Calibrating Mars

The Primary Target

The primary target measures 10 centimeters (4 inches) across. It has an aluminum base with thin silver and gold coatings on top to give it an artistic appearance and to enable laser etching of additional embellishments. Along the outside of the target are 8 circular patches with colors that span the range of sensitivity of the Mastcam-Z cameras. Underneath each is a strong magnet that should repel most of the Martian dust from its center. This is based on similar magnets used on the Spirit, Opportunity, Phoenix, and Curiosity missions.

Inside that are 4 gray-to-black rings that match 4 of the circles on the periphery. These rings, like the grayscales on a photographer’s test chart, provide the main calibration information used by the camera team.

In the middle of the rings is a post called a “gnomon,†which is the name for the part of a sundial that casts a shadow. The grayscale rings surround the gnomon so that brightness can be measured in both sunlit parts of the rings and parts of the rings that are shadowed by the gnomon. The shadows on Mars are not black but reddish because of sunlight scattered by the dusty atmosphere. (Similarly, shadows on Earth are distinctly blueish because of sunlight scattered in our blue sky.) Surrounding the middle rings is a compass rose with points aligned along the rover’s X and Y directional axes. Using the gnomon’s shadow on the compass rose, you can determine either the direction the rover is pointing or the local time as long as you know the other!

The target also contains laser-engraved graphics, a motto, and an inspirational message. NASA calls these embellishments “festooningâ€; they don’t serve a direct engineering or scientific purpose, but they’re meaningful nonetheless. They have artistic value, they help communicate Perseverance’s goals and historical context to the public, and they offer a fun and educational way to connect to the mission.

The Motto

Like Spirit, Opportunity, and Curiosity, Perseverance’s primary calibration target has a motto. The original inspiration for Spirit and Opportunity was the gnomon since sundials often have mottos associated with beginnings, endings, or the passage of time.

The Spirit and Opportunity motto was “Two Worlds, One Sun,†and you can learn more about the calibration target for those missions in the January/February 2004 issue of The Planetary Report. Curiosity’s motto is “To Mars To Explore.†Perseverance’s motto, “Two Worlds, One Beginning,†echoes the Spirit and Opportunity motto and evokes the idea of Earth and Mars growing out of the same initial cloud of gas and dust that formed our solar system. Both planets had similar starting conditions but evolved into very different worlds.

We chose the Eurostile font for its clean forms that are free of sharp points and excessive detail. This also serves as a nod to science-fiction fans like many of us on the team since Eurostile appears in movies like 2001: A Space Odyssey as well as the Star Trek TV and movie series.

The Graphics

Seven icons, which the Mastcam-Z team calls “vignettes,†fill the space between the colored calibration circles and depict the evolution of life, from the formation of our solar system to spaceflight.

• Mercury, Venus, Earth, and Mars orbiting the sun: This symbolizes the formation of the solar system and the steady revolutions of our planets. The positions of Mercury, Venus, and Earth are shown for the time of the launch window in July 2020, while Mars is at its approximate position for the time of the landing in February 2021.
  
• DNA strand: This symbolizes the emergence of life on Earth. Perhaps Perseverance will help us learn that life formed on Mars around the same time.
  
• Cyanobacteria: This symbolizes the early proliferation of microorganisms on Earth and possibly Mars.
  
• Fern: This symbolizes green plants spreading across Earth.
  
• Dinosaur: An apatosaurus symbolizes the great diversity of life on Earth.
  
• Humans: This vignette echoes the image of humans placed on plaques aboard the Pioneer spacecraft in the 1970s, which will eventually reach interstellar space. A similar image is encoded on the Voyager golden records, which are already in interstellar space. On Pioneer, only the man is waving, and on Voyager, the woman is waving. We decided to make both people wave.
  
• Rocket: The rocket is traveling from Earth (the blue dot) to Mars (the red dot). We chose a stylistic rocket because the realistic version looked a little too much like a missile. Additionally, the stylized version is linked to science fiction that was popular at the dawn of the Space Age, which inspired many early scientists and engineers to pursue the exploration of other worlds.

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