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Projects: MarsDials

Basic Facts

Features of the MarsDial

The scientific purpose of the MarsDial is as a calibration target for the camera, providing a known set of colors and gray scales, both in sunlight and in shadow, much like a television test pattern. Calibration shots must be taken at least every day and sometimes more often. The image of the MarsDial, at a distance of 1.35 meters, is out of focus because the camera was designed for looking at greater distances, in particular at the Martian landscape. The dial is made of anodized aluminum and has a mass of 65 grams (2.3 ounces). The post is painted with a highly nonreflective black paint, and the four color patches and three gray scales are pigmented silicone rubber.

There are two mirrors (highly polished aluminum) on the MarsDial, small sectors visible just to the left and right of the white ring. Since the camera looks downwards at the dial, these mirrors show the color of the Martian sky, which changes depending on location of the Sun in the sky and the amount of dust in the atmosphere. Typical colors are brownish-red, salmon, and rose.

The small red dot and larger blue dot on the outside and inside edges of the white ring represent Mars and Earth in their orbits. Their sizes are in correct proportion (Earth's diameter is 1.9 times Mars'). Notice how Earth's orbit is quite circular (centered on the post) and Mars' is much more eccentric (for this reason the post is placed not in the center of the plate).

Inscriptions on the MarsDial

The largest lettering says "MARS 2004" in the back (with the "2004" permanently obscured to us on Earth) and a motto (traditional for sundials) "TWO WORLDS ONE SUN" in the front. Each of the eight gold areas around the edge contains the name of Mars in two languages. In addition, the spelling "Mars" is used for the Red Planet in many other languages besides English.

Around the four sides of the baseplate is inscribed the following message, illustrated by simple line drawings inspired by those of school children:

"People launched this spacecraft from Earth in our year 2003. It arrived on Mars in 2004. We built its instruments to study the martian environment and to look for signs of life. We used this post and these patterns to adjust our cameras and as a sundial to reckon the passage of time. The drawings and words represent the people of Earth. We sent this craft in peace to learn of Mars' past and to prepare for our future. To those who visit here, we wish a safe journey and the joy of discovery."

Time system on Mars

The Martian day has been called a sol ever since the time of the Viking mission in the 1970s. There are, however, no other conventional units of time for Mars, either for sub-sol units or longer periods. In practice scientists and engineers working with Mars missions have adopted an Earth-like system, especially since the sol happens to be only 39 minutes (2.6%) longer than our Earth day. Thus the sol is divided into 24 units (Martian hours), each of which has 60 Martian minutes. These hours and minutes are then 2.6% longer than the comparable units on Earth. But 6 am still means near sunrise, depending on the season and latitude; 12 noon the middle of the day (Sun highest in the sky); 6 pm near sunset; and 12 midnight the middle of the Martian night.

Reading the MarsDial

The 4.4-centimeter (1.7-inch) post casts a shadow on to the plate bolted to the deck of the rover. For times near the middle of the day, when the Sun is high in the sky and therefore the ball’s shadow remains on the plate, the time is read by the position of the center of the shadow of the ball. Earlier in the morning or later in the afternoon, when the Sun is lower in the Martian sky, the ball’s shadow falls off the plate, so we read the time using the center of the shadow of the petal attached to the post at a height of 1.2 centimeters (1/2 inch).

The straight splayed lines are hour lines on the sundial, indicating the hour of the day. The three other lines allow one to read the date. For example, on the winter solstice the track of the shadow will be exactly along the line labeled WS, on the vernal or autumnal equinox along the (straight) line labeled EQ, and on the summer solstice along the SS line.

Why weren’t the hour and date lines inscribed on the dial on Earth?

That would have been a logical way to do it (when the dial was going to be on a fixed platform – see below), but the dial had to be fabricated long before the landing sites were chosen. We therefore did not know the future latitude of the dial, which is essential in order to calculate the hour and date lines. Thus we devised the scheme of superimposing the hour and date lines, which turned out to be even more needed once the MarsDial ended up on a moving vehicle.

Whoever heard of a sundial on a moving platform?!

Besides being the first extraterrestrial sundial, this sundial may well be the first one in the solar system ever mounted on a moving vehicle. This is indeed strange, but came about because of changes in the mission since the MarsDial idea was first hatched in 1998. Originally the dial was to be on a fixed platform, but then the mission concept changed and the main camera (and its calibration target) were roving!

In order for a dial to work, it must have a known alignment with respect to north. (There are portable dials, but they must achieve this alignment either through a built-in magnetic compass, or through other clever means, such as by knowing the date.) But in our case the rover roves in any direction (we also have to keep track of its changing latitude and longitude) and can be oriented in any direction. In order to draw the superimposed pattern of hour and date lines, we must know how the rover is oriented, so we can know the direction of North on the dial plate. We in fact get that information from the mission engineers. And where do they get it from? The Sun! Every time the rover moves, its orientation is determined by using the camera to measure the position of the Sun in the Martian sky at a known time (on an atomic clock). Knowing this, one can determine the rover’s orientation, in turn allowing the MarsDial to close the circle by determining the time from its shadow. Nevertheless, our humble MarsDial does have a useful semi-official backup purpose for this NASA mission. If for any reason the Panoramic Camera ever were unable to determine the direction of the sun and therefore the rover's orientation, then images of the MarsDial could accomplish this by observing the shadow at a known clock time!