Conclusions The Known and the Unknown

Geologists have walked over and inspected the vast majority of the Earth's surface, mapping and describing its rocks, mapping its structures, and collecting samples for analysis and dating. Seismologists measure earthquakes and use the data to understand the structure of the Earth's interior. Petrologists and mineralogists have measured the compositions of terrestrial rocks and meteorites to estimate the composition of the Earth's interior and the timing of its formation.Though far more is known about the Earth than about any other planet, many questions remain about its formation, evolution, and current processes.

1. What will be the future of the Earth's climate, and what will it mean for life on Earth?

Over the history of the Earth, even the recent history of Earth, temperatures have fluctuated violently. Initially this may seem a comforting fact in the face of the dire warnings about mankind's contributions to global warming, but historical temperature fluctuations appear to have wreaked havoc on the Earth's weather and climate. The half- to one-degree increase in average global temperatures that mankind's chemical contributions have thus far added seems already to have added violence and irregularity to weather patterns. Beyond this immediate, controllable concern, long-term and very long-term climate changes caused by orbital variability, solar output changes, and other effects will also change climate in the long term.

Life on Earth has adapted to many unusual circumstances. Single-celled organisms known as extremophiles have adapted to life in otherwise entirely inhospitable places. Some bacteria, called chemolithoautotrophs ("those that live on energy they extract themselves from rock or inorganic chemicals"), live with less than 1 percent oxygen and without sunlight and derive their energy from methane, manganese, iron, or even arsenic. Some eat sulfide minerals and excrete sulfuric acid. Some bacteria live meters or even kilometers deep in bedrock; they may make up half of the Earth's biomass. Bacteria on Earth can live in water with a high-acid pH of 2.3, such as the Rio Tinto River, (the pH of most water is around 7); the organism Ferroplasma can even live at pH equal to 0. Extremophiles on Earth have been shown to live at temperatures ranging from -4°F (-20°C) to 250°F (121°C), that is, from well below freezing to well above the boiling point of water. On Earth life has adapted to every environment except high temperatures still, and extreme dryness.

These examples of single-celled life show that the most extreme climate changes will not easily deter Earth life. Mankind, however, has greater needs for life than do these single-celled organisms. To retain the societies and habits of the developed world and to continue to spread technology, health, and food to emerging countries, mankind must act to limit its own damaging effects on climate.

2. What are the interactions among large igneous provinces, giant impacts, and extinctions?

Though other planets and moons are covered with impact craters from meteorite bombardment, the Earth has relatively few recognizable craters. They have been eroded away over time. Because there are so few to be seen on the Earth, it took centuries for scientists to agree that giant meteorites have in fact struck the Earth. The reigning paradigm for the previous three centuries had been gradualism and uniformitarianism: the ideas that Earth processes happened gradually and incrementally over vast amounts of time, leading in the end to the dramatic formations seen today. Now scientists think there are several good examples of the converse, catastrophism. Sudden catastrophic processes that alter landforms include meteorite impacts, volcanic explosions, giant landslides, earthquakes, and storms. Giant impacts, in fact, are the only known natural disaster that has the ability to entirely sterilize the Earth of life.

Throughout Earth history there have been a number of large extinctions of multiple species, most notably, the extincation at the end of the Permian, at 250 million years ago, when 90 percent of the species on Earth went extinct over a geologically brief time, perhaps a few million years. This extinction, the extinction that killed the dinosaurs at 65 million years ago, and several other extinctions all occurred simultaneously with another phenomenon, eruptions of large igneous provinces. Perhaps 10 times through Earth history a large outpouring of basalt occurred onto a continent, an event in which 1 or more million cubic kilometers of lava poured onto the crust in less than a million years. Several of these large igneous provinces appear to coincide in time with extinctions. The processes that create large igneous provinces are not fully understood, and some scientists believe they may be triggered by giant meteorite impacts. The reasons for large igneous province development, the exact processes that cause extinctions, and any possible links between them and to giant meteorite impacts are not well understood, though they are the topics of heated debates at scientific conferences. While the extinction that killed the dinosaurs is clearly linked to a large meteorite impact, other extinctions have not been explained.

3. Why is the Moon asymmetrical?

The Moon's crust is, on average, thicker on its far side than on its near side. The most obvious large impact basins are on the near side. The vast majority of mare basalts are on its near side, as it the largest quantity of radiogenic elements near the lunar surface. What caused this asymmetry in lunar formation? The early development of the asymmetrical crust may have influenced the other asymmetries, in that the crust on the near side may have been just thin enough that giant impacts could allow the basalt to erupt. Even if this is the case, a reason for the asymmetrical formation of the crust needs to be found.

Even this most studied planet and its large, close Moon are not thoroughly understood. The Earth and Moon had a common formation when the early Earth was struck by a giant impactor, about the size of Mars, and the resulting spray of superheated material partly settled back onto the Earth and partly reformed to make the

Moon, or at least this is believed to be the case. As effectively as scientists can look into the most distant past of the solar system and infer the timing of core formation for the Earth, the process of magma ocean crystallization for the Moon (and quite likely for the Earth as well), and as effective as researchers are at learning about the Earth's mantle, slow-moving processes, atmospheric movements, and climate changes over time, they do not know with any assurance conditions in the Earth's deepest interior, or the complexity of interactions among the weather, climate, oceans, and orbit of the planet, or about the deeply complicated and critically important effects that mankind is having on the Earth's ecosystems and climate. Indications are unequivocal that mankind is damaging the environment and endangering the planet through climate change.

On a geologic timescale, none of this really matters: The Earth will continue to orbit the Sun, plates will continue to move, and heat will slowly leave the planet and move into space. The large-scale processes continue unabated; from a cosmic distance nothing will change. From a human perspective, however, the big picture is unimportant and only the details really matter. If humankind's changes cause more violent hurricanes and a lack of protection from dangerous solar radiation, then coastal houses will be repeatedly destroyed, lives will be lost, and cancer rates will soar. If global temperatures rise, then ice caps will melt and continents will flood. Life as it is now will no longer exist. Even if catastrophes of this size have occurred in Earth's past, the only catastrophes that will matter are those happening to humankind now.

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