Samuel Bell, Amherst College, 2011

103 Pages·2011·2.42 MB·English

Fresh lunar crater ejecta as revealed by the
Miniature Radio Frequency (Mini-RF)
instrument on the Lunar Reconnaissance Orbiter

Samuel W. Bell ‘11

Submitted to the Department of Astronomy of Amherst College in partial fulfillment of
the requirements for the degree of Bachelor of Arts with honors.

Advisor: Darby Dyar

Readers: George Greenstein and Peter Crowley

May 5, 2011

Abstract
On timescales of tens to hundreds of millions of years, micrometeorite, solar wind
ion, and cosmic ray bombardment gradually erode the ejecta blankets that form around
small lunar impact craters. Sensitive to surface roughness on the scale of its 12.6 cm
wavelength, the 30 m/pixel Miniature Radio Frequency (Mini-RF) instrument provides
detailed imagery of the ejecta blankets of small lunar craters with unprecedented
resolution and quality, allowing large numbers of ejecta blankets to be studied with a
higher degree of precision than previously possible (Thompson et al., 1981; Nozette et
al., 2010; Neish et al., 2011). Using well-established crater-counting techniques
(Arvidson et al., 1979; Michael and Neukum, 2010), analysis of the lifetime of the
discontinuous portion of the ejecta blanket at varying crater diameters shows that the
discontinuous ejecta lifetime is proportional to the square of the crater diameter.
Absolute dates of individual craters can be estimated by combining the empirically
derived function for discontinuous halo lifetime with estimates of what fraction of its
lifetime each discontinuous ejecta blanket has lived through. Cosmic ray exposure ages
of craters visited by the Apollo missions provide confirmation of these results: The
resultant lifetime model predicts that the discontinuous ejecta blanket around the 25-30
Ma Cone Crater (Turner et al., 1971) will have vanished after 8.7(+1.5/-1.7) Ma, and the
discontinuous ejecta blanket is indeed absent. This method produces a radiometrically
determined age estimate of 54(+39/-29) Ma North Ray Crater, consistent with the known
age of 50.0±1.4 Ma (Arvidson et al., 1975).

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Table of Contents
Abstract ............................................................................................................................... i
1. Introduction ................................................................................................................... 1
I Dating individual craters ........................................................................................... 4
II Dynamics of ejecta degradation ............................................................................... 4
2. Literature Review ......................................................................................................... 6
I.1 The Moon—history and general features ............................................................. 6
I.2 The Moon—surface water ...................................................................................... 7
I.3 The Moon—erosion processes ................................................................................ 9
II.1 The impact process—the basics .......................................................................... 11
II.2 The impact process—ejecta properties .............................................................. 14
II.3 The impact process—oblique impacts ............................................................... 14
III.1 Radar observation—the basics ......................................................................... 17
III.2 Radar observation—S1 imagery ....................................................................... 18
III.3 Radar observation—CPR imagery ................................................................... 21
III.4 Radar observation—topographic effects on position information ................ 23
IV.1 Optical crater dating—Trask (1971) method .................................................. 25
IV.2 Optical crater dating—erosion modeling method ........................................... 28
IV.3 Optical crater dating—space weathering ......................................................... 30
V Radar-bright halos .................................................................................................. 31
3. Methods ........................................................................................................................ 35
I Crater counting ......................................................................................................... 35
I. Crater counting—R-plot analysis .......................................................................... 35
II.1 Data sets ................................................................................................................ 37
II.2 Datasets—initial search and highlands focus region ........................................ 39
II.3 Datasets—Mare Serenitatis and 2400s focus regions ....................................... 42
III Age quantification ................................................................................................. 42
IV Image processing ................................................................................................... 43
V Radial brightness profiles ....................................................................................... 45
4. Results .......................................................................................................................... 47
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I Estimating Discontinuous Halo Lifetimes From Crater Counting ...................... 47
II Estimating Ages From Discontinuous Halo Lifetimes and Diameters ............... 53
III Morphology Variation of Degrading Craters ..................................................... 53
IV.1 Comparison With Apollo Results—Cone Crater ............................................ 63
IV.2 Comparisons With Apollo Results—North Ray Crater. ................................ 67
V Thompson et al. (1981) comparison ...................................................................... 69
5. Discussion .................................................................................................................... 76
I.1 Secondary Cratering ............................................................................................. 76
I.2 Wells et al. (2010) Ejecta Morphologies—Diagnostic of Secondaries? ............ 77
II Problems With Halo Diameter Quantification .................................................... 80
III.1 The Discontinuous Ejecta Lifetime to Crater Diameter Relationship—
Extrapolation to Small Craters .................................................................................. 81
III.2 The Discontinuous Ejecta Lifetime to Crater Diameter Relationship—
Extrapolation to Large Craters ................................................................................. 83
IV Possible Effect of Lithology .................................................................................. 84
V.1 Variations in the Impact Rate ............................................................................. 85
V.2 Variations in the Micrometeorite Flux and Erosion Rate ................................ 87
6. Conclusion ................................................................................................................... 89
References ........................................................................................................................ 92

