EST-Planets and Moons

Earth Science Today
Russ Colson
Minnesota State University Moorhead

Stars and Planets:

Topic 1: Planets and how they change

      It is hard to imagine what kind of people we would be if we did not have another world hanging in our sky telling us we are not alone, beckoning us to wonder at the heavens, and inviting us to learn what is there. Written on the face of the Moon are stories of immense cataclysms and change. But perhaps the greatest story written there is simply that worlds exist other than our own.
    Planets in space have been mysterious and exciting places for hundreds of years. From canals on Mars at the dawn of the 1900's to the mysterious stone face on Mars at the end of the 1900's, people have wanted to believe that other worlds are inhabited. In the 1800's a hoax was published saying that a well-known astronomer in South Africa had seen waterfalls and strange bat-like people on the Moon. The distance to South Africa and limitations of communication then, combined with the human longing to believe the remarkable, prevented the quick exposure of this hoax. Until the past few decades, we knew almost nothing of other planets, and imagination was free to roam. Only in the last few decades have we explored other worlds with spaceships from Earth. This great and unprecedented period of exploration has given us a new understanding of our own world as we see how our own Earth compares to other planets whose stories are more bizarre than science fiction.
     Our exploration has transformed our understanding of ourselves. We encountered a new understanding of Earth's place among the stars, when, in geologically recent times, humans entered space. The blue and white image from space of the jewel Earth stirred an environmental movement in the 1960's and gave rebirth to Gaia, the mother Earth, in the early 1980's. Recent exploration of our solar system has given us an understanding of planets as dynamic, changing places. Or fragile places. Or dead places where the engines of change and activity have ceased working. And some day, we may reach out for the resources and treasures on other worlds. And the greatest resource and treasure may simply be space to grow into.

Types of Planets and Moons:

Terrestrial Planets: are made of rocky (silicate) material, sometimes with an iron core. Density of these bodies ranges from 3 to over 5 grams per cubic centimeter (with a larger iron core resulting in higher densities). Small rocky bodies, such as asteroids, may have a lower density of 2-3g/cc due to open spaces in the rock that could not exist on larger planets and moons. Volcanism, weathering, and chemical differentiation are some of the processes that may occur on these bodies. However, volcanism and other processes may be quite different from what we are used to on Earth, such as water volcanos on Europa. Examples include Mercury, Venus, Earth, Moon, Mars, Europa, and asteroids.

Gas Giants: (Quick, can you name two ways these planets are different from terrestrial planets!?) They are of course, particularly large planets and are composed mainly of the gases Hydrogen and Helium, as well as derivatives of these such as Methane (CH4), Ammonia (NH3), and water (H2O). Because these planets are so massive, these gases get compressed into very exotic forms in the interior of the planets (such as metallic hydrogen on Jupiter and Saturn). These planets range in density from less than 1 to about 1.7 (with greater mass resulting in higher densities by compression, and greater amounts of rocky material increasing density). These planets experience winds and storms, lightning, magnetic fields and aurora. They have clouds made of a variety of liquid and solid chemicals that condense from the atmosphere. Chemical differentiation is occuring within the atmosphere and within the deeper interior of the planet as chemical reactions occur among the planets' components. In some cases, this chemical differentiation may be releasing energy (like the condensation of water releases energy) that drives the planets weather. Some of these planets (Jupiter for example) are releasing more energy to space than they receive from the Sun, suggesting an internal source of energy. The gas giants in our solar system are Jupiter, Saturn, Uranus, and Neptune. It is likely that the large planets that have been discovered recently around other stars are gas giants.

Icy planets: (Quick! can you guess what these planets are made of?!) Icy planets exist in the parts of the solar system remote from the Sun where cold conditions permit ices of water, methane, ammonia, nitrogen and other gases to exist. They have densities similar to water (1g/cc) ranging up to about 3g/cc depending on the size of the planet and how much rocky material it contains. Icy planets may have water volcanism, or even volcanism of liquid nitrogen. They may have or may have once had liquid oceans or liquid interiors. If an icy body approaches the Sun, the increase in temperature can result in jets and geysers of evaporating ice (comets). Examples are Pluto, Ganymede, Callisto, Triton, Titan, and comets.

