Space Rocks
National Space Centre

Space Rocks

  • 21st Jun 2024
  • Author: Elspeth Lewis

Planet Earth

Our home planet is the 3rd planet from the Sun and the only place we know with life. In an area called the habitable zone, also known as the goldilocks zone. Earth is not too hot, not too cold but just right, for the conditions for life as we know it.

 

The outer rocky layer of the Earth is called the crust. This layer is up to 40km thick. The furthest we have drilled into the crust is 12km.  

 

Below the crust is a layer of mostly solid silicate rocks called the mantle. These rocks flow over millions of years helping shape the Earth’s landscape contributing to earthquakes and volcanoes.

Beneath the mantle, there is an iron nickel core. The outer core is liquid, and the inner core is solid. The movement of the molten material in the outer core is responsible for the generation of Earth’s magnetic field.

The Earth as seen by Apollo 17
NASA

What about other rocky planets?

Earth is a terrestrial planet which is a planet is composed of silicate rocks, iron and water and or carbon. The Earth is the largest of the terrestrial planets in our Solar System. The other 3 terrestrial planets are Mercury, Venus, and Mars. All of the terrestrial planets have a similar interior structure, with a metallic core, a mantle and a crust.

The structure of the terrestrial planets and the Moon
NASA
  1. Mercury
    NASA/ Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington

    Mercury

    At about a quarter of the size of Earth, Mercury is the smallest planet in our Solar System. Mercury is the closest planet to the Sun, and has daytime temperatures of up to 430°C. However, because Mercury does not have an atmosphere to trap heat it is not the hottest planet. Due to this lack of atmosphere at night Mercury’s surface temperature drops down to -180°C.  

    The surface of Mercury is covered in craters, and there may be frozen water at the bottom of some of these craters.

    Mercury has a magnetic field, but it is about 100 times weaker than Earth’s.

  2. Venus
    NASA/ JPL-Caltech

    Venus

    Venus is the 2nd planet from the Sun and the hottest in the Solar System. It has an atmosphere about 90 times heavier than Earth’s that traps heat from the Sun meaning that temperatures can reach about 470°C.

     

    Although the iron core of Venus is a similar size to the Earth, Venus does not have its own generated magnetic field. It has what is called an induced magnetic field which is caused by interaction of the Sun’s magnetic field with Venus’ atmosphere.

    The surface of Venus is geologically young, with many volcanoes covering it.

  3. Mars
    NASA/ edited by Lošmi

    Mars

    Mars is the 4th planet from the Sun and the second smallest in the Solar System. Mars is about half the size of Earth.  

     

    Mars has the largest volcano in the Solar System called Olympus Mons, it is about 3 times as tall as Mount Everest.

     

    Mars does not have a magnetic field although it would have had one in its past. Over time the materials in the core have separated and cooled, meaning that the necessary currents to cause the currents that cause the magnetic field cannot happen.

The Moon

For thousands of years, our moon was the only moon we knew about.

The Moon is believed to have formed about 4.5 billion years ago, when a planet about the size of Mars crashed into the young Earth. This flung out large amounts of rock into space which then formed into the Moon.

Like the terrestrial planets the Moon has a metallic core. This solid core is surrounded by a molten mantel and a crust about 70km thick. Like Mercury, the moon is covered in craters, has a very thin exosphere and a weak magnetic field. However, this magnetic field originates in the crust rather than the core.

The full Moon
Gregory H. Revera

Other Moons

A moon is a naturally occurring solid object orbiting around another celestial body that isn’t a star. There are hundreds of moons orbiting around planets, dwarf planets and even asteroids! However, neither Mercury or Venus have moons.

 

Mars has two small moons called Phobos and Deimos which are thought to be captured asteroids. In about 50 million years Phobos will either collide with Mars or will break up and become a ring.

 

In 1610 Galileo Galilei discovered the first moons around a planet other than Earth. He found four moons around Jupiter. These moons are Ganymede, Callisto, Io, and Europa. Ganymede is the largest moon in the Solar System and both it and Callisto are larger than Mercury. Io is the most volcanic place in the Solar system and Europa likely has a liquid ocean underneath its icy surface. The Galilean moons likely formed from a ring of material left behind after the formation of Jupiter. Jupiter currently has 95 known moons some of those are likely captured asteroids.

 

Saturn was the 1st planet to have more than 100 known moons, and at current count there are 146. The largest is Titan, which is the 2nd largest moon in the Solar System. Titan has rivers and lakes, but rather than water, they are made up of liquid hydrocarbons. Saturn’s 6th largest moon Enceladus produces watery plumes that vent out into space and is also partly responsible for one of Saturn’s rings. Saturn’s 7th largest moon Mimas is the butt of many Star Wars jokes as it is the spitting image of the Death Star.

 

Uranus has 28 moons, all named after characters from Shakespeare and Alexander Pope plays.

 

Neptune has 16 moons, the largest of which, Triton was discovered only 17 days after Neptune.

