Sky and Telescope - February 2017 - 53

N ASA / A RIZON A STATE UNIV.

B

A

20 km

0

Elevation (m)

strictly vertical drop; instead, structural
failure occurs along curved faults that
slide previously flat-lying rocks downward into the hole. In the process, the
slabs rotate so that their surfaces end
up tilting downward away from the
crater's center and toward its rim. These
outward-facing tilts are easy to recognize in high-resolution Lunar Reconnaissance Orbiter (LRO) images, which
show where impact-produced molten
rock fell as a liquid onto a terrace,
flowed downhill, and collected in ponds
against the walls of adjacent terraces.
The inward slumping of terraces
makes the final diameters of big craters
30% to 40% larger than their transient
cavities were initially. And because the
thickness of ejected debris tapers outward from its maximum at the transient cavity's rim, it's not as thick at the
final, terraced-enlarged rim crest. This
means that the contribution of fallback
ejecta to the rim's height should be less
than 50% for large craters.
To test this assumption, Krüger and
Kenkmann used high-resolution images
from LRO and the Japanese Kaguya
lunar orbiter to look in detail in crater
walls. They calculated the amount of
structural uplift of preexisting rocks and
measured the thickness of ejecta at the
rim for 11 craters, ranging in size from
Bessel (16 km) to Pythagoras (130 km).
As an example, they found that the
final rim of 41-km-wide Bürg shows
about 850 meters of structural uplift
and 300 m of overlying ejecta. This
ratio fit the pattern seen in all of the
craters studied: ejecta constitutes about
30% of a rim's height, and uplift makes
up the other 70%.
So now that you know how crater
rims are made, can you observe any
of these details? Start by looking at
Copernicus under a high Sun, when just
a narrow line of shadow hugs the rim.
Just below the rim crest is a steep, bright
scarp, 1½ km high, and below it are the
terraces that widened the original diameter of the crater. This scarp includes
both uplifted preexisting surface and
the ejecta that rained down on top of it.
Interestingly, the bottom of the scarp is
the same elevation as the surrounding,

Rim crest
(+269 m)

Scarp

-1,000
-2,000

Floor
(-3,560 m)

B

Elevation
of original
terrain

Terraces

-3,000
A

0

20

40

60

80

100

Distance from center (km)

S This topographic cross-section shows that Copernicus is nearly 4 km deep from its floor to the
crest of its rim. The dramatic scarp just inside its rim crest (bright in the image) is 1½ km high.

pre-impact terrain well beyond the rim.
For a real challenge, observe Bürg.
It's on the lava-covered floor of Lacus
Mortis and less than half the diameter
of 93-km-wide Copernicus. With high
power you just might notice that the
uppermost part of Bürg's rim scarp
looks brighter than the bottom twothirds. The bright material is ejecta, and
below it is the structural uplift of the
mare lavas. These brightness differences
make sense, because lavas are dark, but
pulverized ejecta (which makes rays) is
highly reflective. High-quality amateur
images show the same brightening atop
the north rim scarp of Plato - though I
have never observed it visually.
To see these rims more in profile,
observe large, fresh craters near the

limb. You can gaze across the near rims
and floors of the limb-hugging craters
Moretus, Pythagoras, Langrenus,
and Zucchius to the bright rim-crest
scarps and the jumble of terraces below
on their far walls. Compare the narrow
extent of the scarps - typically they're
1 to 1½ km high - and the greater
heights of the terraces. This shows how
much farther craters excavate below the
original terrain than they pile up ejecta
on their rims. And it reminds us of the
powerful forces that shaped the Moon's
heavily battered face.
¢ CHUCK WOOD would prefer to study
the rim scarps of lunar craters by repelling down one.
For more information: www2.lpod.org.

