Now with the tools that we've acquired for how to study a planet, let's turn our
attention to the three other terrestrial planets in the solar system, and see how
well they fall into place, with the way we have of
understanding how a planet works. first thing you want to look at is the
way they orbit and the way they spin. And, Mercury is an interesting case.
Mercury orbits very close to the Sun, so it's subject to very powerful tidal
forces from the Sun. We would not be surprised to find Mercury
locked into a resonance between spin and orbit, just as the Moon, is locked when
it orbits the Earth. It turns out things are a little bit more
subtle. And the reason is that Mercury's orbit is
eccentric. the following demo will explain what goes
on. Mercury's eccentric orbit means that it
can't quite synchronize spin and orbit, because the rate at which Mercury spins
cannot adjust to the rate of its angular motion around the Sun.
It moves faster around the Sun when it's nearer to the Sun.
It moves slower when it's farther from the Sun.
the way Mercury [COUGH] resolves this is that tidal forces are most intense when
it's closest to the Sun. So it essentially matches its rotation
rate to the rate at which it moves about the Sun near perihelion, when it's
closest. what this means is that, Mercury spins
faster than it orbits. And it turns out that the ratio that it
manages to do this in is a three to two ratio.
In other words, Mercury completes one and a half rotations about its axis every
time it orbits the Sun once. This allows its,
rate of spin to match the rate at which it moves around the Sun at its nearest
approach, and this is the way in which Mercury is tidally locked between
rotation and spin. It orbits the Sun every 88 days.
Venus orbits the Sun every 224 days. Its orbit is tilted very little, about
three degrees to the ecliptic. the rate at which Venus spins about its
axis was very hard to determine, because as we shall see, Venus is covered by a
deck of clouds that makes it impossible to the surface.
eventually radio, radio telescopes from Earth managed to bounce right off the two
sides of Venus, use the Doppler shift of the reflected light to,
managed to figure out the velocity which with the sides of the planets were
moving, and it turns out, surprisingly, that Mercury rotates about its axis very
slowly, once every 243 Earth days is one sidereal rotation.
And the spin is essentially opposite in direction to the orbit.
Mercury is almost unique among the planets.
In, well, completely unique in this sense, in spinning about its axis, at a
effective axis tilt of 180 degrees. Its rotation is not part of the original
rotation of the solar system. quite possibly some large collision early
in the days of the solar system flipped the planet over and gave it this angular
momentum in the wrong direction. Mars orbits the Sun every two years or
so. its orbit is a little more,
eccentric, than that of the Earth. its day however, is almost exactly an
Earth day, 24.6 hours, and its tilt angle is 25 degrees, its axis is tilted 25
degrees, so that seas, seasonal variations on Mars are somewhat similar
to those on Earth, although accentuated as I said by the, increased eccentricity.
it's interesting if you want that, on Mars, there's a north star like there is
on Earth. But the direction in which Mars's axis is
tilted is not the same as that on Earth. On Mars, Deneb would serve very nicely as
a north star. What do we learn by looking on the
surfaces of these three planets? Well, looking at Mercury,
we see evidence of cratering on the surface,
so we have a relatively old surface, and we see no evidence of tectonic
activity. We do see the graben and the
rilles, that reflect compression of the crust, the shrinking of the crust as the
interior cools. But we don't see any evidence of tectonic
activity. Observing the surface of Venus is
difficult for reasons that will become clear shortly.
But we do have radar imaging through the clouds that gives us an understanding of
the topography and are again, we don't see evidence of plate tectonics in the
sense in which this takes place on Earth. The entire planet seems to be one huge
plate if you wish. The crust is very strong and, does not deform very easily.
So we do not see the kind of features that we see on Earth.
the age of the crust of Mercury is, alarmingly uniform.
It seems that the entire planet underwent a global resurfacing maybe four or 600
million years ago. And the features that we do see that
indicate, the motion and the pass are indicated in
these reconstructions. These are not images from the surface, we
have precisely about three of those but these are
reconstructions of models from three dimensional, radar measurements of the
terrain. And since the terrain on Venus is
extraordinarily flat, these have been enhanced in the vertical direction by a
factor of about 22. And so this is a region called Ovda
Regio, and it shows you these sort of, compression ridges,
this complicated network of, compression ridges that occur when the surface is
shrinking and the sea, other area called Gula Rift shows you these large,
rifts, which, which indicate a extension motion.
And, what we see in the background are two huge volcanoes.
Mercury does exhibit, large volcanoes. the surface of Mars on the other hand,
is, heavily cratered, but only in one hemisphere.
There's this great dichotomy between the north and south hemisphere, suggesting
that perhaps the planet. is constructed out of two plates though
the origins of the dichotomy are not exactly clear.
And right along the boundary runs this huge canyon the Valles Marineris 1500
kilometers long. Dwarfs the.
