Track 1

Track 2

TRACK LIST

1. An introduction to synthetic aperture radar

2. Mini-RF on LRO

3. The moon as seen by radar

4. The search for ice

5. Conclusions

6. Acknowledgements

Track 3

rada

rRadio describes the long-wavelength end of the electromagnetic spectrum

• ~1 cm – 1 km• ~300 kHz – 30 GHz

RADAR stands for RAdio Detection And Ranging. Radar systems emit radio waves that are reflected by a target and detected by a receiver

Track 4

rada

rIt is possible to image planetary surfaces using synthetic aperture radar (SAR), even on planets with opaque atmospheres

SAR

LookAngle

IncidenceAngle [i0]

Suborbital Track

Radar Swath

Track 5

rada

rWhen a SAR system transmits a radio pulse,

some energy is reflected back towards the SAR.

This energy is what SAR measures. It is known as radar backscatter. SAR cannot measure energy reflected in other directions.

SAR

SAR

Track 6

rada

rSAR images provide a wealth of information about the surface being imaged, because radar backscatter depends on three different properties:

1. Topography: Affects local incidence angle– Does the radio wave bounce towards the

receiver or away?

Radar bounces towards receiver

HIGH BACKSCATTER

Radar bounces away from receiver

LOW BACKSCATTER

SAR SAR

Track 7

rada

r SAR images provide a wealth of information about the surface being imaged, because radar backscatter depends on three different properties:

1. Topography: Affects local incidence angle– Does the radio wave bounce towards the

receiver or away?

The volcano Kilauea, as observed by SIR-C SAR

SAR lookdirection

tilted towards

tilted away

Track 8

rada

rSAR images provide a wealth of information about the surface being imaged, because radar backscatter depends on three different properties:

2. Roughness: Affects direction of backscatter– Is the radio wave scattered in many

directions, or is it specularly reflected?

Some signal directed towards receiver

HIGH BACKSCATTER

No signal directed towards receiver

LOW BACKSCATTER

SAR SAR

rough smooth

Track 9

rada

rSAR images provide a wealth of information about the surface being imaged, because radar backscatter depends on three different properties:

2. Roughness: Affects direction of backscatter– Is the radio wave scattered in many

directions, or is it specularly reflected?

smooth

rough

Titan’s north polar lakes, as observed by Cassini SAR

Track 10

rada

rSAR images provide a wealth of information about the surface being imaged, because radar backscatter depends on three different properties:

3. Composition: Affects dielectric constant (ε)– How well does the surface reflect radio waves?

High dielectric constant, more energy reflected

HIGH BACKSCATTER

Low dielectric constant, little energy reflected

LOW BACKSCATTER

SAR SAR

saturated soil dry soil

Track 11

rada

rSAR images provide a wealth of information about the surface being imaged, because radar backscatter depends on three different properties:

3. Composition: Affects dielectric constant (ε)– How well does the surface reflect radio waves?

Fields near Melfort, Saskatchewan, as observed by CCRS Airborne SAR

Wet Field(high ε)

Dry Field(low ε)

Track 12

Min

i-RF On June 18, 2009 the Lunar Reconnaissance

Orbiter launched, carrying with it a miniature radar dubbed Mini-RF

Track 13

To date, we have acquired radar data over ~50% of the non-polar regions of the Moon, and nearly full coverage over the two poles

First radar views of the lunar far side!Min

i-RF

Track 14

nort

h p

ole

70°N

Track 15

Part I: Mini-RF Observes the Moon

Track 16

gera

sim

ovic

h d Mini-RF discovered an impact melt in the crater

Gerasimovich D that is not observable in optical

LROC WAC

Mini-RF

Track 17

linne

cra

ter

Linne is a classic, bowl-shaped, “simple” crater

Low radar return suggests a halo of block-poor ejecta

Disappears over time due to meteoroid

bombardment

Indicates a young crater

High radar return indicates rough, blocky ejecta

Track 18

apol

lo la

ndin

g si

tes Apollo sites provide “ground truth” for radar data

Ap

ollo

16

Ap

ollo

17

Track 19

apol

lo la

ndin

g si

tes

South Massif

Track 20

Centaur

SSC

Pre-Impact (October 9, 2009)

LCR

OS

S im

pact

site

Equipped with its own active source, Mini-RF can “see in the dark”!

ex. LCROSS impact site

Track 21

Post-Impact (March 22, 2010)

LCR

OS

S im

pact

site

Track 22

Part II: The Search for Ice

Track 23

• The Moon’s axis of rotation is nearly perpendicular to the Sun, so there are regions near the poles where the Sun never shines

• These “permanently shadowed regions” are very cold. When comets hit the moon, ice can migrate to these cold craters, possibly collecting there

Image of South Pole from Kaguya

spacecraft

ice

on th

e m

oon?

Track 24

• Ice has unique radar properties

In weakly absorbing media with scattering centers (like water ice) there will be constructive interference between radar signals that follow the same path in opposite directions.

These signals are forward scattered, which preserves the original sense of polarization, leading to large “same-sense” (SC) returns, and high circular polarization ratios (CPR).“rock”

“void”

Adapted from Campbell (2002)

plane wavefro

ntHigh SC signal

CPR = SC/OC > 1

sear

ch fo

r ic

e

Track 25

• Ice has unique radar properties

In weakly absorbing media with scattering centers (like water ice), there will be constructive interference between radar signals that follow the same path in opposite directions.

These signals are forward scattered, which preserves the original sense of polarization, leading to large “same-sense” (SC) returns, and high circular polarization ratios (CPR).

“rock”

“void”

plane wavefro

nt

High SC signal

CPR = SC/OC > 1

Example: High radar return from the north pole of Mercury (Harmon et al. 2001)

sear

ch fo

r ic

e

Track 26

• On October 9, 2009, the LCROSS spacecraft impacted Cabeus crater, located near the south pole of the Moon

• Early reports from LCROSS indicate the presence of water in Cabeus crater

Cabeus

LCR

OS

S

Track 27

Chandrayaan-1 LRO

= approximate location of LCROSS impactor

LCR

OS

S Low radar return indicates that there is no near-surface, thick deposits of ice in Cabeus crater

Track 28

ice

in n

orth

pol

e? BUT there are many “anomalous” craters near the north pole which are good candidates for ice

ex. “Normal” craters like Main L have high CPR inside and outside the crater, indicative of a rough, blocky ejecta blanket

Track 29

ice

in n

orth

pol

e? BUT there are many “anomalous” craters near the north pole which are good candidates for ice

ex. “Anomalous” craters like this one in Rozhdestvensky have high CPR (>1) inside the crater,

and low CPR outside the crater

Track 30

nort

h po

le 78°N

Track 31

conc

lusi

ons

Radar data compliments data obtained at optical wavelengths, yielding information about surface roughness, topography, and composition

In particular, ice has unusual properties that can be observed with radar

The Mini-RF instrument on LRO has found no evidence for large ice deposits at the LCROSS impact site, but has identified some promising candidates in the north polar regions

Track 32

PI Ben BusseyMini-RF Science TeamLRO ProjectNASA