The Goldilocks Problem
DB: This is Earth and Sky for Friday, October 4 – the first day of World Space Week. Venus orbits one step closer than Earth to the sun. Mars orbits one step outward.
JB: Venus – whose surface is submerged beneath thick clouds – heats up to nearly 500 degrees Celsius – about 900 degrees Farenheit. Meanwhile, Mars is a presumably lifeless desert – in winter, much colder than the coldest day in Antarctica.
DB: But Earth is just right for life. And scientists have wondered why. Venus is closer to the sun, and Mars is farther away. But a complete answer involves carbon dioxide or CO2 – a “greenhouse gas.” Carbon dioxide keeps Venus, Earth and Mars all warmer than they would be otherwise.
JB: How each planet recycles CO2 between its surface and atmosphere is what makes Venus too hot, Mars too cold and Earth just right. On Earth, shifting land plates and active volcanoes help cycle CO2 into the air. Martian volcanoes became extinct long ago. Most martian CO2 stays in the ground – and that’s why Mars is so cold. And Venus gets more sunlight than Earth, but its thick clouds reflect most of it back to space. Without CO2 in its atmosphere, Venus would be colder than Earth – instead of hot enough to melt lead. Thanks today to the National Space Grant Foundation – on the web at spacegrant.org. We’re Block and Byrd for Earth and Sky.
Author’s Notes:
To convert Celsius to Farenheit, multiply by 1.8 and add 32. So, 470 C becomes about 890 F.
The following notes come from Dr. Paul Weissman, an astrophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, California:
AN IMPORTANT FACT HERE IS THAT THE EARTH HAS BURIED MOST OF ITS CO2. IT IS BURIED PRIMARILY BY LIFE – BY SMALL SEA CREATURES THAT INCORPORATE THE CO2 INTO THEIR BONES, AND THEN WHEN THEY DIE, FORM LIMESTONE BEDS. IF ALL THE AVAILABLE CO2 WAS RELEASED, THE EARTH WOULD HAVE ITS OWN STRONG GREENHOUSE, MUCH LIKE VENUS, THOUGH NOT QUITE SO WARM. THAT IS THE WHOLE PROBLEM WITH GLOBAL WARMING. BY BURNING FOSSIL FUELS, WE ARE RELEASING SOME OF THAT BURIED CO2 INTO THE ATMOSPHERE, RAISING THE CO2 CONTENT AND INCREASING THE GREENHOUSE.
AT THE SAME TIME, THE CO2 THAT IS IN THE EARTH’S ATMOSPHERE IS IMPORTANT TO KEEPING IT TEMPERATE, AS OPPOSED TO RELATIVELY COOL. THE AVERAGE TEMPERATURE OF THE EARTH, FROM SUNLIGHT ALONE AND WITH NO CLOUDS, SHOULD BE ABOUT 280 K, OR ABOUT 7 C, ABOUT 45 F. WITH THE CLOUDS, WHICH REFLECT BACK ABOUT 30% OF THE SOLAR ENERGY, THE AVERAGE TEMPERATURE SHOULD BE ABOUT 256 K, OR –17 C, OR +1 F; VERY COLD!!! SO THE CO2 GREENHOUSE IS IMPORTANT IN WARMING THE EARTH BACK UP. BUT IT IS NOT TOO WARM, OR THE EARTH WOULD BE UNINHABITABLE LIKE VENUS.
SO THE ANSWER IS THAT VENUS HASN’T BURIED ITS CO2 AND HAS AN INTENSE GREENHOUSE. THE EARTH HAS BURIED MOST OF ITS CO2, BUT HAS JUST ENOUGH IN THE ATMOSPHERE TO MAKE IT TEMPERATE RATHER THAN FROZEN. MARS HAS ALSO BURIED (OR LOST) MUCH OF ITS CO2, AND THUS DOESN’T HAVE ENOUGH TO WARM IT UP. OVERLYING THIS IS THE FACT THAT VENUS GETS TWICE THE ENERGY FROM THE SUN THAT THE EARTH DOES (BUT REFLECTS MORE OF IT BACK FROM THE CLOUDS) WHILE MARS GETS LESS THAN HALF AS MUCH AS THE EARTH, AND HAS FEW CLOUDS TO REFLECT ANY BACK.
