Chapter 9: Mercury, Venus and Mars

Mercury

• 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 M_Earth
• 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 2^nd 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 $=$ 437^oC.
• Very long, cold nights - nighttime temperature drops to 80 K $=$ -193^oC.

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

• 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 M_Venus $=$ 4.9 $\cdot$ 10²⁴ Kg $=$ 0.815 M_Earth.
• Our space probes sent radar signals through the clouds to measure the radius of Venus $=$ 6052 km $=$ 0.949 R_Earth.
• 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 1030^oC $-$ 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)

Mars

• Basic properties are easy to measure:
• Surface is visible through its atmosphere R_Mars $=$ 3369 km $=$ 0.532 R_Earth.
• Mars has two tiny moons to measure its mass, M_Mars $=$ 6.42 X 10²³ kg $=$ 0.17 M_Earth
• Average density of Mars $=$ 3.94 gm/cm³.
• Rotates in about 24.6 hours.
• Rotation tilt $=$ 25.19^o $\rightarrow$ 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 $=$ -55^oC, although at noon it can reach > 0^oC
• 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.