GCSE Astronomy

The Inner Planets of the Solar System

Differentiation

Mercury

Mercury is very hard to see because of its proximity to the Sun. Due to the way the ecliptic is steeply orientated in early spring and early autumn, this is the best time to view the planet. In early spring it can be viewed in the evening sky, and in early autumn in the morning sky.

Mercury has only been visited by one probe, Mariner 10, in 1974/5 who were able to map less than half the surface.

The Surface is very 'moon-like', with craters and large areas resembling the Moon's maria. Its largest crater is the Caloris Basin (diameter 1,300 km). Other craters are named after people who have contributed the the world of culture, like Beethoven, Shakespeare, Cervantes and Tolstoy.

It has the highest density of any planet apart from Earth, indicating high levels of iron and nickel. Like the Earth, it has a detectable magnetic field, which Mars and Venus do not have.

The diagram below showing Venus's orbit has features which are also relevant for Mercury, except that Mercury's maximum elongation is 28°.

Transits of Mercury, when it crosses the disk of the Sun, are much more common than Transits of Venus, separated by 3 to 10 years or so. There were 13 transits in the 20th. Century.

Venus

Diameter   12, 104 km km

Distance from Sun   0.723 AU

Time to Complete One Orbit   224.7 days

Rotation Period   243.16 Earth days (in a retrograde direction)

For the observer, Venus appears as either the 'Morning Star', heralding the rise of the Sun, or as the 'Evening Star', following the Sun below the horizon. Its maximum elongation (i.e. distance from the Sun), is 47°, which corresponds to about 3 hours in time.

Its maximum magnitude is -4.4 (compared to about -2.5 for Mars and Jupiter). So it is extremely bright, and in a telescope will show phases, analogous to the phases of the Moon. It can even be seen in the daytime sky if you know where to look - to do this it is best to watch it as a Morning Star as the sky brightens.

Obviously, Venus can never be at opposition. At conjunction, it can either be on the same side of the Sun as the Earth, which is called inferior conjunction, or it can be on the opposite side of the Sun, which is called superior conjunction. Inferior conjunction occurs every 584 days, at which time Venus is closer to Earth than any other planet will come (in the region of 40 million km).

At western and eastern elongation, Venus will have half its disk displayed, a phase which is known as dichtomy. Note that at this point, the angle between the earth, Sun and Venus is less than a right angle.

Note that the actual visual size of Venus will vary noticeably as Venus goes thru its phases. This is hopefully easy to understand from the diagram above - and the planet will actually be brightest when it is in its crescent phase (as first discovered by Halley). The diagram below attempts to explain this by showing the relationship between the size of the planet and its phase (or aspect).

It becomes really bright when its brightest phase occurs at perihelion in December - which happens about every 8 years.

At conjunction, it will usually pass above or below the Sun. Very rarely, it passes in front of the Sun, in what is called a transit. In the past these transits have been very important and observation of a transit was the given reason for one of James Cook's voyages to the Pacific. More information on the link - Venus transits in history.

Recent Venus Transits

1639	observed by Jeremiah Horrocks
1761
1769
1874
1882
2004, June 8
2012, June 5

In terms of size, mass and density, Venus is fairly similar to the Earth. However, it has experienced a runaway greenhouse effect which has raised the surface temperature to 750 degrees (on the Kelvin scale). Therefore it has no surface water : an initial warming has caused evaporation and this evaporated water has raised the temperature even more which causes even more evaporation and so on - this is the runaway greenhouse effect. Venus's atmosphere nowadays is mostly Carbon Dioxide, which is also a greenhouse gas.

The pressure is extreme, about 90 times that of Earth's.

The rotation is slow (243 Earth days) and also in an opposite sense to most of the other bodies in the Solar System. In other words, the orbit is retrograde. Venus is the only planet with a rotation period greater than the time it requires to make one orbit around the Sun (the sidereal period).

The planet is covered with thick clouds of carbon Dioxide which also contain an appreciable amounts of sulfuric acid.

The surface was mapped using microwave radar by the Magellan probe, which went into orbit around the planet in 1990 and completed its task in 1993. There are two main highland areas, the largest, Aphrodite Terra, runs along the Equator. The other, Ishtar Terra, is in the North.

There are volcanoes - small cone-shaped, shield volcanoes and circular lava pancakes (which do not exist on Earth) about 25 km across.

Other features unique to Venus : arachnoid (circular to ovoid volcanic structures), coronae.

There are many craters but most are volcanic. Most impact craters appear to have been wiped out by volcanic flows. The average age of the surface is around 500 million years old.

The IAU has decided that all features on Venus should be named after females. The only exception is Maxwell Montes, named after James Clerk Maxwell.

Similarities between Venus and Earth

• Diameter (and therefore total volume also) (Venus has diameter 95% of Earth's)

• Average Density

• Total Mass (Venus is 81% of Earth's mass)

Differences between Venus and Earth

• Surface temperature on Venus is about 750K

• Venus's atmosphere is mainly Carbon Dioxide, which is a very small constituent of Earth's atmosphere

• Venus has no surface water

• The atmospheric pressure on Venus is about 90 times that of the Earth.

