Astronomy 101
Spring 2001
2nd Midterm Exam: ANSWER KEY
  1. [9 pts] What three criteria [list them] did we agree are necessary conditions for defining an object as a planet?


    2. answer: 1) object's primary orbit must be around a star [note: orbits around "the sun" are too
                           parochial; definitions that lack "primary" could include moons, which also orbit stars]
                     2) object must be big enough to be round (size controlled by gravity) [note: just saying
                          "round" is insufficient - basketballs are round]
                     3) object must be small enough to not be a star or brown dwarf (no nuclear fusion)
     
     

  2. [3 pts] Why are Centaurs, like Chiron, on unstable orbits?


    3. answer: Centaurs cross or come close to one or more orbits of the outer planets.  The gravitational
                     influence of the giant planets will constantly push and pull a centaur, thus making it's orbit
                     unstable.

  3. [3 pts] What is the approximate size (diameter) of a typical comet?


    4. answer: 5-15 km [note: not meters; not 100s of km]
     

  4. [3 pts] What is the most abundant material in a typical comet?


    5. answer: ice. [preferred answer was water or water ice]
     

  5. [3 pts] Where is theKuiper Belt located?


    6. answer: 40-200 AU; or, from Pluto's orbit outwards to a few 100 AU; or from Neptune's orbit
                     outwards. [note: "around 40 AU" implies too narrow a region]
     

  6. [3 pts] Where is the Asteroid Belt located?


    7. answer: in between the orbits of Mars and Jupiter; or, approximately from 2.2 to 3.3 AU
     

  7. [3 pts] Approximately when were the first Kuiper Belt objects discovered?


    8. answer: early 1990s; or 1991-1992; or the very late 20th century.  Or, if you had a defense of your
                     answer, 1970s or 1978 (discovery of Chiron) or 1930s or 1931 (discovery of Pluto).
     

  8. [3 pts] Approximately when were the first Asteroid Belt objects discovered?


    9. answer: 1801-1802; or early 19th century; or, the 1800s; or ~1800-1810
     

  9. [10 pts] We classify meteorites into three distinct classes: irons, stoney-irons, and stones. How do we explain how these three broad classes of meteorties formed? In this regard, what is the significance of the Allende meteorite?


    10. note: many of you told me lots about Allende, but ignored the words "in this regard" and so your answer had little or nothing to do with why there are three broad classes of meteorites.

    answer: Most likely, they all formed inside a large, asteroid "parent" body that broke into small fragments after a collision.  Fragments from the core became iron meteorites (if/when they hit Earth); fragments from the crust became stoney meteorites.  The inside of the asteroid had a core and crust because it once became warm enough to differentiate, permitting the denser iron to sink to the center and the lighter rocky materials to rise upwards.

    The Allende meteorite contains the isotope 26Mg in places where an aluminum atom, not a magnesium atom, normally should be.  Thus, we are confident that the 26Mg is the radioactive decay product of 26Al. And 26Al would have been an abundant material with a short (700,000) year half-life and thus was likely the heat source for melting fairly small (few km in diameter) asteroids, thus effectively creating a reservoir from which iron and stoney meteorites could be released.  In summary, Allende tells us that 26Al  was live, hot, in the early solar system and was the material that enabled asteroids to heat and differentiate.
     

  10. [10 pts] Why, on Sept 23, 1846, did the German astronomer Johann Galle, working at the Berlin Obsevatory, discover Neptune?


    11. answer: Neptune had been predicted to exist, by the Englishman Couch and the Frenchman Leverrier,  on the basis of problems with correctly predicting the orbit of Uranus.  They used Newton's law of gravity (not the Titius-Bode law) to surmise that another object must exist further from the Sun than Uranus. They predicted exactly where that object must be. Leverrier sent his predictions to Galle who looked and, bingo, found Neptune. [note: Galle didn't make the predictions; he just made the observations.]
     

  11. [10 pts] For over 20 years, the distinguished astronomer Sir Fred Hoyle and his colleague Chandra Wickramasinghe have argued that the great flu pandemics are caused by flu viruses that originate in comets.  These viruses, they claim, are swept into Earth's atmosphere as the Earth passes through the tails or orbital paths of comets. The idea that flu viruses originate in comets is very far out of the scientific mainstream and Hoyle and Wickramasinghe have almost no supporters in the scientific community.  Is this idea completely flawed or can you suggest any reasons why this theory should be taken seriously?


