Earth’s water may have arrived partly through water-rich asteroids, while suitable planetary temperatures allowed vapour to condense and remain as liquid water.
Coverage 2025–2025 · Updated 14 Jul 2026
Within the IB syllabus, the extraplanetary hypothesis is that water-rich asteroids delivered water to Earth. Its long-term presence helped provide conditions in which life could evolve.
Two conditions explain retention: temperatures became low enough for water vapour to condense, and Earth's gravity was strong enough to retain water over geological time.
Use the chain: asteroid delivery -> cooling and condensation -> gravitational retention -> persistent water available during the evolution of life.
If surface and atmospheric temperatures allow water vapour to condense, water can collect as liquid instead of remaining only as vapour; gravity helps keep that water associated with Earth.
Treat asteroid delivery as a hypothesis, not a proven statement that asteroids supplied all water. For this objective, do not expand the origin explanation to comets or internal sources.
This objective is assessed through multiple choice, commonly using Identify.
Identify
Gravity, atmospheric development and continued geological cycling also contributed to long-term retention, but the available tagged exam evidence for this objective directly supports the importance of a suitable temperature range for condensation.
Treating asteroid delivery as the only accepted source of Earth’s water.
Representative question
It has been hypothesized that asteroids were the primary source of Earth's water. What has caused the retention of water on Earth?
The gases of the early atmosphere absorbed water vapour.
High atmospheric pressure prevented water vapour from escaping.
Temperatures on Earth allowed condensation of water.
Water was trapped in the pores of newly formed rocks.