s k y a n d t e l e s c o p e .c o m

* FEBRUARY 2017

53


http://www2.lpod.org http://www.skyandtelescope.com

Sky and Telescope - February 2017

Table of Contents for the Digital Edition of Sky and Telescope - February 2017

Contents
Sky and Telescope - February 2017 - Cover1
Sky and Telescope - February 2017 - Cover2
Sky and Telescope - February 2017 - 1
Sky and Telescope - February 2017 - Contents
Sky and Telescope - February 2017 - 3
Sky and Telescope - February 2017 - 4
Sky and Telescope - February 2017 - 5
Sky and Telescope - February 2017 - 6
Sky and Telescope - February 2017 - 7
Sky and Telescope - February 2017 - 8
Sky and Telescope - February 2017 - 9
Sky and Telescope - February 2017 - 10
Sky and Telescope - February 2017 - 11
Sky and Telescope - February 2017 - 12
Sky and Telescope - February 2017 - 13
Sky and Telescope - February 2017 - 14
Sky and Telescope - February 2017 - 15
Sky and Telescope - February 2017 - 16
Sky and Telescope - February 2017 - 17
Sky and Telescope - February 2017 - 18
Sky and Telescope - February 2017 - 19
Sky and Telescope - February 2017 - 20
Sky and Telescope - February 2017 - 21
Sky and Telescope - February 2017 - 22
Sky and Telescope - February 2017 - 23
Sky and Telescope - February 2017 - 24
Sky and Telescope - February 2017 - 25
Sky and Telescope - February 2017 - 26
Sky and Telescope - February 2017 - 27
Sky and Telescope - February 2017 - 28
Sky and Telescope - February 2017 - 29
Sky and Telescope - February 2017 - 30
Sky and Telescope - February 2017 - 31
Sky and Telescope - February 2017 - 32
Sky and Telescope - February 2017 - 33
Sky and Telescope - February 2017 - 34
Sky and Telescope - February 2017 - 35
Sky and Telescope - February 2017 - 36
Sky and Telescope - February 2017 - 37
Sky and Telescope - February 2017 - 38
Sky and Telescope - February 2017 - 39
Sky and Telescope - February 2017 - 40
Sky and Telescope - February 2017 - 41
Sky and Telescope - February 2017 - 42
Sky and Telescope - February 2017 - 43
Sky and Telescope - February 2017 - 44
Sky and Telescope - February 2017 - 45
Sky and Telescope - February 2017 - 46
Sky and Telescope - February 2017 - 47
Sky and Telescope - February 2017 - 48
Sky and Telescope - February 2017 - 49
Sky and Telescope - February 2017 - 50
Sky and Telescope - February 2017 - 51
Sky and Telescope - February 2017 - 52
Sky and Telescope - February 2017 - 53
Sky and Telescope - February 2017 - 54
Sky and Telescope - February 2017 - 55
Sky and Telescope - February 2017 - 56
Sky and Telescope - February 2017 - 57
Sky and Telescope - February 2017 - 58
Sky and Telescope - February 2017 - 59
Sky and Telescope - February 2017 - 60
Sky and Telescope - February 2017 - 61
Sky and Telescope - February 2017 - 62
Sky and Telescope - February 2017 - 63
Sky and Telescope - February 2017 - 64
Sky and Telescope - February 2017 - 65
Sky and Telescope - February 2017 - 66
Sky and Telescope - February 2017 - 67
Sky and Telescope - February 2017 - 68
Sky and Telescope - February 2017 - 69
Sky and Telescope - February 2017 - 70
Sky and Telescope - February 2017 - 71
Sky and Telescope - February 2017 - 72
Sky and Telescope - February 2017 - 73
Sky and Telescope - February 2017 - 74
Sky and Telescope - February 2017 - 75
Sky and Telescope - February 2017 - 76
Sky and Telescope - February 2017 - 77
Sky and Telescope - February 2017 - 78
Sky and Telescope - February 2017 - 79
Sky and Telescope - February 2017 - 80
Sky and Telescope - February 2017 - 81
Sky and Telescope - February 2017 - 82
Sky and Telescope - February 2017 - 83
Sky and Telescope - February 2017 - 84
Sky and Telescope - February 2017 - Cover3
Sky and Telescope - February 2017 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/newtrack/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com