(Chapter 1; Appendix 2)
Earth exploration was done in the wrong order!
Geological exploration of the Earth was started by scientists who walked around and picked up rocks and minerals and learned a lot about the composition of the upper few kilometers of the continental crust. After World War II, scientists went to sea in ships to make measurements of ocean crust. At about the same time they installed global observatories to collect data for probing the interior of the Earth (e.g., seismometers, magnetometers, GPS receivers). Finally they sent spacecraft onto orbit to examine the Earth from a distance. This progression from detailed studies to the broad overview is probably the least efficient way to study the Earth. Moreover, the continents represent only 30% of the surface of the Earth yet this is where most of the observations are made.
The order of the topics in many introductory textbooks reflects this historical, but backward, sequence in the study of the Earth. Press and Siever is better than most because at least it begins with an overview of how the Earth was assembled from stardust. Then they go on to provide a brief discussion of plate tectonic theory in order to set the stage for the remainder of the book. An additional problem with many textbooks is that they focus on land areas - perhaps because this is where we live.
Growing up on Venus
In the next few lectures we will have the opportunity to explore the Earth without having all of this historical and political baggage. Moreover, we will be equipped with the latest technological tools and computers. Suppose the solar system turned out a bit differently. Suppose the atmosphere of Venus turned out to be conducive for live to begin and flourish and that the human race developed on Venus. Suppose further that life also developed on the Earth but humans or any other type of intelligent life did not develop.
Inference and Remote Observation
Without leaving Venus we could learn quite a lot about the Earth. First we infer that Venus and Earth have about the same bulk composition (link to Lecture 1), structure, and volcanic/tectonic activity because they are about the same size and they formed at the same time and in the same part of the solar system. This provides a working hypothesis for further exploration of the Earth. The second way to learn about the Earth is to examine it with a telescope or some other remote sensing device. (in-class review of origin and composition of the Earth, pages 6 - 13; radiogenic heating of the Earth, pages 197-202; in-class notes on planetary heating)
Properties of the Earth (Appendix 2, page 549)
Known: (please fill in numbers, properties in bold should be known for exam)
approximate radius (m)
distance from sun (km)
rotational period (s)
tilt of spin axis (degrees)
percentage of land and ocean area
polar ice caps
albedo (reflected light /incident light)
surface temperature (šC or šK)
lots of clouds and weather patterns
Can you think of other Earth properties that could be established without leaving Venus?
Unknown (prior to Flyby and Mapping Missions, below):
average density (kg/m**3)
iron core? radius of core (m)?
is the core solid?
what is the average ocean depth (m)?
what is the average elevation of the continents?
why do the outlines of the continents seem to match?
are the volcanoes active? where are most of the volcanoes?
how old is the continental crust (Ma)?
how old is the oceanic crust (Ma)?
how does the Earth lose radiogenic heat?
what is all of the green stuff, plants?
what causes the interesting drainage patterns on land?
There is a limit on how much one can learn about the Earth from such a great distance so bunch of Venusian scientists get together to write a long-term proposal to investigate the Earth with a series of spacecraft. Their objective is to fill some of the gaps in their knowledge of the Earth. NASA thinks this is a great idea because this gives them an excuse to develop high-technology toys. Congress likes it too because it promotes high-tech industry in the US and provides the "high ground" for any potential global conflicts. Military leaders like it because they want NASA to develop high-tech toys for their own surveillance program; but they are especially happy because the scientists toys are sent off to another planet where they cannot be used for military surveillance. The general public likes it because they like to learn about new things and exotic places. Thus Congress allocates a long-term budget for Earth exploration. How do the scientists make the most efficient use of their funds? Do they send people to the Earth to walk around on the continents and pick up rocks? (How Earthlings plan to explore Mars.)
The first experiment is a cheap flyby mission to establish the basic characteristics of the Earth that will be needed for future missions. NASA forms 4 science teams: € gravity team will measure the mass of the Earth € magnetics team will measure the magnetic field of the Earth € remote sensing team will take high-resolution images of the Earth € limb-sounding team will probe the density and temperature of the atmosphere
Gravity - How do we measure the mass of the Earth? How do we measure spacecraft velocity? (in-class notes) Given the mass of the Earth, what can we determine about its internal structure? (density/core notes)
Magnetics - How to we measure the magnetic field of the Earth? What does the magnetic field of the earth tell us about internal structure? (in-class notes, page 441) (link to Galileo web pages)
Remote Sensing Imagery - What are the most important targets on the Earth for high-resolution imagery given the flyby is centered over the north pole? € Geologist - images of Aleutian volcanoes, Siberia. € Glaciologist - images of Greenland ice sheet, glaciers. € Hydrogeologist - images of rivers flowing into Arctic Seas € Oceanographer - images of ocean surface waves and spiral eddies (in-class examples, link to Earth imagery at EROS data center)
Limb Sounding - How can one learn about the density of the atmosphere with a flyby mission? (in-class notes) Observe delay in signals sent from spacecraft to Venus. Use bending of trajectory to sample different layers of the atmosphere.