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What Mars rocks are made of

Radiation and the atom · level 3, lesson 6 of 6

A question to start

Rovers on Mars carry instruments that can measure exactly which elements are in a rock or a patch of soil. What have they found?

The idea

Rocks are made mostly of compounds of a few elements, joined with oxygen. Geologists list them as oxides: silica (silicon and oxygen), iron oxide, magnesium oxide and so on.

Mars Pathfinder's little rover, Sojourner, measured the soil where it landed in 1997. Compared with the rock of Earth's continents, Mars soil has less silica and about three times as much iron oxide:

Out of every 100 g (Mars soil, Earth's continental crust) Mars Earth
Silica (silicon and oxygen) 43.8 g 60.2 g
Iron oxide 17.5 g 6.05 g
Aluminium oxide 10.1 g 15.2 g
Magnesium oxide 8.6 g 3.1 g
Calcium oxide 5.3 g 5.5 g

Rock with less silica and more iron and magnesium is typical of basalt, the dark rock that forms from runny lava, as in Hawaii and Iceland. Mars is covered in it, and its iron is what rusts and turns Mars red.

MarsEarthsilicairon oxidethe rest aluminium oxidemagnesium oxide

Worked examples

1. Three times the iron. \(17.5 \div 6.05 \approx 2.9\): nearly three times as much iron oxide in Mars soil.

2. The most common element. Oxygen. Pathfinder found that about 43 g in every 100 g of Mars soil is oxygen, joined to the other elements.

3. A rover's lab. Curiosity found that Mars soil holds crystals of olivine, feldspar and pyroxene, much like the weathered volcanic soil of Hawaii.

Try it

  1. What element is joined to most of the others in rock?
  2. Which has more iron oxide: Mars soil or Earth's continents? About how many times as much?
  3. Which has more silica?
  4. Why do geologists think Mars's surface came largely from volcanoes?
Answers
  1. Oxygen.
  2. Mars soil, about three times as much.
  3. Earth's continental crust.
  4. Its rock has less silica and more iron and magnesium, like basalt lava.

Common mistake

"Mars is made of different elements from Earth." Both are made of the same elements. The amounts are different: Mars has more iron near its surface and less silica.

Where it leads