The Triple-Point Planet

Earth s silent siblings // eating acetylene on Titan I I three shields to guard the Earth // an arrow through the sky II summing it up with energy rate density II4 billion years of liquid balance H the Earth cracks, moves, and evolves // the limited palette of the first ocean 

Let s move to a vantage point a little quieter the surface of the moon. It is so still that Neil Armstrong s footprints remain undisturbed. The only reason the US flag there appears to fly is that a wire holds it up. 

The moon and Mercury stayed still as Mars, Venus, and Earth moved on down the road of geological development. The moon is a steady environment, a word whose Middle English roots are appropriately tangled with the word for sterile. Nothing moves on the moon, but in its sky Mars, Venus, and Earth move in their orbits, just as they moved on in complexity 4 billion years ago. 

Out of the whole solar system. Mars and Venus are the most like Earth in size, position, and composition. Mars is smaller, but Venus could be Earth s twin in size. If Earth and Venus were separated at birth, then something happened to obscure the family resemblance liquid water brought life. To chemists, liquid is the third phase of matter, between solid and gas, and its presence made all the difference. 

Mars gleams abright blood red even to the naked eye, while Venus is choked with thick yellow bands of clouds. Mars is cold enough to have carbon dioxide snow, while Venus is hot enough to melt tin and boil water. Earth s blue oceans and green continents provide a bright, primary contrast. These three siblings have drastically different fortunes. 

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The triple point is at a temperature of 273.16 K (0.01 °C1 by convention, and at a pressure of 611.73 Pa. This pressure is quite low, about /lee of the normal sea level barometric pressure of 101,325 Pa. The atmospheric surface pressure on planet Mars is remarkably close to the triple point pressure, and the zero-elevation or "sea level" of Mars is defined by the height at which the atmospheric pressure corresponds to the triple point of water. 

Earth is currently balanced on the hquid island east of water s triple point, but this balancing act was difficult to maintain. Any planet left out in the vacuum of space changes in temperature and pressure over time. Air pressure leaked away as hydrogen left the Earth and carbon dioxide was packed into limestone. Gravity and sunlight helped compensate for the loss, but stability can be elusive—just ask Venus and Mars. 

I walked out to the beach and stood at that thin place, between worlds. Something was within reach from each chapter in this story. I stood on the green serpentine rock, the one that may have supplied the chemical hydrogen to the first life on the planet. Waves pounded and surrounded the rock as I stood at a triple point between three phases of matter liquid ocean in front, gaseous air above, and solid earth below. 

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