 Greetings and welcome to the Introduction to Astronomy. In this week's Misconception and Astronomy we are going to look at the planet Mercury and discuss if it is the hottest planet. We know it's the closest planet to the Sun, but we will find out that it is not the hottest planet in the solar system. So let's take a look at this. And what we find is that, yes, Mercury is indeed the closest planet to the Sun, but it is not the hottest. Now, you would think that temperature should decrease as you get farther away from the Sun, and that's true that less energy would be reaching each planet as you get farther and farther away. Now, this so you think that the planetary temperatures would decrease as well, and while it's generally true, this is not always the case. So let's look at what's happening here. When we have an object like Mercury, and in this case we're showing an image for our moon, but the idea is still the same. You have a certain amount of energy coming in and you have a certain amount of energy coming out. In this case, some of it is reflected and some of it is absorbed by the surface. And what you see is the exact amount that leaves is the same amount that is coming in. So the 342 units here are split into 318, which are absorbed. The 24 that are reflected go right back out into space. And those same 318, that amount is then radiated away. Now, while this says moon, the same thing would occur on Mercury. And Mercury can reach a surface temperature of over 300 degrees Fahrenheit. And you'll see a couple of different values here, 333 degrees Fahrenheit, 167 degrees Celsius, or in the scale that astronomers would use, 440 Kelvin. So we can see that it is a very high temperature. That's vastly hotter than anything we get here on Earth. However, we also have to look at other planets. And the planet that actually is the hottest is not Mercury, but is the second closest planet and that would be Venus. So let's go ahead and clear some of this. So let's take a look at the same kind of diagram for Venus. And actually, you'll see it says for Earth, but Venus would be comparable. So again, we have a certain amount of incoming radiation coming in. And then we have how much is coming in and out and back. Now, why is it so much more complicated for Earth or Venus? That's because there's an atmosphere there. So some of that material is reflected in the atmosphere. Some is absorbed by the atmosphere. Some is reflected off the ground. And then some heats up the surface and some is radiated out to the surface. And we do have the greenhouse effect that radiates energy back. Now, in all, everything pretty much balances and that keeps the temperatures relatively stable. However, Venus has a really thick atmosphere. Over 90% of the atmosphere is carbon dioxide. So it has a very intense greenhouse effect. And that means a lot of heat is trapped inside it. Now, Venus is constantly shrouded in clouds. And you may notice if you go out in the morning sometimes and you feel it feels very cold on, say, a nice fall or spring day and you look up and the sky is perfectly clear. That's because there were no clouds there to help hold in that energy and more energy escaped out into space, cooling the atmosphere in that small region. Now, on the other hand, if you go out on one of those days and it's very warm and stuffy, you'll generally see that everything is shrouded in clouds. Those clouds are better able to hold the heat in. And those clouds being made up of water vapor here on Earth are very good at holding in heat. So what this does to Venus is give it an extremely intense greenhouse effect, raising its temperature in this case to over 700 Kelvin or well over 850 degrees Fahrenheit. So if we look at the temperatures here, Venus is easily two and a half times hotter than Mercury even though it is nearly twice as far away from our Sun than Mercury is. And again, part of that is due to the fact that Venus has this thick atmosphere and intense greenhouse effect that help to heat the surface. So let's go ahead and finish up here with our summary and see what we've looked at. And what we've discussed is that Mercury is indeed the closest planet to the Sun, but it is not the hottest planet. We know that Venus has a very thick atmosphere composed mainly of carbon dioxide. And because of this, a runaway greenhouse effect on Venus makes Venus the hottest planet in the solar system. Now that said, and probably not and definitely not the hottest planet in the universe, we discover many other planets that are much closer to the Sun and could be even hotter than Venus in the universe. But within our solar system, Venus is easily the hottest of the planets. So that concludes this discussion on why Mercury is not the hottest planet in the solar system. We'll be back again next week for another misconception in astronomy. So until then, have a great day, everyone, and I will see you in class.