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Astronomy

The Terrestrial Planets - Mercury, Venus, Mars

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Weblecture

The Terrestrial Planets

Mercury, Venus, and Mars

Introduction

Mariner IV was launched November 28, 1964, from Cape Kennedy. It completed its primary mission on August 2, 1965, after transmitting to Earth 21 full pictures and a fraction of a 22nd picture of the Martian surface recorded July 14 when it flew within 6118 miles of the planet.

For 307 days--launch day through October 1, 1965-Mariner IV radioed scientific data to Earth on interplanetary magnetic fields, radiation and micrometeorites. More than 50 million scientific and engineering measurements were obtained.

— NASA News Release, 25 May 19066

Planetary Cores

Pangaea

Source: USGS

As we saw a few units back, in the early 1900s, Alfred Wegener proposed that the continents of earth had once formed a single land mass, which he named Pangaea.

His theory was based on the "puzzle piece" matching coastlines of North America to Europe and South America to Africa, as well as the match between Antarctica and Australia. His ideas about "continental drift" were largely ignored by other geologists until the discovery of sea-floor spreading in the mid-Atlantic and satellites detected the actual drift in the 1960s.

Plates

Source: USGS

On Earth, the more solid continents of the lithosphere or crust ride on top of the asthenosphere, a plastic-like malleable upper layer of the mantle. Radioactive decay of substances in this region produce enough heat to cause convection currents to well up from below. This causes the mid-oceanic ridge in the Atlantic and pushes the European/African and American plates on either side apart, crowding the plates on the other side of the planet, and resulting in the collision of the American Plates with the Pacific Plate. A similar smaller upwelling in the Pacific pushes the Pacific plate westward at its base into the Austrialian plate at New Zealand, and the Nazca Plate east into South America, resulting in the Andes volcanoes. Most volcanic and earthquake activity occurs along the boundaries between colliding plates.

The convection model allows us to explain geological activity, including different kinds of volcanoes, and continental migration of the Earth's plates. Stratovolcanoes, characterized by steep sides and violent eruptions, occur when one plate is subducted beneath another, and the heated crust melts and bubbles up through cracks in the higher plate. Examples include the Cascade volcanoes (Mt. Rainier, Mt. Hood, Mt. St. Helens, Mt. Lassen, Mt. Shasta) of the Pacific Coast of the United States, and Mt. Fuji in Japan. Shield volcanoes occur when the crust of a migrating continental plate moves over a hot spot above a circulation cell in the lithosphere. Examples include the Hawaiian Islands, formed as the Pacific plate passes over a single hot spot in the Asthenosphere, and the Yellowstone caldera and its predecessors in the Pacific Northwest.

Earth appears to be unique in the Solar System in having multiple plates. Our observations so far indicate that even volcanically active planets and moons (Venus, Mars, Io) have a single crustal plate. The volcanic activity on these bodies are currently explained using shield volcano models, and require a molten upper mantle layer. Different models are required to explain how planets and moons of such different sizes, core structure, and current chemical composition can generate the heat necessary to keep the mantle layer molten. Another key factor is the existence and strength of any planetary magnetic field and magnetosphere, since the mechanism for a magnetic dynamo requires a liquid layer of metallic substance capable of carrying a strong electrical current.

Tectonic forces

Source: USGS

The problem now arises: how do we account for the heat necessary to fuel geological activity? Planets receive some energy from the sun, less as we get further away, according to the inverse square law. That is, if Mercury at 0.37AU from the Sun receives 1 unit of energy, the Earth at 1 AU, roughly three times as far from the Sun, receives about (1/3)2 or 1/9 as much energy. Yet the Earth is more geologically active than Mercury, so the core of the Earth must be hotter than Mercury's core.

Planetary mantles such as the Earth's may generate their own heat as radioactive isotopes decay. This is thought to be one source of the Earth's mantle heat. But this doesn't necessarily apply to all planets.

Planetary cores are also under extreme pressure from the weight of the planet above, which creates heat as well. In this case, the planetary core temperature will be the result of a combination of factors: the rate at which the planet radiates heat into space from its surface, which depends on its surface temperature, and the mass of the planet. Smaller planetary bodies (such as Mercury and the Earth's Moon) lose heat more quickly than larger ones because they have a higher surface-to-volume ratio. A cold core and solid mantle may account for the apparent "dead" state of geological activity and lack of magnetic fields on the Moon and Mercury. Liquid iron-nickel "shells" around rocky or iron cores may account for the comparatively large magnetic field of the Earth. In the gas giants, however, where iron-nickel cores are not indicated by temperature and density evidence, the large magnetic fields must have a different source. The current model for gas giants calls for a liquid metallic helium "shell" around a rocky core to provide the dynamo and sustained electrical currents required to generate the magnetic fields detected by satellite probes.