    12. answer: The idea is not completely ridiculous, even if it is wrong.  We think Earth's water came from comets.  Also, the earth does pass through comet tails and comet orbital paths, thereby sweeping up many tons of cometary dust each year.  In addition, we have very strong evidence, in the Murchison meteorite, for the presence of at least 92 amino acids and all five nucleotide bases found in RNA and DNA.  Thus, it is possible that life on Earth was "seeded" by a comet, or cometary debris.  And if this is possible, then it is not completely impossible to imagine that a virus could form or at least survive, frozen, in a comet.  [note: many of you argued that comet dust could absolutely not make it to the surface of the Earth. It is true that small chunks are likely broken up pretty effectively in the atmosphere, and that much of this material is likely torn apart all the way to the level of atoms - thus providing us with "shooting stars."  But we have no way of knowing whether all such material is destroyed, and in fact, we know that meteorites make it to the ground.  We also have collected material believed to be cometary dust by using airplanes flying in the upper atmosphere.  So we absolutely cannot simply dismiss this idea by saying that we know that no cometary materials, let alone a virus, could survive a descent through our atmosphere.]
     

  12. [10 pts] List (do not explain) five lines of evidence that demonstrate that an asteroid impact event occurred 65 million years ago, at the approximate time of the demise of the dinosaurs (the Cretaceous-Tertiary boundary). For one of these five - not including the crater itself - explain the logical connection between an impact event and this piece of evidence.


    13. answer: 1) excess iridium in the boundary clay, 2) shocked quartz grains (stishovite) in the boundary clay, 3) unusual amino acids found in the boundary clay, 4) tektites (glassy spherules) found in the boundary clay, 5) buckyballs containing non-terrestrial ratios of trapped rared gases in the boundary clay, 6) soot and ash in the boundary clay, 7) tsunami deposits around the Gulf of Mexico, 8) the Chixculub crater. [note: simply saying "boundary-layer" is insufficient. ]

    various: see lecture notes
     

  13. [10 pts] The most recently discovered object in the solar system, now named "2001 DC77," was identified on February 20, 2001.  This object has a semi-major axis a = 2.7 and an eccentricity e=0.5. Into what group of solar system objects would we classify 2001 DC77, and why would you classify 2001 DC77 in this way?


    14. answer: This clearly isn't a new planet and is most likely an asteroid.  With a = 2.7 AU and e=0.5, we know that 2001 DC77 comes as close as a(1-e)=1.35 AU from the Sun and goes as far as a(1+e)=4.05 AU from the Sun.  Thus, it is a Mars crossing asteroid ("Amors") (and not an Earth-crosser, or Apollo). [notes: some of you calculated minimum and maximum distances as min = a - e = 2.2 and max = a + e = 3.2.  The right formula, above, would be min = a - ae and max = a + ae. ]
     

  14. [10 pts] The Titius-Bode formula, developed in the 1760s, is an attempt to explain the physical spacings of the planets' orbits (i.e., the sizes of their orbital semi-major axes).  The Titius-Bode formula can be written as distance = 0.4 + 0.3 * 2n, where distance is in units of AU and "n" is the planet number, ignoring Mercury and starting with Venus (n=0, 2n=1, distance=0.7), Earth (n=1, 2n=2, distance=1.0), Mars (n=2, 2n=4, distance=1.6), etc.  In the 1760s, what did the Titius-Bode formula predict that was not already known? The Titius-Bode formula was experimentally tested over the following two centuries.   What were the outcomes of these tests and what do the test results tell us about the value of the Titius-Bode formula?


    15. answer: The Titius-Bode law, c. 1760, predicted the existence of yet undiscovered planets at 2.8 AU, 19.6 AU, 38.8 AU, 77.2 AU etc.

    When Uranus was discovered, it seemed to support the T-B rule, with distance = 19.2 AU, close to the predicted 19.6.  [note: Uranus was not discovered because Herschell was motivated to look by the T-B rule.  Applying the T-B rule to Uranus must be done post-discovery.] Then, when Ceres was discovered, it also seemed to be supportive, at just under 2.8 AU. [note: the search for Ceres was motivated by the T-B rule and the fact that Uranus seemed to have fit the rule.] But then lots of asteroids were discovered at 2.8 AU.  and then Neptune was discovered at 30.1 AU and Pluto at 39.5 AU.  Neptune "should" have been at 38.8 and Pluto "should" have been at 77.2 AU and these errors between prediction and reality are so extreme as to render the T-B rule worthless.  So, the T-B rule really isn't of any value in terms of physics; it's just a fun formula that was created to match the known distances (to within about 10%) of the planets (and planet-like objects like Ceres).
     

  15. [10 pts] Comet Halley, which orbits the Sun in about 76 years, was losing mass at the rate of about 54 tons per second as it passed through the inner solar system in 1986.  What conclusions can we draw about comets and the solar system from this single measurement of the mass loss rate from Comet Halley? You must provide some explanation to justify your answer.


    16. answer: At this mass loss rate, Halley can't last forever.  It might survive for a few thousand passages past the Sun, but must have a finite lifetime that is much shorter than the age of the solar system.  Since new comets are discovered every year, there must be a reservoir of comets from which new comets are derived. Thus, we are immediately led to conclude that something like the Kuiper Belt or Oort cloud must exist.