Astro 202 presentationhosting.astro.cornell.edu/academics/courses/astro202/A202_2008_L… · Tethys - saturnian satellite Diagram courtesy of: saturn.jpl.nasa.gov Tethys - saturnian

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Impact Age DatingImpact Age Dating

ASTRO 202 LectureASTRO 202 Lecture

Thursday, February 14, Thursday, February 14, 20082008

ReviewReview

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 What is relative age dating?age dating?

Before absolute age scalesBefore absolute age scales……

…… relative age scalesrelative age scales

A terrestrial example: layered A terrestrial example: layered depositsdeposits


Before absolute age scalesBefore absolute age scales……

…… relative age scalesrelative age scales

A terrestrial example: layered A terrestrial example: layered depositsdeposits

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The rationale for using crater counts for relative age dating:The rationale for using crater counts for relative age dating:

-- Assume surfaces form without any cratersAssume surfaces form without any craters

Craters are the result of impacts with asteroidsCraters are the result of impacts with asteroids

and cometsand comets

--Two ways to establish relative scale:Two ways to establish relative scale:

(1) The older a surface, the more craters it(1) The older a surface, the more craters it

accumulatesaccumulates

(2) Larger impactors are less common than (2) Larger impactors are less common than

smaller; the more large craters on a surface, the smaller; the more large craters on a surface, the

older it isolder it is


Why arenWhy aren’’t all surfaces in the solar system t all surfaces in the solar system heavily cratered?heavily cratered?

Processes that remove craters Processes that remove craters from a surface:from a surface:

ErosionErosion

-- Rain/water, ice,Rain/water, ice,

windwind

GravityGravity

-- Landslides, Landslides,

““mass wastingmass wasting””

ResurfacingResurfacing

-- Volcanism, tectonicsVolcanism, tectonics


Result is a variety of cratered surfacesResult is a variety of cratered surfaces……Enceladus

Earth

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Mars

VenusModerately/sparsely

cratered…


Martian dichotomy…

Southern Hemisphere

Northern Hemisphere


Heavily cratered:

Mare (young)

Highlands (old)The Moon…


Mercury…

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Tethys

- saturnian satellite

Diagram courtesy of: Diagram courtesy of: saturn.jpl.nasa.govsaturn.jpl.nasa.gov

Tethys

- saturnian satellite

- seems to have

survived a giant impact

Odysseus Crater

Rhea

-large satellite of Saturn

- seems “saturated” with

craters

-one of the most heavily

cratered surfaces in the

Solar System

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Counting CratersCounting Craters

The method:The method:

(1) What is a crater?(1) What is a crater?

--Resolution, various stages of erosion canResolution, various stages of erosion can

make consistent identification difficultmake consistent identification difficult


http://clickworkers.arc.nasa.gov/top

?

??

???????


The method:The method:

(1) What is a crater?(1) What is a crater?

--Resolution, various stages of erosion canResolution, various stages of erosion can

make consistent identification difficultmake consistent identification difficult

(2) How do we keep track?(2) How do we keep track?

--SizeSize--frequency distributions (frequency distributions (SDFSDF’’ss) record) record

both the number and sizes of craters in a both the number and sizes of craters in a

given areagiven area


0 0.5 1 1.5 2 2.5Crater diameter , D @kmD

0

0.02

0.04

0.06

0.08

evitalumuC

ytisned

fo

sretarc

NH>DL

@mk

−2D

All Craters Identified as Secondaries in the Newton Area

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How do we make the How do we make the relative scale absolute?relative scale absolute?

The relative scale is only so usefulThe relative scale is only so useful…… For constraining the history of For constraining the history of planets and Solar System formation, we need something better.planets and Solar System formation, we need something better.

-- Combine relative scale with known absolute agesCombine relative scale with known absolute ages

Apollo program to the rescue!Apollo program to the rescue!

Apollo Lunar Sample Apollo Lunar Sample ReturnReturn

Apollo Program: 1963-1972

11, 12, 14-17 Missions landed and safely returned humans from the Moon…

… and samples!!!

Combining Relative and Combining Relative and Absolute ScalesAbsolute Scales

From the SFDFrom the SFD’’s, we add the s, we add the Apollo sample return absolute Apollo sample return absolute ages to create crater ages to create crater isochrons for the Moonisochrons for the Moon

Extending the Absolute Extending the Absolute Scale Beyond the MoonScale Beyond the Moon

We still donWe still don’’t have the ability to retrieve samples from the other t have the ability to retrieve samples from the other planetary surfaces, so how do we determine absolute ages for theplanetary surfaces, so how do we determine absolute ages for these se bodies?bodies?

