LOLA Laser Radiometry and LCROSS Impact Site Selection

Maria T. Zuber1, David E. Smith2,

Ian Garrick-Bethell1

1Massachusetts Institute of Technology 2NASA/Goddard Space Flight Center

LCROSS Site Selection Workshop

NASA/Ames Research Center

October, 2006

Question:

Can the effect of the LCROSS impact be detected by LOLA radiometry?

Example from Mars

Zuber et al. [2004]

North Polar Region South Polar Region

Ls=180

Ls=284

Ls=0

Ls=104

Passive 1064-nm reflectance of Mars from MOLA.

Rx Telescope

Beam Expander

Detectors (5)2 more on far side

Lunar Altimetry PortLaser

Laser Pulse energy 2.7±0.3 mJ Wavelength 1064.30 ±0.1 nm Pulse width, and rate 6 ns FWHM, 28 Hz Wavelength 1064.30 ±0.1 nm Beam splitting 5-way, >13% total per beam Beam divergence, each beam 100 rad Beam separation 500 rad Receiver Optics Receiver aperture Atel = 0.015 m2 ( = 0.14 m) Field of view 400 rad Optics transmission >70% Optical bandwidth 0.8 nm Photodetector/Preamplifier Detector active area 0.7 mm diameter Detector quantum efficiency 40% Noise equivalent power (NEP) 0.05 pW/Hz1/2 Electrical bandwidth 100 MHz Timing Electronics Timing resolution <0.5 ns Clock frequency uncertainty <1e-7 Laser pulse epoch time accuracy <3 ms Total Instrument: Mass 12 kg Electrical power 30 W Volume, Main housing Electronics unit

45x51x36 cm3 28x17x12 cm3

Date rate 12 kbits/s

Radiator

Earth RangingPort

Schematic & Instrument Parameters

Sun et al. [2005]

S/C Velocity1.6 km/s

Pattern Clock Angle = 26°

D~ 56 m

Spot pattern is clocked 26° wrt s/c velocity vector and provides:– 5 profiles ~ 10 to 15 m apart– 140 measurements/s

LOLA pulse rate = 28 Hz ±0.1 Hz;maintains nominal shot spacingD such that peak deviation < 0.5 m

ds

LOLA Sampling Pattern

~65 m

25 m

5 m

Each 5-m diameter spot contains a measurement of: – elevation; 10-cm precision– surface roughness (rock heights); 30-cm resolution– surface brightness; 10%

LOLA Measurement Team

Surface Reflectance Measurement

LOLA will measure surface reflectance of LOLA laser light via ratio of transmitted (Tx) and echo pulse energies (Rx) to search for ice in permanently shadowed regions.

Mixtures of ice crystals in regolith (4% ice) will exhibit higher surface reflectance ( = 80% ice, = 20% regolith) than surrounding areas.

Simulated regolith-ice mixture detection assuming 2% random noise in Tx and 12% in Rx.LOLA Measurement Team

Discriminating young ejecta from ejecta of a different composition

To determine which bright, young craters have ejecta that is bright due to immaturity (and not composition) compare iron content [Lucey et al. 2000a] and optical maturity (OMAT) [Lucey et al., 200b].

– Both iron and maturity maps use ratios of the band strength at 750 and 950 nm.

Select craters that appear bright in OMAT, but do not show up in iron.

Lucey, P.G., et al., Lunar iron and titanium abundance algorithms based on final processing of Clementine ultraviolet-visible images, J. Geophys. Res., 105, 20,297, 2000a.

Lucey, P.G., et al., Imaging of lunar surface maturity, J. Geophys. Res., 105, 20,377-20,386, 2000b.

Average highlands reflectance at 1000 nm

Analyze large section of farside highlands with few mare units. Average reflectance of all pixels to obtain estimate of average highlands reflectance: 0.241.

– good agreement with laboratory reflectance spectra of highland soils from Apollo 16 at 950 nm [Lucey et al., 2000b].

Data set and regions analyzed

Use calibrated Clementine images at wavelengths of 750, 950 and 1000 nm.

– LOLA detects 1064 nm but spectrum of most lunar materials monotonically increases in region between 1000 -1100 nm [Hawke, B.R., et al., Origin of lunar crater rays, Icarus, 170, 1-16, 2004].– Can detect brightness variations in craters down to 1-km diameter (bigger than expected LCROSS crater but best resolvable from Clementine.)

“Double” region (0oN, 160oE)

750 nm

Double region 1000 nm reflectance showing transects, no stretch.

1 2

3 4

1 2

3 4

1000 nm

south crater

north crater

“Double” region (0oN, 160oE) -- north crater

“Double” region (0oN, 160oE) -- south crater

Reflectance of ejecta is significantly higher than surrounding highlands, by as much as 0.1.

“Far west” region (-1.5oN, 188oE)

750 nm 1000 nm

1 2

3

1

1

“Far west” region (-1.5oN, 188oE) -- A & C

Crater A Crater C

“Far west” region (-1.5oN, 188oE) -- Crater B

Reflectance is higher over craters by as much as 0.1.

“Far east” region (0oN, 236oE)

750 nm 1000 nm

1 2

3 1 2

3

3

“Far east” region (0oN, 236oE) -- -- A & C

Crater C

Crater A

“Far east” region (0oN, 236oE) -- Crater B

“Far east” region (0oN, 236oE) -- Crater D

Crater reflectance ranges from 0.03 to 0.08 above surrounding highlands.

Summary LOLA will measure 1064-nm reflectance of lunar surface to ~10% in each 5-m spot (0.02, 3 sigma); smaller variations detectable by averaging.

Results from Clementine image analysis:– highland reflectance at 1000 nm is ~0.24.– reflectance of fresh, young craters is almost always greater than 10% above surrounding highlands; in some cases reflectance is 50% greater.

LOLA has a chance of detecting a change in brightness of the lunar surface associated with the LCROSS impact, even in a permanently shadowed crater.