Climate Change - page 4
A Martian History LessonEarly geologists used observations of the layers, or strata, in rocks and soil to devise a time sequence of Earth's geologic history. When orbiter missions began returning images from Mars, planetary scientists were able to develop a rudimentary timescale of Martian geologic history, based on the cratering density of various surfaces. An older surface is one that has accumulated more impact craters per unit area.
This THEMIS image shows a close up view of the plains in the Hesperia Planum, a region of surfaces that formed during Mars' Hesperian Era ("middle ages"). The surface shown in the image has a large number of craters that are 1-3 km (0.6-1.9 miles) in diameter, which indicates that the surface is very old and has been subjected to a long period of bombardment. (Image credit: NASA/JPL/Arizona State University)
The Noachian Era is the oldest period and lasted from Mars' beginning, about 4500 million years (MY) ago, to about 3500 MY ago. Climate on Noachian Mars was probably very different than it is today, and geologic features such as dried up river valleys and delta features suggest that the climate may have been warmer and wetter. Erosional processes and volcanic activity took place during this time, and many scientists believe lakes and oceans could have existed.
The Hesperian Era - or "Middle Ages" - lasted from about 3500 MY ago to 2500 MY ago. It was during this time period that Martian climate began to change to drier, dustier conditions. Water that flowed on the Martian surface during the Noachian Era may have frozen as underground ice deposits, and most river channels probably experienced their final flow episodes during this era.
The Amazonian Era began about 2500 MY ago and continues to the present. Although Amazonian Mars has been mostly dry and dusty, there are signs that water is sometimes released onto the surface from underground through sudden floods. Volcanic activity occurs, but at much lower levels than in Mars' earlier history.
This map shows the maximum extent of glacial ice in Earth's north polar area during the Pleistocene Epoch, or Great Ice Age, which began about two million years ago. (Image credit: U.S. Geological Survey)
Ice AgesThe term "ice age" is frequently interpreted to mean a period in Earth's history when vast ice sheets extended into latitudes that, today, have a more temperate climate. In fact, an "ice age" more generally refers to a succession of alternating glaciations and interglaciations, spanning a total time period of 1 to 10 million years.
During a glaciation phase, ice sheets spread outward over many parts of the world, and a long-term decline in the Earth's temperature occurs. During the interglacial periods, the ice sheets recede (melt), and a warmer, more temperate climate prevails on Earth.
At least four major ice ages have occurred in Earth's geologic history, the earliest being about 600 to 800 million years ago. During the past 2.5 to 3 million years, the Earth has been in an ice age known simply as "The Ice Age." Even though Earth's current climate is considered mild, we are still in the ice age because the Greenland and Antarctic ice sheets still exist. Our present position is within an interglaciation period, which began quite rapidly about 15,000 years ago. During the preceding glaciation, called the Wisconsinan Glaciation, ice sheets covered the North American and European continents.