Lecture 15 Crater counting on Mars (Matt Smith, ESS)

Transcription

1 Tuesday, 24 February Lecture 15 Crater counting on Mars (Matt Smith, ESS) Reading assignment: Ch Radar Basics (p ) Ch Passive microwave (p ) Next lecture Forest remote sensing, Van Kane (CFR)

2 Click to edit Master title style IMPACTS! Click to edit Master text styles WHAT ARE THEY AND WHAT CAN THEY TELL US? Second level Third level Fourth level» Fifth level MATT SMITH ESS 421 2

3 Overview Introduction to cratering Applications of craters to (many) other problems

4 Studying Impact Cratering Why do we care? Most abundant planetary feature in the solar system Useful for dating other planetary features and surfaces Can be used to expose hidden rocks

5 Crater Formation: The Impactors Common impacting objects are comets and asteroids During heavy bombardment (> 3.5 Ga), accretional remnants were prevalent Averaged over geologic time, impactors hit terrestrial planets at a near constant rate

6 Crater Formation: The Path to the Ground Planets with Atmospheres Impactors fragment and lose mass by ablation and vaporization Weeds out small impactors Smallest size able to get through Earth 1.5 m (ice) and 0.6 m (stony) Mars m (ice) and m (stony) Planets without Atmospheres No shielding offered

7 Crater Formation: Impact! Crater size is governed by: Energy of the Impactor Size, velocity, & density Density of Target Material Crater Rim Ejecta Excavation Stage

8 Crater Formation: Morphology SIMPLE CRATERS (small craters) COMPLEX CRATERS (larger craters) SECONDARY CRATERS McEwen et al., 2005

9 APPLICATIONS WHAT ELSE CAN YOU USE CRATERS FOR? SURFACE AGES (CRATER DATING) SURFACE PROCESSES (CRATER LOSS) SURFACE DEFORMATION (CRATER ELLIPTICITY) AGE OF FAULTING EVENTS (CUT CRATERS)

10 Crater dating Count craters per unit area on Moon Radiometrically date lunar rock samples Fit the best isochron to crater counts to determine age Derive lunar isochron Transform from the Moon to Mars Count craters on Mars

11 Crater dating

12 Crater degradation and obliteration Unlike the moon, small crater loss through eolian erosion and infilling is commonly observed on Mars The Moon and Mars cannot be simply related at all diameters! 300 m

13 How do martian craters degrade? Crater floor fills with saltating wind-blown fines and boulders falling from rim Rim and ejecta blanket are largely lost through erosion Crater rim becomes rounded as material sheds onto floor 150 m HiRISE PSP_003972_1305

14 Loss of small craters we have found the observed diameter distribution curving slightly toward shallower slopes at small diameters, i.e., a deficiency at small crater sizes, which we attributed to obliteration losses among small craters by erosion, deposition, lava flows, etc.

15 Maximum age: 20 Ma Maximum age: 5 Ma Maximum age: 400 Ma

16 Simple quantitative model Loss rate of craters: λ(d) Production rate of craters: P(D) Number density of craters: N(D,t)

17 Simple quantitative model Loss rate of craters: λ(d) Production rate of craters: P(D) Number density of craters: N(D,t)

18 Simple quantitative model Apply these inputs to a formula to describe constant production and loss: Model Inputs P(D) Production rate derived from Hartmann Production Function (HPF) (2005) λ(d) = Loss time for a crater of diameter D as a function of β (= combined rates of erosion and deposition) Assumptions Crater loss occurs as shown (through ground lowering and infilling) Once crater depth is reduced to zero, crater is invisible to detection Constant production rate for last 3.5 Ga Proportion of secondaries assumed from modeling work of McEwen et al. (2005)

19 Testing the model Test model against calculated measurements of erosion rates at MER landing sites (Golombek et al., 2006) Erosion rates were converted to β by assuming that the volume of eroded material is locally deposited over crater area (an assumption made by Golombek et al.) Model is fit using nonlinear regression to independent crater counts to derive β

20 Testing the model Spirit Opportunity Smith et al., in review Calculated β (Golombek et al., 2006) nm a nm a -1 Model-fit β 4.72 ± 2.58 nm a ± 6.0 nm a -1

21 Martian erosion rates

22 Model results Modeled effect of obliteration (β) on crater curves Roll-off of crater-abundance curves at small diameters for surfaces of different ages Deposit age = 3 Ga 100 Ma Smith et al., in review

23 Model results Quantin et al., 2004 MOC-NA E We reevaluated one crater-counting study on a Coprates Chasma landslide (Quantin et al., 2004) Calculated age = 400 Ma 30 m-relief flow lines are given as evidence of minimal surface modification since deposition 500 m

24 Model results Age (Quantin et al., 2004) = 400 Ma Model Fit β = 54 ± 37 nm a -1 (Opportunity Rover β = nm a -1 ) Model age = ~3 Ga Smith et al., submitted

25 Crater eccentricity On deformed and flowing terrain, surface features (like craters) can become stretched Long axis of stretching should point toward flow direction 1 km

26 Crater eccentricity Control

27 Age of faulting events 111 0'0"W 109 0'0"W 107 0'0"W 105 0'0"W 103 0'0"W 101 0'0"W 99 0'0"W 98 0'0"W 97 0'0"W 96 0'0"W 95 0'0"W 19 0'0"S 20 0'0"S 20 0'0"S 21 0'0"S 21 0'0"S 22 0'0"S 22 0'0"S How do you date the age of a faulting event? Use crater counts to get the ages of surfaces cut by faults 23 0'0"S 24 0'0"S 25 0'0"S 26 0'0"S 27 0'0"S 28 0'0"S 29 0'0"S 23 0'0"S 24 0'0"S 25 0'0"S 26 0'0"S 27 0'0"S 28 0'0"S 29 0'0"S 30 0'0"S 30 0'0"S But what if all faults are restricted to very old rocks? 31 0'0"S 32 0'0"S 33 0'0"S 34 0'0"S 31 0'0"S 32 0'0"S 33 0'0"S 34 0'0"S 35 0'0"S 35 0'0"S 36 0'0"S 36 0'0"S 37 0'0"S 37 0'0"S 38 0'0"S 38 0'0"S 39 0'0"S 39 0'0"S 40 0'0"S 40 0'0"S 41 0'0"S 41 0'0"S 42 0'0"S 42 0'0"S 113 0'0"W 111 0'0"W 109 0'0"W 107 0'0"W 105 0'0"W 103 0'0"W 101 0'0"W 99 0'0"W 97 0'0"W 95 0'0"W 93 0'0"W

28 Age of faulting events Count craters cut by faults and those which aren t Use it to date faulting!

29 Age of faulting events

30 Age of faulting events

31 Planetary Mineralogy Spectral identifications of granitic (quartzofeldspathic) materials along with hydrated minerals (zeolites, phyllosilicates, and hydrated silica indicate a complex geologic history

32 Conclusion Craters can reveal much more about planetary surfaces than just the impact Surface ages Erosional/depositional history Flowing terrain Ages of tectonic events Interesting and important exposed mineralogy