Planetary Observations

 

Part I

 

The next property to discuss is temperature.  As a consequence of making radio observations of the planet, we learn its temperature.  Also the planet, because of its surface temperature of a few hundred degrees, must emit infrared light.  So if we observe the infrared spectrum, we can use the blackbody principles to learn the temperature.

 

More interesting is the variation of the temperature.  There are two types of variations that will concern us: Annual (seasonal) and diurnal (daily) temperature changes.  The causes of annual temperature variation are

 

 

The Earth has seasons by virtue of the inclination of the pole (23.5°).  Several planets have similar inclinations and thus have seasonal temperature variations like Earth.  Others have inclinations that are very small and thus have no seasons.  Only Mars, Mercury, and Pluto have eccentricities large enough to introduce significant annual temperature changes.

Diurnal temperature variations are affected by:

 

 

To see the effects of rotation consider the Earth and the Moon.  Both objects receive the same amount of sunlight, but the Earth rotates in one day whereas the Moon takes one month.  Normal diurnal temperature changes on Earth are 20-25 °F, but on the Moon the change is 450 °F.

 

The clouds in an atmosphere can trap heat near the surface.  Sunlight doesn't penetrate as readily and the daytime high temperature is not as great.  Likewise the surface heat cannot radiate away to space at night and the nighttime low is not as low.

 

Simply having an atmosphere affects the temperature profile for a planet.  We have learned much about the Greenhouse Effect from studying Venus.  Our sister world (so called because the Earth and Venus are similar in radius, mass, density, and distance from the Sun) has an atmosphere composed of 95% CO2.  Carbon dioxide is transparent to the visible light emitted by the Sun.  That energy reaches the surface and heats the ground.  In order for energy to be conserved, an equal amount of energy must be radiated by the planet.  But planets are much cooler than stars and must radiate in the infrared.  Carbon dioxide is fairly opaque in the infrared.  So the energy can get into the atmosphere, but cannot readily escape.  Venus has a runaway Greenhouse Effect.  If the Earth were moved to the location of Venus, the global temperature would increase about 50°F.  We would not live in Texas but we could survive.  The temperature on Venus, however, is 900°F (hot enough to melt lead).

 

The Earth also has a small Greenhouse Effect (about 25°F) that maintains our global average temperature above the freezing point of water.  (A little Greenhouse is a good thing!)  Since the beginning of the Industrial Revolution, humans have about doubled the CO2 content of the atmosphere.  "Global warming" is the predicted result.  Serious consequences of global warming include:

 

 

Part II

 

Also affecting the diurnal temperature variation is the presence of oceans.  Water has a large heat capacity that moderates the temperatures for coastal cities.  The high temperature will not be as high near the coast since the water of the ocean is absorbing much of the Sun's energy.  At night the water slowly releases its energy, keeping the nearby land warmer.  The effect of the oceans on the temperature variations is referred to as a maritime climate.  Inland the temperature variations are more extreme (continental climate).

 

The presence of water vapor in our atmosphere also has an effect on the way the temperature changes during the course of the day or year.  Astronomically, the hottest time of the day should be when the Sun is highest in the sky - at local noon.  Likewise the hottest time of the year should be around the Summer Solstice.  But it takes time to warm the water (both in the oceans and in the atmosphere), so that the hottest part of the day is mid-afternoon and the hottest time of the year is late July.  This effect is called thermal lag.

 

We now begin working toward an understanding of the interior structure of the planet.  We can learn much by studying magnetic fields because the field originates in the core region of the planet.  Circulating charges cause magnetic fields.  Using the Earth as a prototype, the interior of the Earth is composed of a two-section core, the inner, solid core and the outer, molten core.  The core composition is thought to be mostly iron and nickel.  The composition is suggested by the average density of the planet (5.5 g/cm3) coupled with the fact that the Earth formed in a molten form.  Heavy material would sink toward the center while the lighter material floated on the top (differentiation).  The density of surface material is about 3.3 g/cm3 so the interior must be made of denser material.  As the iron and nickel circulates in the outer core, friction in the molten flow can strip charges off of the atoms.  Then the rotation of the planet causes these free charges to circulate, producing a magnetic field.  So the essential elements for the formation of a planetary magnetic field are

 

 

A quick tour of the planet system reveals some interesting surprises.  Mercury should not have a magnetic field.  It is a small planet whose interior should have cooled long ago and at 59 days, its rotation is not rapid.  The Mariner flybys of the planet, however, revealed a small (possibly remnant) field.  Two possibilities exist:

 

 

Venus is so similar to the Earth that it probably has a molten core.  But its rotation rate of 243d is too slow to cause a magnetic field.  And none is observed.

