14 DAY TRIAL // ESA's probe, Mars Express, has surveyed in the last two years more than 90 percent of the Red Planet's surface. The scientists have now analyzed the data and pieced together the mineral history of the planet. The results showed that Mars had three distinct geological eras.
Jean-Pierre Bibring, an astrophysicist from the Institut d'Astrophysique Spatiale (IAS), Orsay (France), led the team of scientists from France, Italy, Russia, Germany and the United States. They analyzed the data taken by the OMEGA (Observatoire pour la Min?ralogie, l'Eau, les Glaces et l'Activit?), an instrument on board of Mars Express. This instrument is capable of determining the mineral composition of the rocks from the spectral analysis of the visible and infrared sunlight reflected from the planet's surface. Different types of minerals absorb different sets of wavelengths and by analyzing the spectral composition of the reflected light one can deduce the mineral composition.
For example, the team managed to determine why the planet is red (why it absorbs such wavelengths that only red light is reflected - i.e. a set of wavelengths that is interpreted by our brains as red light). Mars is red because of tiny grains of red hematite and also possibly maghemite, both containing iron. This discovery was somewhat surprising as scientists suspected the planet became red due to the action of liquid water. Not quite so.
The history of Mars
Based on the comprehensive mineral information gathered by OMEGA, the team put together the following geologic history (the eras are named after the Greek words for the predominant minerals formed within them):
"The earliest era, named the 'phyllosian' era, occurred between 4.5-4.2 thousand million years ago, soon after the planet formed. The environment was possibly warm and moist at this time, allowing the formation of large-scale clay beds, many of which survive today.
The second era, the 'theiikian', took place between 4.2 and 3.8 billion years ago. It was prompted by planet-wide volcanic eruptions that drove global climate change. In particular, the sulphur these eruptions belched into the atmosphere reacted with the water to produce acid rain, which altered the composition of the surface rocks where it fell.
Finally, there was the 'siderikian', the longest lasting of the Martian eras. It began sometime around 3.8-3.5 billion years ago and continues today. There is little water involved in this era; instead, the rocks appear to have been altered during slow weathering by the tenuous Martian atmosphere. This process gave Mars its red color."
The scientists deduced this history from the minerals: The clay minerals, such as chamosite and nontronite, need abundant water, moderate temperatures and low acidity to form. These minerals were found in the oldest rock which was exposed by erosion, impact or faulting. The image above shows Syrtis Major, a large dark area right of center on the hemisphere view of Mars. OMEGA identified here the presence of water-bearing clay minerals. Blue indicates small amounts and orange-red indicates large amounts.
Minerals of the second era, such as gypsum and grey hematite, were found in Meridiani and in Valles Marineris. These rocks, traced by sulfates, mark a dramatic shift from a moist and alkaline environment to a dry, acidic one. The cause of this shift must have been massive volcanic eruptions that spewed sulfur into the atmosphere, which then rained back down on the planet's surface. The most recent minerals, which are iron-rich, dominated by ferric oxides, were not formed with, or altered by, liquid water. These were found across most of the planet and reflect the cold, dry conditions that persist on Mars.
Thus, it appears that Mars started out relatively wet and temperate, underwent a major climate shift, and evolved into a cold, dry place strewn with acidic rock.
"Starting about 3.5 billion years ago, conditions on Mars became increasingly dry and acidic - not a pleasant place for any form of life, even a microbe," said John Mustard, a Brown University geologist and a primary author of the Science paper where these findings are presented.
Not all scientists think this view is so straightforward. Some say that Mars' surface may always have been cold and that the clay minerals may have formed underground: "Hydrothermal activity below the surface, the impact of water-bearing asteroids, even the natural cooling of the planet could all have promoted the formation of clay below Mars's surface. If so, the surface conditions may always have been cold and dry," said Professor Jean-Pierre Bibring.
Where is the life?
If any organisms ever lived on Mars the evidence would probably be found in the clay-rich rocks and soil north of the Syrtis Major volcanic plateau, in Nili Fossae and in the Marwth Vallis regions. The conditions necessary for the formation of these rocks (warmth and moisture) are also as favorable as you can get for the formation of life (at least on Mars). According to Mustard, these areas make compelling targets for future lander missions.
In the meantime, the Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM, aboard the Mars Reconnaissance Orbiter, will begin sending mineralogical data on these clay-rich regions in September. The images will be 20 times more precise compared with those captured by Mars Express.
"I'm eager to get the CRISM data and explore the deposits found by OMEGA, as well as discover new sites and minerals," Mustard said. "OMEGA shows that some of the most interesting sites are small - and CRISM is designed to find and characterize small deposits."
This wealth of information gathered by the Mars Express is incredible. I wonder what the Venus Express probe, which is virtually identical to Mars Express and which has just entered Venus' orbit, will bring.