--Know something about the distribution of impactors that cause thKnow something about the distribution of impactors that cause the e cratering we see on the Mooncratering we see on the Moon

--Determine how this distribution differs at other places in the SDetermine how this distribution differs at other places in the Solar olar SystemSystem

Some other factors that affect cratering distribution:Some other factors that affect cratering distribution:

--Atmosphere, gravity, surface strength, erosion andAtmosphere, gravity, surface strength, erosion and

resurfacing ratesresurfacing rates

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Example martian

isochrons


ControversyControversy

There are a number of controversies when it comes There are a number of controversies when it comes to dating the Solar Systemto dating the Solar System……

--The Late Heavy BombardmentThe Late Heavy Bombardment

--Secondary CrateringSecondary Cratering

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Late Heavy Late Heavy BombardmentBombardment

What is the late heavy What is the late heavy bombardment?bombardment?

--Some scientists believe Some scientists believe that the number of that the number of impactors has undergone a impactors has undergone a steady exponential decrease steady exponential decrease since the formation of the since the formation of the Solar SystemSolar System

--Others cite the Apollo Others cite the Apollo sample returns as evidence sample returns as evidence that there was a burst of that there was a burst of impacts around 4 impacts around 4 GaGa known known as the late heavy as the late heavy bombardmentbombardment

Why does the late heavy bombardment matter?Why does the late heavy bombardment matter?

--Did it happen everywhere in the Solar System?Did it happen everywhere in the Solar System?

--Are the extrapolations of the absolute scale Are the extrapolations of the absolute scale

to other surfaces correct?to other surfaces correct?

--What are the implications for models of Solar What are the implications for models of Solar

System formation?System formation?

Late Heavy Late Heavy BombardmentBombardment

Secondary CrateringSecondary Cratering

Shapes of the crater isochrons:Shapes of the crater isochrons:

“Steep” slope

“Shallow” slope


Shapes of the crater isochrons:Shapes of the crater isochrons:

Eugene Shoemaker identified two shapes or Eugene Shoemaker identified two shapes or ““branchesbranches”” of the of the cratering isochrons for the Mooncratering isochrons for the Moon

Fit powerFit power--laws (SFDlaws (SFD’’s) of the form:s) of the form:

N(>D) = c DN(>D) = c D--bb

Steep powerSteep power--law index (b~3,4) dubbed law index (b~3,4) dubbed ““secondarysecondary”” branch; branch; shallower (b~2) index called shallower (b~2) index called ““primaryprimary”” branchbranch

Secondaries footprints of ejecta from large primary impactsSecondaries footprints of ejecta from large primary impacts

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Dominance of secondary and primary branches of SFDDominance of secondary and primary branches of SFD--Some groups claim that secondaries dominate at D < 1 kmSome groups claim that secondaries dominate at D < 1 km--Others claim that secondaries are at most 10% of small Others claim that secondaries are at most 10% of small Martian cratersMartian craters

Are secondaries even Are secondaries even ““secondariessecondaries””??

-NeukumNeukum, , IvanovIvanov and others argue that the secondary branchand others argue that the secondary branch

is formed by a smallis formed by a small--diameter asteroid populationdiameter asteroid population

ReviewReview

--Even on planets and satellites where we canEven on planets and satellites where we can’’t use surface samples t use surface samples to derive absolute ages, we can establish a relative age scale fto derive absolute ages, we can establish a relative age scale from rom crater sizecrater size--distribution countsdistribution counts

--From lunar samples returned to Earth by the Apollo astronauts, From lunar samples returned to Earth by the Apollo astronauts, we can construct a viable absolute age scale for surfaces on thewe can construct a viable absolute age scale for surfaces on theMoonMoon

--Using what we know about the surface strength, gravity, Using what we know about the surface strength, gravity, resurfacing and impactor rates on other planets and satellites, resurfacing and impactor rates on other planets and satellites, the the absolute age scale for the Moon can be extrapolated to a wide absolute age scale for the Moon can be extrapolated to a wide variety of Solar System surfacesvariety of Solar System surfaces

--Controversial issues such as the existence of the late heavy Controversial issues such as the existence of the late heavy bombardment and secondary cratering need to be resolved to allowbombardment and secondary cratering need to be resolved to allowfor more accurate absolute dating of surfaces in our Solar Systefor more accurate absolute dating of surfaces in our Solar Systemm

Example crater isochrons for Mars and the Moon from:Example crater isochrons for Mars and the Moon from:

http://www.psi.edu/projects/mgs/chron04b.htmlhttp://www.psi.edu/projects/mgs/chron04b.html

Documents

Astro 202 presentationhosting.astro.cornell.edu/academics/courses/astro202/A202_2008_L… · Tethys - saturnian satellite Diagram courtesy of: saturn.jpl.nasa.gov Tethys - saturnian