Heaven and Hell
The second, third, and fourth planets nearest the Sun are all rocky, terrestrial worlds with atmospheres and similar surface gravity. They are represented here by metalic plates set into an artificial rocky surface, at a museum on the third planet.
Venus (right), has an atmosphere some ninety times denser than that of Earth (center), and it is so choked with carbon that a rampant greenhouse effect has raised the planet's temperature far too high for anything like life. When inhabitants of the third planet, Earth, wished to send instruments to Venus, they had to engineer their housings to cope with unimaginable pressures. On one such craft, an immense carbon diamond was carved into a window so that one of its instruments could "see out" from the spacecraft without succumbing to the presures and chemically torturous environment, too quickly.
The Earth, accompanied by its binary, the Moon, stays somewhat more liquid in its interior due to gravitational tides. It's rotation and liquid interior has produced a magnetic field which protects the atmosphere and surface from harmful solar rays. Whereas Venus has carbon in its atmosphere, the Earth has much more of it's store bound up on the surface in rare symbiotic lifeforms. Plants and animals are made of water and carbon, and a trace amount of a few other elements. Plants consume the carbon dioxide in the armosphere and, with liquid water and sunlight, metabolize it to support their growth. The parasitic animal life has arisen to consume the products of this process--oxygen and plantstuff--to support its own metabolism (which produces more carbon dioxide for the plants). Life thrives here, and while the plants depend on a certain amount of atmospheric carbon, if it is allowed to reach critical levels, the greenhouse effect may accelerate to the Venusian levels. Recently, an upstart species of animal life has come about that is dramaitically shifting the balance of this relationship. It is rapidly destroying the plantlife, and meanwhile producing forms of atmospheric carbon that are not only unusable to the plants, but harmful to the animal life as well.
The fourth planet from this star in this system, Mars (left), has two small companion satelites which are not represented here, and which are too small to provide a significant gravitational tidal force like the liquid Earth enjoys. Its magnetic field, while once present, is now extremely weak. Mars is large enough and heavy enough to keep an atmosphere, like the Earth and Venus, but without a magnetic field to protect it, solar wind has stripped away much of what it had. The orbit of this planet is near enough to its sun to keep liquid water on its surface, but without atmospheric carbon to lock in heat, liquid water is impossible. The water of mars is frozen to its poles. If Mars had plant life--and there are a few places on the Earth, which are also supporting life, that are nearly as cold as some of the warmer places on Mars--then Mars would have a means of absorbing more sunlight (with dark plant matter), and producing more atmospheric carbon to trap more warmth. Plant life is precious.
Thank you Carl Sagan.