India's Chandraayan-1 looks back at Earth from lunar orbit.

The Mini Synthetic-Aperture Radar (SAR) is a lightweight radar imaging instrument flying currently on the Indian Space Research Organization's Chandrayaan-1 mission. A modified version of this instrument will fly on NASA's Lunar Reconnaissance Orbiter mission in 2009.

This composite shows Rozhdestvensky K, a moderately sized (42-kilometer [26-mile] diameter) impact crater on the southern rim of the larger crater Rozhdestvensky, near the moon's north pole.

Mini-SAR uses a different analytical approach to look for ice. Traditionally, the key parameter used to determine if ice is present is the circular polarization ratio (CPR). This quantity is equal to the magnitude of the same sense (i.e. the left or right sense of the transmitted circular polarization) divided by the opposite sense polarization signals that are received. Mini-SAR uses a hybrid dual polarization technique, transmitting a circular polarized signal (either Right or Left Circular Polarization) and then receiving coherently the linear Horizontal and Vertical polarization signals. This hybrid architecture preserves all of the information conveyed by the reflected signals and ultimately serves to determine if the returned signal is caused by an ice-regolith mixture, or simply dry rocks on the lunar surface.

The principal goal of Mini-SAR on Chandrayaan-1 is to conduct systematic mapping polewards of 80? latitude for both poles. Mini-SAR uses S-band (2380 MHz), has an illumination incidence angle of 35?, and image strips have spatial resolution of 75 meters per pixel. During the observation opportunities given to the instrument, it will image in SAR mode both poles every 2-hour orbit, covering both polar regions in a single 28-day mapping window.

These regions close to both poles contain some of the most promising sites for potential water deposits. By operating Mini-SAR during orbits of maximum inclination, the scientists will be able to obtain SAR strips of permanently shadowed regions within 2? latitude of both poles.

The Mini-SAR instrument was activated on November 17, 2008 and acquired SAR images of both poles during a commissioning test (Fig. 1).

Scientists:

1. Lunar and Planetary Institute, Houston TX 77058 (spudis@lpi.usra.edu) 2. Applied Physics Laboratory, Laurel MD 20723 3. NRAO, Socorro NM 4. NASM, Washington DC 5. Univ. Hawaii, Honolulu HI 96822 6. ISRO, Bangalore, India 7. JPL, Pasadena CA 8. ASU, Tempe AZ 9. USGS, Flagstaff AZ

Implications

Here's a very young, fresh impact crater (3 kilometers [nearly 2 miles] in diameter) on the western limb of the moon near the crater Sylvester, taken by the Mini-SAR instrument aboard India's Chandrayaan-1 lunar orbiter.

While no remote measurement can definitively answer the question of whether ice exists at the lunar poles, an orbiting SAR provides the most robust method of obtaining a positive indication of ice deposits. With an orbital SAR, ALL areas on the Moon can be seen. All permanently shadowed regions will be imaged multiple times by an orbiting radar with incidence angles favorable for determining their scattering properties.

Significance to Solar System Exploration
The presence of water ice on the Moon has the potential to completely change the space flight paradigm. Currently, space probes must be supplied and equipped on Earth and launched complete; this limits the amount of material, and thus capability, of future space probes. In contrast, if the Moon's resources can be used, specifically the water ice at the poles to make rocket propellant, the rules of space exploration will be forever changed. Use of lunar generated propellant will create an Earth-Moon transportation infrastructure, with which we can not only access any point in space, but also voyage to the planets beyond.

Written by Samantha Harvey

For more information about NASA Science Highlights and information on publication, please contact Samantha Harvey, Samantha.K.Harvey@jpl.nasa.gov.

Last Updated: 23 February 2011