Published 2 years ago
- 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 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 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.
- 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.
- 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.
- Without an atmosphere, Mercury has no erosion 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.
- 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.
- 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 no magnetic field???
- Our space probes to Mercury have found that it does have a magnetic field, about 1% as strong as Earth’s.
- 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 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?
- 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 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
- 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 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 H and O
- Role of biology on earth to create O2.