As I write these lines, Venus is setting in the west and Mars is rising in the east. They are part of a grand planetary observing opportunity that also includes Jupiter and Saturn. That opportunity will continue for a couple of months.
They are certainly worth a look because they represent just how lucky we are to be where we are.
Many planetary characteristics were necessary for the existence of life on Earth. One of them is the correct distance from the star the planet orbits. Our planet, the third from the sun, orbits a stable yellow-dwarf star that produces a relatively uniform amount of energy in its mature middle age. Earth’s orbit is nearly (but not quite) circular. It is thus bathed in a relatively uniform amount of energy over time.
Astronomers have dubbed that life-producing zone the Goldilocks Zone. You know, “This planet is too hot. This planet is too cold.” Our planet is just right at about 93 million miles away.
The GZ can be defined many ways, but the most obvious characteristic is that the sun heats Earth just enough to allow for an abundance of liquid water. Life on Earth was born from the sea, and as it moved to the land, it carried the sea with it. Thus, humans have been defined as “bags of water.”
Mars, the fourth planet, and Venus, the second, are both in the Goldilocks Zone, even if they do seem to define the outer and inner boundaries of the zone, respectively.
In fact, during the early part of the 20th century, it was widely assumed that both planets harbored life, but Mars captured the imagination of scientists and the public at large.
Percival Lowell saw evidence of intelligent life in the “canals” he spied on the Martian surface. The conception of alien life, which used to be called the “bug-eyed-monster” or “BEM,” we see in popular culture stems from the low Martian gravity, its distance from the sun, and its thin atmosphere.
However, Mars provides ample evidence that being in the Goldilocks Zone is a necessary but not a sufficient condition to sustain life.
For one thing, Mars suffers through extreme changes in temperature that make life difficult. At the equator, where the warmest and most life-giving temperatures exist, temperatures get as high as 70 degrees Fahrenheit, which would be more than adequate to make water liquid. However, at night the temperature can plummet to about minus 100 degrees. At the poles, conditions are even worse. The surface water there is frozen solid by the minus 200-degree temperature. It’s so cold, that carbon dioxide is forever frozen as dry ice.
Part of the problem is that the atmosphere of Mars is exceedingly thin. The atmospheric pressure on Mars is equal to less than one percent of Earth’s atmospheric pressure at sea level. From the point of view of human physiology, that’s the equivalent of being in the vacuum of space.
You’d think that the lack of oxygen would be the first thing to get you if you were on Mars. Hardly. Without a spacesuit, your eyeballs would squirt out your eye sockets, and the rest of you would eject from any orifice that wasn’t tightly closed.
Here’s another reason to feel extremely lucky to have such thick atmosphere. It holds in the heat from the sun like a thermal blanket and makes our surface temperatures relatively stable. Mars quickly leaks most of the sun’s heat into space, which accounts for the extremes of temperature .
Perhaps, some argue, Mars stood a chance for life when the sun was hotter billions of years ago. Perhaps. But Mars is only half the diameter of Earth and produces only about 38 percent of our planet’s gravity. That fact makes it difficult to hold on to an atmosphere for very long.
It took billions of years to go from one-cell life on Earth to complex forms like us. The atmospheric conditions for life on Mars were likely far too brief for that to happen. Its potentially thick atmosphere has long since spun off into space. Its surface liquid water, if it ever had any, has done the same or has been frozen into permanent permafrost at its poles.
Still, hope springs eternal. Radar scans from the European Space Agency’s Mars Express orbiter suggest a 12-mile-wide lake of salty, liquid water beneath the southern polar cap of Mars. It is in just such places where bacterial life could have survived over the billions of years since Mars had better conditions for life.
No such hope exists for Venus. Astronomers used to think that Venus was Earth’s twin sister. At about 8,000 miles wide, the planet is about the same size as Earth. At 67 million miles from the sun, it’s only 25 million miles closer than our planet. However, despite the similarities, we’ll find no life on Venus.
Venus turns out to be Earth’s evil twin. The problem is its atmosphere, which is mostly a very thick layer of carbon dioxide covered over completely with sulfuric-acid clouds. The air pressure is so great that an unwary astronaut who landed there would be crushed flat like a bug under a heavy atmospheric boot.
The clouds let in only about two percent of the sun’s light, which suggests that Venus should be cold, despite its proximity to the sun. The trouble is that the dense atmosphere is very efficient at trapping the heat from the sun’s feeble rays. On Venus, we find an out-of-control greenhouse effect with temperatures of 900 degrees Fahrenheit everywhere on its surface. You wouldn’t want to visit a planet where sulfur lies in molten, steaming pools on the ground.
Despite being in the Goldilocks Zone, neither planet has life of any kind, or at least so it seems.
As I watch Venus set and Mars rise, I contemplate just how lucky we are. Our planet is small — but just big enough to hold on to an atmosphere with its delightful mix of nitrogen, carbon dioxide and oxygen. It’s atmospheric pressure helps hold us together but does not crush us. The atmosphere is warm enough to give us an abundance of liquid water.
Although it might not seem so on a hot summer day or a frigid winter night, over large parts of the Earth, the temperatures remain relatively stable. Mess with any of these intricate balances, and your whole species will soon regret it.
Venus sets in the west, and Mars rises in the east. They are beautiful together, and we are alive to see them and realize their beauty. What a shame it is that we are intelligent enough to interfere actively with the delicate balance of factors that keep us alive. What a joy it is to be who we are and know where we are on our tiny, irreplaceable speck of the cosmos. It is that joy I feel as I watch Venus set in the west and Mars rise in the east.
Programs at Perkins
In August, the Friday night programs resume at Perkins Observatory. Come and observe Venus, Mars, Jupiter and Saturn, weather permitting. Programs start at 9 p.m. Advanced reservations are strongly suggested.
Also, during the program on Aug. 3, I’ll be giving my “farewell” talk during the program.
Please preorder your tickets by calling 740-363-1257.
Tom Burns is director of the Perkins Observatory in Delaware.