Thicknesses of and Primary Ejecta Fractions in Basin Ejecta Deposits

Larry A. Haskin and William B. McKinnon

Department of Earth and Planetary Sciences, Washington University, St. Louis

Why would a geochemist attempt to doejecta deposit modeling?

From where on the Moon did the materials sampled by the Apollo and Luna missions come? Mostly beneath the sites? Or mostly from a long way off?

Did Th-rich KREEP form as a global layer on the Moon? Or was most of the Th we find at the Moon’s surface ejected from the Procellarum KREEP Terrane when the Imbrium basin formed?

Which basins did the samples of crystalline breccia dated by geochronologists come from? Several? Or mainly from Imbrium?

Our approach to ejecta deposit modeling:

Desired output: ejecta deposit thickness and the fraction of ejecta in the deposits.

Assume ballistic cratering (Oberbeck, Morrison, Hörz).

Concatenate results from several types of cratering studies to estimate average properties of ejecta deposits.

Steps in the modeling:

1. Select a basin, select a sampling site, and find the distance between them.

2. Estimate the total ejected volume as that of a paraboloid using the transient crater radius of the basin and d/D = 0.1, less ~10%, e.g., Melosh.

3. Estimate the ejecta thickness at the sampling site: Housen et al.; map to sphere using ejecta angle and velocity.

4. Estimate the mass distribution of primary fragments: MT

-0.85, from Hartmann, Melosh, Turcotte.

5. Constrain the largest fragment size: MT0.8, O’Keefe &

Ahrens; decrease with distance: v-2, Vickery.

Steps, continued:

6. Calculate the mass and number of primary fragments in each size range.

7. Secondary crater diameters from Schmidt-Holsapple scaling; excav. volumes as paraboloids with d/D = 0.10

8. Determine the fraction of the area excavated as craters of each size range; Garwood (bomb craters).

9. Estimate excavation efficiency on the basis of the largest primary fragment to excavate in any spot; calibrate to data for Orientale and Ries.

10. Result: the areal distribution of deposit thicknesses and % of primary material in deposits around the site of interest

diam. of secondarycrater from largestejecta fragment inthe corridor

corridor of ejecta defined by thedominant ejecta fragment strikingthat spot

Geometry of ejecta fragments that mix with substrate excavated by the largest fragment

-1

0

-3 -2 -1 0 1 2 3

point of impact

-2

kilometers from center of crater

overall excavation,cylinder, 3 X depthof excavation cavity

kilo

met

ers

dep

th

transient crater,displaced rock,lining of melt and shocked debris,d/D~0.35

excavation cavityof largest crater, d/D~0.1

Cross section through the largest crater formed at this location within the SOI

0

1

2

3

4

5

6

400 800 1200 1600

km from the center of Orientale

de

po

sit

th

ick

ne

ss

(k

m)

0

CL10%

CL50%

CL90%

filled cratersrim heightspartly filled craters

Moore et al., 1974Two points per crateron this diagram; theydo not mutually agree.

ejec

ta d

epo

sit

thic

knes

s (m

)

0%

20%

40%

60%

80%

100%

% P

riF

rag

s in

eje

cta

dep

osi

t

km from center of Ries crater

1

10

100

1000

0 10 20 30 40 50 60

c

trans crater diam = 6.5 kmRies ejecta fragments10% coverage level50% coverage level90% coverage level

Horz et al.

eje

cta

dep

osi

t th

ick

nes

s (k

m)

coverage level

% e

ject

a fr

agm

ents

in d

epo

sit

0.00% 20% 40% 60% 80% 100%

0.5

1.0

1.5

2.0

2.5

3.0

3.5

ejection angle = 45 degrees

ejection angle = 35 degreesejection angle = 55 degrees

5%

10%

15%

20%

25%

0% 20% 40% 60% 80% 100%0%

Apollo 16 landing site,1600 km from Imbrium

Craters near the Apollo 16 site (from Jeff Gillis)

Crater diam. (km) fill (m) Crater diam. (km) fill (m)Abulfeda 65 1668 Kant G 26 884Kant D 50 1889 Zollner D 24 547Descartes 48 2628 Unnamed 17 992Zollner 47 627 Abulf. C 17 2049Taylor 42 2156 Kant B 16 1705Taylor A 40 109 Dolland Y 14 1310Andel 35 1544 Andel A 14 1810Dolland B 33 1561 Unnamed 13 2059Lindsey 32 1463

For fresh craters Average 1500 700For degraded craters Average 750 350

Modeled: 2.2 km (CL10%) 1.1 km (CL50%) <0.50 km (CL90%)

0

5

10

-6 -5 -4 -3 -2 -1 0 1

Imbrium to Apollo 16 site, b=0.85

log

# o

f p

rim

ary

frag

men

tsp

er s

qu

are

kilo

met

er

7

8

9

10

log primary fragment radius, kmlog

mas

s p

rim

ary

frag

men

tsp

er s

qu

are

kilo

met

er

-6 -5 -4 -3 -2 -1 0 1

% p

rim

ary

frag

men

ts

in e

ject

a d

epo

sit

ejec

ta d

epo

sit

thic

knes

s (k

m)

a

1

10

100

b = 0.85b = 0.95b = 1.05

0a

km from the center of Imbrium

0%

20%

40%

60%

80%

100%

0 1000 2000 3000 4000 5000 6000

c

Effect of fragment size distribution exponent b

Feldspathic Highland Terrane

0

2

4

6

8

10

12

14

0 1000 2000 3000 4000 5000 6000

Lu

nar

Pro

sp

ect

or

Th

(p

pm

)

kilometers from Imbrium

terramixedPKT terraSPA Imbrian

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

0 5 10 15 20 25

approximate transient crater diameter (km)

nu

mb

er o

f cr

ater

s p

er 2

-kilo

met

er

dia

met

er r

ang

e

Wilhelms et al., Imbrium secondary craters, 1800to 3600 km from Imbrium

b = 0.85

b = 0.95

b = 1.10

1800 km3600 km

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6 7 8

crater diameter (km)

Imbrium at Apollo 16, b=0.85%

of

area

sat

ura

ted

wit

h e

xcav

atio

n

cavi

ties

th

is d

iam

eter

an

d la

rger

50% of area saturated with cavities >2.9 km dia.; excavate to depth of >870 m, eliminate craters 8.7 km dia.

90% of area saturated with cavities >1.4 km dia.; excavate to depth of >520 m, eliminate craters 5 km dia.

If most ejecta fragments arrive simultaneously,

Conclusions:

1. The model gives reasonable deposit thicknesses (after empirical calibration).

2. The model gives reasonable estimates of the fraction of ejecta in those deposits.

3. The results of the modeling are somewhat sensitive to ejection angle and to the size distribution exponent.

3. The model overpredicts the density of observed secondary craters and underpredicts their size range.

successive basin-forming events

ejec

ta d

epo

sit

thic

knes

s (k

m)

preN

Nect

Humr

Cris Sern

Imbr

Ornt

% b

asin

pro

ven

ance

of

mat

eria

l in

ej

ecta

dep

osi

t at

th

e A

po

llo 1

6 si

te

0

20

40

60

80

100

0

0.5

1.0

1.5

2.0

2.5

preN

Nect

Humr

CrisSer

nIm

brOrn

t

50% coverage level

Ap 16