Geophysics - Introduction

Geophysics is the study of the Earth through the observation of various forms of radiant, reflected, and transmitted energy. The most obvious form of such energy is reflected sunlight in the visible spectrum - that is the geophysical phenomenon that our eyes are adapted to sense. The famous "Blue Marble" photograph of the Earth from the Moon is the classic example of such an observation. On it we can see the shapes of continents and ocean basins - the shapes that lead, over 400 years ago, to the initial formulation of what would become Plate Tectonic theory.

Each of the segments of today's lab looks at one of the many ways to view the Earth, not only with our eyes, but with other sensors capable of observing and measuring other forms of energy. An excellent summary of several of these methods is in Dr. John Louie's (Mackay School of Mines, University of Nevada, Reno) "Earth's Interior" Web Page (by permission).

Browse the "Earth's Interior" page. Identify and examine the techniques discussed there and identify the part of the Earth's interior each technique is used to examine. The parts of the Earth's interior are listed below.

Some other time, if you are interested, consider reading the Goddard Space Flight Center's Tutorial on Remote Sensing at http://rst.gsfc.nasa.gov/ - it explains how the geophysical technologies pioneered and ground-truthed on Earth are being applied to exploration of other planets, solar systems, and galaxies.

Geophysics - Electromagnetic Spectrum

Visible light - emitted by an object or reflected from it - is a primary source of Earth surface data. The best source of visible light images of Earth is NASA's Johnson Spaceflight Center's "Earth from Space" website, where thousands of images, mostly from hand-held cameras on Space Shuttle missions, are archived.

Exercise: Go to the Earth from Space page and investigate the Craters of the Moon (Idaho) volcanic field. You can navigate the page in several ways. One is to chose the Clickable Map, point at the area of the western United States with only two isolated white dots in it, click (and wait) and select the Craters page from the choices that come up. Load the Lo-resolution image - http://earth.jsc.nasa.gov/sseop/efs/lores.pl?PHOTO=STS41G-36-95.

1. The basalt lava flows are depicted in several shades of dark gray and black. Why might they display different colors (list several options)? How might you test one or more of these options?

2. In the upper part of the image are numerous pale rectangles. What are these rectangles, and why do you think they are pale?

3. Use any of the navigation tools from the main page to find one additional page with visible-light images with geological significance, describe the page by title, and describe the geological information and how visible light transmits it.

Infrared radiation - wavelengths longer than those of visible light and often summarized as "heat" - is a secondary source of Earth surface data.

Infrared sensors "see" light at wavelengths slightly longer than those our eyes can see. Some emissions in the infrared wavelengths are excellent for examining, for example, vegetation patterns - see the USGS Fact Sheet t http://erg.usgs.gov/isb/pubs/factsheets/fs12901.html. Other infrared sensors are used to look at cloud temperatures, see the NOAA and Defense Meteorological Satellite Program pages at http://dmsp.ngdc.noaa.gov/. A good example of thermal infrared imaging is that of Hurricane Mitch (1998), which was responsible for over 9000 deaths in Central America. Infrared sensors are similar to visible light sensors, but are effective at night as well as in the daytime. Use the Hurricane Mitch images to answer the following questions.

1. Given the nature of convection and of latent heat transfer, explain why the cloud tops in the Hurricane Mitch image are so bright. (Hint: are they warmer or colder than surrounding air and why?)

Geophysics - Gravity

Gravity exerts a precisely measurable force based on the masses of two objects and the inverse square of their mutual separation. Gravity is typically measured on the ground, from ships, from aircraft, and from spacecraft. Because spacecraft orbits are the result of the interplay between their velocity (tending to throw them into space) and gravity (pulling them earthward), even minor local gravity differences can change a spacecraft's orbit measurably! Such local gravity differences are termed "anomalies" and may be stronger (positive) or weaker (negative) than the expected field at that point.

One interesting application of gravitational theory is the use of precise satellite altimetry to map tiny fluctuation in the elevation of the ocean surface, from which can be inferred the topography of the underlying seafloor. The map of world's topography produced by that method has a resolution unmatched by any other techniques.

1. Describe the Hawaiian Island chain as viewed from this perspective. Explain the origin of that chain in the context of Plate Tectonics.

2. Explore one other location on the Earth and discuss how satellite altimetry helps you to understand the regional geology.

The GEOID link (http://atlas.geo.cornell.edu/webmap/) brings up a World map, from which various data sets are accessible. Use the Set Map Extent button to select United States, then Show Map. Use the Show Data Sets button to get a choice of types and maps, then Images/Grids and finally double-click US Gravity. Clicking on Submit Request should yield a colored map of the United States with blues (negative gravity anomalies - "low gravity") coloring the western interior of the US. [Note that, for more precise reference, you can select Geography, US State borders.] The western gravity low is the result of hot, buoyant (and less dense) rock at shallow depths beneath the Rocky Mountains. Observe the sinuous ("snake-like") band of bright green (positive anomaly - "high gravity") from Nebraska NE to Minnesota/Lake Superior.

1. What type of rock might result in such an anomaly? How might it have formed? [Hint: consider the present mid-ocean ridges!] When might it have formed?

