FAQ1
ES10
February 24, 2003

1. What is a planetisimal?
1. A planetisimal is a small body that is the precursor of a planet in the early stages of solar system development, or the fragmented result of a cataclysmic collision between a planet and another celestial body. 
2. Meteorite is a solid mass of mineral or rock matter that has fallen to the earth’s surface from outer space without being completely vaporized in the atmosphere
3. Asteroid is a rocky body, less than 1000 kilometers across, in orbit around the sun
4. Comet is composed mainly of ices and dust, that usually follows a highly elliptical orbit and displays a long tail of gas and dust when warmed by the sun.   

The earth is constantly bombarded by extraterrestrial objects.  These objects may deliver minute amounts of water, organic material and other materials to Earth.  The amounts are under intense debate.  

2. What is ozone?  

Ozone = O₃, a tri-atomic form of oxygen.  Ozone is a pungent unstable blue gas.  In the upper atmosphere, ozone forms a protective layer against excess ultra-violet radiation. In the lower atmosphere, ozone is an ingredient of photochemical smog.  It is used in purification of drinking water and is an oxidizing agent.

3. U-shaped versus V-shaped valleys:  http://wrgis.wr.usgs.gov/docs/usgsnps/glacier/uvalley.htm

4. What is firn?  What is it made up of?

Firn is the loose permeable granular material transitional between snow and glacial ice.  Firn can be considered as the top layers of a glacier, where air is freely mixed within the pore air spaces in the snow.   At greater depths in the glacier, the increasing pressure of the snow on top causes air bubbles to form.  The air bubbles thus contain a mixture of air of different ages – from the snow fall to the bubble closure.  Analysis of the composition of air bubbles trapped in glaciers yields information about atmospheric composition in the past.  How far back in time we can go depends on the depth of the glaciers. 

5. What is the Red Shift?

Light and sound waves are described in terms of their spectra.  When an ambulance approaches, the sound appears to have higher pitch (higher frequency).  The pitch is lower (lower frequency) when it is moving away.  This effect, which occurs for all wave motion, including sound and light waves, is called the Doppler effect.  In the case of light, when a source is moving away, its light appears redder (longer wavelength, lower frequency) than it actually is.  “Red Shift” is the observation in 1929, by Edwin Hubble, that most galaxies have Doppler shifts toward the red end of the spectrum, indicating that the galaxies are receding from the Earth.  Dimmer galaxies are probably further away from brighter galaxies.  Using estimated distances based on relative brightness and the observed Doppler red shifts, Hubble discovered that galaxies that exhibit the greatest red shifts are the most distant (dimmest).  The observations show that all galaxies are moving away from us.  An expanding universe can adequately account for the observed red shifts.     The concept of an expanding universe led to the Big Bang Theory.

 A good explanation of the Red Shift is found at this site:   http://www.pbs.org/deepspace/classroom/activity2.html

6. What is an isotope? 

An isotope is a member of a chemical-element family that has two or more nuclides with the same number of protons (same atomic number) but a different number of neutrons (different mass number = protons + neutrons). They have the same chemical attributes (the same number of protons = the same number of electrons), but they often display different physical attributes. ¹H has one proton and no neutrons, and ²H (deuterium) has one proton and one neutron in the nucleus. Both are stable, while ³H (tritium) undergoes radioactive decay. ¹²C and ¹³C are stable while ¹⁴C is radioactive. ¹⁶O and ¹⁸O are two stable isotopes of oxygen.  

7. What is fractionation and why is it important for environmental science? 

Fractionation refers to a process that separates the components of a mixture into fractions. Isotopic fractionation would result in a change in the isotopic composition of a substance.

