Why is Mars so cold?
JB: This is Earth and Sky. Bentley Walton in North Carolina writes, “If carbon dioxide is a greenhouse gas, then why is Mars, which has an atmosphere predominantly made up of CO2, so cold?”
DB: Mars is colder than Earth. While temperatures do rise above freezing during the martian summer, they’ve been known to dip to minus 100 degrees Celsius – that’s about minus 200 degrees Fahrenheit – at the winter pole of Mars. Mars is cold relative to Earth because it’s farther from the sun – we’re the third planet out – Mars is the fourth. But it’s also the case that Mars lacks significant global warming – even though its atmosphere has plenty of carbon dioxide, or CO2.
JB: The greenhouse effect is created when gases like CO2, ozone and water vapor trap solar trap heat that strikes a planet’s surface. It’s a lot like when a pane of glass traps heat inside a greenhouse. On Mars, this greenhouse is relatively flimsy. That’s because the atmospheric pressure on Mars – the density of the gases – is very low. Mars’ atmosphere is mostly carbon dioxide, but this martian air is so thin that there’s no significant greenhouse warming.
DB: On Earth, artificial production of greenhouse gases enhances a natural greenhouse effect. But without a little greenhouse warming, Earth’s surface would be frozen solid. Thanks for your question, Bentley, and with thanks to the National Science Foundation, we’re Block and Byrd for Earth and Sky.
Web links to related information:
“Nine Planets – Mars”
http://seds.lpl.arizona.edu/nineplanets/nineplanets/mars.html
University Corporation for Atmospheric Research learning module
“”Global Warming: Focus on the Future””:http://globalwarming.enviroweb.org/ishappening/greeneffect/index.html by Knauer Communications
The following individuals were interviewed for today’s show. Our thanks to:
Dr. Richard Young
Research Scientist
Space Science Division
NASA Ames Research Center
Moffett Field, CA
Author’s Notes:
Mars mean surface temperature is ~ 53 degrees centigrade
Earth’s atmosphere has 3/10 of 1 millibar of CO2; Mars’ has 6 millibars, but if you doubled the amount of atmospheric CO2, it would only raise the global temperature 1–2 degrees centigrade
Venus also has an atmosphere made up completely of CO2, but it has 100 bars. Venus’ greenhouse effect creates surface heat (450 degrees C.) capable of melting lead.
Mars has 20X as much CO2 as Earth, but Earth has 200X as much atmospheric pressure due to the presence of other gases.
