What made the earth reach an average temperature of 20 degree celcius if it was a hot blob of molten mass?

Answer 1

You are right in stating that the earth began as a largely molten mass of rock. That's why the first billion or so years were known as the Hadean period, as the surface temperature was well over 1000 degrees C.

Why has the earth cooled? Simple, it radiated the heat away into space. If it wasn't for our sun, earth would be a lot like pluto, basically barely above the 3 Kelvin background temperature of intergalactic space.

The reason the earth has a nice, pleasant 20C surface temp is that there is a nice dynamic balance of incoming radiation and outgoing radiation/reradiation. Essentially, the earth absorbs relatively short wavelength light and reradiates it back into space as long wavelength light. So, although the earth may be radiating heat away at an average of 600 Watts per square meter, we also receive an average of 600 Watts per square meter of solar radiation.

By the way, by radiation I do not mean ionizing radiation like gamma or x-rays, I mean radiant energy. Visible light, infrared light, radio waves, and even gamma rays are all forms of electromagnetic radiation. However, in this case, the radiant energy involved is ultarviolet, visible, and infrared light, though we do receive a SMALL amount of x-ray and gamma from the sun (if it wasn't small life couldn't exist!).

So anyways, to answer your question, the loss of heat to take the molten rock down to less than 20C was radiant heat loss over a period of a billion years. However, most of earth's rock is still molten! Only the relatively thin crust (~15-100 miles thick) is solid, the rest is either molten (mantle), or very hot but under extreme pressure and thus a superheated solid (core). In fact, by now, the earth should have radiated away nearly all of the heat needed to cool the mantle to solid, and thus there is a major scientific effort underway to understand why this is the case.

Answer 2

Well, the earth gives off (loses) much more heat than it recieves from the sun, so gradually over millions of years the earth became solid (mostly). The solid crust is a very good insulator and protects the surface of the earth from the intense heat of the inner earth, allowing the surface to be a comfortable 20 degrees on average.

edit:
I'm tired of people saying the earth is mostly molten. The mantle is not molten - it is solid rock. It is very hot and there is a lot of pressure so the solid rock will deform sort of like silly putty - but it IS NOT MOLTEN. The only compositional zone of the earth that is molten is the outer core - which is composed of liquid metal. That is the ONLY part of the earth that is liquid (besides the oceans) - the inner crust is (mostly) solid metal, the mantle is (mostly) solid rock, and the crust is (mostly) solid rock.

Answer 3

Heat travels in three ways: Radiation (which is the conversion of thermal energy into light), conduction (as when two objects touch one another), and convection (when a fluid medium carries heat away, like cooling off with a fan). Since the earth is isolated, the latter two are not possible. The answer is that the heat radiated away from the surface.

All hot bodies emit light at a frequency that depends on their heat. When you see infrared vision, with thermal sensors, this is a way of visualizing this light-energy which is normally at a frequency too low for human eyes to see directly.

-Apologies to the next answerer, I was misinformed on the composition of the earth's interior.

Answer 4

The average temperature of the planet is a little under 15°C not 20°.

There is only one "c" in Celsius. Anders Celsius was the Swedish scientist who invented the scale.

Answer 5

It's a cold universe out there. Heat flows from high concentrations to low concentrations.