1. Introduction

This review article synthesizes research from several disciplines by conceiving of the Earth as an open system significantly influenced by inputs of matter from outer space.[1] While this would not be a scientific revolution on par with the Copernican, it would nonetheless be a fundamental paradigm shift. A university level astronomy textbook published in 1978 stated, "Asteroids and meteoroids are other residents of our solar system. We shall see how, along with the comets, they may prove to be storehouses of information about the solar system's origin."[2] Treating asteroids as relics has been forcefully revised by the well-publicized findings that an asteroid impact triggered the extinction of the dinosaurs and cleared the way for mammals. In 1985, an eminent geophysicist commenting on mass extinction wrote of asteroids, "A fraction of these are large enough to survive the trip through the Earth's atmosphere."[3] But during the 1990s, research into interplanetary dust and micrometeorites revealed that a highly relevant question is whether the object is small enough to survive the trip (see section 3).

The current dominant paradigm is expressed by a recent Earth System Science text which explains that it covers:

"1. The cycles of matter (Earth is a closed system with respect to matter);

2. The flows of energy (Earth is an open system with respect to energy); and

3. The web of life (Earth is a networked system with respect to life.)"[4]

Yet almost simultaneously, geology professor Csaba Detre was writing that, "Today it is obvious that the main events in the Earth's history have cosmic origin, because the Earth is not a closed system, but a mirror of cosmic events, an inseparable part of the Universe."[5] And professors James and Jessie Miller, pioneers of system science, were writing that, "The Earth is an open system, interacting with its atmosphere and with matter and energy in space."[6]

Textbook thermodynamics informs us that an isolated system is one with a boundary that allows neither energy nor matter to pass. This is a concept that exists in the ideal realm, but a well-insulated thermos is an approximation. A closed system is one that allows the exchange of energy, but does not allow matter to pass. A battery is an example. An open system is one whose boundary passes both energy and matter.[7]

It is fundamental that the Earth is at least a closed system. The input of solar energy in the form of visible light and infrared radiation is essential for photosynthesis and a habitable planet temperature. Without these the planet would feature a severely hobbled biosphere, atmosphere and hydrosphere. The surface would be frozen, pitch-black, lifeless, with almost no wind. Life would still exist up to 10 kilometers below the surface[8] and around deep ocean hydrothermal vents,[9] which are the only places where water would flow freely, thawed by the Earth's internal heat. In these places chemosynthetic life, which derives energy from chemicals such as hydrogen sulfide, is at the bottom of the food chain. Conversely, the escape of excess infrared radiation prevents the planet from becoming an oven like Venus.

Recent thinking is pointing to the conclusion that perhaps the Earth is an open system with respect to matter- a direct reversal of point 1 above. Inputs of extraterrestrial matter into the terrestrial sphere may have made a continuing vital contribution throughout the existence of the planet. This manuscript assesses the following areas of open systems research: mass extinctions, interplanetary dust/micrometeorites, delivery of complex pre-biotic organic molecules from space, ice ages, small comets and panspermia.[10] It does not cover cosmic rays,[11] solar wind particles and the contribution of supernovae to mass extinctions[12] due to the need to limit the scope. The areas covered are very interesting findings that deserve to be thoroughly considered.