Question

document - ess august 2025 set 5 - how did earth get its water? scientists are unsure about where and how earth got its water. early earth lacked an atmosphere and was very hot, so water would not have formed in such an environment. one theory is that water was brought to earth when comets collided with earth, since comets are made of frozen water, gases, and dust. another theory centers around asteroids. earth is believed to have formed from the clumping together of space debris. these early solar system materials contained the elements hydrogen and oxygen. the chemical proportions of the water found on asteroids are similar, though different, from earths water. the most recent discoveries have come from studies of dust particles brought back to earth from the hokawa asteroid. the particles were found to have water and an oxygen - hydrogen ion. the source of the hydrogen is believed to be from solar wind - hydrogen ions ejected from the sun streaming through space and lodging onto the surface of dust particles. these particles have been falling to earth since the solar system formed. the space dust is believed to have provided about 50% of earths water. the graph below shows the relative deuterium - to - hydrogen ratio (d/h) in water found in solar system objects. diamond symbols are measurements obtained by satellites. dots are estimated values. deuterium is a heavier form of hydrogen. this ratio is an important factor in determining where in the solar system an object formed and how asteroids, comets, and the solar wind contributed to the water in earths oceans. a student makes a claim that the water on our planet is most similar to the water found on comets based on its d/h ratio, so comets must be a major source of water on early earth. which statement, based on evidence from the graph, would refute this claim? 1 the source of earths water is the four largest planets in our solar system because these planets have water with higher d/h ratios than earths oceans. 2 the water on all the comets has the same d/h ratios except halley, so halley is not a source of water for earths early oceans. 3 many asteroids have water that is very close in d/h ratios to earths oceans, so asteroids could be a source of water on early earth. 4 water on enceladus has a similar d/h ratio to comets, so enceladus could also be a source of water for earth

Explanation:

The student's claim is that comets are a major source of Earth's water because Earth's water has a similar D/H ratio to comets. To refute this, we need evidence that another source (not comets) could be responsible. Option 3 presents evidence that asteroids (another potential source) have water with a D/H ratio very close to Earth's oceans. This suggests asteroids (not just comets) could be a source, weakening the claim that comets are the major source based solely on D/H ratio similarity.

Answer:

1. Many asteroids have water that is very close in D/H ratios to Earth's oceans, so asteroids could be a source of water on early Earth.