Geophysics - Magnetism

Like gravity, the Earth has a magnetic field. Also like gravity, that field displays local variations termed anomalies. If you still have the US Gravity map displayed from the previous (Gravity) exercise, click on US Gravity in the Data Sets box, then Clear Selected to delete it. Now return to the Images/Grids menu and select US Magnetic, then Submit Request.

1. Why is the anomaly showing up on both the magnetism and gravity survey? What property of the rock present causes both a magnetic and gravity anomaly?

One of the major applications of magnetic survey is the study of magnetic stripes on the ocean floor. This topic is covered in detail in your textbook and we will not examine it further here.

Geophysics - Seismic Waves

Seismic (sound) waves travel through any material - air, water, and rock. In air they weaken rapidly (you can't hear thunder from much more than about ten miles away). In water and rock, seismic waves travel efficiently over long distances. However, a very powerful source of energy is needed to generate the initial wave.

On Earth, we use the energy released in earthquakes as the signal source. Seismic waves from earthquakes are described as ringing the Earth "like a bell". Like a bell, the Earth vibrates for a long time, however, it is the first arriving waves that carry the most information.

Examine the IRIS poster (in the classroom or on the wall in the hall outside). [Note: there is a lot of material here! Much of it will be discussed later in the context of earthquakes and earthquake hazard!] Notice how the first arrival (the direct P wave) seems to accelerate (travels farther in a shorter time) with distance for the first 135° around the Earth, then abruptly jumps sideways (arrives later) by several minutes at that distance.

1. What characteristic of the Earth, and particularly the deep Earth, might explain the higher average velocities with increasing distance? What other characteristic might explain the marked delay of first arrivals beyond 135°? [Note - examine the wave paths that arrive at those distances for clues.]

As with gravity and magnetic fields, minor differences from an expected value of seismic velocities can provide valuable clues to material properties. Dense datasets of delay of seismic waves (through hot material) and acceleration (through cold material) have enabled the 3-D reconstruction of such features in the mantle (as seen on Dr. Louie's page in the Introduction) and crust. Yellowstone National Park is described as a "window on the Earth's interior" because of the detailed geophysical studies, including seismic tomography, by Dr. R. B. Smith of the University of Utah and many others.

Geophysics - Summary

The objective of this question is to discover how a geologist might use astronomical remote sensing to estimate the mass of another planet, and then combine that mass with other data to arrive at a “ballpark” estimate of the internal composition of that planet. You will only need a calculator and your brain, the textbook will not be of much help (but you will need it to help answer the final question).

Method

The mass of a planet (or primary body) can be measured by the effect of its gravity on the velocity of anything in the immediate vicinity, such as natural satellites or space probes. The smaller the mass of the effected object (or secondary body) compared to the primary, the more accurate the result. We can thank Kepler and Newton for this!

Mars has two very small satellites, Phobos and Deimos, which have almost circular orbits. You have a colleague who is an astronomer and she assures you that she can supply you with accurate values for the following data:

• Orbital radii for Phobos & Deimos
• Orbital periods for Phobos & Deimos
• Diameter of Mars

Unfortunately your astronomer friend has supplied you with values in less-than-ideal units - they must be converted to kg, m, and s or these calculations will not work!

Notes: orbital radii are always quoted as being measured from the center of the primary body, so you do not need to add the radius of the planet. 1 mean solar day = 24 hours. Mars is much less spherical than Earth, Venus or Mercury, so do not expect your value for density to be extremely accurate, we are aiming for first-order calculations.

Some constants and formulas:
π = 3.14159… (dimensionless transcendental ratio)
G = 6.67x10-11Nm²/kg² (universal gravitational constant)
v = 2πr/t (circular velocity in meters per second)
t = time (orbital period in seconds)
r = radius (mean distance from center of planet in meters)
M_Å = Mass of Earth (5.976x10²⁴kg)
Æ_Å = Diameter of Earth (12756km)
M♂ = Mass of Mars (?)
Æ♂ = Diameter of Mars (6794km)

Orbital radius: 9.377x10³km	= m
Orbital period: 07h39.2m	= s
Orbital velocity = 2πr/t	= m/s

Substitute into the formula
M = rv2/G	M♂ = kg

Orbital radius: 2.3436x10⁴km	= m
Orbital period: 01d06h17.9m	= s
Orbital velocity = 2πr/t	= m/s

Substitute into the formula
M = rv2/G	M♂ = kg

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11. So now you have a mass for the planet Mars – Bravo! How close were the masses obtained by the two sets of calculations (in percent)?

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12. Why do you think they might not be exactly the same? List at least two reasons.

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13. Calculate the density of Mars, using the formula for the volume (V) of a sphere:

V = 4/3πr3

D = M♂/V

=

How does this compare to the density of Earth, using the same method?

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14. Assume for a moment that Mars has only a rocky outer lithosphere with an average density of 3300kg/m3 (similar to terrestrial basalt) and a metallic nickel-iron alloy core with a density of 9000kg/m3 (slightly higher than pure nickel). This is a simplified but not unreasonable assumption based on the composition of Earth and meteorites. Using the mass you obtained for Mars and the chart on the next page, estimate the maximum size that the core could be.

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Extra credit question: Based on these results, would you expect Mars to have a strong magnetic field? Why or why not? Do you think this is a trick question?

(Use the back of the next page to write your answer)