• Venus is permanently covered by thick cloud

• Rotation of Venus = 243 Earth days

• Sense of Rotation - Venus rotates in a retrograde motion in comparison with most other bodies of the Solar System, and definitely opposite that of the Earth No Moon

Earth

Data

Diameter   12,756 km (at equator), 12,709 (thru poles)

Distance from Sun   147 million km (perihelion), 152 million km (aphelion)

Time to Complete One Orbit   365.256 days

Rotation Period 23h 56m (sidereal),   i.e. to rotate completely about its own axis  

More than 70% of the surface is covered by water.

It has a magnetic field, which implies the existence of an iron core. Further this core has to be molten because solid unmoving iron cannot produce a magnetic field.

It is tilted at 23.5° (to the nearest half a degree). The tropics are parallels of latitude 23.5° either side of ther equator. These mark out the limits within which the Sun will be directly overhead at noon sometime during the year. Parallels of latitude 90-23.5 = 66.5° define the arctic circle and antarctic circle. Within these circle, the Sun will never set on Midsummer Day and will not rise on Midwinter Day.

The Troposphere is that part of the atmosphere where we find all our weather (up to a height of between 8 and 11 kms). The calmer stratosphere lies above this, as experienced by anyone who has flown in a jet airliner.

Space starts at about 100 km.

Mars

Diameter   6,787 km

Distance from Sun   1.524 AU

Time to Complete One Orbit (Sidereal Period)   687 days

Synodic Period   780 days

Rotation Period   24h 37m

Axial Tilt   23° 59' (similat to Earth, so seasons are of same general type as Earth)

At opposition, when Mars is closest to Earth (which occurs approximately every 26 months), the distance betwen Mars and the Earth can vary by virtue of the eccentricity of orbits. When Mars comes close to Earth, it is classed as a favorable opposition.

The most favorable oppositions will occur when Mars is at perihelion during opposition. Then it will be about 56 million kilometers distant, a situation which occurs every 15 to 17 years. In August 2003, Mars was closer to Earth than for over 5,000 years - more information on this link.

I have a bit of information on Johann Heinrich Mädler who did some notable research on Mars in around 1830 - no mention of Martian canals is made here.

That in fact came later and seemed to have been fuelled because of a mistranslation of the Italian word canali, used by the observer Schiaparelli. Neverthess the erroneous belief in Martian canals achieved ludicrous proportions. This was followed by Orson Welles' War of the Worlds of 1938, reaction to which showing that we are not as far from the Middle Ages as some like to imagine.

Mars has noticeable ice caps which vary in extent according to the season.

The first flypast by a spacecraft was by Mariner 4 in July 1965. In 1976, two Viking landers set down on the planet, 7500 km apart.

In the general Tharsis region lie four of the biggest volcanoes in the Solar System, two of which can be seen clearly at the left of the image of Mars at the top of this section. The biggest of them all (although just out of view on the image just mentioned) is Olympus Mons with a height of 24 km (cf. 8450m for Everest, and a comparable height from the sea floor for the similar Hawaii volcanoes). The adjacent image shows this volcano, which is 600 km across. Its gentle slopes imply that its lava was basalt.

Tharis itself is a ridge rising several kilometers above the above elevation. It appears to be a bit of a mystery as to why Tharis rises so high, instead of sinking into the crust.

Mariner Valley is 5,000 km long and occupies the central region of the image at the top of this section, around the equatorial region. It is named after Mariner 9, which discovered it in 1972. It has an average depth of 6km and is up to 400 km wide. It is a geological fault, it has not been formed by erosion.

Syrtis Major near Equator

In Southern Hemisphere, two great craters formed by impact. Hellas, 1600 km across, and Argyre

The atmosphere is mainly Carbon Dioxide (96%) but very thin (atmospheric pressure less than 1% of that of Earth's), so it becomes very cold (about 0° C at maximum and -120° C at minimum). The wind can nevertheless be very noticeable, whipping up large-scale dust storms. The 'weather' is affected by the inclination of the axis of rotation. The inclination of this axis is 25°, only slightly greater than Earth's, so seasons are similar except that they are longer on Mars due to Mars' longer period of revolution. However, they are more unequal because of the effects of Mars' greater orbital eccentricity.

The planet has two tiny moons - Deimos (16km diameter) and Phobos (28km diameter). Neither is large enough to graviatationally contract to a spherical shape. Deimos has a sidereal period of 30 hours 18 mins, slightly less than the period of rotation of the planet. Phobos, on the other hand, has a sidereal period of only 7 hours 39 mins. As viewed from the planet's surface, Phobos will appear to move in an opposite direction to other celestial bodies. It will rise in the West and set in the East twice every Martian day.

Consult the following link for information on Martian meteorites .

1997 Mars Global Surveyor, Mars Polar Lander