Relative sizes of selected moons of the Solar System
NASA by Bricktop / edited by Deuar, KFP, TotoBaggins, City303, JCPagc2015
The four Galilean moons
NASA
Saturn's moon Mimas
NASA

Dwarf Planets

In 2005 an object called Eris was discovered. It was similar in size to Pluto. This led the International Astronomical Union (IAU) to form a committee to define what a planet was. The definition agreed upon was:

A planet

  • orbits around a star
  • is nearly round in shape
  • has cleared the neighbourhood around its orbit.

 

The last point is why Pluto was reclassified as a dwarf planet in 2006, as the area around Pluto, known as the Kuiper Belt has many other objects. 1 of Pluto’s 5 moons, Charon, is about half the size of Pluto, further reinforcing that Pluto has not cleared the neighbourhood. Pluto is in good company though, with 4 other official dwarf planets: Ceres, Eris, Haumea, and Makemake. 

 

Ceres is a rocky object in the asteroid belt between Mars and Jupiter. The other dwarf planets are icy rocky bodies beyond the orbit of Neptune.

Pluto and two of its moons, Charon and Hydra
Camille Gévaudan

What are the objects in the asteroid belt and the Kuiper belt?

  1. The asteroid Vesta
    NASA

    Asteroids

    Asteroids are rocky remains from the formation of the Solar System. They are often unusual shapes as they are not big enough to form into spherical shapes.

    Most asteroids are found in the asteroid belt between Mars and Jupiter, although there are some that also occupy the orbit of Jupiter called Trojan asteroids. There are 1.3 million known asteroids and the largest is Vesta which is 525km in diameter.

  2. Comet Hale Bop
    E. Kolmhofer, H. Raab

    Comets

    Comets are made up of ice, rock and dust. They are remnants of the formation of the Solar System. Some of the frozen material they contain include water, carbon dioxide, methane, oxygen and amino acids.  

    They range in size from a few kilometres to 10s of kilometres.

    They have very elliptical orbits which brings them closer to the Sun. This means they warm up, which causes dust and gas to be spewed out into space. This gives comets their characteristic tails. There are likely billions of comets, but there are 3949 confirmed.

  3. Hole in Space Shuttle Atlantis caused by a meteoroid
    NASA

    Meteroids

    Meteoroids are smaller rocks in space that range in size from 2mm to 1m. Anything smaller than 2mm is classed as micrometeoroid.  Most of them are broken up bits of asteroid or comet debris. 

    Around 44,000 kg of meteoroid material hits Earth every day, most of it vaporising in the atmosphere. Meteoroids can be dangerous for space craft as they are travelling very fast in space.

What happens when space rocks come close to planets?  

  1. The general orbit of a quasi-satellite
    DIrohrer2003

    Quasi-Satellites

    Quasi satellites are asteroid like objects that orbit the Sun, but stay close to a planet while orbiting. The 1st discovered was Venus’ quasi-satellite Zoozve. Zoozve was discovered in 2002 but it is likely it has been a companion of Venus for over 7000 years. The name Zoozve came about from a misreading of the designation 2002 VE. With 2002 the year of discovery and the VE representing Venus, however the name Zoozve became official in February 2024.  

    Earth has 7 known quasi-satellites.

  2. A meteor pictured from the ISS
    NASA/ Ron Garan

    Meteors

     

    When meteoroids enter Earth’s atmosphere and burn up, the fireballs are known as meteors, or shooting stars.

    Increased meteor activity is known as a meteor shower and is the result of Earth passing through the trail of debris left by a comet.

  3. A meteorite from the National Space Centre's collection
    National Space Centre

    Meteorites

    Meteorites are rocks from space that have survived burning up in the atmosphere and landed on a planet or moon. Meteorites can be rocky or metallic. In our galleries we have several meteorites on display, including one that came from Mars! Meteorites can help us understand objects in space are the closest most of us will ever get to space rocks other than Earth.

Full references / credits:

(Banner) Space rocks image, National Space Centre 

(1) The Earth as seen from Apollo 17, NASA, Public Domain 

(2) The structure of the terrestrial planets and the Moon, NASA, Public Domain 

(3a) Mercury taken by MESSENGER, NASA/ Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington, Public Domain 

(3b) Venus, NASA/ JPL - Caltech, Public Domain 

(3c) Mars, NASA/ Edited by Lošmi, Public Domain 

(4) The Moon, Gregory H. Revera, CC by SA 3.0 

(5a) Relative sizes of selected moons of the Solar System, Orginially uploaded by NASA by Bricktop/Edited by Deuar/ KFP/TotoBaggins City303 JCPag2015/ Public Domain 

(5b) The four Galilean moons, NASA, Public Domain 

(5c) Saturn's moon Mimas, NASA/ Public Domain 

(6) Pluto and 2 of its moons Charon and Hydra, Camille Gévaudan, Public Domain

(7a) The asteroid Vesta, NASA, Public Domain 

(7b) Comet Hale Bop, E. Kolmhofer / H. Raab, CC by SA 3.0 

(7c) Hole in Space Shuttle Atlantis caused by a meteoroid, NASA, Public Domain 

(8a) The general orbit of a quasi-satellite, DIrohrer2003, Public Domain

(8b) A meteor pictured from the ISS, NASA/ Ron Garan, Public Domain

(8c) A meteorite from the National Space Centre Collection, National Space Centre