Telling Time on Mars
By Michael Allison
Accurate solar time keeping on Mars is essential to the study of its weather and climate. No other planet in the Solar System exhibits such extreme daily and seasonal variations in temperature, atmospheric pressure, wind, and frost. For this reason the scientists working with the Pathfinder and other Mars spacecraft are careful to tag their measurements in terms of the Mars "local solar time".
While an average Mars solar day (or "sol") is only 39 minutes, 35.2 seconds longer than the terrestrial 24 hours, a Mars solar year is 1.881 Earth years, or 668.59 sols. The 25.2° tilt of the planet's equator with respect to the plane of its orbit imposes an Earth-like progression of the seasons. The approximate alignment of the Martian northern winter (southern summer) solstice with the planet's closest approach to the Sun appears to trigger occasional global dust storms sometimes lasting up to several weeks. But accurate solar time keeping on Mars is further complicated by the planet's orbital eccentricity, over five times larger than Earth's, implying a nearly 40% seasonal variation in its incoming sunlight and a fifty minute variation in the timing of local noon (as measured on a 24 "hour" Mars clock).
Until now, accurate Mars solar timing has depended on elaborate computer programs, usually run separately or only crudely approximated in atmospheric models and data analysis studies. I recently developed a simple and efficient method for the accurate calculation of Mars solar time in the form of trigonometric series. My published report of Mars solar timing methods was featured on the cover of the 1997 August 15 issue of Geophysical Research Letters. The cover figure, shown here, depicts the seasonal variation of the apparent latitude of the Sun on Mars and the associated time of sunrise at the Pathfinder landing site, along with the "equation of time" for the accurate reckoning of true solar time from the evenly advancing clock-time. (The tear-drop pattern for this seasonal/solar relationship is the Mars analog to the figure-8 "analemma"
I have built this new solar timing method into a Mars-adapted version of the GISS general circulation model as a further step in my work toward the development of a weather mapping program for the upcoming Surveyor 98 mission to Mars. For the current solar time anywhere on Mars, see the interactive Goddard Institute MarsTerminator clock developed by Robert Schmunk.