Question

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. What is the likely reason for Venus's lack of water in any form? (a) The planet accreted little water during its birth. (b) The water is locked away in the crust. (c) The water was in the atmosphere, where molecules were broken apart by ultraviolet light from the Sun.

Short answer

The most likely reason is option (c): Water was in the atmosphere and broken apart by ultraviolet light.

Step-by-step solution

Evaluate the Options

Let's review the given options: - **(a)** The planet accreted little water during its birth. - **(b)** The water is locked away in the crust. - **(c)** The water was in the atmosphere, where molecules were broken apart by ultraviolet light from the Sun.

Analyze Option A

Option **(a)** suggests Venus accreted little water during its formation. Given Venus's similarity in size and composition to Earth, it is unlikely that it would have accreted vastly different amounts of water.

Analyze Option B

Option **(b)** proposes that water is locked in the crust. However, geological studies and surface exploration don’t support significant water reserves in the crust.

Analyze Option C

Option **(c)** posits that the water was in the atmosphere and broken apart by ultraviolet light. This is supported by the high concentration of heavy isotope deuterium in Venus's atmosphere, which suggests that water vapor was indeed lost to space after being dissociated by solar UV rays.

Draw a Conclusion

Based on the analysis, option **(c)** is most plausible because scientific evidence (such as isotope analysis) points to the fact that Venus's water was lost when it was broken apart in the atmosphere by ultraviolet light.

Key concepts

Planetary Formation

Planetary formation is a grand cosmic process that shapes celestial bodies like planets and moons. It begins in the rotating disks of dust and gas surrounding young stars, where gravity pulls particles together to form planetesimals. Over time, these planetesimals collide and merge, growing into larger bodies known as protoplanets.
In the case of Venus, questions arise regarding its water content during formation. Venus is similar to Earth in size and composition, suggesting it likely accreted a significant amount of water during its birth. However, unlike Earth, Venus shows an absence of water, presenting a mystery that delves into deeper atmospheric processes.

Ultraviolet Light

Ultraviolet (UV) light is a form of electromagnetic radiation emitted by the Sun. It is more energetic than visible light and has profound effects on the atmospheres of planets.
When it comes to Venus, ultraviolet light played a crucial role in the loss of water from the atmosphere. Water molecules, exposed to the Sun's intense UV rays, absorbed energy and broke apart into their constituent hydrogen and oxygen atoms. This process is called photodissociation. Since hydrogen is a light element, it escaped Venus’s gravity easily into space, leaving oxygen behind. This escape of hydrogen effectively reduced the water content on Venus over time.

Atmospheric Science

Atmospheric science is the study of the layers of gases surrounding celestial bodies. It examines processes such as weather, climate, and the dynamics of atmospheric composition.
The atmosphere of Venus offers a fascinating study due to its extreme conditions. Its thick atmosphere, composed mainly of carbon dioxide, creates a severe greenhouse effect, leading to surface temperatures hot enough to melt lead. Understanding Venus's atmosphere involves exploring how it evolved to become so different from Earth’s. A critical component in this evolution is the role of UV radiation in stripping away lighter elements like hydrogen, contributing to the planet's dry and inhospitable climate today.

Isotope Analysis

Isotope analysis is a technique used to study the chemical elements in substances by analyzing different forms of the same element. In the context of planetary atmospheres, isotopes can provide clues about processes like water loss.
For Venus, scientists have studied the ratio of deuterium (a heavy isotope of hydrogen) to regular hydrogen in its atmosphere. A higher ratio of deuterium suggests significant water loss over time because as lighter hydrogen is lost to space more easily, the heavier deuterium becomes enriched in the atmosphere.
This evidence through isotope analysis confirms the theory that Venus once had much more water, which was dissociated in the atmosphere by UV light and eventually lost to space, leaving behind a dry planet.