Grand Canyon by a large margin. And this might be the fault line between
two plates would be reasonable. All three planets show evidence of
volcanic activity. one is not surprised to find that all
volcanoes on Mars are extinct. Mars is a small world,
and as we saw on the moon, one would imagine that Mars would cool more quickly
than the others. The fact that vulcanism on Mercury seems
still to be active, is, perhaps, explained by tidal friction.
The strong tidal forces of the sun are maintaining the heat in the planet.
So, judging from this, we can try to understand something about the interior
of the planet, so Mercury Based on what we know about its density,
it has a huge core. Its uncompressed density, in other words
if you relieved gravity, it would have almost the density of iron, 5300 kilo per
meter cubed. Essentially the density of earth would be the density of Mercurym if
you relieved gravitational pressure. there is a, the, some of the, core at
least must be liquid, because of features we observe in the dynamics of
the planet Also it has much too
Astronomers' surprise missions to Mercury discovered dipolar magnetic field so
there seems to be a geodynamo at work on Mercury.
That is still something that people are studying.
The conjecture for how it is that Mercury came to be, dominated by essentially a
large core with a very thin mantle is that back in the days of, heavy
bombardment, Mercury underwent a rather dramatic collision and, the effect of
this collision was, to essentially strip the outer layers of the planet leaving,
essentially an exposed core with a small, covering.
And so here's Mercury orbiting. Here is another,
proto-planet coming in. And the collision vaporizes the mantles
of both planets. these, are ejected and what's left is
basically an almost exposed core. This is our, conjecture to understanding
of why it is that Mercury is so dense and is so dominated by a large core.
What we know very little about the interior of Venus.
We can barely see the surface of Venus. As I said it's covered with clouds and,
we can see through them with radar but, we do not have good on-the-ground
measurements on Venus. We presume that the internal structure is
rather similar to Earth. The density and the size are similar.
But, Venus does not show any geodynamic magnetic field.
there are. Various conjectures as to why this might
be. one of them, as demonstrated in this
image, is that much of its core has already solidified, and therefore there's
no condensation of a liquid core and no room for convection.
It may be that temperatures throughout the core are uniform because resurfacing
of the crust, as we saw, has now basically sealed the crust completely.
There are no venting. There's no room for energy to be vented,
so the entire core is heated to a uniform temperature and there's no convection
currents going. On, our understanding of the interior
geology of Mercury, of Venus is incomplete in general. Venus is a hard
planet to study. moving on to our friend Mars Mars has a,
very low density, essentially, slightly higher than the.
Density of rocks. It has a mantle that seems to be
inactive. The interpretation is that its core is
small, it never accumulated lots of iron. but there are traces on Mars of the
existence of a geodynamo in the past. We can see the magnetic field frozen into
the rocks. we'll talk about why Mars is such an
underdeveloped planet in as, as we go on with the our investigation.
But the similarity in many ways between Mars and Earth, in terms of orbits and so
on, is belied by the fact that Mars is both smaller and less dense.
So for example, the gravitational field on Mars is much weaker than that on
Earth. This is what we see on the sur, on the
interior of the planets, this is what we know about it, we can move on and talk
about atmospheres. That was very important to our discussion of Earth
Mercury is an easy answer. There's not much of an atmosphere.
Mercury actually is a very dark planet. Albedo is much smaller than that of
Earth. on the other hand, absent an atmosphere
and with the slow rotation, you get this huge dichotomy between daytime and
nighttime temperatures. 700 kelvin on the day side.
100 kelvin, rather cold, in, on the night side.
And again there are craters that are in perpetual shadow.
And recently, it seems quite clear that there is substantial quantities of ice.
In Merc-, on Mercury, very close the sun, in these, shadowed craters.
A, a calculation, if you followed our
calculation for the mean surface temperature of Mercury,
treating it as, as a black body closer to the sun.
We would get 428 degrees. So the average is about right.
Venus is a more interesting object. Venus has an extremely dense atmosphere.
the mass of Venus's atmosphere is almost 100 times the mass of Earth's atmosphere,
which means pressure on the surface is about 100 times the pressure on Earth.
this corresponds to being about a mile underwater.
there is, the atmosphere is mostly carbon dioxide.
So we predict a substantial, greenhouse effect.
There are, this dense deck of clouds on the surface
of Mercury, of Venus. This is what Venus looks like.
It's covered with this dense deck of clouds.
The, these clouds are actually sulfur dioxide. They are not water ice clouds.
It turns out there's almost no water on Venus.
These clouds give Venus a huge albedo, lowering its temperature the temperature
is because of the thick atmosphere, almost uniform around everywhere on the
planet at about 730 degrees. notice that given that albedo, the fact
that only 10% of the Sun's, radiation, penetrates the clouds, our calculation
would have predicted 184 degrees Kelvin as the planet's temperature.
So this is a serious greenhouse effect going on.
We'll talk about this. these, sulphur dioxide clouds actually
lead to, lightning storms and rain. what rains is sulphuric acid.
the surface of Venus is a very harsh environment.
the Russian spacecraft, Venera which landed on Venus, survived for about 25
minutes, before succumbing both to a lightning strike and to the corrosive,
and high pressure atmosphere. And this makes studying the surface of
Venus a very difficult task. Mars, on the other hand.