 

The Earth possesses both of the requirements - the outer core is molten iron and nickel and the rotation rate of 24 h is fast enough to cause the field.  We also know that the solar wind greatly influences the structure of the magnetic field near the Earth, pushing it in on the daytime side and stretching it out on the night face.  Charged particles can become trapped inside the field structure (van Allen radiation belts) and the aurorae are the direct result of particle interactions with the Earth's magnetic field.  We will be looking for these effects as evidence of magnetic fields on other planets.

 

The other planets that have magnetic fields are the Jovian planets.  The fluid centers of these planets are thought to be liquid hydrogen (for Jupiter and Saturn) and possible water for Uranus and Neptune.  The magnetic fields of Uranus and Neptune are quite asymmetric.

 

Since a fluid interior is one requirement of a magnetic field and terrestrial planets form hot, it is reasonable to ask why the inside of a planet would still be hot.  If the planet is massive enough, it can retain much of its original heat.  The Earth and Venus fall into this category.

 

A second heating mechanism is radioactive decays.  Every terrestrial planet formed with a certain percentage of its material in the form of radioactive elements.  When these elements decay, the reaction produces helium gas and heat.  This form of heating was enough to have melted the surface of the moon in its early history.

 

Lastly, tidal heating may be important on some worlds.  By this I mean the frictional heating generated by the presence of a nearby massive object.  Consider the Galilean moons of Jupiter.  Innermost Io has tides raised in the crust due to the combined influence of Jupiter and Europa, that its surface shifts up to 100 yards each orbit of Jupiter.  Clearly, much interior heat can be generated in this way and we observe that tiny Io has active volcanoes.  Europa is somewhat farther from Jupiter - tidal heating is less - the interior is warm but not hot.  We strongly suspect a global, subterranean ocean exists under the icy surface.  By contrast to these inner moons of Jupiter, Ganymede and Callisto have very little tidal heating and show surfaces that have clearly been exposed to space for a very long time (saturated in craters).

 

Part III

 

We turn now to planetary atmospheres.  The detection techniques work best for inferior planets and include:

 

 

Twilight is sunlight scattering in the atmosphere of the planet.  We may expect the terminator to be less distinct on planets that have atmospheres.  The same effect extends the intersection of the terminator with the limb into the so-called "horns."  Occasionally, an inferior planet at inferior conjunction with cut across the face of the Sun.  Careful observations of such transits for Venus gave the first evidence of an atmosphere.

 

Having determined the existence of the atmosphere, spectroscopy can be used to determine the composition.  In taking the spectrum of a planet, we must first of all remove the spectrum of the Sun.  It is best to look in the UV since the molecular constituents of a planetary atmosphere do not have prominent features in the visible spectrum.  For this reason a UV spectrometer is part of the instrument package of planetary probes.

 

There are only a few types of atmospheres in the solar system.  The Jovian planets all share a hydrogen atmosphere.  Hydrogen can be retained by these planets because of the great mass and, therefore, higher gravity.  The Jovian atmospheres are primary, having existed with the planets from the beginning.

 

Mars and Venus have CO2 atmosphere, but there the similarity ends.  The atmosphere of Venus is 90 times thicker than Earth's, whereas Mars has an atmosphere only 1% the Earth's.

 

Earth has a nitrogen atmosphere, as does Titan, the large moon of Saturn.  Our atmosphere is not primary, but is the result of geologic activity.  The original atmosphere of the Earth looked much like the atmosphere of Venus: CO2, NH3, CH4, H2, and H2O.  The hydrogen is too light to be retained and escaped into space.  The CO2 was, in large part absorbed by the oceans.  Once plant life developed, the process of photosynthesis removed even more CO2.  The photosynthesis also gave us the O2 content of the atmosphere.  The N2 came from out-gassing of the many volcanoes, photodissociation of the NH3, and maybe even from cometary impacts. 

 

Mercury and Pluto have not atmospheres.