You can find many of the terms used in this page in the glossary.
Many of the surface and atmospheric characteristics of Mars described by astronomers e.g. bright and dark (high and low albedo) markings, dust storms, the seasonal growth and decay of polar caps and the presence of clouds are controlled by the planet's current orbital characteristics. Cyclic variations over tens of thousands to millions of years in these orbital characteristics have also led to changes in the climate of Mars over time. This has implications for the possible past presence of liquid water at the planet's surface as we will see later. The orbital characteristics of Mars are shown together with those of the other planets in Table 1. These parameters can tell us a lot both about Mars's structure and climatic conditions.
The overall mass of Mars is also about one tenth that of Earth and the diameter is half. This is a reason for the different ways the planets have cooled since their formation in the early stages of the Solar System. Earth, being relatively large has cooled slowly and this has allowed the subduction of cool lithosphere and hence the development of plate tectonics. In contrast Mars cooled more quickly with intense volcanism concentrated in the early and middle parts of Mars's history.
|Object||Diameter(km)||Density g/cm3||Rotation Period (days)||Obliquity||Revolution Period (yr)||Semimajor axis (AU)||Orbit Inclination||Eccentricity|
The axis along which Mars spins is tilted at 25 degrees. This tilt is called the planet's obliquity and is similar to that of the Earth (Table 1, 23.5o). It is this obliquity that leads to the seasonal variations of incoming sunlight and temperature on Earth and Mars. For instance, in summer of the southern hemisphere on Mars, the South Pole is tilted towards the Sun. When it is summer in the southern hemisphere, it is winter in the northern hemisphere (and vice versa).
The seasons on Mars are denoted by Solar Longitude (Ls) values (see figure below). Solar Longitude is the angle between the Mars-Sun line and the martian line of equinoxes. Ls = 0o corresponds to the spring or vernal equinox (when day and night time are of equal length) in the northern hemisphere of Mars. Ls = 180o is autumn equinox in the northern hemisphere and vernal equinox in the southern hemisphere.
The orbit of Mars is also highly elliptical (elongation of a planet's orbit is called eccentricity) compared to that of the Earth, which is nearly circular. This means that the seasons on Mars are of unequal duration. For instance, southern summer is about 150 martian days whereas southern winter is 180 martian days. The martian year lasts 669 days (or 687 terrestrial days, the martian rotation period or day is only slightly longer than that of Earth, Table 1). At the perihelion of this orbit (the closest point to the Sun in the orbit of Mars) the southern hemisphere happens to be tilted towards the Sun. Therefore, not only is southern summer of short duration compared to northern summer it also receives significantly more sunlight. This affects surface temperatures (which are about 30oC higher during the summer in the south than the northern summer), wind velocity and the transport of dust (see Dust, Ice and Wind)
Solar Longitude Ls and the Orbit of Mars
An opposition is the close approach of Earth and another one of the 6 planets that is further out in the Solar System. For instance, when Earth passes between Mars and the Sun, Mars is said to be at opposition. Mars oppositions occur every 787 terrestrial days and so it is approximately every two years that conditions are best for telescopic observations and sending space probes from Earth to Mars. The closest approaches of all occur when opposition coincides with Mars's perihelion; the distance between the two planets is then about 56 million km. The next such close approach is in 2003. In 2003/4, three landers, Beagle 2 and the two NASA Mars Environmental Rovers are taking advantage of this.
These orbital parameters change slowly over time. For instance, the obliquity of Mars varies from 15o to 35.5o with a period of 1.2 x 106 years. At times of high obliquity more sunlight falls on the polar regions leading to a decrease in the size of the ice caps. Another slow change in Mars's orbital parameters is due to precession (which is the slow conical rotation of an axis of rotation). Both the spin axis of Mars and its elliptical orbit around the Sun undergo precession, and the combined effect is to change the hemisphere which is pointing towards the Sun every 25 000 years.
MOC image PIA01333 of Phobos (approx. 10 km field of view) showing grooves and craters
Mars has two small moons called Phobos (21 km diameter) and Deimos (13 km). These are primitive, asteroidal-like bodies which were captured by Mars at an early stage in the planet's formation. The two moons have low albedo, low density and spectral reflectance patterns similar to carbonaceous chondrites (and so can be classified as C-type asteroids). An unusual feature of Phobos is the presence of grooves, which are typically <15 km long, 10-20 m deep and 100-200 m wide, radiating from the largest impact crater. They may have formed as radial fractures during the impact.