In our context, we focus on the isotopic fractionation of water. Water (H₂O) is a mixture of ¹H₂ ¹⁶O (molecular weight [MW] 18, the most abundant form), ¹H²H ¹⁶O (MW 19), ¹H ₂ ¹⁸O (MW20), etc. Different isotopes of the same element have different mass, and so different rates of diffusion, evaporation and chemical or biological reaction. Slower rates are associated with the heavier isotope. Suppose we start out with a mixture of (liquid) ¹H₂¹⁶O and ¹H₂¹⁸O and let it evaporate. The ratio of ¹H₂¹⁸O : ¹H₂¹⁶O in the vapor would be lower than the ratio in the original liquid because of the slower rates of the heavier isotope. Hence we say that there is isotopic fractionation associated with evaporation. Similarly for the other processes. Diffusion, evaporation and condensation rates are temperature dependent. We therefore use the relative abundance of stable isotopes is used to diagnose temperature. Past temperatures are deduced from variations of ¹⁸O/¹⁶O ratios in ocean sediment cores and of ¹⁸O/¹⁶O and ²H/¹H ratios in ice cores.

The relative abundances of the stable isotopes of carbon are used to diagnose biological activity. For photosynthesis, the diffusion rate of ¹³CO₂ from the ambient atmosphere to the stomates is slower than the diffusion rate of ¹²CO₂. The biological fixation rate of ¹³C is also slower than that of ¹²C. There is thus a typical isotopic fraction associated with photosynthetic products or organic materials. Finding the typically lower ¹³C/¹²C in rocks as old as 4 billion years old is how we infer that there were biological activities then.  

8. What dating methods have been used to determine the age of the Earth?

 Radioactive isotopes are those whose nuclei spontaneously decay, usually by emitting some neutrons or protons.  The decaying radioactive isotope, or parent isotope, evolves into a decay product, the daughter isotope.  Radioactive decay rates are constant, unaffected by changes in temperature, pressure or by chemical reactions involving the parent isotopes.  The decay rate is characterized by the half-life of the parent, i.e. when X atoms of the parent decays into X/2 atoms of the parent, and X/2 atoms of the daughter.    Different radioactive isotopes have different half-lives, and are useful for dating.

9. Can you explain plate tectonics?  

 In the Earth’s interior, there is residual heat from the formation of the Earth, and heat from radioactive decay.  The lower mantle is thus hotter than the upper mantle.  This temperature gradient drives convection cells in the mantle.  Convection is the major way to deliver heat away from the center of the Earth so the heat can escape.  These heat-driven currents within the mantle are a major driving mechanism for plate movements. 

10. At what rate are various parts of California moving due to plate tectonics?

An average plate velocity is 4 cm/yr.  Of course this varies from plate to plate.  San Francisco Bay Area is part of the complex plate boundary system between the Pacific and Northern American Plates.  The San Andreas Fault is the dominant expression of the collision between these plates in California.  San Franciso, on the eastern side of the San Andreas fault, is on the North American Plate.  Los Angeles, on the western side of the fault, is on the Pacific Plate.  Relative to the North American Plate, the Pacific Plate is moving northwest; the Juan de Fuca Plate is moving eastward. 

 There is a superb movie of plate motion:  http://animations.geol.ucsb.edu/

11. Describe atmospheric oxygen and banded iron formation 

How much oxygen (or any gas) is in the atmosphere depends on how fast oxygen is produced versus how fast it is removed. Obviously excess of production over removal will allow oxygen to accumulate in the atmosphere. In the atmosphere now, oxygen is production by photosynthesis nearly balances removal by chemical and biological processes. Before 3.8 Billion years ago, we conjecture that atmospheric oxygen was very low, and that removal of oxygen was faster than the production.

One piece of geological evidence is the occurrence of banded iron formations (BIF). These formations are laminated sedimentary rocks that consist of alternating layers of iron-rich (dark-colored) and silica-rich (light-colored) minerals. Most BIF’s were formed in the Proterozoic era, from 2.5 to 1.9 billion years ago. None is produced today or in the recent past. Absence of BIF’s older than 2.5 billion years is suggestive of the lack of oxygen in the early atmosphere – iron probably existed in un-oxidized states. Absence of BIF’s younger than 1.9 billion years ago is suggestive of the near-complete oxidation of the available iron. And so the BIF’s indicate the rise of oxygen in the early atmosphere.

How BIF’s were formed remains a subject of debate. One hypothesis goes as follows: