Chapter 9: Mercury, Venus and Mars


  • Mercury is hard to study from Earth because:
  • It is the planet nearest to the Sun, so the sunlight makes it hard to observe.
  • It is the smallest major planet.
  • It does not have a moon.
  • Therefore, it was not until we sent a satellites to Mercury that we started to learn about it.

Mercury’s Mass and Radius

  • Mercury’s radius can be measured from Earth using its angular size and distance (Ch 2) - although its small size limits the accuracy - 2440 km.
  • Finally in 1974 our Mariner 10 space probe measured Mercury’s mass using the method from Ch 3 == 3.3 \cdot  1023 kg == 0.055 MEarth
  • Using the mass and radius we find Mercury’s average density == 5.43 gm/cm3 very similar to Earth’s average density.

Mercury’s Rotation

  • Mercury is 0.0387 AU from the Sun and orbits in 87.97 days.
  • Because Mercury is so close to the Sun’s powerful gravity it is similar to our Moon being so close to Earth.
  • Therefore, we expected Mercury’s rotation to be synchronized to its orbit = both 87.97 days - WRONG!
  • Mercury’s rotation CANNOT synchronize because its orbit is too elliptical.
  • Mercury’s distance from the Sun varies by plus or minus 21% while our Moon varies by only plus or minus 5.5% - we knew this but did not pay attention to it.
  • Kepler’s 2nd law (Ch 2) found that a planet’s orbit speed varies with distance from a body.
  • Mercury’s rotation cannot speed up and slow down to match its changing orbit speed.
  • Therefore, Mercury’s rotation cannot synchronize to its orbit.
  • Mercury’s rotation cannot speed up and slow down to match orbit speed. Mercury does the next easiest thing, which is to speed 3 spins every 2 orbit.

Mercury’s Temperature

  • On Mercury’s surface, the Sun rise once every 176 days.
  • Very long, hot days - daytime temperature reaches 710 K == 437oC.
  • Very long, cold nights - nighttime temperature drops to 80 K == -193oC.

Mercury’s Atmosphere

  • Because Mercury’s mass is only 0.055x Earth’s mass, its escape velocity (Ch 3) is only 4.3 km/s (Earth’s is 11.2 km/s and the Moon’s is 2.4 km/s).
  • Because Mercury’s day side is 710 K, the speed of any atmospheric gas is so fast that it escapes to space.
  • Therefore, Mercury has no atmosphere.

Mercury’s Surface

  • Without an atmosphere, Mercury has no erosion \rightarrow its surface is very heavily cratered, like our Moon.
  • There is evidence of massive impacts such as those that created the Moon’s maris.
  • Estimates (not radioactive dating) of their age is about he ages of the Moon’s maria.
  • Mercury has one unique surface feature: enormous “scarps” -== cliffs.
  • Up to 3000 m high, measured from its shadow (CN tower is 500 m tall)
  • Up to 1000 km long.
  • Possibly created by the solid surface cracking as it rapidly cooled and shrank.

Mercury’s Interior

  • We have no seismic measurement to study Mercury’s interior.
  • We do know Mercury’s average density of 5.4 gm/cm3, which is almost the same as Earth’s, 5.5 gm/cm3.
  • We conclude that Mercury has a metallic core and a rocky mantle like Earth.
  • However, because Mercury is less massive than Earth, its gravity is weaker and its interior is less compressed than Earth’s.
  • This tells us that Mercury’s metal core must be a larger fraction of its volume.
  • Maybe Mercury has a larger metallic core because rock could not condense as easily so close to the Sun.
  • Or , maybe Mercury has a collision that blasted off much of its rocky mantle.

Mercury’s Magnetic Field

  • Recall (CH 6) Earth’s magnetic field is explained by a combination of:
  • 1) metallic core ++ 2) molten core ++ 3) rotation.
  • Mercury
  • Has a metallic core.
  • Has a small radius so its core should have cooled and solidified, but some observations suggest it might still be partly molten.
  • Rotates very slowly \rightarrow no magnetic field???
  • Our space probes to Mercury have found that it does have a magnetic field, about 1% as strong as Earth’s.
  • Maybe:
  • Mercury’s large metallic core compensates for its slow rotation.
  • Or it rotated much faster long ago, and the magnetic field “froze” in when it slowed down.


  • Venus is the planet that comes closest to Earth and it is the brightest planet.
  • But it has been difficult to study because:
  • It does not have a moon.
  • It’s atmosphere is permanently cloudy, preventing us from observing its surface.
  • As with Mercury, we need space probes to help us learn its properties.
  • Using the method from Ch 2, our space probes found MVenus == 4.9 \cdot  1024 Kg == 0.815 MEarth.
  • Our space probes sent radar signals through the clouds to measure the radius of Venus == 6052 km == 0.949 REarth.
  • Using the mass and radius we found the average density == 5.24 gm/cm3.

Are Venus and Earth Twins?

  • NO!!!!!
  • It’s atmosphere is VERY different.
  • 96% CO2 and only 3.5% N2.
  • Very thick - clouds extend from 30 km to 60 km above the surface and are sulfuric acid, not water vapor.
  • Very dense - atmosphere’s pressure at the surface of Venus is 1000x Earth’s surface pressure - like being 1000m underwater.
  • It’s temperature is VERY different.
  • At the surface of Venus the temperature is about 650 K which is about 1030oC - hot enough to melt lead and even hotter than Mercury’s surface.
  • The thick atmosphere makes the nightside temperature the same.

