Mercury, Venus, Mars: Comparative Planetary Geology and Atmospheres

Mercury

Mercury is always much closer to the Sun than Venus is, and yet it never appears brighter than Venus, even at maximum brightness.

Mercury's surface

Mercury is small, has a dark surface, and has no reflecting clouds.

Craters on Mercury

Craters on Mercury appear to have been produced by impacts from objects from space early in the planet's history.

Caloris Basin

The Caloris Basin is a multiringed impact basin on Mercury.

Formation of Caloris Basin

The Caloris Basin appears to have been produced by the impact of a massive object in the early phases of the planet's formation, soon after the initial cratering period.

Jumbled terrain on Mercury

A distinct area of unusually jumbled, hilly terrain has been found on Mercury, believed to have been formed by seismic waves generated by the Caloris impact.

History of Mercury

The history of Mercury can be summarized as thin crust forming at first, followed by extensive bombardment, the craters from which were then covered by lava flows that produced extensive lava plains between the remaining craters.

Mercury's characteristics

Mercury can be characterized as having a Moonlike surface and an Earthlike interior.

Mercury's core

Mercury's average density and the fact that it has a (weak) magnetic field lead to the conclusion that its central core is probably composed of solid and/or molten iron.

Internal structure of Mercury

The internal structure of Mercury is a dense iron core taking up almost half of the volume of the planet and a rocky mantle surrounding the core.

Plate tectonics on Mercury

There is no evidence of plate tectonics on Mercury, likely because the planet cooled too rapidly and the mantle became too rigid.

Solar wind deflection

The physical feature of the planet Mercury that causes the deflection of solar wind particles away from its surface is its magnetic field that produces a magnetosphere surrounding the planet.

Mercury's magnetic field

Compared with that of the Earth, Mercury's magnetic field is weak but clearly present.

Mercury's rotation

Mercury alternately turns one side (Caloris Basin) toward the Sun at one perihelion and the opposite side toward the Sun at the next perihelion.

Spin orbit coupling of Mercury

Mercury has become locked into a 3-to-2 spin orbit coupling instead of a 1-to-1 coupling like the Moon around the Earth because its orbit is very eccentric.

Water ice on Mercury

The unexpected chemical compound recently discovered on Mercury is water ice at the north and south poles.

Source of water ice

The source of the water found in the form of ice near Mercury's north and south poles is unknown at the present time.

Mercury's atmosphere

Mercury's atmosphere consists of traces of hydrogen, helium, potassium, sodium, and oxygen.

Venus's mass and radius

The physical properties of Venus that are very similar in value to those of the Earth are mass and radius and hence average density and surface gravity.

Brightness of Venus

When Venus is at its brightest as seen from the Earth (near greatest elongation), it is the brightest celestial object in the sky other than the Sun and the Moon.

Reflective clouds on Venus

Venus appears to be very bright in our skies at certain times because it is covered by very reflective clouds.

Surface conditions on Venus

The conditions on the surface of Venus are a high-pressure, high-temperature, carbon dioxide atmosphere.

Cloud structure on Venus

The cloud structure in the atmosphere of Venus can be described as a clear layer at the surface, a haze layer above it, and then a high, thick layer of permanent cloud.

Sulfuric acid clouds

The sulfuric acid clouds on Venus are confined to a narrow layer about 60 km above the planet's surface and cover the whole planet.

Atmospheric conditions on Venus

The severe atmospheric conditions that quickly destroyed spacecraft that soft-landed on the surface of Venus were high temperatures, high pressures, and corrosive acid clouds and mist.

Very high temperature on Venus

The main reason for the very high temperature (750 K) on the surface of the planet Venus is thought to be absorption of visible radiation by the Venusian surface and the subsequent trapping of infrared radiation emitted by the surface by the atmosphere and clouds.

Greenhouse effect on Venus

The greenhouse effect has been much more effective in raising the surface temperature on Venus than in raising the surface temperature on the Earth because carbon dioxide, which traps heat from the planet's surface, is the major component in the very dense Venusian atmosphere, while it is a only a minor constituent of the Earth's.

