When near opposition the brilliant red jewel is unmistakably king of the late night skies.

At favorable oppositions Mars subtends about 25 arcsec and a good telescope can resolve about 100km so, about the same as the moon with the naked eye. So only major features are observable, no mountains valleys or craters. But some features like the lunar maria are evident and the polar ice caps which grow and recede with the seasons are evident. Some larger features like Syrtis Major and Hellas and even Olympus Mons (Nix Olympia) are visible but not even Valles Marineras is discernable much less the fabled canals of Barsoom.

The real exploration of Mars began in 1965 when Mariner 4 flew by and returned 22 photos. One surprise was a bleak surface pocked by craters. Mariners 6 and 7 confirmed the findings. Then in 1971 Mariner 9 became the first spacecraft to orbit another planet. Mariner 9 mapped Mars with a resolution approaching a kilometre and revealed volcanoes, layered polar caps, channels that suggest water flows and a huge valley (Valles Marineris) that extends nearly a quarter of the way around Mars. Mariner 9 set the stage for the Viking landers and the recent Mars landers (Sagan station). Still higher resolution will be received by the current Mars orbiter when it attains mapping orbit in 1999.

Mars is a little over half the diameter of the Earth and so has a surface area about the same as Earth's continental area. The mass is 11% Earth's and the uncompressed density 3800kg/m³ intermediate between that of Earth (4500) and the Moon (3200). This suggests a solid FeS core 2400km across and a large silicate mantle. Absence of a magnetic field suggests the core is not liquid. We really need some seismic studies to give us a better idea as to the internal structure of the red planet.

Mars has a rather eccentric orbit e=0.093 which led Kepler to the conclusion that planetary orbits were elliptical. Mars distance to the sun ranges between a(1-e)=1.382 and a(1+e)=1.665AU so the distance at opposition can range from 0.382 to 0.665AU, a factor of 1.74 in distance and 1.742 times (1+e)2/(1-e)2, a factor of 4.4 in apparent brightness or 1.6 magnitudes. The next favorable opposition will be in 2003:

The sidereal period is 687d and the synodic period is 780 days so oppositions occur every 26 months. This same 26 month interval applies to favorable launch windows to Mars.

Perihelion advance 72,000 years

Spin axis precession 175,000 years

Polar maximum radiation: pole - pole 51,000 years

e =e _0± 0.03 in 95,000 years

0.01 < e ₀ < 0.14 in 2x10⁶ years

i=i_0± 1/2° in 1.6x10⁵ years i is now 25° for the earth.

i₀=i₀’± 2.5° in 1.2x10⁶ years

the axial tilt varies from 15° to 35°.

The surface of the planet is divided into two topographic hemispheres (inclined about 35 degrees to the equator) with plains and volcanoes in the northern hemisphere and ancient cratered terrain in the south. The volcanic plains, like Earth's oceans, are several kms lower than the older uplands in the south. There is an immense bulge straddling the division between uplands and plains rising some 10km above its surroundings, the Tharsis bulge, a region that includes four large volcanoes Arsia, Ascraes, Pavonis and Olympus Mons.

Topology:

Topographic elevated plains

• Thaumasia, Elysium, Planitia and Tharsis

Topographic basins

• Hellas, Argyre and Isidis possibly originating from an impact.
• Chryse and Tharsis

a) Plains. Tharsis, Elysium and Hellas (oldest). Found in the North. Possibly due to volcanic flooding (We can see the volcanic flow fronts (obate scarps)).

Note the difference between:

Both are due to lava flows (isostatic) but the Martian plains are more like the continental flood basins found on earth.

b) Volcanic shields. 12 huge volcanoes. The biggest is Olympus Mons (600km x 21km). The largest on earth is Mauna Loa (200km x 9km)

c) Grooved terrain, ridges, etc.

Cratered terrain. Ancient, like the lunar highlands. Most craters are shallow with flat floors.

Mars has 3 large multi - ring and 20 smaller 2 - ring impact basins, all in the ancient south. Hellas is some 1800km across and 6km deep, larger than the Imbrium basin. Argyre basin is 700km across. Both of these are surrounded by mountains raised by the impact.

Valles Marineris. 5000km. The central region is certainly due to faulting.

To the west, Labyrinthus Noctis, a maze of interconnecting valleys.

To the east , merges with chaotic terrain, meandering sinuous valleys which empty into the plains of Chryse.

Separating the low northern hemisphere and the high southern is a topographic break, a slope going about 2/3 of the way around mars.

Consisting of Hummucky deposits:

• Chaotic terrain
• Knobby terrain
• Fretted terrain

Chaotic terrain: in low regions. Some are drained by channels, indicating fluid flow.

Fretted terrain: lying between the Southern cratered terrain and the Northern plains. 500 km wide and stretching around ~1/2 of mars. Contains irregular blocks, several 100 km on down to < 1 km. Due to a massive break up of the crust to a great depth.

Knobby terrain: Similar to fretted terrain but the blocks are worn down. Between Tharsus and Elysium. Due to a break up of older crust.

Furrowed terrain: in cratered terrain in equatorial regions. 2-3km x 30km furrows. Possibly evidence of water, or even rainfall.

Undivided plains: Hellas at first appeared smooth and featureless (M3, M6). Later, M9 revealed rugged scarps. Much of the time Hellas’ floor is obscured by haze or dust.

