Signs and symptoms are both signals of injury, illness, disease â signals that something is not right in the body. A sign is a signal that can be seen by someone else â maybe a loved one, or a doctor, nurse, or other health care professional.
Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, magnesium, aluminum, calcium, and potassium. Oxidation of the iron dust is what gives the surface its reddish hue. Composite image showing the size difference between Earth and Mars. Credit: NASA/Mars Exploration.
Mars has two small moons, called Phobos and Deimos. The planet Mars is made of rock. The ground there is red because of iron oxide (rust) in the rocks and dust. The planet's atmosphere is very thin and contains a lot of carbon dioxide and a very tiny amount of oxygen. The temperatures on Mars are colder than on Earth, because it is farther away from the Sun and has less air to keep warmth in. There is water ice and frozen carbon dioxide at the north and south poles. Mars does not have any liquid water on the surface now, but signs of run-off on the surface were probably caused by water. The average thickness of the planet's crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi).