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.

C.

Thinning Atmosphere

Regardless of the specific details of the climate of early Mars, it seems clear to most astronomers that beginning around 2.5 billion years ago Mars began to evolve into the frozen, dry world it is today. It became a planet with little volcanic activity and a carbon dioxide atmosphere too thin to allow liquid water on the surface or to sustain a significant greenhouse effect. Exactly what could have happened to most of the thicker early atmosphere that might have existed is still a mystery. One theory is that loss of the planet’s magnetic field after Mars’s liquid core began to turn solid allowed charged particles from the solar wind to collide with the atmosphere, knocking atoms and molecules off into space. Other theories propose that part of the atmosphere may have been blown away in a catastrophic impact event, or that the gases reacted with water and are chemically combined in rocks and minerals on the surface and in the subsurface.

D.

Loss of Liquid Water

Scientists also wonder where the liquid water that appears to have formerly existed at the Martian surface went. Some astronomers believe that it seeped into the ground and is still there as ice or possibly liquid water in the subsurface today. Others think that much of it may have evaporated and slowly trickled off into space as sunlight broke apart the water vapor molecules over long periods of time. Determining the history of the Martian atmosphere and finding out whether sizable quantities of water still exist below the surface are among the most important goals of Mars exploration today.

E.

Shifting Axis Tilt

A major difference between Earth and Mars is the stability of the tilt of the planet’s axis. Due to gravitational effects of the Moon, Earth maintains a relatively stable 23.5 degree tilt on its axis, with only a slight wobble (obliquity) between about 22.1° to 24.6° over a timescale of millions of years. Without a large moon, and being closer to the strong gravitational pull of Jupiter, Mars has a much more dramatic wobble to its axis, resulting in a tilt that may shift from 0° to 60° and causing major climate shifts over time, between massive ice ages and relative warm periods. When the tilt of Mars is nearly vertical, the polar caps may expand almost to the equator. At its greatest tilt, one pole is warmed almost directly by the Sun, likely releasing the deposits of frozen water and gas into the atmosphere.