Welcome to GEOL212 - Planetary Geology. Be excited. The study of the planets embodies more than just the excitement of a positive and progressive future for humanity. It manifests the passage of the study of the solid bodies of the Solar System into the realm of Geology with the proliferation of robot planetary probes.
- When this course was taught in 2014, our best image of the dwarf planet Ceres was from the Hubble Space Telescope. In April 2015, NASA's Dawn spacecraft entered orbit around it, returning the linked image.
Hubble Space Telescope image of Pluto from Starts with a Bang
- When this course was taught in 2014, our best image of the dwarf planet Pluto was this heavily processed Hubble Space Telescope image. On July 17, 2015, NASA's New Horizons spacecraft executed a fly-by of the Pluto system, returning the linked image.
Mars viewed by Wilson Observatory from marswatch.sese.asu.edu
Ceres viewed by Hubble Space Telescope from NASA
This is definitely a geology course addressing:
- General geologic concepts developed for the study of the Earth.
- Their application to the study of the planets.
- Planet: In 2006 a formal definition of planet was adopted for the first time by the International Astronomical Union, a planet is a celestial body that:
- is in orbit around the Sun
- has sufficient mass to become rounded by its own gravity
- has cleared its orbital neighborhood of smaller bodies or controls their motion through its gravity.
This definition encompasses the major worlds of the Solar System but excludes many objects of interest:
- Dwarf planets such as Pluto or Ceres don't satisfy the third criterion.
- Many minor bodies such as Eros don't satisfy the second criterion.
- Large worlds like Ganymede or recently detected exoplanets circling other stars don't satisfy the first criterion.
- Geology: A definition that has evolved over the last two centuries and has no definitive standard form, but for most people it is:
The study of the physical Earth, it's history, and the physical and chemical processes that have shaped it.
But this is insufficient, because in the last four decades, as human and robot exploration of the Solar System has returned increasing data (including actual samples) of the type that geologists are used to analyzing, the study of the solid worlds of the Solar System has slipped from the realm of astronomy to that of geology. Thus, a better contemporary definition might be:
The study of the planetary bodies, their history, and the physical and chemical processes that have shaped them. This revised definition, of course, encompasses the traditional one.
Saturn's moons Hyperion (left) and Mimas (right) to scale.
- "Worlds:" Planetary bodies massive enough to pull themselves into a (more or less) spherical shape through their own gravity. (E.G. Mimas, right - a small world)
- "Rocks" or "potatoes:" Planetary bodies insufficiently massive to pull themselves into a (more or less) spherical shape through their own gravity. (E.G. Hyperion, left - a large spud.)
Planetary Geology is the branch of geology specifically devoted to the study of objects other than Earth. In it we bring to bear many of the major geologic disciplines used to study Earth, including:
The Moon (left) and Triton (right) to scale.
The big questions:
What do we actually care about in this course? Consider the two worlds at right: Earth's moon and Triton. They are about the same size, yet they are different. What are the differences and why are they there?
- Moon: mean density - 3.34 103kg m-3 (Note: liquid water has a density of 1 103kg m-3.)
- Triton: mean density - 2.05 103kg m-3
- Moon: 0.074 x 1024 kg
- Triton: 0.0215 x 1024 kg
- Moon: 250 K (= deg. C+273)
- Triton: 34 K
- Relationship to their primary (the planet they orbit)
- Moon: Orbits in same direction as Earth's rotation
- Triton: Orbits in opposite direction as its primary's rotation
- Evidence of geologic activity
- Moon: Pretty dead for the last 3 ga (billion years)
- Triton: Streaked with the deposits of erupting geysers
- Moon: None
- Triton: Thin atmosphere supports clouds.
Comparison of the Solar System with the Kepler-186 system from NASA
Why are the worlds of the Solar System different from one another?
How does our Solar System differ from others?
These question will form the central focus of the semester. In addressing it, we will inevitably be led to the biggest question:
Why do the worlds of this or any Solar System exist at all?
To address these, we will need to take on a short list of other important questions including:
- What are the physical characteristics of planetary bodies?
- How do planetary bodies interact with one another?
- How do planetary bodies change over time?
In pursuit of these answers, we will encounter more interesting questions than can possibly be encompassed in a single world. For example:
Io from Wikipedia
Venus as seen by Venera 9 from Project Avalon.
Why do physically similar worlds like Earth and Venus, have such different surface environments?
Mercury and Ganymede to scale.
Why are some worlds made mostly of rock and metal (Mercury - left at 0.055 Earth masses), and others mostly of ice (Ganymede - right at 0.025 Earth masses) even though they are nearly the same size?
Indeed, why do even the rocky worlds differ so much in their internal structure?
The Solar System from Wikipedia.
Why do the worlds of the Solar System occur in so many different sizes?
Subducting plates revealed by seismic data from MantlePlumes.org.
What actually goes on inside the planets?
Comet Shoemaker-Levy fragment impacts Jupiter
from Arizona Skies Meteorites
Is there any practical reason to care about our Solar System neighbors?
But first, a more fundamental preliminary issue:Key concepts and vocabulary. Understand these or you're toast:
- Dwarf planet
- Minor body
- Planetary body
- Planetary geology
- "World" vs "rock/potato/spud"- non-technical but meaningful descriptors