How it all began Mars was formed along with the other planets in the Solar System about 4.5 billion years ago. The Sun's formation left a "proto-planetary" (pre-planet) disk of dust and gas that, over millions of years, accreted into big lumps of matter called "planetesimals", or "little planets". Many of these planetesimals collided, sticking together to form the rocky cores of all the planets. These cores continued to gather more and more material onto them until most of the leftover material from the Sun's formation had been swept up into the formation of the planets. Jupiter was the first planet to form, and continued to gather the majority of material in the early Solar System. Mars, as the planet closest to Jupiter, was deprived of its "fair share" of dust and gas and thus ended up much smaller than Earth. Portrait of a young planet The heat of the newborn Sun caused much of the lighter, gaseous elements, such as hydrogen and helium) to escape from the inner part of the Solar System. This prevented the inner planets, Mercury, Venus, Earth, and Mars, from forming thick atmospheres. Covered only by thin veils of leftover gases, or in the case of Mercury, no gases at all, the inner planets are known as "rocky" or "terrestrial"-type planets. In its early years, Mars had a liquid surface for tens of millions of years, caused by the heat generated by sweeping up Solar System material. Heat in the rocks, caused by the decay of radioactive elements, also added to the internal heating of Mars. As a terrestrial planet cools, it releases stored heat through geologic activity, such as volcanoes or earthquakes. The bigger the planet, the more heat has been retained, and the longer it takes for the planet to cool down completely. Because Mars is so much smaller than Earth or Venus, it has a smaller heat supply, and has already lost most of its heat. Mars clearly has had an active geologic past - its surface has many volcanoes, but now, they are thought to be dead. What craters tell us about Mars' past Meteors hit planets a regular rates, especially small meteors. Why? Because there are many more tiny meteors than large ones left over from the formation of the Solar System. Large meteors strike a planet like Mars about once every hundred million years. If you have a planet with many craters on its surface, it must have been exposed to bombardment of meteors for a long time, undisturbed by processes on the planet such as weathering. A surface with only a few craters must have been undisturbed for a short time. Mars probably isn't volcanically active anymore because it is absolutely covered with craters. Lava flows tend to bury craters! There are a few areas on Mars that look like they have been covered by lava, because they have fewer craters. If you want to find the age of a region on Mars, you can count the number of craters of a given size in that region and compare it to the same number in surrounding regions. This is called relative dating. Relative dating, unfortunately, is inexact. It is only good for making vague, general comparisons. For some real numbers, we must know the exact cratering rate over time. Fortunately, we have other bodies in the Solar system that have been meteor victims and are geologically dead. The Moon and Mercury are great examples. By studying these worlds, scientists new believe that the early Solar System was flooded with leftover chunks of material that continuously rained down on the infant planets, leaving large numbers of craters behind. This is known as the "Heavy Bombardment" period. Heavy Bombardment was at its peak about 3.9 billion years ago; the rate of cratering has been tapering off drastically ever since. Given what we know, and through the information gained from studying Moon rock samples, we can break down Martian geological history into 3 epochs marked by the numbers of craters counted on different regions of Mars. Each era is named after the region of Mars where the crater count was made. The dates are pretty uncertain and can only be narrowed down when we get more rock samples from Mars. The Martian geological eras are: Noachian - This spans the time between the formation of Mars and about 3.5 billion years ago, when Mars had much crater impacting and formation of channels called "small valley networks". Hesperian - 3.5 to 2 billion years ago. There was heavy geologic activity, such as crust folding, faulting, volcanism, and flooding. Amazonian - 2 billion years ago to the present. Volcanism gradually stopped during this period. Eventually, the Martian surface quieted down to become the dry dustbowl we see today. Global Geology Mars has had an active history. The geological features on the planet have been molded over time into their current shapes by four basic processes: Tectonism - Heat and motions deep inside a planet cause the ground above to respond by bending and folding or breaking along fault lines if there is enough force, like in an earthquake. This process makes hills and valleys. On Mars, enormous faults surround the Tharsis region, the location of Mars' large volcanoes in the northern hemisphere, while folded hills, called "wrinkle ridges", occur across most of the older terrain. Volcanism - Heat and motions deep inside a planet melt part of the crust, causing pockets of liquid rock, or "magma", to rise and then freeze. Magma that erupts through the surface is called "lava" and forms volcanoes. Mars has two volcanic regions Elysium and Tharsis. Tharsis contains Olympus Mons, the largest volcano in the Solar System. Surface processes - Erosion agents, such as wind and water, remove material from high ground and deposit it in low ground until a planet's surface is completely flat. Each process makes characteristic shapes on a planet's surface. Today on Mars, the surface is shaped by gravity, wind, and some ice. In the past, Mars was also shaped by liquid water. Where did all the water go? Some of Mars' water is now frozen in the polar ice caps, some is in very thin clouds, but we think that most of Mars' water is hidden as underground ice or "permafrost". Many impact craters have puffy edges, probably created by meteors exploding into a soft, wet surface instead of a hard, dry one. Why did Mars dry out? No one really knows. It is still a mystery. Future explorers will survey ancient lakebeds to try and find the answers to this perplexing question. Impact Cratering - Meteors from space fall and hit the surface of Mars. The resulting explosion creates a large hole, called a "crater". On Mars, craters vary in size from a few yards across to the Hellas Basin, which is the size of the western United States.