Twilight at Mars. It's 6:20 p.m. local time at Gusev Crater, Mars, on April 23rd, 2005. The Sun has just sunk under the horizon. This beautiful and evocative view of a Martian twilight was taken by Spirit, one of both twin rovers which NASA sent to Mars in quest of a watery past and of life. Both rovers are working at opposite locations on Mars since last January 2004 as, due to their good condition and their aptitude to continue to make science, their 3-month primary mission has been extended. They are now scheduled to work until September 2006. Such twilight images are not taken for the pleasure but they are occasionally acquired by the rovers to determine how high into the atmosphere the martian dust extends, and to look for dust or ice clouds. One of the characteristic of the Martian twilight glow is that it remains visible for up to two hours before sunrise or after sunset. This is long compared to what is seen at Earth and it's caused by sunlight scattered around to the night side of the planet by abundant high altitude dust. picture site 'Amateur Astronomy' based on a picture NASA/JPL/Cornell