ASTRONOMICAL PHENOMENA FROM MARS






Like the night sky on the Earth, the martian sky hosts a plethora of astronomical phenomena occurring around the clock in a neverending space opera played out for the benefit of robotic and future human observers alike. The following is a rough stargazers' guide to celestial spectacles on Mars.







Stars

Because the stars are so far away, their positions and constellations are exactly the same as on Earth. However, star ``twinkling'' would be all but absent at Mars due to the much lower atmospheric pressure, less than one percent that of the Earth. Moreover, Mars' rotation axis does not point in the same direction as on Earth, the result being that the ``pole star'' on Mars is not Polaris in the constellation of Ursa Minor or Little Bear, but instead the bright star Deneb in Cygnus the Swan. The brightest star in the sky would naturally be the Sun. Its disk size will vary between 19 and 23 arcmin from orbital aphelion to orbital perihelion, its brightness between -25.6 and -26.0 and magnitude. For comparison, the apparent solar disk as seen from the Earth is 32 arcmin across.








Moons

Mars has two moons, Phobos and Deimos . At 20 and 10km respectively they are much smaller than the Earth's moon (3,500 km), but they partially make up for their small size by orbiting the planet very close to the surface, at 6,000 and 20,000 km respectively.
Size.

Phobos shows an apparent disk 11 arcminutes across as viewed from the surface,considerably smaller than the moon's (over 30 arminutes). To a human eye,it should be easily resolvable as a very small disk. Deimos would be more of a challenge, since its apparent size is only 2 arcminutes across and thus only discernible to exceptionally sighted individuals.



Motion.



The orbital periods of the two moons are very different. Phobos revolves around Mars in 7.6 hours, much faster than the planet rotates (24hr 37min). As a result, Phobos would appear to rise in the west , racing across the sky to set in the east a little more than four hours later. Deimos, on the other hand, has a 30.0 hour revolution period, almost keeping pace with Mars. It would rise in the east and move very slowly in the sky, setting in the west approximately 2 1/2 days later. In that period it would go through its phase cycle twice, varying in brightness considerably during that time.



Brightness.


Due to their small sizes, neither of the martian moons can be as bright as our own Moon (mag. -14). However, they are quite prominent in the sky. When Phobos is at full phase and at the zenith, it will have a magnitude of -9, about that of a very bright Iridium flare and the brightest object in the sky after the Sun. Under the same conditions, Deimos would be as bright as -5, somewhat brighter than the planet Venus appears in Earth's sky.

Eclipses.








As with all satellites in the solar system, Phobos and Deimos occasionally pass in front of the Sun as seen from a location on Mars. Such events are best described as annular eclipses or transits. Eclipses by Phobos would last only 30 seconds, and Deimos about 2 minutes. The latitude of these shadows on the planet is very closely related to the season, with the shadow footprint always falling on the winter hemisphere, while at spring and autumn equinox the shadow crosses the martian equator. The moons are also eclipsed by Mars itself (a situation analogous to lunar eclipses at the Earth). As it climbs near the zenith, Phobos would enter the martian shadow and disappear over a period of 10 seconds or so, to reappear after culmination approximately an hour later.Eclipses of Deimos would last about twice as long depending on season. There would be periods centred on the equinoxes during which both moons would appear to undergo eclipses on every night that they are visible on the sky and at every revolution.
Finally, the too moons can occult (ie pass in front of) each other, although such occultations are visible only from a very narrow latitude band around the equator of the planet.









Planets

The planets would generally look different at Mars. Saturn and the outer planets would be about as bright as from Earth. Jupiter would be somewhat brighter, reaching a magnitude of about -3 at opposition. Venus and Mercury would be dimmer and closer to the Sun than they appear from the Earth. Mars' morning and evening star would undoubtably be the Earth itself. it will shine at a magnitude of about -2.5 (about as bright as Jupiter appears from the Earth) and never stray further than 40 degrees from the Sun. The Moon would be about as bright as the star Aldebaran in Taurus (mag. +1), staying to within a third of a degree of its parent planet.



Page Credit : Agnieszka Drewniak   Text Credit : Apostolos Christou