Thursday, February 23, 2012

First Civilization Part 1 Chapter 4

 

The geologic record of the Proterozoic (2,500-570 Ma) is known much better than that for the preceding Archean. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas; furthermore, many of these rocks are less metamorphisized than Archean-Eon ones, and plenty are unaltered. Study of these rocks show that the period featured massive, rapid continental accretion (unique to the Proterozoic), super continent cycles, and wholly modern organic activity. The first-known glaciations occurred during the Proterozoic, one began shortly after the beginning of the period, while there were at least four during the Neoproterozoic, climaxing with the Snowball Earth of the Varangian glaciation.

1st ice age

The Proterozoic Ice age Begins

During the Solar System's formation, Mars had been created out of the protoplanetary disk that orbited the Sun as the result of a stochastic process of run-away accretion. Mars has many distinctive chemical features caused by its position in the Solar System. Elements with comparatively low boiling points such as chlorine, phosphorus and sulfur are much more common on Mars than Earth.  It also had a healthy atmosphere that cloaked it with clouds.

early Mars atmophere

Early Atmosphere of Mars

At this time Mars was just inside the habitable zone but on its outer edge, while Earth was a little outside of the inner edge of the zone itself.

mars earth hab zone

Early Positions of Earth and Mars in the Habitable Zone

Much of a planet’s history can be deciphered by looking at its surface, asking what came first, and what came next. For example, a lava flow that spreads out and fills a large impact crater is clearly younger than the crater, and a small crater on top of the same lava flow is younger than both the lava and the larger crater. This principle, is called the law of superposition, and other principles of stratigraphy (the study of the strata found in rocks) are used in determining the age of rocks and their strata such as are seen in mountainsides and canyons.

super strata 

Since there is no overturning, the rock at the bottom is older than the rock on the top by the Principle of Superposition.

 

The same methodology was later applied to the Moon and then to Mars.  Another stratigraphic principle used on planets where impact craters are well preserved is that of crater number density. The number of craters greater than a given size per unit surface area (usually a million km2) provides a relative age for that surface. This latter principle was used to figure out the age of Mars, but some look upon this method with skepticism.

martian crators

Large Crater Density on Mars

Assigning absolute ages to rock units on Mars is much more problematic. Numerous attempts have been made over the years to determine an absolute Martian chronology (timeline) by comparing estimated impact cratering rates for Mars to those on the Moon. Martian meteorites have provided datable samples that are consistent with ages calculated thus far, but the locations on Mars where these meteorites originated are unknown, limiting their value as chronostratigraphic tools.