In December 2015, NASA's Curiosity Mars rover reported high concentrations of which substance in Martian rocks and soils, helping scientists learn more about the planet's ancient wet environment?

Introduction / Context:
NASA's Curiosity rover has been exploring the surface of Mars to understand the planet's geology and past environmental conditions, including whether it once had liquid water that could support life. One of the important discoveries made by Curiosity in December 2015 involved detecting unusually high concentrations of a particular substance in Martian rocks and soils. This substance is often associated with water-related processes on Earth and can preserve clues about past environmental conditions. The question asks you to identify that substance.

Given Data / Assumptions:

• The discovery occurred in December 2015 and involved NASA's Curiosity rover.
• The context is learning about an ancient wet environment on Mars.
• Options include magnesium, cadmium, silica, and calcium.
• Silica is known to be associated with water-related processes and can form deposits like opal.

Concept / Approach:
Silica is a compound of silicon and oxygen (SiO2) and is a major component of many rocks and minerals on Earth, such as quartz. On Mars and Earth, elevated silica concentrations can form in environments where water has interacted with rocks, leading to chemical alteration and the possible precipitation of silica-rich deposits. Silica can sometimes preserve evidence of microbial life or other environmental features. Curiosity's discovery of high-silica deposits suggested that water had once interacted with these rocks, providing an important clue about Mars' ancient climate. Magnesium and calcium are also present in Martian rocks, but the 2015 discovery specifically highlighted silica. Cadmium is less relevant to the typical geochemistry of Mars in this context.

Step-by-Step Solution:
Step 1: Recall that the Curiosity rover was sent to study whether Mars ever had conditions suitable for life, including the presence of liquid water. Step 2: Remember that in December 2015, NASA announced that Curiosity had found unusually high levels of silica in some Martian rocks. Step 3: Understand that silica-rich deposits often form through chemical processes involving water, such as hydrothermal activity or long-term water-rock interaction. Step 4: Compare this with the other options: magnesium and calcium are common elements but were not the focus of that particular announcement, and cadmium is not prominently associated with the Curiosity findings. Step 5: Recognise that silica as a compound of silicon and oxygen fits the description of the substance mentioned in the question. Step 6: Conclude that silica is the correct answer.

Verification / Alternative check:
Press releases and scientific reports from NASA and associated research teams describe Curiosity's observations of high-silica rocks in areas such as the "Marias Pass" region of Gale Crater. They explain that these silica-rich deposits were identified using instruments on the rover, and they discuss how such deposits can form in the presence of water. Articles emphasise that understanding the origin of these silica concentrations could reveal whether the environment was once hospitable to microbial life. Magnesium, cadmium, and calcium are not highlighted in the same way in these December 2015 discoveries, confirming that silica is the substance referred to.

Why Other Options Are Wrong:
Magnesium, a light metal element, is present in rocks and minerals but was not the specific focus of the December 2015 announcement concerning high concentrations linked to an ancient wet environment.
Cadmium, a toxic heavy metal, is not commonly mentioned in the context of Curiosity's major findings and is not central to interpretations of past Martian water.
Calcium, a metal associated with carbonates, is important in some Martian minerals, but the question specifically refers to the well-publicised discovery of high silica content.

Common Pitfalls:
Because all the options are chemical elements or compounds known on Earth, students may guess based on familiarity rather than recalling the specific rover result. Some may associate calcium with limestone and water on Earth and mistakenly choose it. To avoid confusion, focus on what was highlighted in December 2015 reports: silica-rich rocks. Remember that silica is chemically SiO2 and is frequently discussed in relation to Curiosity's findings about ancient Martian environments.

Final Answer:
In December 2015, Curiosity found high concentrations of Silica, a compound of silicon and oxygen in Martian rocks and soils.