NASA's Rover Captures a Martian Sunset

Prior to the arrival of aerosol monitoring satellites, sunrises and sunsets provided atmospheric scientists with data about the presence of aerosols in the atmosphere and their height. Scientists are now using martian sunrises and sunsets to study the presence of dust in the red planet's sky.

Figure 1 is a crisp photograph of the sun setting on Mars at about 6:07 in the evening on 19 May 2005. The image was made by the Mars Exploration Rover Spirit's panoramic camera. The image was made through the camera's 430 nm (blue), 530 nm (green) and 750nm (far red) filters to provide an image that resembles what a human observer would see.

According to a media release from the Jet Propulsion Laboratory, "... the bluish glow in the sky above the Sun would be visible to us if we were there, but an artifact of the Pancam's infrared imaging capabilities is that with this filter combination the redness of the sky farther from the sunset is exaggerated compared to the daytime colors of the martian sky."

The bluish glow is caused by sunlight scattered by dust high in the martian sky. Previous images from Mars have revealed twilight glows that last as long as two hours after sunset. A twilight glow lasting this long on Earth from a location 40 degrees north of the Equator would suggest aerosols higher than 100 kilometers over the surface! Only an extraordinarily powerful volcanic eruption can place aerosols this high in the sky.

If the Rover's camera were used to make a similar photograph of a sunset from Earth, the solar disk would be around a third larger than its size as viewed from Mars.

Forrest M. Mims III