As I write this article (a week before it comes out), it’s a frosty record-tying 8 degrees out my window in French Camp. The coldest recorded temperature in Mississippi was in Corinth in 1966, minus 19 degrees. For the US, the record coldest temperature was set in Alaska in 1971, minus 80 degrees. For the Earth, the record was set in Antarctica in 1983, minus 128.6 degrees.
This brings me to wonder about why the Earth is considered the “Goldilocks” planet in our solar system. We’re not too hot and not too cold. We’re “just right”. Let’s look at why.
A planet’s average surface temperature depends on several variables. The most important thing is distance from the Sun. The closer to the Sun, the warmer you’ll be. This is because the amount of energy received depends strictly on distance according to the “inverse square law”. Go twice as far from the Sun and you get exactly one-fourth the energy. We know the luminosity of the Sun, so we can calculate how much energy any planet will get.
A second factor is the reflectivity of the planet. If a planet were pure white, it would reflect all the Sun’s light and absorb none of it. A completely black planet would absorb all the energy it receives. No planet has 100% or 0% reflectivity. The Moon and Mercury are quite dark at 12% reflectivity. Venus is very bright at 75% reflectivity. Earth reflects 29% of the sunlight energy it receives, and absorbs 71%. When the expected surface temperature of each body is calculated, based on solar distance and reflectivity, you don’t get the actual surface temperatures measured by space missions! Something is missing.
Several additional factors need to be considered to get better predictions. For Mercury and the Moon, you have to take into account their rotation rates. They rotate so slowly that they get extremely hot during their weeks-long sunlit days and bitter cold during the long nights.
For Venus, Earth and Mars, the discrepancy in predicted versus measured temperature is due to their having atmospheric warming. Mars has a thin carbon dioxide atmosphere, so it only traps a little heat that raises the predicted surface temperature by only 10 degrees Fahrenheit. It’s still very cold though, since Mars is so far from the Sun. Venus should be a cold -40 degrees due to its high reflectivity, but its thick atmosphere of heat trapping carbon dioxide raises its actual temperature by over 900 degrees. That’s extreme “greenhouse” heating!
How about our Earth? With our medium reflectivity and distance from the Sun, Earth’s average surface temperature should be about 3 degrees Fahrenheit. Brr! Fortunately, our tiny 0.04% of greenhouse gasses in Earth’s atmosphere raises this by 56 degrees to a comfortable average of 59 degrees. A trace of CO2 is a good thing, but even a little change and we could be back in an ice age or a hot house climate. Geology tells us that this has happened several times in the past. Science gives us knowledge to “husband” the Earth. Do we have the wisdom/courage to use it?
Questions or comments: James Hill, NASA/JPL Solar System Ambassador, jhill6333@gmail.com
