Retrograde Motion of Mars in the Sky
The planet Mars, as well as the other outer planets to lesser degrees, appears at times to move towards the east in the sky among the constellations (called direct motion) then briefly stop its eastward motion and begins to go backwards towards the West for a short period of time and then finally resume its normal eastward motion in the sky. This forward and backward motion of Mars and the outer planets in the sky is known as retrograde motion. Retrograde motion is due to the fact that the outer planets orbit the sun at lesser speeds then the earth in its orbit. This is analogous to two runners racing on a track and the runner on the outside lane (the planet Mars orbiting the Sun beyond the earth) appears to be ahead but as the runner on the inside track (the Earth within its inner orbit around the Sun) has a shorter track he is able to catch up to the outside runner and pass him on the track. If the motion of Mars is traced in the sky it may produce a zig-zag or or looping motion depending upon the position of the planet above or below the ecliptic.
The Planet Mars
The planet Mars is the 4th planet from the sun. Mars orbits the sun at an average distance of 146 million miles 235,000,000 kilometers, 1.52 astronomical units (AU, one astronomical unit equals approximately 93 million miles or 150 million kilometers). The orbital period of Mars equals 687 days 1.88 years, or the sidereal period (orbital period of a celestial body required to return to the same point among the stars). The synodic period of the planet Mars is equal to 780 days (or 2.1 years) Which is the time required for a celestial body to return to a position in relation to another celestial body, or the time required between successive oppositions of the planet. The diameter of Mars is equal to 4221 miles (6792 kilometers, or 53% the earths diameter). The rotation period of Mars is equal to 24 hours 37 minutes 22.7 seconds (or 1.03 days, also known as a Sol). The axial tilt of Mars is 25.2 degrees compared to the earths axial tilt of 23.5 degrees. The mean surface pressure on the planet Mars is 600 pascals (Pa, or 0.087 PSI). This ranges from 30 pascals (0.0044 PSI) and the Olympus Mons to 1155 pascals (0.1675 PSI in Hellas Planitia). The main surface pressure of Mars is therefore 0.6% that of the earth's (101.3 kilopascals or 14.69 PSI). The surface gravity of Mars is 38% the earths. The atmosphere of Mars is composed primarily of carbon dioxide (96%, CO2), argon (1.93%, Ar), nitrogen (1.89%, N2), oxygen (0.146%, O2), and water vapor (0.021%, H2O). Mars has two satellites named Phobos (“Fear” in Greek) is the inner potato-shaped moon (mean radius of 7 miles (11 km)) of Mars that orbits the red planet at an average distance of 3,700 miles (6,000 km) from the Martian surface. It orbits Mars in a period of 7 hours 39 minutes and appears to rise in the west and move across the Martian sky in 4 hours 15 minutes or less, and set in the east, twice each Martian day. Deimos (“Terror” in Greek) is the outer moon (mean radius of 3.9 miles (6.2 km)) that orbits Mars at 14,580 miles (23,460 km) in a period of 30.3 hours rising in the east and setting in the west. Since its period is longer than the Martian day (24.7 hours (1 Sol)) an observer on the Martian equator would see it rise and set after ~2.5 days. Both Martian moons were discovered by the American astronomer Asaph Hall, III (1829-1907) at the United States Naval Observatory in Washington, D.C. in 1877 (August 12 for Deimos and August 18 for Phobos).
A map of the classical albedo features over the planet Mars that I produced. Not all features noted on the map are visible to an observer at any one apparition/opposition as it depends upon the hemisphere of the planet that is pointing towards the earth (Northern Hemisphere during aphelic oppositions and Southern Hemisphere during perihelic oppositions) as well as the presence of dust and clouds over the surface of the planet over time. The darker albedo features shown are typically visible during the majority of apparitions/oppositions. The North and South Polar Caps (NPC and SPC) will change in size according to the current martian season over that hemisphere. The canal-like albedo features are due to a contrast effect between the light and dark zones over the surface.
Download a free copy of the classical albedo map of Mars for the observer to use as reference at the telescope.
Martian Dust Storms
The formation of Martian dust storms (and dust storms on the Earth’s surface as well) are due to the surface of Mars being strongly heated by incoming solar radiation that heats up the surface. The air closer to the surface is heated and cooler air is present above it. The heated and cooler air becomes unstable, and the heated air then rises taking dust up into the Martian atmosphere with it, like thunderstorms on the Earth. Rising plumes of heated air and dust form dust devils (small cyclonic whirlwinds) that grow as more dust is added to them. The dust devils them coalesce to form massive dust storms over a period of hours to days. Depending upon the size of the dust storm the dust may remain aloft in the Martian atmosphere for days, weeks, or months which then obscures the surface features below. Illustration by Carlos E. Hernandez©
Hellas Basin Dust Storm
The Hellas Basin on Mars is huge impact basin located over the southern hemisphere of the planet (42.40S, 70.50E) and measures 1,400 miles (2,300 km) across and up to 4.4 miles (23,465 feet or 7.2 km) in depth, the deepest point on the planet Mars. The Hellas Basin contains the highest surface pressure on Mars at 12.4 millibars (mbar; 1,240 Pascal (Pa) or 0.18 psi). This surface pressure in the Hellas basin is 2x higher than the average surface pressure on Mars (6.1 mbar, 610 Pa, or 0.09 psi). The surface pressure within the Hellas Basin could theoretically support liquid water under certain conditions of temperature, pressure, and dissolved salt content (but only for brief periods of time). The Hellas Basin is a very common site for the origin of Martian dust storms. My painting of the Hellas Basin before and during a dust storm©.
