Question

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. We can recognize the oldest surface regions of Mars by the fact that they have (a) the most impact craters; (b) the most volcanoes; (c) the most evidence of past water flows.

Short answer

(a) the most impact craters

Step-by-step solution

Understanding Surface Features of Mars

Mars has various surface features such as craters, volcanoes, and channels formed by water. Older surfaces are generally more exposed to impacts, leading to distinct characteristics that help in identifying their age.

Analyzing Impact Craters Significance

Impact craters are created when meteoroids collide with the surface of a planet. Over time, older surfaces accumulate more craters. Thus, the presence of many craters typically indicates an ancient surface.

Evaluating the Role of Volcanoes

While Mars has many volcanoes, their presence does not directly correlate with the age of the surface. Volcanic activity can occur at various stages of a planet's history, so a high number of volcanoes doesn't necessarily imply an old surface.

Assessing Evidence of Past Water Flows

Evidence of past water flows can reveal ancient or relatively recent surface changes. While significant, they're not as straightforward an indicator of age as impact craters because water can re-shape surfaces at different times.

Choosing the Best Answer

Considering all features, the most reliable indicator of an ancient surface on Mars is the number of impact craters. Unlike volcanoes and water flow evidence, craters provide a consistent method to determine surface age.

Key concepts

Impact Craters

Mars is peppered with impact craters, which are essential clues to understanding its geological history. An impact crater forms when a meteoroid crashes into the surface of the planet, releasing a massive amount of energy. This collision creates a bowl-shaped depression characterized by raised rims and sometimes central peaks. The number and size of these craters help scientists determine the age of the surface.

- More craters indicate an older surface, as these areas have been exposed to meteoroid impacts for a longer time.
- Fewer craters suggest either a younger surface or geological processes that may have erased older craters.

Impact craters are one of the most reliable methods for dating the surface of Mars because their presence and accumulation over time provide a clear and consistent record of the planet's exposure to space debris.

Volcanoes

Volcanoes on Mars are a testament to its geologically active past. Unlike on Earth, Martian volcanoes can grow to enormous sizes due to the lack of plate tectonics, which on Earth prevents such massive buildup. Olympus Mons, the largest volcano on Mars, is an example.

However, the presence of volcanoes does not necessarily indicate the age of a surface. Here’s why:

• Volcanic activity spans various periods of Mars' history, from billions of years ago to possibly recent times.
• New volcanic material can resurface an area, covering older surfaces and potentially making them appear younger than they are.

Thus, while volcanoes provide critical insights into the environmental and thermal history of Mars, they are not the best indicator of geological age.

Evidence of Water Flows

Mars showcases fascinating evidence of past water flows through features like dry riverbeds, channels, and lake beds. These features suggest that liquid water once flowed, altering the Martian landscape in a striking way. Signs that water was once abundant include:

• Valley networks resembling dry riverbeds.
• Outflow channels indicating massive floods.
• Sediment deposits from ancient lakes or seas.

While this evidence is significant for understanding Mars' climate history, it doesn't consistently reveal the surface's age. This is because water flows could have occurred at different times, altering the landscape sporadically. Hence, while indicative of Mars having had liquid water, they provide scattered clues rather than a continuous timeline like impact craters do.