No single model of cores and layers can account for the variation in observed planetary conditions such as surface temperature, forms of geological activity, magnetic fields, density, chemical composition. It is also clear that assuming that all planets followed a similar pattern of formation by condensing from the proto-sun cloud of gas and dust does not account for the variation in composition and density of the planets, or even for the orbital characteristics such as inclination of rotation to the ecliptic plane.

We can note some interesting facts about the terrestrial planets and moons with similar structures in our solar system. With the exception of Earth, the other planets and moons with true surfaces have features that indicate a single tectonic layer. No other planet or moon has the multiple drifting and colliding plate structures found on Earth. So the mechanisms that drove volcanic activity on Venus, and that now drives volcanic activity on Io, and the forces that caused scarps on Mercury and the great rift valley on Mars must have other explanations than those we use to explain similar features on Earth. This poses a challenge for scientists: we cannot assume that what we know best from direct observation will serve as the final model for other planet's features. In fact, rather than displaying the "norm", Earth may be the exception in a number of ways.

Characteristics of Terrestrial Planets

MercuryVenusEarthMars

Naked Eye Observation
Close to the sun; difficult to observe from earth/orbital telescopes. Surface features obscured by atmosphere. Direct observation on the planet's surface. Earth-based telescopes able to detect surface features, including polar caps and variation in color.

Exploration
1973-75: Mariner 10
2004-2011: MESSENGER		 1960-1970s: Soviet Venera
1990-94: Magellan		 1960s-current: LANDSAT 1960s: Mariners 4-7
1970s: Viking
Current: Multiple Rovers, Global Surveyor, etc.

Orbital and Rotational Characteristics
Orbit: Highly eccentric (0.206)
3:2 spin-orbit synchronization
(58.646d:87.969 d)
Inclination: 0.5°		 Retrograde rotation.
(243.01d:224.70d)
Inclination: 177.4°		 Normal rotation, revolution
(1d:365.25d)
Inclination: 23.5°		 Normal rotation, revolution
(1.0216d:686.98d)
Inclination: 25.19°

Magnetosphere
Approx. 1/100 Earth (300nT)
Explained as thermal dynamo		 No significant magnetic field.
Magnetosphere due to ions in clouds.		 30000nT-60000nT Estimates from 30nT to 400nT
Lack of field due to impact event?

Atmosphere

Pressure:

Approximately 10-15 bar.

Composition:

Traces of hydrogen, helium (from solar wind), outgassed SO4 and O2. Hydrogen interaction with oxygen produces water vapor.

Pressure:

90 bar.

Composition:

CO2 - about 97%, nitrogen about 3%, traces of sulfur dioxide that form sulfuric acid rain.

Pressure: 1.0 bar


Composition: 77% N2, 22% O2, trace Ar, CO2, H2

Pressure:

0.006 bar

Composition:

carbon dioxide - 95%, 2.7% nitrogen.

Surface
Old and heavily cratered; marked by scarps (planetary-wide cooling), Caloris basin. Surface relatively young, due to geological activity.
Homogenous elevation, local Ishtar, Beta Regio highlands.		 Multiple tectonic plates; surface young due to geological activity, erosion forces. Crustal dichotomy. Older and moderately cratered; Olympus Mons, Valle Marineris main features; crustal dichotomy.
TerrestrialPlanets

Source: NASA

Core
Iron core 75% diameter; mantle, crust. Crust thin: ~0.5% diameter. Iron core 55% diameter; mantle, thin crust ~0.5% diam. Iron core 44% diameter; silicate mantle; thick crust ~ 4% diameter.
Planeary Cores

Source: NASA

Planetary formation
Atmospheres, upper layers torn away by close proximity to sun; accounts for large iron core. Surface same age overall due to thin crust (flake tectonics). Development of life changed atmosphere; allowed liquid water retention, oxygen, as erossive agents. Periodic impacts match fossil evidence for mass extinctions. Possible massive impact heated surface to point where temperature difference too small to sustain convection required for magnetic dynamo.

Discussion Questions

  1. How do the size, composition, and density of cores affect the surface features and magnetosphere of a planet? How could you use these trends to predict characteristics for a planet of a given mass and diameter?
  2. How does the composition of the terrestrial planets differ as we move outward from the sun?
  3. Science fiction writers often depict Mars as similar to Earth in many ways. How would Mars' size and amount of energy from the sun cause it to differ significantly from Earth?

Optional Web Reading

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