Expert: Jayendra Upadhye - 10/16/2004

Question
The planet Saturn appears yellowish to the unaided eye, while the planet Mars appears reddish. The reddish surface of Mars is much warmer than the yellow clouds of Saturn. Yet the full spectrum of electromagnetic radiation emitted by each is a good match to the readiation expected from a perfect blackbody. How can these factors be true without contradicting one another?

Answer
Hi Sara,

The contradiction is because, apparent color is decided by "reflection" in case of the planets.

While black body emissions for planets are essentially infra-red in nature (thermal emissions)!

Now by definitions,
The black body is defined as under:
black body:-
-------------
An ideal or imaginary body that is absolutely black when cold, but is a perfect absorber of radiation and at the same time a perfect radiator. Black body temperatures are used to work out the theoretical laws of radiation and to calculate the temperature of the sun.

In case of the planets, the "black-body" part of the emission is in the infra-red!

For mars to glow red based on temperature, mars would have to have an 800 deg centigrade surface temperatrure! the red we see on mars is "reflected" light. Not part of the emissions spectrum related to temperature.
That applies to Saturn too.

If you were to look at the infrared area of the emissions spectrum for each of these two planets, you would see that that these two would exhibit plank's curves peaking at their average temperatures. [plank's curve].

The Sun is approximately a black-body at a temperature of about 6000K .
Planets emit most of their infrared energy as black-bodies, and the peak of the spectrum tells us the planet's average temperature.

pls use this link that has a beautiful java applet to plot a planck's curve for a wide temperature range:
http://csep10.phys.utk.edu/guidry/java/planck/planck.html

pls use this link for the black body curve:-
http://csep10.phys.utk.edu/guidry/java/blackbody/blackbody.html
Try guessing what temperature is the blue plot and enter the temperature in the textbox, to see if the curve "fits".
I could guess in 5 tries. (a perfect fit!) I used successive approximation.
Jayen.