Mars (planet)

VII

The Martian Past

Space probes have provided scientists with enough information to decipher some of the history of Mars as a planet. Surface features indicate that the environment on the surface of Mars has changed dramatically over time, from geologically active early periods when a relatively thick atmosphere and liquid water may have been present to today’s frozen world with a thin atmosphere.

A

Formation of Mars

The chemical composition of Mars is similar overall to that of the Earth, although there are important differences in the abundances of iron and of volatile elements like water and sulfur. These differences probably exist because Mars likely formed further away from the Sun than the Earth, in a different region of the disk of rock, ice, and gas from which all of the planets formed.

Like Earth, Mars went through a period of massive bombardment from asteroids and comets from its formation about 4.6 billion years ago until about 4.2 billion years ago. Conditions would have been hostile to the rise of life, but the bombardment also provided Mars with some of the same chemical building blocks that made life possible on Earth. These substances include organic carbon compounds.

B

Climate History

While there is still intense debate and scientific study of Martian climate change, a number of models of the planet’s evolution have been proposed to try to match the steady stream of new observations.

---

---

B

1

Earthlike Mars Model

In one model, Mars appears to have had more Earthlike surface conditions between about 4.2 and 3.5 billion years ago. A thick CO₂ atmosphere may have trapped more solar heat through the greenhouse effect, allowing the surface to warm up. Along with higher atmospheric pressure, the warm temperatures allowed water to remain liquid for long periods of time and to possibly cover extensive areas of the surface. Water reacted with the surface and subsurface rocks, creating clays and other hydrated minerals and possibly also carbonate rocks.

Volcanic eruptions over time released large amounts of sulfur dioxide (SO₂) into the atmosphere, slowly changing the chemical environment. The added sulfur turned the surface water acidic, dissolving most of the clays and any carbonate rocks that may have formed during earlier more Earthlike periods. When the sulfur-rich surface water evaporated it left behind deposits of salty sulfate minerals.

B

2

Dry Mars Models

Other models of the Martian past paint a “drier” picture of early Mars. In these scenarios, liquid water may have existed on the surface or in the shallow subsurface for perhaps only intermittent periods or only in small regions of the planet. These models lead to less optimistic implications for the possibility of life on Mars but are still consistent with the information that space probes have obtained. Astronomers focus significant efforts on trying to distinguish between these very different models for the Martian past, including designing future missions to try to resolve the controversy.