling within the astronomical definition of habitability, which is related to the characteristics of the stars around which the planets orbit. Thirdly, detection of atmospheric components in extrasolar planets (e.g., particularly water vapor, O₂, O_3, and CH₄) is clearly helpful in assessing the likelihood of a planet harboring life—or at least life similar to Earth’s. Lastly, whether seas, land, glaciers, and organic matter exist on the surface of extrasolar planets are useful pointers in considering habitability. Recently, liquid water and atmospheres (mainly consisting of hydrogen and helium) have been discovered on super-terrestrial planets. Whereas, it is still difficult to determine whether these planets have plate tectonics similar to the Earth. In the future, the fundamental question of “What are the basic conditions required for life to exist in the universe?” is one that must be answered. Our understanding of this most basic question is still evolving. It is now known, for example, that atmospheric air is not necessary for life in submarine environments, although, as far as we know, it is a universal feature of life on Earth’s surface. What, in fact, are the true necessities of terrestrial life? It is a fundamental question which must be answered if we are to successfully predict and detect life in deep space. The apparently essential conditions of life on Earth’s surface are not necessarily the essential conditions for life on other planets, or even in other spheres of the earth.

2.9 The Human-Earth System and Sustainable Development

Global climate change and anthropogenic activities have intricated plenty issues in environment, resources and ecology. These problems are continuously challenging the sustainable development of human society and the ecological civilization construction in our country. To resolve these problems, the critical basis is to comprehensively understand the interaction between human and Earth systems, and further to uncover the dynamic mechanisms of human-Earth systems and the mechanisms required for sustainable international, national, and regional development (Fu 2020).

The human-Earth system is considered to be a dynamic combination of human society and natural systems; thus, its structure and feature are different from either single system. The human-Earth system has many possible stable states, it allows conversion between different stable states when critical thresholds are surpassed. Understanding of the dynamic evolution of the overall human-Earth system and the feedback mechanisms between its subsystems will require study of the elasticity, adaptability, and transformative capacity of the entire system (Fig. 2.9). The human-Earth system has multiple challenging properties, including complexity, nonlinearity, uncertainty, and multi-sphere nesting. Driven by global change and human activities, both social and natural systems are now in accelerating states of dynamic variation. Description of the evolutionary laws of human-Earth system structures and functions, and uncovering the dynamic mechanisms that support them, will provide the scientific basis for maintaining and enhancing the elasticity and adaptability of the system and for promoting regional sustainable development.

Fig. 2.9

Preferred strategic areas for the study of the development of the human-Earth system

In the human-Earth system, the natural geographic system is the basic parameter of socio-economic dependency. The combination of the multiple elements and processes of water, soil, air and ecology determines the functionality of entire regions and their capacities to bear demands on their resources, which present as complete ecosystem services.

Long-term observation, field test, and numerical simulation studies are essential to understand the ecological structures and the relationship between the important ecological processes and its functions and services. This will enable clarification of how changes in ecologic system structures and processes affect the generation, transmission, and actualization of ecological system functions and services. Revealing the connections between ecologic system services and human well-being is the basis for comprehending the values of ecological system services, the demands that are placed on them, and their internal associations with human social well-being (Wang et al. 2013). To improve and optimize the ecosystem services, it is essential to know the linkage of the demands put on the ecosystem services and the social well-being, to distinguish the interaction mechanism between the dynamic changes of ecosystem services and human well-being and sustainability.

2.10 Global Environmental Change and the Evolution of Biology and Human Culture

Since Earth was born, the terrestrial environment has been continuously changing, driven by a combination of internal and external forces to form the fundamental environmental framework of Earth as it exists today. Since the Industrial Revolution, the influence of human activities on Earth’s environment have accelerated and intensified. The global biogeochemical cycle is affected by the changes to land surface system mainly induced from human activities (e.g., surface alteration and construction, and pollutant discharge). The ultimate aim of research on global environmental change is to avoid or mitigate the destructive impact of human activities on Earth’s ecology and environment while promoting the sustainable development of human society.

To achieve these vital objectives, future studies must focus on: (1) identifying the mechanisms by which human activities influence global environmental change, particularly in their effects on biogeochemical cycle between the multiple surface spheres and connecting media; (2) investigating the interactions and mutual influence of regional and global environmental change; (3) predicting the future influence of human activities on the environment and the reciprocal influence of environmental change on human society, especially for the prediction of the critical thresholds of destructive and catastrophic environmental changes; (4) developing strategies to allow human society adapting to the environment change and mitigating the impact of human activities on the global environment.