It goes the other way. There is a very tenuous atmosphere.
The mass of Mars's atmosphere is half a percent of the mass of Earth's
atmosphere. And again the atmosphere is primarily
carbon dioxide, presumably the oxygen generating, processes happened on neither
Venus nor Mars. given, Mars's albedo of 25,. and its
distance from the Sun, we predict a temperature of 210 degrees Kelvin.
In fact, again absent a dense atmosphere, there are huge differences between day
and night. And, temperatures range from 308 on the
equator during the day to 130, near the poles.
there are these, very serious seasonal changes.
So it's 130 in winter at the pole, and 308 in summer at the equator.
Mars has large polar icecaps, made, we think, primarily of water ice.
Now, These are, this is a beautiful image,
There's some, carbon dioxide ices in the poles.
But mostly, there's an active hydrosphere on Mars.
again, with such a tenuous atmosphere, you don't expect liquid water,
but you do get ice and water vapor in Mars' atmosphere.
And these polar ice caps, sublimate.
The ice evaporates in the summer, to a large extent, and then reforms in the
winter, and we can follow the seasonal changes.
The atmosphere is very dusty, because the surface is dusty, and we have these huge
dust storms. And.
There are tantalizing hints of something like water, this sequence of images from
the Mars orbiter, shows something that. Were it on Earth, would definitely look
like water flowing. You cannot possibly have an actual liquid
water flowing on the surface of Mars. We saw that water does not exist under
such low pressures, but it may be that some heavily salted brines are able to
flow either on or just below the surface, and the, the attempt to understand
exactly how the hydrosphere of Mars operates is of course, a area of intense
interest about which we are hoping to learn from the current curiosity rover
and many Mars orbiting systems. The difference in these atmospheres,
the fact that, Venus has this very dense atmosphere of carbon dioxide, and these
extremely high temperatures. Mars has essentially no atmosphere,
and in between them Earth has an atmosphere, is something that's, worth
understanding. a comparative history of the atmospheres
of the three terrestrial planets will, probably be worth pursuing.
So we have to imagine that initially. All three planets would have lost
hydrogen and helium, and there would have been some outgassing of carbon dioxide as
rocks cooked. And there would be some water import as,
heavy bombardment, imported ice from the outer parts of the solar system.
So the initial conditions for Venus, Mars, and Earth are reasonably similar.
The differences are, the smaller gravity on Mars and the slightly different
distance from the Sun. But note, these are distances of a factor
of two at most. And so,
despite this rather modest change, this makes a huge difference in the history,
and it explains what I meant when I said there's this sensitivity,
to non-linear sensitivity, to details of the greenhouse mechanism effect.
So, on Venus, what we think happens is the water lands on the surface and the
planet is hot. the CO2 is generating a greenhouse
effect. The water evaporates.
Because Venus is closer to the Sun, the water never condenses.
You never get rain. Water just evaporates right up through
the atmosphere to the upper atmosphere, and there, ultraviolet light from the
intense sunlight, dissociates it. The hydrogen is lost to space.
The oxygen binds with, things like sulphur to make sulphur dioxide.
without rains, you don't build up oceans. In fact, the oceans of Mercy, of Venus,
evaporate. There's no tectonics.
There's no continents building up. There's no way to recycle carbon dioxide
into the rocks, which then
dump it back into the magma so the carbon dioxide atmosphere continues to build up
under volcanic activity out-gassing carbon dioxide from the mantle, and
there's no way to get rid of it. Once all of the water has essentially
boiled away and disappeared, you have a pure carbon dioxide atmosphere, and you
have a runaway greenhouse effect which boils away the oceans and leave a baked,
parched, high-pressure, high-temperature planet
This is a story for Venus. Turn the dial a little bit down from
where the Earth is and go out to Mars and start with it again.
Similar initial conditions. Here, water that evaporates, it's cooler
out on Mars. The water does rain.
This removes some of the greenhouse gases from the atmosphere.
Again Mars is not blessed with active plate tectonics.
So what this means is that gas that gets trapped in rocks stays there.
As Brain, er, er, removes carbon dioxide
from the atmosphere, water that is the planet is cool enough that water trapped
on the surface becomes trapped as permafrost we think, underneath the
surface. Or if it evaporates up, again, evaporates
out into space and is then dissociated by the ultraviolet light absent a strong
magnetic field, there is no protection, and the solar wind removes the remnant of
the atmosphere. In some sense you can describe the story
of Mars as a runaway icehouse effect in the same way that Venus's atmosphere is a
runaway greenhouse effect. If you start too hot you lose your
oceans. There's no water, carbon dioxide builds
up and the greenhouse effect takes away. You start too cold, the water freezes,
does not generate liquid water to trap carbon dioxide.
And again in both cases, the absence of tectonics makes the system unstable.
We are very lucky to live on a planet that splits the difference.