Earth and Space: Planet Earth

The Earth is not a solid ball of rock; it is made up of four distinct layers. From the outside in, they are the crust, the mantle, the outer core, and the inner core. The crust is the thin, solid, rocky layer we live on. Beneath this is the mantle, the thickest layer, made of semi-molten rock called magma that flows very slowly. Deeper still is the outer core, a layer of liquid iron and nickel. The movement of this liquid metal generates the Earth's magnetic field. At the very centre is the inner core, a solid ball of iron and nickel. It is incredibly hot but remains solid because of the immense pressure from the layers above.

The Earth behaves like a giant bar magnet. This is because the constant churning and flowing of the liquid iron in the outer core creates electrical currents, which in turn generate a vast magnetic field. This field extends thousands of kilometres into space, creating a protective bubble around our planet called the magnetosphere. This field is crucial for life, as it deflects most of the harmful charged particles from the Sun (the solar wind) that would otherwise strip away our atmosphere. A compass works because its magnetic needle aligns with the Earth's magnetic field lines, pointing towards the magnetic north pole. It's important to know that the magnetic north pole is not in the same location as the geographic North Pole (the point on the Earth's axis of rotation).

The air you breathe today is very different from the Earth's early atmosphere. When the Earth formed, intense volcanic activity released gases, forming an atmosphere mainly of carbon dioxide (CO2), with some water vapour, methane, and ammonia, and almost no oxygen. As the Earth cooled, the water vapour condensed to form oceans. About 3.5 billion years ago, simple life forms like algae evolved in these oceans. They began to carry out photosynthesis, a process that uses CO2 and sunlight to produce energy, releasing oxygen as a waste product. Over billions of years, this process dramatically changed the atmosphere, reducing the amount of CO2 and increasing the amount of oxygen to the levels we see today. The current atmosphere is approximately 78% nitrogen, 21% oxygen, and small amounts of other gases like argon and carbon dioxide.

The greenhouse effect is a natural and essential process that keeps our planet warm enough for life to exist. It works in two main steps. First, energy from the Sun (short-wavelength radiation) travels through the atmosphere and warms the Earth's surface. Second, the Earth's surface radiates some of this energy back towards space as heat (long-wavelength infrared radiation). Greenhouse gases in the atmosphere, such as water vapour, carbon dioxide (CO2), and methane, trap some of this outgoing heat, preventing it from escaping into space. This keeps the Earth's average temperature at about 15°C; without it, the average would be a freezing -18°C. However, human activities like burning fossil fuels (coal, oil, gas) and deforestation are releasing extra CO2 into the atmosphere. This leads to an 'enhanced greenhouse effect', trapping more heat and causing the Earth's average temperature to rise, a phenomenon known as global warming.

The Earth's climate is not static; it has naturally cycled between cold and warm periods for millions of years, long before humans existed. An 'ice age' is a long period (millions of years) of generally cold global temperatures, where large ice sheets cover parts of the Earth. We are currently in an ice age that began about 2.6 million years ago. Within an ice age, the climate fluctuates between two states: 'glacial periods' and 'interglacial periods'. Glacial periods are the coldest phases, lasting tens of thousands of years, where ice sheets grow and advance from the poles to cover large parts of continents. Interglacial periods are the warmer phases in between, like the one we are in now, where ice sheets retreat and the climate is milder. These natural cycles are thought to be caused by small, predictable changes in the Earth's orbit around the Sun, known as Milankovitch cycles.

Scientists can study the Earth's past climate using several clever methods that act as natural archives. The most important is the analysis of ice cores. By drilling deep into the ice sheets of Antarctica and Greenland, scientists extract long cylinders of ice that have built up over hundreds of thousands of years. These cores contain tiny air bubbles that are a trapped sample of the atmosphere from the past. Scientists can measure the concentration of carbon dioxide and other greenhouse gases in these bubbles. They can also analyze the isotopes of oxygen in the water ice itself, which gives a very accurate estimate of the temperature when the snow fell. Other methods include analysing pollen grains preserved in layers of mud in lakes and bogs, which tells us what plants were growing and therefore what the climate was like. Similarly, the width of annual growth rings in old trees can indicate past temperature and rainfall.