To study the co-evolution of life and environment, it requires intersection and deep integration of multiple disciplines. For example, what is the relationship of biodiversity to variations in the concentrations of CO₂ and O₂ in the atmosphere and ocean? How do deep-Earth activity and continental variations impact the evolution of Earth’s ecological systems? How do extreme climate events affect the stability of Earth’s ecological systems? What is the current and future influence of global warming on biodiversity in the ocean and on land? Can a sixth mass extinction be avoided? What are the differences in adaptability to environment change between biomes? How contemporary environmental change driving spatiotemporal variations in biodiversity? Current studies in biology, molecular biology, evolutionary developmental biology, and epigenetics indicate that environmental factors can both directly and indirectly influence biological phenotypes by controlling their genetic expression. Hence, besides the effects of environment as a major factor in natural selection, the interactions between genetics and the environment and their influence on biological evolution will become major research directions.

2.11 Sphere Interactions and Earth System Processes

The Earth’s plate tectonics are significant pathways for materials and energy exchange among Earth’s spheres and are essential to Earth’s evolution as a habitable planet. The study of sphere interaction is an effective approach for understanding the past, present, and future of Earth habitability (Fig. 2.10). For instance, Earth’s magnetic field, generated by a geomagnetic ‘dynamo’ process driven by Earth’s inner and outer cores, can protect our atmosphere from erosion by the solar wind. The dynamic processes of Earth’s interior maintain the oceans in which the complex submarine life systems generate and further originate life on Earth. In addition, at different evolutionary stages of the habitable Earth, the deep Earth processes and the connections between internal and external systems may have varied (Brune et al. 2017; Campbell and Allen 2008; Smit and Mezger 2017). This variation could increase the difficulty in explaining the mechanisms of interactions between interior and exterior Earth systems. To address this challenging issue, we can focus priory on the cycle behavior of the vital life elements (C–H–O–S) in the deep Earth at several major geological events. Specifically, its necessities to investigate the laws governing the cycle and re-distribution of the elements (C–H–O–S) in the deep Earth and underlying mechanisms. It is also vital to understand the exchange of materials and energy between the super-deep and upper zones of the Earth’s interior as well as their roles in both gradual global evolution and the abrupt change. The synthesis of multi-disciplinary studies, big data analysis and Earth dynamic simulation will clarify the mechanisms controlling the interactions between inner and outer Earth spheres and their effects on the planet’s habitability. In this way we will be able to construct theories which will reveal the deep controlling mechanisms of Earth habitability.

Fig. 2.10

Working model of linkage mechanism of internal and external Earth system

Materials circulation in the deep Earth may play a critical role in shaping Earth habitability, mainly through the long-term reconstructive effects of volcanic activity, plate tectonic activity, and fluid circulation in surface systems (Sleep and Zahnle 2001; Foley and Fischer 2017). Crust-scale fluid activity occurs widely in inner ocean plate before it enters subduction zones (Fig. 2.11). Oceanic subduction zones are sites for carbon exchange between the exterior and interior of the Earth. Some carbon enters the mantle, while the remainder is returned to the surface along with island arc magma (Kelemen and Manning, 2015). This process raises a number of questions: Does plate subduction contribute to the generation of convective driving forces in the deep mantle? How do carbon and water circulate in the deep Earth and on the surface? To get innovative thoughts for us to recognize the sphere interactions in Earth’s systems, we need to consider the interactions between planets and the interactions of multiple spheres on Earth, to explore the fundamental laws governing the periodic fluctuations of cold and warm conditions on Earth, to investigate major geologic events and their internal systemic relationships with thermal fluctuations in Earth’s evolution and to quantify the relative contributions of natural and human activities on global climate change.

Fig. 2.11

Schematic of submarine hydrothermal fluid circulation. Submarine hydrothermal fluids and cold springs are important channels for materials and energy exchange between the oceans and the oceanic crust. Hydrothermal activity mainly occurs in mid-ocean ridges, with extensive mineralization and development of hydrothermal ecological systems. In areas away from mid-ocean ridges, methane leakage in sediments is the principal factor in the formation of cold springs. The hydrogen and methane generated in serpentinization were the primary cold springs in Earth’s early history