Surface features of Venus

The surface features and overall topology of Venus have been determined primarily by radar methods from Venus-orbiting spacecraft, measuring radio echoes from the surface.

Geology of Venus

The geology and geography of the surface of Venus is best described as mostly volcanic plains, with two continent-sized uplands and a number of large volcanoes.

Surface flatness of Venus

Compared with the surface of the Earth, the surface of Venus is almost completely flat and relatively smooth, except for two high volcanic mountain ranges.

Impact craters on Venus

The reason that very few impact craters are seen on Venus compared with the Moon is believed to be that lava flows and surface melting have covered all but the most recent craters.

Maximum age of Venus features

The maximum age of features on the surface of Venus appears to be about 700 million years, due to constant resurfacing of the planet by lava flows.

Magnetic field of Venus

Venus has no magnetic field.

Rotation of Venus

Venus rotates in the opposite direction from the Earth but very slowly.

Retrograde rotation of Venus

The most likely explanation for the retrograde rotation of Venus is the impact of a massive object on it early in its history.

Length of solar day on Venus

The length of a solar 'day' (time between successive sunrises) on Venus is 117 days.

Observations of Martian surface

The observations of the Martian surface that led Lowell to conclude that intelligent life-forms existed on Mars include melting icecaps, a network of linear features that look like canals, and varying dark surface markings assumed to be vegetation.

Geography of Mars

The overall geography of Mars can best be summarized as major volcanoes in the northern hemisphere and extensively cratered plains in the southern hemisphere, separated by one major valley system.

Location of extinct volcanoes on Mars

Most of the extinct volcanoes are located in the northern hemisphere of Mars.

Olympus Mons

Olympus Mons is a volcano on Mars.

Size of volcanoes on Earth vs. Mars

The principal reason we have no volcanoes on the Earth anywhere near the size of Olympus Mons is because of tectonic activity on the Earth, which prevents a volcano from sitting over the same spot for long periods and continuing to grow.

Caldera

A caldera is a crater formed by collapse at the summit of a volcano.

Hot-spot volcanism

Hot-spot volcanism is a process that produces gigantic volcanoes on Venus and Mars and chains of smaller volcanoes on the Earth (e.g., the Hawaiian Islands).

Production of volcanoes on Mars

The major volcanoes on Mars and the Hawaiian Islands were both produced by hot-spot volcanism resulting from the upflow of heat from below.

Valles Marineris

The Valles Marineris are a large rift valley system associated with the great volcanoes on Mars.

Red color of Mars

The distinctive red color of Mars is probably caused by iron oxides or rust in the soil.

Martian magnetic field

The Martian magnetic field is weak and localized, not at all like the global magnetic field of the Earth.

Craters on Mars

Craters on Mars have flatter bottoms and their rims appear to be more worn down than those on either the Earth's Moon or Mercury due to erosion by wind and infilling by dust storms.

Atmosphere of Mars

The major constituent of the atmosphere of Mars is CO2 (carbon dioxide).

Tilt of Mars

The tilt of the equator of Mars to its orbital plane is very similar to that of the Earth, about 25°.

Seasonal changes on Mars

Mars experiences similar seasonal changes to those on the Earth because its spin axis is tilted at about the same angle to its orbital plane as is the Earth's axis.

Similar properties of Earth and Mars

The two very similar properties Earth and Mars possess are length of solar day and inclination of equator to the ecliptic.

Atmospheric features of Mars

A major feature of the atmosphere of Mars is occasional strong winds and dust storms.

Polar caps on Mars

The polar caps on Mars are most probably made up of water and carbon dioxide ices.

Evidence of water on Mars

Significant evidence for the idea that large quantities of water once flowed on the planet Mars includes deep, winding canyons and flood plains.

Water on Mars

We know that water exists on Mars, but it does NOT exist flowing in river valleys.