Like the earth, Mars has winter solstice near perihelion so the south pole has hot short (160d) summers and long (199d) bitter cold winters. The wide seasonal swings cause the southern icecap to shrink from latitude 55 down to a diameter of about 350km. The residual southern cap is a thick layer of CO2 and water ice?

The milder northern seasons result in a larger northern permanent water ice cap some 1000km in diameter. The large seasonal changes in both polar caps is due to the freezing and evaporation of CO2. Both polar caps have quasi-linear swirl-like features and the edge of the permanent water cap shows a cliff and bench topology with 100m thick layers.

Sedimentary layers (wind blown dust?)

Etched plains: irregular pits 1 - 10 km.

Permanent Ice: The North pole is larger than the South pole. This is due to e and i.

Craters

On the Earth we have Mauna Loa, Monte Somma (Vesuvius), Mt. Pelee, Plato’s Atlantis. Volcanoes are a result of plate tectonics.

Lunar volcanoes occur mainly in the lowlands of maria.

• maria: long sinuous scarps (flow fronts related to volcanic activity)
• sinuous rilles (e.g. Hadley (Apollo 15)). Tens of m to km’s wide and several 100 kms long.
• buried structure, collapsed craters,
• volcanoes: consider typical cinder core, earth vs. moon: no air drag and smaller g.

On Mercury the Caloris basin could possibly be of volcanic origin, although this is a bit uncertain.

Mars: pre M9, observers claimed to have seen bright flares.

Olympus Mons (the largest volcanic mountain in the solar system) lies on the NW slope of the Tharsis uplift and is over 500km in diameter and rises 25km above the surrounding plains. It is the biggest volcano in the solar system, 100 times the volume of Mauna Loa, earth's largest volcano, and rivaling Maxwell Montes on Venus. These are shield volcanoes consisting of many overlapping layers of basaltic lava and topped by calderas. The number of impact craters on Olympus Mons suggests it is less than a hundred million years old and some flows could be 100 years old for all we know. Surrounding the shield of Olympus Mons is a 5km scarp which needs explaining.

Did the flows push a thick blanket of ash (tuffs) which later blew away?

Olympus Mons is surrounded by a ring of grooved terrain, extending to 1000 km from the centre. It is unknown whether this is a response to the load, or the eroded roots of a more ancient volcano.

The other large volcanoes are Elysium Mons lying in the plain of Elysium and Ascraeus Mons, Pavonis Mons and Arisa Mons on the Tharsis ridge. They are ~ 200 My old and should have been active for ~ 130 My.

The distribution of Martian volcanoes is non - random.

They are composed of Basathic (low viscosity) rock, similar to the shield volcanoes on earth.

Wind (Eolian processes)

Large yellow dust storms can be seen on the planets surface. The largest occur shortly after perihelion, when it is summer in the South. Perihelion is the point in it’s orbit where it is closest to the sun, aphelion is the point in its orbit when it is furthest from the sun. Input at perihelion increases by 45° over that at aphelion.

Bright and dark streaks and "splotches" are relatively stable features associated with craters. They are possibly thick deposits of a fine material. These features are caused by deposition and not erosion. The dark areas may be "bedrock".

Polar sediments (see book).

Water in the form of ice at the poles and vapour in the air are confirmed. Mars at present cannot have free surface water but there is strong evidence that water did create some features seen by Mariner 9, the Viking orbiters and the current MGS. There are a number of sinuous channels the largest of which are some 1500km long and up to 100km wide. Outflow channels lead from the upland regions down to the great basins and valley networks occur in the uplands and lead from the sides of some volcanoes, possibly the result of subsurface flows. Sandbars and all of the features associated with running water are everywhere.

Rain will not occur, even with variation of the insolation. (The earth’s oceans are not in equilibrium with the poles).

Reservoirs:

• Regolith - 10gm CO₂ cm^-2 liberated when the temperature goes from -110° C to -70° C.

Sources:

• Primitive?
• Volcanoes?
• Polar caps
• Ground ice
• Permafrost

Water sinks:

• H₂O + g ® OH + H
• H + H ® H₂ (escapes)
• CO⁺₂ + b ® CO + O (escapes)
• O⁺₂ + b ® O + O "
• CO⁺ + b ® C + O "

Mars has two moons, Phobos (fear) and Deimos (panic) discovered by Asaph Hall in 1877 which enables accurate determination of Mars mass. Phobos is 9380km from Mars and has a period of 7h39m, so it rises in the west and sets in the east (in 5.5 hours) just like a slow version of Mir or the space shuttle. Phobos is a potato-shaped 28x23x20km moon with the long axis pointing to Mars so when it passes overhead at a distance of 9830-3397=6433km it would have an apparent diameter of 206265"(22/6433)=705" or 11.8', about a third the size of the Moon. It would be pretty dim too since sunlight is on average 1.62=2.5 times fainter out here and the Martian moons, most likely captured asteroids, have reflectivities of about 2%. At aphelion the sun's diameter is about 32/1.665=19.2' so total eclipses are out. But if you wait a few million years Mars will have total eclipses since tides are dragging Phobos towards Mars and eventually Phobos will be called the Phobos basin.

Formation by accretion (Differentiation). A short time after 10⁹ years the meteor/asteroid flux decreased to the present levels.

Expansion of the mantle (olivine - spinel) breaks the crust.

North (low) hemisphere (thin crust) - fractures

South (high) hemisphere (thick crust) - radial faults (Tharsis uplift, plume?)