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Observations and the Solar System : Lecture 5

The Solar System

Information on the planets is contained in the

planet data sheet

The solar system consists of the eight planets orbiting the sun plus a variety of minor or dwarf planets. In describing the solar system it is very useful to compare distances and sizes to that for earth. Then
  • 1 A.U. = average distance from sun to earth (=150x106 km)
  • 1 earth mass (=6x1024 kg )
  • density- here we use kg/m3 Note the following values for typical materials:
    Information for the planets has been gained by observation but also by send probes to visit these planets.
    Planet Probes/Explorers
    Mercury Mariner 10(1974)
    Venus Mariner 10, Venera 7, Magellan
    Mars Viking, Mars Pathfinder Mars Science Laboratory mission and its Curiosity rover
    Jupiter Voyagers+ Galileo
    Saturn Voyager I+II , Cassini-Huygens
    Uranus Voyager II
    Neptune Voyager II
    Examining this data we could split the planets into 3 groups
    1. The four inner Terrestrial planets: Mercury, Venus, Earth and Mars These are small composition is largely rock and they are close to the sun
    2. The four Jovian planets : Jupiter Saturn, Uranus and Neptune. These are large , composed of low density material and are far from the sun
    3. The dwarf planets : Pluto, Ceres, Eris, Haumea, MakeMake

    The Earth

    It is useful to see how some of the earths features appear from space

    Mercury

    The planet nearest the sun. Consequently it might be expected to be hot! The vital stats are
  • surface temp. min -170C max 350C
  • heavily crater
  • no volcanoes
  • no atmosphere
  • no possibility of life. Note that Mercury has very extreme temperature variations. This is due to two factors. Firstly the lack of atmosphere means Mercury heats up very quickly and cools down quickly. Secondly Mercury rotates very slowly. Wheras the earth rotates in about 24hrs, it takes Mercury 58 days to rotate. This means the nights on Mercury are very long (and cold).

    We have some pictures of Mercury's moon-like surface

    Sidereal and Synodic Periods

    We discussed the difference between sidereal and synodic periods. The sidereal is the `real` period as would be observed by someone off-planet. The synodic period is from noon to noon and so is the day observed by someone on the planet. These are different because of the rotation of the Earth/Mercury around the sun. For the Earth and Mercury

    Planet Sidereal period Synodic Period
    Mercury 57 days 115 days
    Earth 23.935 hours 24 hours

    Venus

    x Venus is, superficially, very similar to Earth. It is 0.72 A.U. from the sun so we would expect it to be a warmer than earth. It is about Earth sized and appears to be covered in clouds. (Again not a great telescope object). Its cloud cover is highly reflective making it the brightest sky object after the sun and moon. It is difficuly to see any features other than ``phases'' with a telescope. Most info. hence comes from probes.
    • The Venutian atmosphere is different from Earths. It comprises 96% Carbon Dioxide (CO2) 4 % Nitrogen and virtually no Oxygen.
    • Venus has a slow retrograde orbit . Most planets rotate in the same direction as they circulate the sun - venus rotates backwards. This is called retrograde. If on Venus, sunrise would be in the west.
    • Venus is surprisingly hot. Mean Temperatures of 450C are measured.
    • The surface pressure is 70At. That is the density of the gas at the surface is 70 times that on earth. Thick stuff. It also appears that the clouds contain droplets of surphuric acid.
    • Venus has many volcanos and volcanic surface features.

    It is thought that Venus is very hot due to a Greenhouse Effect caused by the CO2 in the thick cloud layers. Venus does not look pleasant for life! Some images mostly taken by the Magellan space probe which orbited above the clounds and used infra red imaging to ``see'' the surface are

    • Image Venus showing clouds (not Magellan!)
    • Image of the Volcano Sapas Mons, the area shown is a square of side 650 kilometers
    • Image of the Ushas Mons Volcano a 2-kilometer-high volcano in the southern hemisphere of Venus
    • 3-D Perspective of Sif Mons, a volcano with a diameter of 300 kilometers and a height of 2 kilometers
    • 3-D Perspective of Three Craters on Venus -names Howe, Danilova and Aglaonice. Howe, in the foreground is a crater with a diameter of 37.3 kilometers Danilova, a crater with a diameter of 47.6 kilometers appears above and to the left of Howe in the image. Aglaonice, a crater with a diameter of 62.7 kilometers is shown to the right of Danilova.
    • Composite image of Venus
    • Composite image of Venus from another view
    • link to the Magellan images at Nasa website

    Mars x

    Known as the red planet. Surface features visable through telescope. (No canals!!!!) Rather smaller than Earth and further away fom the sun. Surface conditions look very much like a desert

  • Surface Temperature -140C min, 20C max
  • tilted by 25degree and so has seasons like earth.
  • Rotation period just 37 minutes longer than earths.
  • only extinct volcanoes
  • atmosphere like Venus 95% CO2 4% Nitrogen. However it is very thin - at surface 6mb or 6/1000 of that on earth.
  • Polar Ice-caps (composed of CO2/Water?)
  • Evidence suggests Mars was once more hospitable to life. There is evidence of surface water flowing.
  • There is evidence that Life might have existed on the Martian surface. We postpone discussion of this to a later point.

    Some images of Mars from a variety of sources

    © Dave Dunbar 2020