Video Transcript: Mercury

Now Mercury is the closest planet to our sun, that's why we're looking at it first, but you can see it in our sky.  Now, Mercury is one of the trickiest planets to see because it is so close to the sun.  As you can imagine, if here's the sun and Mercury's in a tight orbit around it, well then that means Mercury is only really going to be visible in the daytime when the sun is up, or just after sunset, or just before sunrise because it's got to be close to the sun in the sky.  And that's what makes it so difficult to see.  You'll never really see Mercury when it's dark out; it's always going to be kind of this Twilight when you see Mercury. 

So the first picture in our gallery is a picture of Mercury but it's not the bright star like thing you see in the picture - that's actually Venus.  Mercury is the fainter star like thing you see next to Venus - that would be Mercury.  And so many people don't even know that they're seeing Mercury when they do see it because you really got to know to look for it, but when you see it, it's relatively bright but it looks just like a star, like all the other planets do.

Seeing Mercury has not been easy, you know, you might think, to send a spacecraft to Mercury would be really simple, but it's actually not.  And another challenge with Mercury is that it's really quite small. In fact, if you were to compare the size of Mercury to our own Earth, or the Moon, you'd see that Mercury is actually just slightly larger than the Moon.  And it's really far away, so when we try to take pictures of it through telescopes, you really can't see any details.  Even with really high, powerful telescopes like the telescopes in space, Hubble Space Telescope, you can't capture really great pictures.  So how can we learn what Mercury actually looks like?  

Well, we send spacecraft to actually pass by Mercury or to orbit around it.  Now the very first spacecraft that ever visited Mercury was in the 1970s - this is the Mariner mission.  And what they did is they just flew past it; they didn't even orbit it, they just flew past and captured a bunch of pictures of the surface of Mercury while they flew past.  And these were the very first images that were ever captured of the surface of Mercury.  This is what it looked like.  And you can kind of tell, this is the third picture in the gallery, you can kind of tell that this larger picture is really a mosaic of smaller pictures that have been kind of stitched together.  Well, these are all pictures that were taken by these Mariner missions.  And for a while this was this was pretty much all we could see of the surface of Mercury.

When you look at this, what does it remind you of?  My guess is it makes you think of the moon, right, because the surface is pretty much completely barren.  It's just totally covered in craters.  And in fact, that's a pretty good analogy, Mercury and the moon, because neither of them have an atmosphere. You know, we would see an atmosphere and the atmosphere would cause a wind that would erode these craters, and neither of them looks to be geologically active.  And by that what I mean is on the Earth, you know, we have an active planet, there's molten rock beneath the surface, this magma which is always moving, and that causes the tectonic plates of the earth to be moving.  It causes volcanoes to erupt and make new land.  And so when we get a crater on the Earth, and eventually it's covered up or subducted beneath the surface; things are always changing.  While on places like Mercury, it's really what we'd say is geologically dead, that is, there's no magma right beneath the surface that's moving the surface around, it's not reshaping and covering up these craters, and so it looks like not a lot has changed on the surface of Mercury for a really long time.  

Now these weren't the only pictures that we've ever captured of the surface of Mercury.  We have more recent pictures - I'll show you an example of that in just a minute.  But Mercury is a very strange place for some other reasons. Being the closest planet to the Sun, it's one of the hottest places in the solar system.  

As you can imagine, you're really close to the sun, you're getting baked by that heat all the time and so it can reach temperatures well over 400 degrees Fahrenheit, which is about 200 degrees Celsius.  But it doesn't just sit there with day and night like we experience.  It has an unusual rotation.  And so it's kind of tricky to understand but the gravity of the sun is so strong and Mercury is so close that the same side of Mercury wants to face the sun most of the time, not all the time, but there's a relationship between how Mercury orbits the Sun, and how it rotates on its axis.  

And we see this with Earth's moon as well.  We always see the same side of the Moon. Think about that. When you look at the night sky and you see the moon, it doesn't look like it's rotating, you always see the same side, because the moon is gravitationally locked to the earth. And so as the moon orbits around, it rotates slowly, and the same side is always facing the Earth.  

Well, Mercury does something very similar with the sun except Mercury rotates around twice for every like three times it orbits.  So it has this weird relationship.  But as a result, the day on Mercury is very, very long, much longer than a day on Earth because it rotates so slowly as it's kind of locked to the sun's gravity.  As a result, a day takes an enormous amount of time on Mercury.  And so the sun is shining for like 80 days the sun is shining on Mercury, and then the sun is down for like 80 days.  But because of this weird motion, where it rotates as it orbits, the sun takes a really weird path.  

So if you were if you were standing on the surface of Mercury, and watching the sun rise, you know, you'd expect it just to rise go across the sky and set but it doesn't do that.  Because of this relationship between the rotation and the orbit, from Mercury the sun appears to rise and then it kind of stops in the sky for a while, kind of goes backwards a little bit, and then keeps going and sets.  So it's like it does this retrograde motion that we see of the planets.  Very bizarre, very bizarre.  Now, oh, and I can show that to you.  I'm sorry; the fourth picture in the gallery kind of illustrates this bizarre relationship where the rotation, you'll see from one to two, Mercury has rotated about a half a day, from two to three, it's rotated all the way around now for a full day, and then four to five was the second day. And then from six back to one was a third day.  So there's this relationship like two years on Mercury matches three days on Mercury. Very weird. Very weird. 

Now we have better pictures of Mercury than just from the Mariner spacecraft.  Recently, like in the past 10 years, a new space mission was sent to Mercury that would pass Mercury several times and then eventually enter into an orbit around Mercury so that it could take pictures of the whole surface of the planet.  And what we see is some really cool things.  So here's a more up to date picture showing at least one side of the surface of Mercury.  And you can see it looks an awful lot like the moon.  

There's one or two surprising features on Mercury.  One of them is called the Caloris Basin, and it appears to be an enormous impact crater from a long, long time ago; so big that you can just see the boundary of it, barely; it’s covered up with other craters but we can see this ancient history on the surface of Mercury as it's been impacted by all these craters.

Now it's worth maybe taking a minute just to mention the value of these craters.  You know, when we look at the moon, or when we look even at Mars and some of the other planets, but especially at Mercury, the craters can tell us something about how our solar system has changed over time.  What we noticed when we look at craters is that we see the biggest craters that are there tend to look older.  And what do I mean by a crater looking older?  Well, there's a couple things. 

One is that the edges are less well defined, it looks like they've kind of eroded away over time.  You know, if you were to take a golf ball, and or a ball and just drop it in the sand and then take it out, you'd see that there's a nice circle in the sand, especially if it's wet sand, you'd see a perfect circle there.  But if you came back a week later, chances are you might still see a little divot, but the edges are going to have eroded away. Well, you see the same thing on the surface of a planet.  Some craters look relatively new and some of them look relatively old.  The other thing that you can see in craters is that craters land on top of craters, which land on top of craters, and so you'll see a large crater with several smaller craters around it.  And that's a general pattern that we observed throughout the solar system.  The first craters, the oldest craters are big, and the newest craters are small.  And what that suggests is that earlier in our solar system, there were bigger objects flying around that were running into the planets, and more recently in our solar system, the objects have gotten smaller and smaller.  So maybe these big objects have all gotten kind of swallowed up by the planets and now all that's left are smaller objects flying around in space. 

So by looking carefully at the surface of a planet you can try to understand some of the history of the surface of that planet and in turn some of the history of our entire solar system. 

Pretty cool. All right. Next time we're going to take a look at Venus.