UPSC Express

In Brief: Features of the moon

The distinctive features of moon when observed from a telescope or even naked eye are:

• Impact craters
  • Lunar craters, large and small, are the result of the bombardment of meteorites on the lunar surface.
  • That is why they are called impact craters
• Light-coloured highlands
  • Most of the lunar surface constitutes light-coloured highlands.
  • About 85% of lunar surface is light-coloured highlands.
• Dark-coloured Maria
  • The large dark areas on the lunar surface are called maria (singular form-mare for “sea” in Latin) (see figure below)
  • These dark coloured features are the remains of huge lava flows which have solidified to give it a distinctive dark color.

Other Features

• No Atmosphere
  • Moon is too small to have an atmosphere.
  • Its surface gravity is so low, (only about 1/6th of that of Earth), that any gas molecules easily escape into space.
• No global magnetic field
  • Moon has no global magnetic field because it lacks in substantial amounts of moving molten material in its interior. (Earth’s magnetic field is due to moving iron core)
  • However Moon does have weak magnetic fields in certain pockets when the rocks solidified billions of years ago.

In-Focus: Lunar Swirls

• Besides, lunar surface also has another distinctive feature called lunar swirls.
• They look like bright, snaky clouds swirled over the surface of the moon. (See figure)
• Imagine it to look like cream swirled in a cup of coffee.

Origin of lunar swirls

• It is seen that the location of lunar swirls coincide with the location of localized magnetic fields on moon’s surface.
• The localized magnetic fields on the lunar surface are due to magma ejected during volcanic eruptions.
• The localized magnetic fields deflect particles from the solar wind and cause some parts of the lunar surface to weather more slowly.
• This has resulted in bright-and-dark patterns swirling the lunar surface.
• In summary, the localized magnetic fields act like ‘sunscreen’ to regolith on the lunar surface.

ARTEMIS mission

• The ARTEMIS mission is NASA’s mission to study effects of solar wind from a lunar orbit.
• ARTEMIS it deployed primarily to observe Moon’s interaction with the solar wind.

Basics: Solar Wind

• Sun’s powerful gravitational attraction keeps most of the gases of the photosphere, chromosphere, and corona from escaping.
• Corona is the outermost region of the Sun’s atmosphere.
• Due to high temperature of corona, its atoms and ions are moving at very high speeds (about a million kilometers per hour).
• As a result, some of the coronal gas escape is called the solar wind.
• The solar wind is composed of electrons and nuclei of hydrogen and helium.
• Each second the Sun ejects about a million tons of material into the solar wind.
• Solar winds engulf the entire solar system including Pluto.

Interaction of solar wind and magnetic field

• Earth is protected from solar winds due to its global magnetic field that acts as a shield.
• The aurora seen at far northern or southern latitudes on Earth are produced when electrons and ions from the solar wind enter our upper atmosphere.
• On lunar surface the localized magnetic fields interact with solar winds to give rise to distinctive lunar swirls.