On May 19th, 2005, NASA's Mars Exploration Rover Spirit captured this
stunning view as the Sun sank below the rim of Gusev crater on Mars. This
Panoramic Camera (Pancam) mosaic was taken around 6:07 in the evening of
the rover's 489th martian day, or sol. Spirit was commanded to stay awake
briefly after sending that sol's data to the Mars Odyssey orbiter just
before sunset. This small panorama of the western sky was obtained using
Pancam's 750-nanometer, 530-nanometer and 430-nanometer color filters.
This filter combination allows false color images to be generated that
are similar to what a human would see, but with the colors slightly
exaggerated. In this image, 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. Because Mars is farther from the Sun
than the Earth is, the Sun appears only about two-thirds the size that it
appears in a sunset seen from the Earth. The terrain in the foreground is
the rock outcrop "Jibsheet," a feature that Spirit has been investigating
for several weeks (rover tracks are dimly visible leading up to
"Jibsheet"). The floor of Gusev crater is visible in the distance, and
the Sun is setting behind the wall of Gusev some 80 km (50 miles) in the
distance.
This mosaic is yet another example from MER of a beautiful, sublime
martian scene that also captures some important scientific information.
Specifically, sunset and twilight images are occasionally acquired by the
science team to determine how high into the atmosphere the martian dust
extends, and to look for dust or ice clouds. Other images have shown that
the twilight glow remains visible, but increasingly fainter, for up to
two hours before sunrise or after sunset. The long martian twilight
(compared to Earth's) is caused by sunlight scattered around to the night
side of the planet by abundant high altitude dust. Similar long twilights
or extra-colorful sunrises and sunsets sometimes occur on Earth when tiny
dust grains that are erupted from powerful volcanoes scatter light high in
the atmosphere.