Puzzle: Which type of planet is Jupiter's moon Io? You are able to measure the diameter of Io to be about 3640km By watching the way a spaceship is attracted to it, you determine that its mass is 8.89x10²²kg. (answer)

Processes that bring planets alive:

A variety of processes produce change on planets, creating mountains or erasing them, and changing the rock or landscape. See how many processes you can think of that might change Earth or other planets. Which of these would happen on Earth but not on the Moon? Which might happen on both Earth and the Moon? (a couple of examples)

Cratering: Meteorite impact craters both alter the surface of a planet and provide a means for determining its age. The rims of the craters become mountain ranges. Some of the mountain ranges formed on the Moon by this means are 15-20 thousand feet high. Other features of planets also reflect cratering. The large dark circular features on the Moon that give us the face of the Man in the Moon (or a rabbit, which is what I see!) are impact crater basins flooded by lava that cooled.

Other processes, such as lava flows, weathering, or even the viscous flow of a planet's crust (like the oozing of thick cornsyrup over long periods of time) can erase craters. Thus, the number of craters is a measure of how long it has been since a planet was resurfaced by some process. It is a measure of whether the planet is geologically alive, or dead. (Age of a surface-puzzle, est3b2.html). The rate at which craters form has changed through time because no new asteroids or meteorites are being made (evidence: radioactive dating reveals that the vast majority of them are the same age). (How is cratering rate changing-puzzle, est3b3.html).

There are a variety of different types of craters. Smaller craters are often simple circular excavations with a raised rim. Larger craters may include a central rebound peak or multiple rings of mountain ranges. (illustration). Very young craters on the Moon, like Copernicus and Tycho, have bright rays radiating away from them.

Volcanism: This process, as it happens on Earth, was discussed previously in this course. There are differences between Earth and other planets, including differences in the composition of "lava" and differences in the source of energy that provides partial melting inside a planet. On Earth, the source of energy is radioactive decay, which melts rocky (silicate) material. On Triton, sunlight provides energy to melt extremely cold ices, perhaps of nitrogen. On Io, tidal friction due to the nearness of Jupiter (think about Jupiters gravity flexing Io like you might flex a piece of wire, making the wire hot) produces heat that melts rocky material making the most volcanically active body in the solar system. Tidal interaction with more distance moons of Jupiter (Europa, Ganymede) have produced water volcanism when ice melted inside the planets.
Many planets were volcanically active long ago when they were younger but have now cooled off enough so that melting doesn't occur. In general, larger planets remained volcanically active longer than small planets because they retained their heat longer because of their size. Earth, Io, and Triton are still volcanically active. Other planets including Venus may be active as well, but eruptions have not been observed there. Mars was volcanically active in geologically recent times. How can we know how long ago a planet was volcanically active?

Tectonics: Tectonics refers to compressional, extensional, and shear forces in a planet that cause faulting and folding of rocks. On Earth, tectonics is often caused by movements of large plates (called Plate Tectonics) but this is not true on most other planets we have explored. Tectonics can also be caused by shrinking or expansion of a planet as it freezes, by distortions of a planets crust due to tidal forces, by convection or other movement of material inside the planet, or by shifting in a planet's crust such as might be due to weight of a mountain range.
Faults are a major type of tectonic feature. The type of fault reflects whether tectonic forces on the planet were compressional (pushing together) or extensional (pulling apart). (Illustration of types of faults)
Tectonic features, such as faults, show up on images of a planet as linear or slightly curved features. Tectonic forces can also produce mountain ranges and valleys. Grabben are a type of linear tectonic valley with normal faults on either side. (illustration of grabben) Tectonic valleys tend to be straighter than typically winding water-formed valleys.

Weathering: Weathering refers to changes in the surface of a planet due to interaction with an atmosphere or with the solar wind. Sunlight most often provides the energy sources for weathering processes. Processes include rain or snow (of water, carbon dioxide, methane, or other chemicals), movement of liquid (such as water) within or on the crust of a planet (such as in rivers or ground water), movement of ice, effects of wind, and chemical reactions between air and rock. On Earth, biological processes enhance changes and weathering. Features that can result from weathering include river channels, sand dunes, beach ridges, and so on. Weathering reactions show up in the chemical composition of the surface rocks. For example the red color of Martian sediment is due to presence of hematite (rust), a weathering product. Metallic iron in Lunar sediment is formed in part from interaction with hydrogen particles in the solar wind.

Chemical differentiation: This planetary process was discussed extensively previously in this course. (planetary differentiation experiment that has been done with 8th graders)

Experimental impact cratering lab, making predictions (est3b5.html)
Planetary features and processes (in MSword)

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Image credits: unknown earth observatory