Characteristics and Origins of the Solar System

Lecture 9

September 19, 2001

 

>>>>> Astronomical event of the week: the autumnal equinox at 6:02 PM on Saturday.  Think about what it means.

 

The Earth as a Planet, Continued

 

             There are some remarkable aspects of the Earth from an astronomical perspective.  These have to do with the atmosphere of the Earth and its oceans.  This material is well-described in the book in Chapter 7. 

 

A.     The Atmosphere of Earth

>>>>>>> Look at Figure 7.11, showing profile of atmosphere.

 

·        The composition is primarily N2 and O2.  Next Argon, water vapor.  Everything else is trace gases.

·        Height: the lowest level of the atmosphere is the troposphere.  Its scale height is 9.5 kilometers.  At one scale height of altitude, the atmosphere pressure is down to 37% of what it is at sea level, so you are well on your way to outer space.  Commercial airliners routinely fly at an altitude of one scale height or higher.

·        The chemical composition of the Earth’s atmosphere is unusual by planetary standards.  The other two terrestrial planets have atmospheres which are dominated by CO2,  with almost no oxygen.  The presence of oxygen as a major gaseous constituent is a cosmic oddity.  See explanation below. 

·        The Earth’s atmosphere is the arena for some of the most important science policy issues of the present day.  These are the phenomena of stratospheric ozone depletion  and global warming ,  or the Greenhouse Effect.

 

  1. The Ocean

 

·                        Earth is the only solar system object where liquid water is stable on  the       surface.  Earth has lots of it.  About 70% of the surface is covered by water, up to a depth of several kilometers.

·                                      How did the water get here?  Oddly enough, this is something of a mystery in planetary astronomy.  We can understand why it is that rocky,  as opposed to gaseous planets, formed in the inner solar system.  How a volatile material like water formed here is not so well understood.  To understand this, we need to consider the remote prehistory of the Earth, and we come face-to-face with astronomical phenomena. 

 

  1. Geological/Astronomical History of the Earth

 

>>>>>>  Picture from McKay article with history of the Earth.

·        We infer that the Earth is 4.5 – 4.6 billion years old.  This result comes from radioisotope dating of rocks, but not Earth rocks.  No Earth rocks are that old. 

·        Water was apparently brought to Earth by ancient comets.  The Deuterium to Hydrogen ratio for seawater is similar to that of comets;  quite different from that of Jupiter. 

·        The original atmosphere of the Earth was primarily CO2.

·        Evidence of first, microbial life forming 0.5 – 1.0 Gyr after the formation of the Earth.  Very nice description on pp157,158 of textbook. 

·        First forms of life on Earth: colonies of blue-green algae called Stromatolites.  Look at gripping picture in Figure 7.13 of book. 

·        Stromatolites produced oxygen as product.  For first couple of Gyrs,  this oxygen oxidized iron in seawater, so there was no buildup in the atmosphere.  Starting about 2 Gyr ago,  O2 content of atmosphere began to increase.  We are so certain that the presence of oxygen was a product of the biology of Earth that future spacecraft will search for life on planets around other stars by looking for evidence of oxygen in the atmospheres of those planets.

·        580 million years ago was the Cambrian Explosion,  with the apparently sudden emergence of multicellular life. 

·        The story of ancient (and continuing) impacts of large objects on the Earth is an interesting story, but one better delayed until we have studied the Moon and planets more.