Chapter 6 The Terrestrial Planets

Chapter 6 The Terrestrial Planets

6.1 Orbital and Physical Properties
The orbits of Venus and Mercury show that these planets never appear far from the Sun.

6.1 Orbital and Physical Properties
The terrestrial planets have similar densities and roughly similar sizes, but their rotation periods, surface temperatures, and atmospheric pressures vary widely.

6.2 Rotation Rates Mercury can be difficult to image from the Earth; rotation rates can be measured by radar.

6.2 Rotation Rates The Italian astronomer Giovanni Schiaparelli observed Mercury through a telescope in the 1880s, and reported that it was in synchronous rotation with the Sun

6.2 Rotation Rates But in 1965, radar measurements at the Arecibo radio telescope like those on the previous slide showed this to be false.

6.2 Rotation Rates In fact, Mercury has a 2:3 orbital-rotational resonance… The Sun’s tidal force makes it rotate three times while going around the Sun twice.

6.2 Rotation Rates In fact, Mercury has a 2:3 orbital-rotational resonance… The Sun’s tidal force makes it rotate three times while going around the Sun twice. The Sun’s tidal force also makes Mercury’s rotation axis perpendicular to its orbit.

6.2 Rotation Rates Venus Venus’s surface is not visible due to thick clouds

6.2 Rotation Rates Venus Venus’s surface is not visible due to thick clouds Venus’s rotation rate was also found by radar observations

6.2 Rotation Rates Venus Venus’s surface is not visible due to thick clouds Venus’s rotation rate was also found by radar observations It was found to rotate very slowly (about 243 Earth days)

6.2 Rotation Rates Venus Venus’s surface is not visible due to thick clouds Venus’s rotation rate was also found by radar observations It was found to rotate very slowly (about 243 Earth days) The rotation is retrograde

6.2 Rotation Rates Mars The surface of Mars is not obscured by clouds, which makes finding its rotation easy

6.2 Rotation Rates Mars The surface of Mars is not obscured by clouds, which makes finding its rotation easy It rotates once in about 24.6 Earth hours – nearly the same as Earth

6.2 Rotation Rates Mars The surface of Mars is not obscured by clouds, which makes finding its rotation easy It rotates once in about 24.6 Earth hours – nearly the same as Earth Its axis is tilted nearly the same as Earth, too, but its seasons are more extreme due to its orbit

6.2 Rotation Rates All the planets except Venus rotate in a prograde direction

6.2 Rotation Rates All the planets except Venus rotate in a prograde direction; Venus rotates in a retrograde direction

6.2 Rotation Rates What do you suppose the reason is for Venus’s unusual rotation?

6.2 Rotation Rates What do you suppose the reason is for Venus’s unusual rotation? Most likely a GIANT impact!

End 3/5 lecture

6.3 Atmospheres Mercury has no detectable atmosphere

6.3 Atmospheres Mercury has no detectable atmosphere Any idea why?

6.3 Atmospheres Mercury has no detectable atmosphere
It is too hot, too small, and too close to the Sun

6.3 Atmospheres Venus has an extremely dense atmosphere

6.3 Atmospheres Venus has an extremely dense atmosphere Mostly CO2

6.3 Atmospheres Venus has an extremely dense atmosphere Mostly CO2
The outer clouds are similar in temperature to the Earth, and it was once thought that Venus was a “jungle” planet

The outer clouds are similar in temperature to the Earth, and it was once thought that Venus was a “jungle” planet Clouds are H2SO4

The outer clouds are similar in temperature to the Earth, and it was once thought that Venus was a “jungle” planet Clouds are H2SO4 Surface is hotter than Mercury’s, hot enough to melt lead

6.3 Atmospheres The atmosphere of Mars is similar in composition to that of Venus, but very thin and cold

6.4 The Surface of Mercury Mercury cannot be imaged well from Earth

6.4 The Surface of Mercury Mercury cannot be imaged well from Earth
The best pictures are from Mariner 10.

6.4 The Surface of Mercury Mercury cannot be imaged well from Earth
The best pictures are from Mariner 10. They show that cratering on Mercury is similar to that on Moon

6.4 The Surface of Mercury Mercury has some distinctive features:
Scarps (cliffs), several hundred km long and up to 3 km high, thought to be formed as the planet cooled and shrank.

Caloris Basin, very large impact feature; ringed by concentric mountain ranges

Caloris Basin, very large impact feature; ringed by concentric mountain ranges Mountain ranges are 3 km high

Caloris Basin, very large impact feature; ringed by concentric mountain ranges Mountain ranges are 3 km high Basin spans more than 1000 km

6.5 The Surface of Venus This “topographic” map of the surface features of Venus from Pioneer Venus is on the same scale as the Earth map below it.

6.5 The Surface of Venus This “topographic” map of the surface features of Venus from Pioneer Venus is on the same scale as the Earth map below it. Aphrodite Terra and Ishtar Terra have mountains comparable to Earth’s.

6.5 The Surface of Venus This “topographic” map of the surface features of Venus from Pioneer Venus is on the same scale as the Earth map below it. Aphrodite Terra and Ishtar Terra have mountains comparable to Earth’s. No apparent plate tectonics.

6.5 The Surface of Venus Venus as a globe, imaged by Magellan

6.5 The Surface of Venus Venus as a globe, imaged by Magellan
Aphrodite Terra is shown

6.5 The Surface of Venus Venus as a globe, imaged by Magellan
Aphrodite Terra is shown Volcanism appears to resurface the planet every few hundred million years

6.5 The Surface of Venus Top: Lava domes on Venus (L), and a computer reconstruction (R) Bottom: the shield volcano Gula Mons

6.5 The Surface of Venus Aine corona, with lava domes

6.5 The Surface of Venus Aine corona, with lava domes
Volcanism probably continues today

Volcanism probably continues today SO2 levels fluctuate

Volcanism probably continues today SO2 levels fluctuate (due to volcanic activity?)

Volcanism probably continues today SO2 levels fluctuate (due to volcanic activity?) Radio bursts

Volcanism probably continues today SO2 levels fluctuate (due to volcanic activity?) Radio bursts (from lightning above caldera?)

Volcanism probably continues today SO2 levels fluctuate (due to volcanic activity?) Radio bursts (from lightning above caldera?) No eruptions have been seen, though

6.5 The Surface of Venus A photograph of the surface, from the Venera lander:

6.5 The Surface of Venus Impact craters can be distinguished from volcanic craters by their rough ejecta blankets, which appear light in radar images

6.5 The Surface of Venus Above: multiple-impact crater.

6.5 The Surface of Venus Right: Mead, Venus’s largest impact crater.

6.6 The Surface of Mars Tharsis bulge is the size of North America and 10 km above surroundings

6.6 The Surface of Mars Tharsis bulge is the size of North America and 10 km above surroundings Tharsis has minimal cratering

6.6 The Surface of Mars Tharsis bulge is the size of North America and 10 km above surroundings Tharsis has minimal cratering This suggests that it is the youngest surface on Mars at 2-3 billion-years-old

6.6 The Surface of Mars Valles Marineris is another major feature

6.6 The Surface of Mars Valles Marineris is another major feature
Much larger than Grand Canyon

6.6 The Surface of Mars Valles Marineris is another major feature
Much larger than Grand Canyon Not formed by running water, but probably by cracking of the crust when Tharsis bulge formed

6.6 The Surface of Mars Northern hemisphere (left) is rolling volcanic terrain Southern hemisphere (right) is heavily cratered highlands; average altitude 5 km above northern; probably original crust Assumption is that northern surface is younger than southern; 3 billion years versus 4 billion years Means that northern hemisphere must have been lowered in elevation and then flooded with lava; no one knows how

6.6 The Surface of Mars This map shows the main surface features of Mars. There is no evidence for plate tectonics.

6.6 The Surface of Mars Mars has largest volcano in Solar System: Olympus Mons

6.6 The Surface of Mars Mars has largest volcano in Solar System: Olympus Mons 700 km diameter at base

6.6 The Surface of Mars Mars has largest volcano in Solar System: Olympus Mons 700 km diameter at base 25 km high

6.6 The Surface of Mars Mars has largest volcano in Solar System: Olympus Mons 700 km diameter at base 25 km high Caldera is 80 km in diameter

6.6 The Surface of Mars Mars has largest volcano in Solar System: Olympus Mons 700 km diameter at base 25 km high Caldera is 80 km in diameter Three other Martian volcanoes are only slightly smaller

6.6 The Surface of Mars Was there running water on Mars?
Runoff channels resemble those on Earth. Left: Mars Right: Louisiana

6.6 The Surface of Mars Outflow channels: No evidence of connected river system; features probably due to flash floods.

6.6 The Surface of Mars This feature may be an ancient river delta.

6.6 The Surface of Mars Much of the northern hemisphere may have been ocean

6.6 The Surface of Mars Present water on Mars appears to be contained mostly in two places:

6.6 The Surface of Mars Present water on Mars appears to be contained mostly in two places: The permanent polar ice caps (left)

6.6 The Surface of Mars Present water on Mars appears to be contained mostly in two places: The permanent polar ice caps (left) Permafrost at a depth of about 1 meter below the surface (right)

6.6 The Surface of Mars Recently, gullies have been seen that seem to indicate the presence of liquid water, but this is controversial

6.6 The Surface of Mars Left: Viking 1 photo
Right: Mars rover Sojourner, approaching “Yogi”

6.6 The Surface of Mars Landscape and close-up by Opportunity rover

6.7 Internal Structure and Geological History
Internal structure of Mercury, Mars, and the Moon, compared to Earth

Mercury

Mercury Has a weak magnetic field, ~1% of the Earth’s

Mercury Has a weak magnetic field, ~1% of the Earth’s Since no liquid core and slow rotation, the source is not clear

Mercury Has a weak magnetic field, ~1% of the Earth’s Since no liquid core and slow rotation, the source is not clear Might be “fossil” magnetization

Mercury Has a weak magnetic field, ~1% of the Earth’s Since no liquid core and slow rotation, the source is not clear Might be “fossil” magnetization As core cooled back in the day, it shrank and the crust collapsed, forming the scarps

Mercury Large metallic core might be due to the location in the solar nebula where Mercury formed

Venus (not shown)

Venus (not shown) Is probably similar to Earth inside

Venus (not shown) Is probably similar to Earth inside But does not have a magnetic field

Venus (not shown) Is probably similar to Earth inside But does not have a magnetic field…why?

