Question

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. A planet that is not within a habitable zone cannot have (a) life: (b) subsurface oceans; (c) abundant liquid water on its surface.

Short answer

(c) abundant liquid water on its surface.

Step-by-step solution

Understand the Question

The question asks which condition or characteristic cannot exist for a planet that is not within a habitable zone.

Define the Habitable Zone

The habitable zone, sometimes called the 'Goldilocks Zone,' is the region around a star where conditions are right for liquid water to exist on the surface of a planet. This is often thought to be essential for life as we know it.

Analyze Each Option

Examine each option individually: - (a) Life: Life could potentially exist in forms that do not require conditions similar to Earth. - (b) Subsurface oceans: Planets or moons outside the habitable zone can have subsurface oceans, potentially heated by internal forces. - (c) Abundant liquid water on its surface: If a planet is not in the habitable zone, stable liquid water on the surface is unlikely due to temperature extremes.

Select the Best Answer

Based on the analysis, the presence of abundant liquid water on the surface is highly dependent on the planet being in the habitable zone. Therefore, (c) is the correct answer.

Key concepts

Liquid Water

Liquid water is an essential ingredient for life as we know it. It serves as a solvent, allowing chemical reactions necessary for biological processes. On Earth, water's unique properties, such as its ability to dissolve various substances, enable life to flourish. In the context of planetary science, the presence of liquid water is often taken as a key indicator of a potentially habitable environment.
This is because water helps regulate temperatures and transport nutrients. Scientists are particularly interested in finding liquid water elsewhere in the universe because it increases the likelihood of discovering life. While it is easy to find water in the form of ice or vapor in space, liquid water requires a specific range of conditions, especially temperatures, which is why it is so significant in the search for life. As a result, determining the presence of liquid water often involves understanding a planet's climate and atmospheric conditions.

Goldilocks Zone

The Goldilocks Zone, or habitable zone, is a term used to describe the area around a star where conditions are just right—not too hot and not too cold—for liquid water to exist. This zone varies depending on the star's size and brightness. For example, larger stars have wider habitable zones, while smaller stars have narrower ones.
In the Goldilocks Zone, planets are at a distance where they can support liquid water, given an atmosphere of sufficient pressure. This doesn't guarantee life but increases the possibilities, as seen with Earth. To put it simply, it's the sweet spot where temperatures allow water to remain liquid rather than freezing into ice or boiling into vapor. While the concept is simple, many factors, like atmospheric composition and planetary rotation, influence whether a planet within this zone can sustain water and potentially life.

Subsurface Oceans

Subsurface oceans are bodies of liquid water located beneath a planet's or moon's surface, usually under layers of ice. These oceans are intriguing because they might exist outside of the traditional habitable zone.
Subsurface oceans could be heated by geothermal processes like radioactive decay or tidal forces. This internal heating can maintain liquid water below the surface, even if the surface is freezing. Moons like Europa and Enceladus are great examples, where evidence suggests oceans under thick ice. These hidden oceans could harbor life, protected from harsh space environments by ice shields. The thought of life existing in these conditions broadens our understanding of where life could potentially thrive in the universe.

Conditions for Life

Conditions for life don't only refer to temperatures amenable to liquid water. They include a range of factors that create a stable environment for life. These conditions encompass having a source of energy, nutrients, and the right chemical composition.

• Energy provides the power needed for life processes, often derived from a star or internal planetary forces.
• Nutrients need to be available for organisms to build and maintain biological structures.
• A protective atmosphere can shield life from harmful radiation and help maintain the right surface temperatures.

While planets within the Goldilocks Zone might seem most likely to support life, life could exist in diverse environments, like around hydrothermal vents deep under oceans. The diversity of potential life-friendly environments inspires the search for life beyond Earth, requiring scientists to expand their views on possible habitability in the universe.