Observing Equipment
The tools and techniques applied by astronomers using quality instruments and accessories is vital to be able to detect low-contrast albedo features across the surface of the planet Mars and other extended celestial bodies (Moon, planets, and nebulae). An observer hoping to detect faint detail over the surface or atmosphere of the planet Mars must prepare beforehand to have a successful observation. The prospective Mars observer needs to be using an instrument that is diffraction-limited (1/4 wave peak to valley surface error allowable of a lens or mirror), properly collimated, and cooled to ambient temperature before being able to detect the fine albedo features over the red planet. Many excellent telescope designs are available for high-resolution observation and imaging of the planets that include refractors (doublet (achromatic) or apochromatic (triplet) lenses), reflectors (multiple mirrors of different configurations and surface figures), and catadioptric ( a combination of lenses and mirrors).
The refractor employing a doublet (achromatic) or extra-low dispersion (ED) triplet lens is considered by many to be the classic planetary instrument. The design using a lens that is properly figured (diffraction-limited), centered, and baffled (placed within the tube to reduce stray light from affecting the final image) will produce the sharpest and highest contrast image possible for the observer. The refractor, unfortunately, becomes very expensive as the lens diameter exceeds 4 inches/10.2 cm that will require a larger and heavier mount to sustain it unless using a short-focus ED triplet lens. The view of Mars under good seeing (atmospheric) conditions in a 4 to 6 inch (10-15 cm) refractor is impressive and memorable.
Reflectors come in many designs but the most popular one is the Newtonian reflector that was created by the eminent English scientist Sir Isaac Newton (1643-1727) in 1668. The Newtonian reflector employs a primary parabolic mirror at one end and a small flat secondary mirror at the other end of the tube that the deflects the reflected light to the side of the tube in order to observe the object. A well-constructed Newtonian reflector with a diffraction-limited primary and secondary mirrors and with the secondary mirror’s diameter being no greater than 20 to 25% of the diameter of the primary mirror may provide outstanding images of the moon and planets. Some of my best views of the moon, planets, and deep sky objects have been through quality made Newtonian reflectors.
The catadioptric reflector employees both a lens and a mirror to form the final image. An example of the catadioptric design is the Gregory Maksutov-Cassegrain reflector developed by Russian/Soviet optical designer and amateur astronomer Dmitry Dmitrievich Maksutov (1896-1964) in 1941. A popular type of Maksutov-Cassegrain telescope is the Gregory or “Spot” Maksutov-Cassegrain that uses all-spherical surfaces and have, as secondary, a small, aluminized spot on the inner face of the corrector. This design allows “fixing” of the secondary mirror (typically an aluminized spot on the inner surface of the weakly-negative lens corrector) and eliminates the need for a spider that would produce diffraction spikes. the difficulty in building the Gregory Maksutov-Cassegrain is that the full aperture lens corrector that is required it becomes large and heavy and more expensive as the aperture increases. The full aperture corrector lens also requires extra time for cooling down to ambient temperature which depending upon the aperture of the instrument will take a significant amount of time to do so. Maksutov-Cassegrain designs are typically not built larger than 180 millimeters (7 inches) in aperture due to these constraints. Another popular catadioptric telescope design is the Schmidt-Cassegrain telescope that includes the Celestron Schmidt-Cassegrain (developed by Thomas “Tom” J. Johnson (1923-2012) of Celestron Pacific in 1970) and the Meade Schmidt-Cassegrain (developed by John C. Diebel in 1972). The Schmidt-Cassegrain reflector is a very popular instrument that is used by amateur astronomers around the world for observation and imaging of the moon, planets, and deep sky objects. Whatever telescope design instrument that the planetary amateur astronomer is going to use must be diffraction-limited, collimated, and allowed to reach the ambient temperature at the observation site.
Eyepieces for Observing Planets
About the author:
Ambassador Carlos Hernandez has contributed his planetary observations to worldwide organizations including The Association of Lunar and Planetary Observers (ALPO, United States), British Astronomical Association (BAA, Great Britain), Oriental Astronomical Association (OAA, Japan), and many other excellent planetary amateur astronomer groups over many decades.