Venus (not shown) Is probably similar to Earth inside But does not have a magnetic field…why? Slow rotation

Mars

Mars Has very weak magnetic field, ~1/800th of Earth’s

Mars Has very weak magnetic field, ~1/800th of Earth’s Has rapid rotation, so lack of magnetic field suggests core is not liquid, not metallic, or both

Mars Has very weak magnetic field, ~1/800th of Earth’s Has rapid rotation, so lack of magnetic field suggests core is not liquid, not metallic, or both Best evidence is that Mars’s core is nearly solid and composed of iron sulfide

6.8 Atmospheric Evolution on Earth, Venus, and Mars
At formation, Earth had a primary atmosphere – hydrogen, helium, methane, ammonia, water vapor – which was quickly lost through thermal motion.

At formation, Earth had a primary atmosphere – hydrogen, helium, methane, ammonia, water vapor – which was quickly lost through thermal motion. Secondary atmosphere – water vapor, carbon dioxide, sulfur dioxide, nitrogen – comes from volcanic activity.

At formation, Earth had a primary atmosphere – hydrogen, helium, methane, ammonia, water vapor – which was quickly lost through thermal motion. Secondary atmosphere – water vapor, carbon dioxide, sulfur dioxide, nitrogen – comes from volcanic activity. Earth now has a tertiary atmosphere, 20% oxygen, due to the presence of life.

Earth has a small equilibrium greenhouse effect providing a comfortable surface temperature (for us).

Earth has a small equilibrium greenhouse effect providing a comfortable surface temperature (for us). Plate tectonics assists by returning CO2 to the atmosphere after it combines with the surface

Venus’s atmosphere is much denser and thicker – and hotter – than Earth’s

Venus’s atmosphere is much denser and thicker – and hotter – than Earth’s The greenhouse effect on Venus maintains its present surface temperature of 730 K

Venus’s atmosphere is much denser and thicker – and hotter – than Earth’s The greenhouse effect on Venus maintains its present surface temperature of 730 K This is too hot for CO2 to combine with the surface

This is the result of a runaway greenhouse effect Venus’s location closer to the Sun meant temperatures too high for liquid H2O to exist Water is a very effective greenhouse gas Led to even higher temperatures And this led to higher and higher temperatures as more H2O – and CO2 – was forced into the atmosphere

Mars may have suffered a reverse runaway greenhouse effect

Mars may have suffered a reverse runaway greenhouse effect As Mars cooled, more and more CO2 was bound up in carbonate minerals in liquid water on the surface, removing gas from the atmosphere

Mars may have suffered a reverse runaway greenhouse effect As Mars cooled, more and more CO2 was bound up in carbonate minerals in liquid water on the surface, removing gas from the atmosphere This led to more cooling, removing more gas

Mars may have suffered a reverse runaway greenhouse effect As Mars cooled, more and more CO2 was bound up in carbonate minerals in liquid water on the surface, removing gas from the atmosphere This led to more cooling, removing more gas Water froze out of the atmosphere, leading to even more cooling

Mars may have suffered a reverse runaway greenhouse effect And we end up with Mars as it is today, with a very thin atmosphere and very cold

Mars may have suffered a reverse runaway greenhouse effect And we end up with Mars as it is today, with a very thin atmosphere and very cold Stripping by the solar wind probably helped as well

Summary of Chapter 6 Mercury is tidally locked in a 3:2 ratio with the Sun Mercury has no atmosphere; Venus has a very dense atmosphere, while the atmosphere of Mars is similar in composition but very thin. Mercury has no maria, but does have extensive intercrater plains and scarps

Summary of Chapter 6 Venus is never too far from Sun, and is the brightest object in the sky (after the Sun and Moon) Many lava domes and shield volcanoes Venus is comparable to Earth in mass and radius Large amount of carbon dioxide in atmosphere, and closeness to Sun, led to runaway greenhouse effect and very hot surface

Summary of Chapter 6 Northern and southern hemispheres of Mars are very different South is higher and heavily cratered North is lower and relatively flat Major features: Tharsis bulge, Olympus Mons, Valles Marineris Strong evidence for water on Mars in the past

Summary of Chapter 6 Mercury has very weak, remnant magnetic field
Venus has none, probably because of very slow rotation Neither Venus nor Mars show signs of substantial tectonic activity

Chapter 6 The Terrestrial Planets

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