Impact Age Dating. ASTRO 202 Lecture Thursday, February 14, Review. What is relative age dating? What is relative age dating?

Transcription

1 Review Impact Age Dating ASTRO 202 Lecture Thursday, February 14, 2008 Carbon-14, Potassium-Argon isotopic age determination: (1) Parent decays to daughter at some predictable rate (2) How much now? (3) How much then??? What is relative age dating? What is relative age dating? Before absolute age scales relative age scales Before absolute age scales relative age scales A terrestrial example: layered deposits A terrestrial example: layered deposits 1

2 What is relative age dating? The rationale for using crater counts for relative age dating: - Assume surfaces form without any craters Craters are the result of impacts with asteroids and comets -Two ways to establish relative scale: (1) The older a surface, the more craters it accumulates (2) Larger impactors are less common than smaller; the more large craters on a surface, the older it is Processes that remove craters from a surface: Result is a variety of cratered surfaces Enceladus Erosion Gravity - Rain/water, ice, wind Resurfacing - Landslides, mass wasting - Volcanism, tectonics Earth 2

3 Moderately/sparsely cratered Venus Martian dichotomy Mars Southern Hemisphere Northern Hemisphere Mare (young) Heavily cratered: Mercury Highlands (old) The Moon 3

4 Tethys - saturnian satellite Diagram courtesy of: saturn.jpl.nasa.gov Odysseus Crater Tethys - saturnian satellite - seems to have survived a giant impact Rhea -large satellite of Saturn - seems saturated with craters -one of the most heavily cratered surfaces in the Solar System 4

5 Counting Craters Counting Craters The method: (1) What is a crater? -Resolution, various stages of erosion can make consistent identification difficult?????????? Counting Craters Counting Craters The method: (1) What is a crater? -Resolution, various stages of erosion can make consistent identification difficult (2) How do we keep track? -Size-frequency distributions (SDF( SDF s) ) record both the number and sizes of craters in a given km 2 D Cumulative densit y of crater s NH>DL All Craters Identified as Secondaries in the Newton Area Crater diameter, 5

6 How do we make the relative scale absolute? Apollo Lunar Sample Return Apollo Program: The relative scale is only so useful For constraining the history of planets and Solar System formation, we need something better. - Combine relative scale with known absolute ages Apollo program to the rescue! 11, 12, Missions landed and safely returned humans from the Moon and samples!!! Combining Relative and Absolute Scales Extending the Absolute Scale Beyond the Moon From the SFD s, we add the Apollo sample return absolute ages to create crater isochrons for the Moon We still don t t have the ability to retrieve samples from the other planetary surfaces, so how do we determine absolute ages for these bodies? -Know something about the distribution of impactors that cause the cratering we see on the Moon -Determine how this distribution differs at other places in the Solar S System Some other factors that affect cratering distribution: -Atmosphere, gravity, surface strength, erosion and resurfacing rates 6

7 Extending the Absolute Scale Beyond the Moon Extending the Absolute Scale Beyond the Moon Example martian isochrons Extending the Absolute Scale Beyond the Moon Controversy There are a number of controversies when it comes to dating the Solar System -The Late Heavy Bombardment -Secondary Cratering 7

8 Late Heavy Bombardment Late Heavy Bombardment What is the late heavy bombardment? -Some scientists believe that the number of impactors has undergone a steady exponential decrease since the formation of the Solar System -Others cite the Apollo sample returns as evidence that there was a burst of impacts around 4 Ga known as the late heavy bombardment Why does the late heavy bombardment matter? -Did it happen everywhere in the Solar System? -Are the extrapolations of the absolute scale to other surfaces correct? -What are the implications for models of Solar System formation? Secondary Cratering Secondary Cratering Shapes of the crater isochrons: Shapes of the crater isochrons: Eugene Shoemaker identified two shapes or branches of the cratering isochrons for the Moon Steep slope Shallow slope Fit power-laws (SFD s) of the form: N(>D) = c D -b Steep power-law index (b~3,4) dubbed secondary branch; shallower (b~2) index called primary branch Secondaries footprints of ejecta from large primary impacts 8

9 Secondary Cratering Dominance of secondary and primary branches of SFD -Some groups claim that secondaries dominate at D < 1 km -Others claim that secondaries are at most 10% of small Martian craters Are secondaries even secondaries? -Neukum, Ivanov and others argue that the secondary branch is formed by a small-diameter asteroid population Review -Even on planets and satellites where we can t t use surface samples to derive absolute ages, we can establish a relative age scale from f crater size-distribution counts -From lunar samples returned to Earth by the Apollo astronauts, we can construct a viable absolute age scale for surfaces on the Moon -Using what we know about the surface strength, gravity, resurfacing and impactor rates on other planets and satellites, the absolute age scale for the Moon can be extrapolated to a wide variety of Solar System surfaces -Controversial issues such as the existence of the late heavy bombardment and secondary cratering need to be resolved to allow for more accurate absolute dating of surfaces in our Solar System Example crater isochrons for Mars and the Moon from: 9