How are planetary maps made?

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The maps of planetary bodies are compiled from several photos and altimetry data. If you want to know more about their terrain, the easiest way is to take photos when there are long shadows in the pictured area, that is, right before the sun sets there. The shadows illustrate well the height of surface formations. These photos are then placed exactly next to each other (to make a mosaic), just like a puzzle. This used to be done manually, but now computers do it. Shadows are barely visible in the photos taken in midday, but they can show us how reflective (shiny) the material covering the area is.

fig1

Basho crater, Mercury. MESSENGER Mercury Dual Imaging System (MDIS). NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

There are generally two types of cameras on the space probes. One is a wide-angle camera, i.e. it can make photos of a larger part of a planet or moon. The other one has a telephoto lens, it is almost like a telescope. They take close up photos of smaller areas. If possible, more photos of the same area are taken with different sun positions, that is, in different lighting conditions. These images do not cover the entire planet. The researchers select the areas they think are interesting in advance, and there the camera is directed.

viking1

Mars Viking Orbiter 1 Mosaic. Jet Propulsion Laboratory and the Astrogeology Branch, U.S. Geological Survey, ca. 1968; gelatin silver print mosaic, San Francisco Museum of Modern Art

Relief maps can be made by laser or radar altitude measurements. The space probe orbiting around the planet sends a laser or radar signal to the area underneath, which is then reflected, and the probe’s instruments measure how much time it takes for the signal’s echo to get back. If it takes a long time then the area beneath is deep set, that is, the signal travelled a long way going back and forth; if it takes a short time, then there is a high area (mountain) below the probe.

google-mars

Google Mars

Surfaces of some planets are covered with an atmosphere that is always foggy over the entire planet (Titan) or cloudy (Venus), so the surface cannot be seen from the space. In these cases, the probe emits radar waves towards the planet, which can penetrate the thickest clouds, and radar images can be made about the surface with the help of the reflected signals.

A nice map can be made about the side of the Moon which faces the Earth, but for mapping other planetary bodies the “cameras” should be sent right to them. In fact, the space probe is a remote-controlled camera (but many other instruments are also packed on it).

voyager

Voyager 1 http://northessexastro.files.wordpress.com/2012/06/voyager-003.jpg

When visiting a planetary body for the first time, the space probe only flashes past the body, and during this short time takes as many photos as it can (e.g. Voyager probes). In the next phase of discovery , space probes  are placed in an orbit around the planetary body and orbiting there for years they can make many close-up images (e.g. Galileo, Cassini probes), or can take pictures of the same area in different seasons (e.g. MGS, MRO) . When the researchers find a planet to be very interesting, first they are discussed for a few years to decide where to land on it, that is, which areas seem to be the most interesting (e.g. an ancient lake or dried up river bed), and which can be reached safely, that is, the probe would not tumble on them, or have no steep or rocky parts in the landing area. Then remote control probes land at these places, which can examine the surface with a microscope. More diverse results are obtained when the landed probe has wheels (rover, e.g. Lunohod, MER [Spirit and Opportunity], Curiosity) because it is able to survey large areas instead of standing still, until they stop working (e.g. Venera, Viking, Phoenix, Huygens).

curiosity_pan

Curiosity Rover

To make researchers better oriented, a committee of astronomers has the task to give names to the surface formations, e.g. craters, mountains, plains. There are various kinds of names: including names of scientists, artists, actors, earthly city names and fabulous locations from the Lord of the Rings to Shakespeare’s works.

mars_nomenclature

http://planetarynames.wr.usgs.gov/

Laser and radar altitude measurements, distant photos and close-ups were then given to children’s book illustrators, graphic designers, who , on the basis of these data drew the maps, inhabiting the desert, inanimate surfaces with fabulous make-believe creatures.

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