Impact Craters AST 1022L

Transcription

1 Impact Craters AST 1022L

2 Crater Cross- Section *Breccia: rock made of shattered fragments cemented back together

3 Terrestrial Craters I Meteor Crater, AZ 1.2 km across 170 m deep 50,000 years old Impactor width: ~ 50 m

4 Terrestrial Craters II Chicxulub Crater, Mexico 180 km across 20 km deep (mostly buried) Impactor width: ~ 12 km 66 million years old Likely caused extinction of non-avian dinosaurs Crater Earthquakes > mag. 10 Fireball Ejecta Earthquakes Tsunami

5 Terrestrial Crater Map 204 confirmed impact structures & 3 airbursts (white)

6 Craters on the Moon Rayed craters: craters with splatters (e.g. Tycho) Maria (singular mare): dark basins where lava filled in extraordinarily deep early craters

7 Impacts in the Solar System: Mars Note the difference in number of craters, & elevation, between the northern & southern hemispheres

8 Impacts in the Solar System: Mercury Left: Mercury s north pole as seen by MESSENGER + color overlay Red: areas in permanent shadow Yellow: radar indications of water ice on a planet where sunlit surfaces can reach 700 K!

9 Jupiter: Comet Shoemaker- Levy 9 (July 1994)

10 Impact Physics Kinetic energy: the work (force times displacement) needed to accelerate an object from rest to its given velocity Energy as a function of mass Energy as a function of velocity

11 Uncertainty & Error Analysis A BIT ABOUT STATISTICS

12 Precision Vs. Accuracy Accuracy: how well the measured value agrees with an accepted/ theoretical value. Precision: degree of consistency among independent measurements of the same quantity; reproducibility or reliability of the result

13 Error (a.k.a. Uncertainty): The Reason for Sig Figs RANDOM Statistical fluctuations (+ or -) due to precision limits of measuring tools & chaos inherent in nature Statistically analyzable Reducible by averaging over many observations SYSTEMATIC Consistent, reproducible inaccuracies that tend to follow a pattern Cannot be statistically analyzed, or reduced via repetition & averaging If found while calibrating against a standard, a corrective factor may help

14 Possible Sources of Error Instrument/visual resolution (random) Parallax/moving vantage point (systematic or random) Environmental factors (systematic or random) Physical changes in the system (systematic or random) Incomplete/ambiguous definition of what is being measured (may be systematic or random) Failure to account for a factor (usually systematic)* Improper calibration of an instrument (systematic)* *These can be mistakes depending on the limits of our understanding of the physics & equipment involved

15 Human Error, i.e. Mistakes Poor adherence to proper procedures Misuse of lab equipment Arithmetic mistakes Personal Bias (trying to get a particular result) If asked to identify sources of error in an experiment, human error is NOT a valid response! You are being graded on your ability to perform the experiment correctly.

16 Normal/Gaussian Distributions (Nature Loves Em) Standard Deviation (σ): a measure of spread in data randomly distributed about an average x i = an individual measurement n = total # of measurements x = mean (average)

17 Experiment Part #1 Drop each of the 8 impactors from a height of 1 meter (find a way to keep track of those you ve dropped so you don t do the same one twice) Plot crater diameter vs. impactor diameter AFTER you have the data for parts 2 & 3 Impactor Diameters (ignore error)

18 Experiment Parts #2 & #3: Choose one midsized sphere (e.g. the 15.1 mm) & PART 2 Drop from a height of 1 meter, 10 times, recording the crater diameter each time Compute the standard deviation ONLY AFTER you have the data for part #3 PART 3 Drop from 100 cm, then 90 cm, then 80, decreasing the height in 10 cm increments until you get to 10 cm. Plot crater diameter as a function of drop height Hint for Part 3

19 For Next Week First formal lab report due: Impact Craters First night lab on Sept. 12 Target TBD Read the manual on How Big is the Sun? Also read You Can Weigh Jupiter, in case it s too cloudy to observe the Sun Now let s go play in the sand!