Question

Recent data from the spacecraft Lunar Prospector indicate the presence of large quantities of water in shadowed areas of lunar impact basins. Describe the hypothesis that scientists have developed to explain how this water reached the moon and how it might be preserved.

Short answer

Water on the moon likely arrived via comets and asteroids, and it is preserved in permanently shadowed craters and the lunar regolith.

Step-by-step solution

Introduction to the Hypothesis

Scientists have developed a hypothesis to explain the presence of large quantities of water on the moon, particularly in shadowed impact basins. According to the hypothesis, this water did not originate from the moon but was delivered by external sources.

Delivery by Comets and Asteroids

The primary explanation is that comets, which are composed mostly of ice, and water-bearing asteroids collided with the moon. These impacts are thought to have happened over billions of years, continually delivering water to the moon's surface.

Preservation in Permanently Shadowed Craters

Once the water reached the moon, it was preserved in permanently shadowed craters. These areas, located at the lunar poles, are never exposed to sunlight, maintaining extremely cold temperatures that allow the water ice to remain stable and preserved for extended periods.

Role of Surface Regolith

Lunar regolith, a layer of loose, fragmented material covering solid rock, may also play a part in preserving this water. It can trap water molecules, protecting them from evaporation or sublimation into space.

Key concepts

Lunar Impact Basins

Lunar impact basins are large depressions on the surface of the moon caused by massive collisions with asteroids and comets. These impacts have created vast craters that shaped the moon's terrain over billions of years. The significance of these basins, in the context of lunar water, is their potential to harbor water deposits.
The intense heat generated at the moment of impact might suggest it would vaporize any water present. However, the subsequent cooling and trapping of material could provide conditions suitable for water ice to eventually accumulate. These basins are essential targets for study because they help us understand how our moon has interacted with other celestial bodies and how it has preserved volatile substances like water over long periods. This preservation is crucial for determining whether any water remains accessible today.

Cometary and Asteroidal Delivery

Cometary and asteroidal delivery propose that many of the moon's water resources arrived through impacts from comets and asteroids. Comets, known for their icy compositions, and certain water-rich asteroids, when colliding with the lunar surface, left traces of water in the form of ice. This hypothesis suggests that these celestial collisions have happened over an extensive timescale.
Each impact released water into the lunar environment, where some could be trapped in shadows or beneath the surface. Scientists believe this process has been ongoing since the early solar system, contributing significantly to the lunar water inventory over millions of years. Understanding this theory helps explain how the moon could harbor water, even without an atmosphere that supports liquid water conditions like Earth's.

Shadowed Craters

Shadowed craters on the moon, especially at the poles, play a pivotal role in preserving its water. These are areas never exposed to sunlight due to their orientation and the moon's axial tilt. Because they remain perpetually dark, temperatures within these craters can dip to extremely low levels.

• This cold environment allows water molecules to remain frozen, effectively preserving them over enormous time scales.
• Without solar radiation to cause sublimation, water ice can remain with little loss or movement.

These shadowed regions are thus critical in maintaining lunar water reserves. They act as natural deep-freezes, safeguarding ice that could provide insights into the history of water in the solar system and potentially offer resources for future lunar exploration.

Lunar Regolith

The lunar regolith is the thin, dusty layer covering the moon's solid rock. It consists of tiny fragments of mineral and rock materials that have accumulated over aeons of micrometeoroid impacts and other processes.

• This surface layer is vital to the preservation of lunar water.
• It can trap water molecules that land on or near the surface.

Lunar regolith insulates and shields the trapped water from solar winds and loose exposure to space, preventing its evaporation or sublimation over time. This protective quality means that even areas that are not permanently shadowed might still retain water, hidden just beneath the surface. Understanding this interaction between regolith and water is essential for future exploration, as it can guide us toward potential water extraction methods on the moon.