Why is Venus so Hot?

  • In Chapter 6 we learned that Earth has a mild greenhouse effect.
  • Earth’s greenhouse effect is controlled by rain that washes CO2 out of the air.
  • The atmosphere of Venus is 96% CO2, so it has a VERY extreme greenhouse effect.
  • Venus is too hot for rain, so there is no control - runaway greenhouse effect.

Surface of Venus

  • The thick clouds hide the surface of Venus.
  • Spacecrafts landing in Venus fail rapidly because of the heat - just a few pictures.
  • Use radar from orbiting satellites to map the surface.
  • Some of the surface is similar to ocean floors, so maybe the young, cooler Venus had oceans.
  • Large volcanoes that could have emitted the CO2 and may still be active because we detect variable amounts of sulfur in the atmosphere,
  • Very few signs of continental drift. (surprise!)

No Continental Drift?

  • Some possible explanations:
  • The lack of water raises the melting temperature of the rock, making it harder to break apart, preventing continental drift.
  • Water is needed to “lubricate” the crust, making it slide across the mantle
  • The high temperature has made the crust thicker and harder to break into pieces so it cannot drift.

Rotation of Venus

  • The cloudy atmosphere of Venus kept us from measuring its rotation.
  • Radar measurements solved the problem.
  • Venus rotates once in 243 days! This is longer than its orbit period = 224.7.
  • Venus rotates in the “backward” direction!!
  • Maybe caused by a massive collision?? (idea that people are exploring)
  • Maybe why Venus doesn't have a magnetic field (lacks the rotation to set up currents in the core)
  • (Our closest neighbor but it is very different from us in many ways and we’re like wtf)


  • Basic properties are easy to measure:
  • Surface is visible through its atmosphere RMars == 3369 km == 0.532 REarth.
  • Mars has two tiny moons to measure its mass, MMars == 6.42 X 1023 kg == 0.17 MEarth
  • Average density of Mars == 3.94 gm/cm3.
  • Rotates in about 24.6 hours.
  • Rotation tilt == 25.19o \rightarrow Seasons like Earth (but a little longer)

Interior of Mars

  • We have very few seismic measurements of mars quakes.
  • We use the size, mass, and average density to learn about the interior.
  • Metallic core about half the volume of Mars and probably solid because it has cooled.
  • Rocky mantle about half the volume.
  • No magnetic field is measured.

Surface of Mars

  • From Earth we see some detail.
  • Our many space probes have given us a very detailed survey of Mars.
  • The southern hemisphere is higher and heavily cratered, like our Moon.
  • The northern hemisphere is lower and has few craters (which means there has been much more erosion in the northern hemisphere than the lower hemisphere).
  • There is an enormous rift 4000 km long, 100 km wide and 7 km deep.

Mars Has Volcanoes

  • Mars has the largest volcanoes in the solar system - can be seen from Earth but could not be identified from here.
  • The volcanoes have few craters, so they may be “younger” than 250 millions years.
  • The enormous size may be caused by the volcanoes not “drifting” off a deep plume of lava.

Atmosphere of Mars

  • The atmosphere is 95% CO2 and only 3% N2 - like Venus.
  • The atmosphere’s density is only 1% of Earth’s atmosphere, but there are winds that create dust storms, planet wide
  • Because of the low density there is very little greenhouse effect - average temperature is only 218 K == -55oC, although at noon it can reach > 0oC
  • Occasionally the temp of mars gets up to 0 and the absence of greenhouse effect is very important

Did Mars Ever Have Water?

  • Mars is too cold for liquid water
  • But images from our space probes show that mars had water in the past, therefore the planet was warmer

Does Mars still have water?

  • Yes, but it is frozen ice, not liquid water.
  • Polar Caps
  • White polar caps can be seen from Earth and they expand and shrink with the seasons.
  • The surface of the caps is frozen CO2, “dry ice”, freezes out of the atmosphere.
  • But under the surface the polar caps is frozen water ice.
  • Frozen into the Martian soil, like Earth’s permafrost.
  • Exposed when the surface dust is blown off (there is water-ice underneath the surface dust)
  • Slight melting during the summer time ( during the summer time it gets up to 0 degrees Celsius so water can melt for a brief time)
  • Earth UFOs have arms that dig, found ice, confirmed aliens.

  • Aliens love to dig
  • Aliens disguise themselves as water

  • The evidence shows that mars was much warmer in the distant past, and has cooled off in its life time so that the water that existed is now ice.
  • Therefore Mars had a much denser atmosphere when it was young.

Did Life Develop on Mars

  • Yes
  • On earth liquid water was essential for the development of life - did this happen on Mars when it was warmer and wetter?
  • The Curiosity rover on Mars is currently studying the bottom of a crater that once held water looking for clues.
  • No detention of life yet but the search continues.

Trends and Questions

  • The terrestrial planets have similarities and differences. Studying the properties helps us understand these planets.
  • Role of mass, radius, and average density.
  • Planets change and cool at different rates
  • Role of internal activity.
  • Plate tectonics
  • Volcanoes \rightarrow atmospheres, they emit lots of gases
  • Roles of distance from the sun and sunlight.
  • Role of water.
  • Extremely important so it may be important for planets who show evidence that they had it too, (Mars and possibly Venus).
  • Frozen, liquid
  • Dissociated H2O \rightarrow H and O
  • Role of biology on earth to create O2.

Note Created by
Is this note helpful?
Give kudos to your peers!
Wanna make this note your own?
Fork this Note