Lecture Notes:

• HALE-BOPP
  Problem set #4 is due on April 17 and deals with the path of the comet across the sky and length of its tail. Look at the photos of the comet at http://lyra.colorado.edu/sbo/hale-bopp/halebopp.html

• PLANETS
  There are two groups of planets that are distinguished by albedos and density which are the terrestrial planets and the gas giants. Low albedos of the moon and Mercury are due to scorching by sunlight; low albedos of satellites of the gas giants or of the asteroids come from primordial carbon. The terrestial planets lost their gaseous atmospheres due to baking by the early sun and hot inner solar nebula.

• EARTHS MOON
  The albedo of the moon is something of a surprise: it is like a chunk of a charcoal briquette in its albedo of .07.

  Major craters and other features of the moon that I would like you to learn are Mare Imbrium, Serenetatis and Crisium, and craters: (from young to old) Tycho (500 million years), Copernicus (1.1 billion years), Eratosthenes (3.2 billion years), and Archimedes and Plato. The latter two were form between the Imbrium impact (3.85 billion years) and the subsequent flooding of basins that continued until 3.2 billion years. Their floors are flooded; others such as ghost craters were almost entire inundated by the basalt.

  Eratosthenes is clearly older than Copernicus because all its rays have faded. Tycho is the youngest large crater judging form its extensive ray system. The rays are bright because of "gardening" of soil by a swarm of small projectiles ejected at the time of impact; dirt has been turned over and is therefore brighter but with time the fresh dirt also darkens due to sunlight.

  The superposition of features can be used to date them relative to one another: from the oldest to the youngest:

  1. Highland features present before the Imbrium impact marked by scratches produced by hurtling rock thrown out by the impact.
  2. Imbrium basin
  3. Craters produced in the basin prior to flooding
  4. Flooded maria
  5. Eratosthenes: post Imbrium flooding but with faded rays
  6. Copernicus extensive ejecta blanket and rays across Imbrium
  7. Tycho: extensive and bright rays.

• VENUS
  The run-away greenhouse effect has established much of characterizes this sister of the earth. It may have started it life as a near twin of the earth, with oceans resulting from comet nuclei that impacted on it. Some 85% of its surface is now lowland plains; in the past that area may have been flooded by vast oceans. Because Venus is just slightly closer to the sun than the earth, its higher temperature caused an early and powerful atmospheric greenhouse, produced by water vapor and carbon dioxide. Carbon dioxide is now absorbed in the earths rocks and oceans, but in an earlier and hotter Venus, there would have been more carbon dioxide in the atmosphere. Its elevated temperature due to the greenhouse caused even more carbon dioxide and water vapor to enter the atmosphere, increasing the temperature even more. Higher temperature released more carbon dioxide and water and so it went, up up up the temperature rose until all the oceans were vaporized and carbon dioxide was outgassed from the surface rocks. Now its temperature is 700 K or 900 F; the atmospheric pressure is 92 times that of the earth. Upon each square inch of its surface, the atmosphere presses down with the weight of 2/3 of a ton.

  Volcanic eruptions have injected sulfur and fluorine into the atmosphere which has combined with carbon, hydrogen, and oxygen to produce some of the most corrosive liquids known by chemists. Suspended in its atmosphere at a height of 50 km are droplets of sulfuric, hydrochloric( HCl) hydrofloric (HF) and fluorosulfuric acids. The latter, especially, dissolves lead, tin, and most rocks. Below the layer of acid droplets it is a clear, hot, and dense atmosphere.

  The greenhouse effect is produced by a layer of gas (the so-called green house gasses that are carbin dioxide, water vapor, and methane) that is transparent to visible light and opaque to infrared light. Ordinary glass has the same properties, hence cars heat up during the summer when light enters through their windshields. Energy enters as visible light, but is trapped when it is transformed to infrared light.

  We hope such is not the fate of the earth as we pour more and more carbon dioxide into our atmosphere. Venus, bright and beautiful from a distance, shines brilliantly in the sky as a dreadful warning to us about misusing our planet.

  The surface of the planet is dominated by volcanic features. There are very few impact craters, although many must have been formed in the past during times of intense bombardment. Their evidence must have been erased by volcanic activity. There are no impact craters smaller than 10 km on Venus; meteorites that would have produced them would have been burned up in passing through its thick atmosphere.

  The remaining 15% of the surface area of Venus is composes of highlands, such as Ishtar Terra and Aphrodite Terra. Ishtar contains the hightest mountain on Venus, Maxell Mons which rises some 12 km above the lowland plains. Ishtar also contains Lakshmi planum, Cleopatra, Sacajawewa, and Colette.

  Our knowledge of Venus comes from radar mapping by Pioneer in 1979 and Magellan in 1990-3. The first glimpses we had of its surface were from the Soview Venera space craft which landed on its surface in 1983-84, and functioned only briefly before being destroyed.