10.2 Earth & Space Science

The Water Cycle

Water continuously moves between Earth's surface and atmosphere in the water cycle. The four main processes are:

1. Evaporation — liquid water turns into water vapor and rises into the air (energy from the Sun drives this).
2. Condensation — water vapor cools and turns back into liquid droplets, forming clouds and fog.
3. Precipitation — when droplets in clouds grow heavy enough, they fall as rain, snow, sleet, or hail.
4. Collection — water gathers in oceans, lakes, rivers, and groundwater; some flows back to the ocean.

Transpiration is a related process: plants release water vapor through their leaves, adding to the moisture in the air. A class terrarium is a great way to make the water cycle visible — droplets form on the glass (condensation) and run down (collection) on a sunny day.

Weather Versus Climate

• Weather is the short-term condition of the atmosphere at a particular place and time — today's temperature, cloud cover, and chance of rain.
• Climate is the long-term average pattern of weather in a region over many years (usually 30+). Atlanta's humid subtropical climate has hot, humid summers and mild winters, even if a single January day is unusually warm.

A helpful analogy for children: weather is your outfit today; climate is your whole wardrobe.

Weather Instruments

A class weather station that uses all five instruments over a month lets students compare weather data with long-term climate averages for their Georgia county.

Day and Night

Earth rotates on its axis once every about 24 hours. The side of Earth facing the Sun experiences day, while the side facing away experiences night. The Sun appears to move across the sky, but it is really Earth turning. Rotation also explains why the Sun rises in the east and sets in the west.

Seasons and Earth's Tilt

Earth revolves around the Sun once every about 365 days (one year). Earth's axis is tilted about 23.5° from vertical. This tilt — not the distance from the Sun — is what causes seasons:

• When the Northern Hemisphere tilts toward the Sun, it gets more direct sunlight and longer days: summer.
• At the same time, the Southern Hemisphere tilts away: winter.
• Six months later, the roles reverse.

Near the equator, seasons are much less noticeable because sunlight strikes about the same way year-round. Georgia's mild winters and hot summers come from living between the equator and the mid-latitudes.

Phases of the Moon

The Moon orbits Earth about every 29.5 days. We see phases because sunlight lights only the half of the Moon facing the Sun, and our viewing angle changes as the Moon orbits:

New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Last Quarter → Waning Crescent → New Moon.

A classroom moon journal where students sketch the moon each night for a month makes this cycle concrete.

The Solar System

Our solar system has one star (the Sun), eight planets, more than a hundred dwarf planets, millions of asteroids (mostly between Mars and Jupiter), and countless comets (balls of ice and dust that grow tails when they near the Sun).

Stars and constellations: a constellation is a recognizable pattern of stars in the night sky, such as Orion or the Big Dipper. Stars differ in size, color (related to temperature), and distance from Earth; the Sun is a medium-sized, fairly ordinary star that looks bright only because it is so close.

Rocks and the Rock Cycle

Rocks are classified by how they form, and the rock cycle slowly recycles Earth's rocky material:

The rock cycle connects all three: igneous rock can weather into sediment that becomes sedimentary rock, which can be buried and heated into metamorphic rock, which can melt into magma and cool again as new igneous rock.

Soil Formation and Types

Soil forms slowly as rock is weathered into smaller pieces and mixed with dead plant and animal material (humus) and living things. Soils have three main particle sizes:

• Sand — large particles, drains quickly, holds few nutrients.
• Silt — medium particles, feels smooth and powdery when dry.
• Clay — tiny particles, holds water and nutrients tightly but drains slowly.

Loam is a balanced mix of sand, silt, and clay that is ideal for growing most plants. Georgia's red clay soils come from weathered igneous and metamorphic rocks rich in iron.

How Fossils Form

Most fossils form in sedimentary rock. The process usually requires quick burial and the removal of oxygen:

1. An organism dies in or near water and is quickly covered with mud or sand.
2. Soft parts decay, but hard parts (bones, shells, wood) may be preserved.
3. More sediment piles on, turning to sedimentary rock over long times.
4. Minerals seep into the remains and replace them bit by bit (petrification), or a mold is left when the remains dissolve and a cast forms when new minerals fill the mold.

Trace fossils are not the remains themselves but evidence of activity: footprints, burrows, or coprolites (fossil dung). Fossils tell us what past environments were like and give evidence that life on Earth has changed over time.

Constructive and Destructive Processes

Earth's surface is constantly shaped by both:

Constructive processes build up landforms:

• Volcanism — lava and ash pile up to form volcanoes and islands (Hawaiian Islands).
• Deposition — rivers, waves, and wind drop sediment to build deltas, beaches, and sandbars.
• Folding — Earth's plates push together, bending rock layers into mountains (Appalachian and Himalayan ranges).

Destructive processes break down landforms:

• Weathering — the breakdown of rock at or near Earth's surface, either physical (ice wedging, root growth) or chemical (acid rain, oxidation).
• Erosion — moving water, wind, ice, or gravity carries weathered sediment away (the Grand Canyon was cut by the Colorado River).
• Earthquakes — sudden movements along faults can crack land and shift ground, sometimes causing damage far from the epicenter.

Constructive and destructive processes work together — a volcano builds a new island while waves and weathering slowly wear it down.

Earth's Layers

Earth is divided into three main layers based on composition:

• Crust — the thin, rocky outer skin. Continental crust is thicker and less dense than oceanic crust.
• Mantle — the thickest layer, made of hot rock that flows very slowly, allowing the plates above to move.
• Core — mostly iron and nickel. The outer core is liquid and produces Earth's magnetic field; the inner core is solid due to extreme pressure.

A hard-boiled egg is a classic elementary model: the shell is the crust, the white is the mantle, and the yolk is the core — but remind students that the real layers are much hotter and under far more pressure than any egg.