Types of Planetary Data Available (not all-inclusive list)

I. Camera.

A. Data available for most planets and moons.

Notable exceptions: Venus, Titan, and Pluto

B. However, these are not cameras like normal cameras. Instead the camera is an array of photoconductors (CCDs, charge-coupled devices). The photoconductors simply measure intensity of light, and color images are obtained by using filters. Thus, reconstructing a true color image is often impossible.

C. Some of these cameras can also be sensitive to, and have filters in, the ultraviolet and infrared.

II. Topography - Data available for Venus, the Moon, Mars, and Eros.

A. Older missions - Radar altimeters - factors that affect resolution:

1. Footprint size increases with increasing altitude

2. Footprint spacing also affects resolution

B. Laser altimeters have a very small footprint and only footprint spacing determines resolution.

III. Gravity - Doppler shifts in the tracking data occur as the planet's gravity field moves the spacecraft up and down in its orbit. From this the gravitational acceleration in the Earth - spacecraft (line-of-sight, or LOS) direction.

A. The resolution of this data is roughly the spacecraft altitude

B. Global data available for the moon, Venus, Mars, and Eros.

4. Magnetics – Will not go into how a magnetometer works. Typical method involves creating a magnetic field in the instrument and seeing how it's decay is affected by the planet's magnetic field.

A. A significant magnetic field requires convecting magnetic material. The Earth has a liquid-iron outer core.

B. When rock cools it retains a record of the magnetic field at the time it cooled. Because of this we know the Earth's field has reversed over time.

V. Spectroscopy - A diffraction grating separates different wavelengths and the instrument scans in two directions to build up on image.

A. Data availability - the moon, Mars, asteroids, and Galilean satellites.

B. While photographs are better at low sun angles, spectroscopy best at high sun angle.

C. Spectroscopy can be used to detect particular minerals, but is only sensitive to material in the upper cm of the planet (problem of dust on Mars).

VI. Synthetic Aperture Radar (SAR).

A. Data availability:

Venus - global coverage at ~100m resolution

B. How does it work:

1.The spacecraft sends a pulse of a few cm wavelength out to the side as it orbits the planet.

2. The spacecraft records the reflected pulse. The recorded signal arrives over a period of time and a range of frequencies. The higher (lower) frequency returns are from point ahead (behind) of the spacecraft in the long tract direction, and the earlier (later) arriving signals are closer (farther) away from the spacecraft.

3. After processing, the end result looks very similar to a black and white photo.

C. What is SAR sensitive too:

1. Topography - topographic slopes facing the radar are bright, facing away are dark. Also those facing the spacecraft are foreshortened in appearance, and those away lengthened.

2. Reflectivity - the inherent ability of the rock, dependent on composition and porosity, to reflect the radar return.

3. Roughness - if the surface is blocky at radar wavelengths it will appear radar-bright.

VII. Landers.

A. Very high-resolution, well-calibrated pictures at localities on Venus, the moon, and Mars.

B. Some primitive elemental composition analysis, typically from x-ray fluorescence.

C. A network of seismometers existed on the moon, and plans exist to establish one on Mars.

VIII. Returned samples exist only for the moon. Much more chemistry, petrology, etc., can be done with returned samples than can be performed in situ, but the cost is much greater.