Each planet is unique in its characteristics and properties, which help shape and define it. The location of a planet in the Solar System and its distance from the Sun are key factors that allowed each one to evolve into what it is today. The inner planets were closer to the hot Sun and this played a vital role in how they coalesced during their creation. Having condensed out in the inner region of the solar nebula early on, heavy elements and compounds such as include iron, silicates, water, carbon dioxide, and oxygen were incorporated into the inner planets: Mercury, Venus, Earth, and Mars. Today, even among them, there are distinguishing properties, often extreme, that sets each apart.

Mercury: The closest planet to the Sun has the distinguishing property of being very rich in iron, and geologically features giant scarps. Since Mercury formed so close to the hot Sun, it could not hold onto lighter materials such as hydrogen, oxygen, and water. The intense heat quickly boiled these away leaving Mercury an airless, rocky world with a huge iron core. Iron cannot melt or boil away even at Mercury’s close distance to the Sun, so this heavy element gradually sank towards the center creating a huge core. This gives Mercury the added characteristic of a magnetic field like Earth, but unlike Venus and Mars, which have none or are weak.

At first glance Mercury looks very much like the Moon with its countless craters and lack of an atmosphere. It appears dead with no active volcanoes or weather, but a closer look reveals that it was a dynamic world early in its life. As Mercury was cooling, it was also contracting, and this slow shrinking created stress in the crust. In several areas the pent up stress was released in the form of giant scarps that stretch hundreds of miles and are up to two miles high. Huge other areas slumped creating giant cliffs that slice through hills and craters. These distinguishing surface features will be a target of study for NASA’s MESSENGER orbiter when it arrives on March 18, 2011. Mercury also has the oddity of possibly having pockets of water ice in the permanently shaded craters at the poles where temperatures fall as low as -300ºF on an otherwise extremely hot world that bakes at 800ºF. A huge iron core and giant scarps are two distinguishing features that set Mercury apart from the other planets.

Venus: No other planet is as hostile as Venus with an average surface temperature of 870ºF and a perpetually cloudy, thick carbon dioxide atmosphere that is 90 times denser than Earth’s and laced with sulfuric acid mist and lightning. Such a fearsome environment conjures up images of Hades and it is even made more frightening by the eerie orange light that bathes the surface due to the dense atmosphere. The intense heat is being carefully mapped by the ESA’s Venus Express orbiter, which has recorded temperatures as high as 891ºF in the lower plains and as cool as 837ºF in the rolling uplands in a broad area of the southern hemisphere. It may exceed 900ºF in the lowlands, possibly approaching 1000ºF in the deepest canyons and basins. Maxwell Montes, the highest mountain on Venus, soars approximately seven miles into the gloomy acidic skies and is the coolest place on Venus at about 700ºF. Elsewhere it makes no difference whether it is day or night, or at the poles or equator, it is always an inferno. To make it even more frightening the rocks may glow dull red at night due to the intense heat as they are nearly half way to their melting points.

A fitting feature for Venus would be volcanoes and this happens to be a true fact. It is uncertain if active volcanoes exist today, but unusually high concentrations of sulfur dioxide in the atmosphere in the late 1970’s hinted at a possible eruption back then. The convincing evidence is that the concentration later dropped and has since been stable for decades. A few isolated spots on Venus are as hot as 1300ºF and may be fumaroles, or active volcanic vents, where hot gases are escaping from beneath the surface. The surface is covered with volcanic features such as calderas, domes, channels, and ancient lava flows. Massive flows covered Venus perhaps as recently as 500 million years ago creating one of the youngest surfaces seen anywhere in the Solar System. At that time, Venus might have been even hotter than today with the lava relentlessly heating the surface. The only redeeming outcome of this massive global eruption would be the clearing of the solid cloud cover to reveal the stars and Sun even though the air temperature would be a searing 1200ºF! The clouds would quickly return as Venus cooled below 1000ºF. Intense heat and widespread volcanoes are two distinguishing features that set Venus apart from the other planets.

Earth: By far the unique planet within the Solar System, and maybe the whole Milky Way, due wholly to the presence of liquid water and life. Water is not found freely in liquid form anywhere else in the Solar System. It is the driving force behind the weather and geology because it dissolves many compounds and also aids in creating new ones. It has the power to erode mountains and build up new land. As water evaporates into the atmosphere it takes heat with it that powers storms. These storms help regulate the Earth’s temperature by delivering warm air towards the poles and cold air towards the equator, thereby preventing any one area from getting too extreme. All of these dynamics are vital for another unique characteristic of Earth: life.

The search goes far and wide to find life anywhere beyond Earth, but it appears that this is the lucky planet. The right temperature, climate, geology, chemicals, atmosphere, and the big ingredient, water, set Earth apart from any other planet. A fairly quiet, stable Sun also helps keep Earth alive and evolving to the extent that life can be found everywhere on the land, in the sea and in the air. The ozone layer high in the atmosphere protects life from the deadly radiation from the Sun. The average temperature on Earth is 59ºF with the extremes being from -129ºF to 136ºF. Liquid water and flourishing life are two distinguishing features that set Earth apart from the other planets.

Mars: A rusty desert planet that is the most earth-like, although frigid with an average temperature of -64ºF and extremes from as cold as -190ºF to as balmy as 80ºF. It is more like a frozen desert with red soil that is truly rusted due to iron oxide. There is evidence that water once flowed and formed ponds where possibly life could have existed, but so far the evidence indicates that Mars never had life. Across the rusted plains are towering volcanoes higher than any in the Solar System, canyons larger than any on Earth, and two polar ice caps of carbon dioxide and water. The predominantly carbon dioxide atmosphere is 100 times thinner than Earth’s, but is still capable of producing weather, which includes a characteristic phenomenon all its own: global dust storms.

The red, rusty soil is a mixture of dark rock, lighter sand, and even lighter and finer dust. Mars is famous for its small dust storms as it does not take much wind to get the dust airborne. Under certain conditions a dust storm can grow so large and fierce that it can cover all of Mars blocking everything from view. No other planet has storms that grow so enormous that they completely engulf the planet for months. The worst one on record was in 1971 when nothing more than the tops of the highest volcanoes could be seen for nearly six months. It was all repeated again in 2001 when another relentless global dust storm enveloped Mars. Red rust and howling dust storms are two distinguishing features that set Mars apart from the other planets.

The inner planets are as individual and as extreme as can be, but the outer planets, Jupiter, Saturn, Uranus, and Neptune, are in a class by themselves when compared to the smaller, rocky, inner, Terrestrial Planets. The outer Jovian Planets are even different from one another, and while Saturn’s rings definitely set it apart from all other planets, there are other surprises ahead in Part 2 of Planetary Personalities.