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Acknowledgements
I would like to thank Amherst College, the Applied Physics Laboratory intern program,
and NASA for sponsoring and funding this research. I would like to thank Josh Cahill,
G. Wesley Patterson, and Ben Bussey for general support and answering my questions
this summer; Mike Zanetti for help with LROC image processing; and Catherine Neish
for teaching me ISIS, C-shell, and the basics of radar observation. I would also like to
thank all of my friends for their help (especially with Word formatting) and support
throughout this process. And finally I would like to thank Brad Thomson and Darby
Dyar for all their help. I cannot say how much I appreciate everything they’ve done for
me.
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Introduction
The Moon may not have the cryovolcanic plumes of Enceladus and Triton, the
hydrocarbon seas of Titan, the subsurface ocean of Europa, the constant volcanic activity
of Io, or the sheer size of Ganymede, but it has two virtues that make it one of the most
scientifically important bodies in the solar system.
To begin with, it is close. Eleven years after NASA was founded, astronauts had
landed on the Moon; forty-one years later, no other extraterrestrial body in the solar
system has felt the bootprints of an astronaut. It can take more than half an hour for
robotic mission controllers to send a signal to a Mars and back but only a few seconds to
send one to the Moon. The Moon is, therefore, the second most easily accessible body in
the universe (after Earth).
Even more importantly, the Moon is a highly simplified system. It has only three
rock types (basalts, anorthosites, and impact breccias), exceptionally few water
molecules, no atmosphere, no active volcanism, no plate tectonics, and no evidence of
extraterrestrial life. The lunar surface is so quiet that the footprints the astronauts left
behind forty years ago remain pristine and virtually undisturbed today. All the
complexities that plague terrestrial geology are dramatically reduced on the Moon. These
qualities make the Moon the perfect place to study impact processes.
Space debris periodically collides at high speeds with the Earth, the Moon, and all
other planetary bodies. When an impactor strikes the lunar surface, it usually makes a
roughly circular depression known as an impact crater. But it also kicks up a cloud of
debris, which spreads out on ballistic trajectories and deposits itself in a blanket of rubble
extending out to more than ten crater radii. Planetary geologists call the boulders,
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pebbles, and powder that make up these debris “ejecta,” and they form one of the most
significant types of landform on the Moon. An incessant rain of microscopic impactors
obliterates the optical signature of the ejecta within a few tens of million years (rapidly,
geologically speaking) for craters with diameters in the range of a few kilometers (Swann
and Reed, 1974). The visibility of ejecta depends strongly on the viewing and
illumination geometry, with high sun-angle photographs displaying the ejecta most
clearly (Schultz, 1976). Only the ejecta around the most recent craters, a tiny fraction of
the total, are evident in visible light images, and even then the appearance of the ejecta
varies heavily with the observation conditions. Fortunately, visible light is not our only
choice for imaging the Moon.
Earth-based radio observatories have long been observing the near side of the
Moon using radar wavelengths (3.8 to 70 cm) (e.g. Lincoln Laboratory, 1968; Thompson
et al., 1971; Ghent et al., 2005). Until recently, high-resolution coverage remained quite
limited, and the far side of the Moon lay hidden from even the best Earth-based radio
telescopes. All that changed in June of 2009 when NASA launched the Lunar
Reconnaissance Orbiter, carrying with it a radio experiment known as Mini-RF, which is
short for “miniature radio frequency.” (A preliminary version of Mini-RF known as
Mini-SAR flew on Chandrayaan-1, an Indian spacecraft that launched in October of 2008
and failed in August of 2009). Because the orbiter passes over the entire Moon, it can see
a hemisphere impossible for Earth-based radar to image. Taking advantage of the orbital
motion of the spacecraft, Mini-RF produces images with 30 m resolution (Nozette et al.,
2010). At these wavelengths, numerous craters with ejecta preserved in bright “halos”
can be seen spreading out from crater rims, as seen in the example in Fig. 1.1. The
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A B

Figure 1.1: Two examples of fresh craters with ejecta displayed in Mini-RF total
backscatter. Both craters show the difference between the blocky inner continuous ejecta
and the fine outer discontinuous ejecta. Image A has a clear zone of avoidance in the
direction the indicator came from, indicating and oblique impact. Image B has an
asymmetry in the ejecta that may be due to a moderately oblique impact. Unlike most
other images in this thesis, these images have not been gamma-corrected, and they have
been contrast-enhanced to bring out subtle features in the ejecta blankets.
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brightness in a radar image of a simple body like the Moon is largely due to the
roughness of the surface on the scale of the wavelength of the radar. This means that the
12.6 cm Mini-RF antenna is sensitive to the abundance of blocks on the order of 12.6 cm
across, so it will be able to detect small boulders in impact ejecta long after space
weathering has darkened their surfaces to the color of the rest of the Moon.
I Dating individual craters
There are few things more important to understanding any planetary body than
knowing the ages of its surface features. However, dating capabilities on bodies other
than the Earth are quite limited. By counting the number of impact craters superimposed
on a feature since it was formed, planetary geologists can roughly estimate its age
(Arvidson et al., 1979; Michael and Neukum, 2010). But dating individual impact craters
is harder because their small sizes make statistically significant crater counting difficult
(Guinness and Arvidson, 1977; Craddock and Howard, 2000). Furthermore, crater
counts of small craters are less reliable because a significant fraction of these craters may
be secondary craters (craters formed by pieces of ejecta from much larger impacts), and
the crater counting methodology assumes only limited secondary cratering (Gunness and
Arvidson, 1977; Rodrigue, 2011). One of the most exciting possibilities offered by
studies of lingering radar signatures from crater ejecta is quantification of how much the
ejecta have faded since the crater first formed, thus constraining the crater’s age.
II Dynamics of ejecta degradation
Degraded impact ejecta dominate the lunar surface. Over the course of the 4.5
billion years since the lunar crust crystallized, nearly every part of the lunar surface has
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