This chart shows the time history (or "chronology") of Mars as derived from interpretations of Viking orbiter images. Model 1 and Model 2 are based on different interpretations of impact cratering of the surface of Mars. For comparison, the geochronology of Earth is shown also -- recall that all the planets were formed at the same time about 4.6 thousand million years ago.
Since we are trying to unravel the history of Mars we are always mentioning dates in the past when we think certain events ocurred. It is helpful, therefore, to note a few terms that photo-geologists use when talking about Mars. The first period of martian history ("morning" if you will) is called the Noachian period -- Noachis happens to be a good example of (ancient cratered highland) terrain that dates back to that time of heavy asteroidal bombardment (about 4.3 billion years ago to 3.5 billion years). The second period of martian history ("afternoon") is called the Hesperian period, the period after the heavy bombardment had tapered off. Hesperia Planitia is, naturally, a good example of (plains) terrain that dates back to that time (about 3.5 billion years to 1.8 billion years). The third period ("evening") is called Amazonian -- obviously, Amazonia Planitia is the type example of the (least cratered volcanic) plains that typify the last billion and a half years of martian history.
When you turn to the atlas of Mars you will find the Noachis region around 40 to 300 degrees west, 15 to 83 degrees south. Hesperia Planitia around 258 to 242 degrees west, 10 to 35 degrees north. Amazonia Planitia is around 168 to 140 degrees west, and 0 to 40 degrees north.
The ability to assign ages to different rocks and thus their associated geologic formations is a pretty basic requirement if we are ever to work out the sequence of events that describe the history of a planet. Geologists today rely on sensitive measurements (possible only in specially instrumented laboratories) of the abundances of naturally occurring radioactive isotopes of certain atoms (like uranium, thorium and potassium) found within all rocks. They base their calculations of age on the fact that the radioactive decay of these "parent" atoms leads to specific "daughter" atoms and that the rate of decay is constant. Thus the more daughter products there are in comparison to the parents the older the rock.
Probably you have heard of the way in which the age of plants is determined by "carbon dating". This is the same approach as the radio-isotope dating of rocks. In the case of plants, their carbon content includes a natural isotope of carbon (carbon 14) which is radioactive (i.e. spontaneously breaks apart over time) and the daughter product can be measured. For rocks that may be billions of years old we need to use isotopes that decay much more slowly than carbon 14.
Since we haven't figured out how to build a spacecraft instrument with the necessary dating capability we can only make the age measurements of rocks in laboratories. We have successfully applied the radio-isotope dating technique to rocks returned from the Moon and to rocks that fall on Earth from space (meteorites). It will probably be towards the end of the next decade (say 2007) before we can get martian rocks back to Earth to date them accurately.
Probably you know that every so often the Earth is struck by an asteroid that makes a big hole in the ground. Meteor Crater near Flagstaff is a good example of a quite small impact. The famous asteroid that caused the extinction of the dinosaurs is a good example of a very big impact. These events have been going on ever since the planets were formed (indeed, accretion of smaller bodies is the very process by which planets came into being) and so we use the technique of counting craters (number of craters of a given size for a given area) as a means of estimating the age of particular geologic terrains -- the more craters the older the terrain.
The technique doesn't work too well on Earth because there aren't very many craters to count (the Earth's surface is continually remaking itself and in doing so the craters are erased) so the statistics are poor. On Mars and the Moon the technique works pretty well though it has its complications (which we won't get into here).
Note that the crater counting technique only provides relative ages -- we know that one terrain is older or younger than another and more or less by how much. Absolute age measurements (e.g. "this event took place 2.9 billion years ago") require some kind of calibration. In the case of the Moon we have the calibration of the radio-isotope dating technique (which is an absolute dating technique because we know the radio-isotopic decay rates very accurately) for rocks collected by the Apollo astronauts. For Mars we try to tie the rate of cratering there to the rate of cratering on the Moon and thus make a link to the absolute dating of lunar rocks. We will know the history of Mars much better when we have returned samples of martian rocks to Earth --about the year 2007 according to present plans. Stay tuned!
Symbols interpretation for Mars Models :
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