Grade 7 Science: Solar System, Earth’s History and Water

Step 1: The Sun is a star at the center. It holds more than 99 percent of all the mass in the solar system.

Step 2: The four inner planets, Mercury, Venus, Earth and Mars, are small and rocky. Earth comes first in our list.

Step 3: The asteroid belt, a ring of rocky and metal chunks, circles between Mars and Jupiter.

Step 4: The four outer planets, Jupiter, Saturn, Uranus and Neptune, are giants made mostly of gas and ice. Jupiter is the closest giant.

Step 5: Past Neptune lies the Kuiper belt, a ring of icy bodies that includes the dwarf planet Pluto. Far beyond it, the Oort cloud is a giant shell of icy objects where many comets come from.

Step 1: Earth moves forward very fast, about 30 kilometers every second. By Newton’s first law, it would keep going in a straight line.

Step 2: The Sun’s gravity pulls Earth toward the Sun all the time.

Step 3: The forward motion and the inward pull combine into a curved path, an orbit, that bends around the Sun again and again.

Step 4: Gravity depends on mass and distance. The Sun is so massive that its pull reaches every planet, but the pull is weaker farther away, so distant planets move more slowly and take longer to go around.

Step 1: Distance from the Sun: Earth orbits in the habitable zone, the band where a planet can be warm enough, but not too warm, for liquid water. Its average temperature is about 15 degrees Celsius.

Step 2: Water: Earth has liquid water at its surface, which every living thing needs. Venus is far too hot for it, and on Mars water is frozen or underground.

Step 3: Atmosphere: Earth’s air holds the oxygen we breathe and traps enough heat to keep nights from freezing. Its ozone layer blocks harmful ultraviolet light, and most small space rocks burn up in it.

Step 4: Venus is not the closest planet, but the graph shows it is the hottest. Its thick carbon dioxide atmosphere traps heat, so atmosphere matters as much as distance.

Step 1: Superposition: in undisturbed layers, the bottom layer formed first, so it is the oldest, and the top layer is the youngest.

Step 2: The bottom layer holds trilobites, ancient sea animals. So this land was once under an ocean.

Step 3: Higher layers hold fern fossils and then dinosaur bones: the area later became land with plants and animals.

Step 4: The same fossils turn up on continents that are now far apart. Fossils of the plant Glossopteris are found in South America, Africa, India, Antarctica and Australia, evidence that these continents were once joined and have moved apart: plate tectonics.

Step 1: Divergent boundary: plates pull apart. Magma rises and hardens into new crust, building mid-ocean ridges and slowly widening ocean basins like the Atlantic.

Step 2: Convergent boundary: plates push together. An ocean plate sinks under another plate, making a deep trench and a line of volcanoes, like the Andes. When two continents collide, they crumple into tall mountains, like the Himalayas.

Step 3: Transform boundary: plates grind past each other sideways. They stick, then slip, causing earthquakes, like along the San Andreas Fault.

Step 4: A hot spot is a plume of hot rock rising from deep in the mantle. As a plate slides over it, a chain of volcanoes forms, like the Hawaiian Islands. The Yellowstone supervolcano sits over a hot spot too.

Step 1: Some rain runs over the ground as runoff into streams. Streams join into rivers and lakes: surface water.

Step 2: Some rain soaks into the soil and down into the spaces in sand, gravel and cracked rock. Rock that holds water is an aquifer, and the water in it is groundwater.

Step 3: People pump groundwater from wells for drinking and farming, and take surface water from rivers and reservoirs.

Step 4: People harm a watershed with fertilizer, oil and trash in runoff, with pavement that stops water soaking in, and by pumping wells faster than rain refills them. They help it with strips of plants along streams, rain gardens and saving water.

Step 1: We depend on it: the ocean gives us food, tiny ocean plants called phytoplankton make about half the oxygen we breathe, currents move heat around Earth and shape our weather, and ships carry most of the world’s goods.

Step 2: We change it: the graph shows that the share of fish populations caught faster than they can recover grew from about 10 percent in 1974 to about 38 percent in 2021. That is overfishing.

Step 3: Plastic trash, oil spills and fertilizer in runoff pollute the water. Fertilizer feeds algae that use up the oxygen, creating dead zones, like the one that forms off the coast of Louisiana and Texas each summer.

Step 4: We can protect it: fishing limits, marine protected areas and keeping trash and runoff out of rivers all help, because rivers carry pollution to the sea.