Mars Inside And Out May Have Had Differing Geological Histories
November 20, 2012

Meteorites May Help Find A Common Denominator Between Mars And Earth

April Flowers for - Your Universe Online

A team of scientists led by the Carnegie Institution for Science recently studied the hydrogen in water from the interior of Mars and found that Mars was formed from similar building blocks to that of Earth. There are differences, however, in the later evolution of the two planets, which implies that terrestrial planets such as Earth have similar water sources — chondritic meteorites.

Unlike Earth, however, rocks on Mars that contain atmospheric volatiles such as water do not get recycled into the planet's deep interior.

The origin, abundance and history of water on Mars are subjects of much controversy. Although the sculpted channels of the Martian southern hemisphere speak loudly of flowing water, the terrain is ancient, leading planetary scientists to describe early Mars as "warm and wet" and current day Mars as "cold and dry."

The focus of the debate is on how the interior and crust of Mars formed and how they differ from Earth. The team of scientists, including members from NASA's Johnson Space Center and the Lunar and Planetary Institute, studied water concentrations and hydrogen isotopic compositions trapped inside crystals within two Martian meteorites to understand the history of Martian water and other volatiles. These meteorites are called shergottites. They are of the same primitive nature, but one is rich in elements such as hydrogen, and the other is not.

The two meteorites, which are pristine samples of various Martian volatile element environments, contain trapped basaltic liquids. One, which has a hydrogen isotopic composition similar to that of Earth, appears to have changed little on its way from the Martian mantle up to the surface of Mars. The second meteorite, however, appears to have sampled Martian crust that had been in contact with the Martian atmosphere. The two meteorites represent two very different sources of water: one sampled water from the deep interior — representing water that existed when Mars formed — and the other sampled the shallow crust and atmosphere.

"There are competing theories that account for the diverse compositions of Martian meteorites," said Tomohiro Usui. "Until this study there was no direct evidence that primitive Martian lavas contained material from the surface of Mars."

The team inferred that Martian surface water has had a different geologic history than Martian interior water because the two meteorites have such different hydrogen isotopic compositions. They claim that the difference is mostly likely because atmospheric water has preferentially lost the lighter hydrogen isotope to space, and has preferentially retained the heavier hydrogen isotope (deuterium).

An important mystery could be solved by the fact that the enriched meteorite has incorporated crustal and atmospheric water. Scientists have been wondering if Martian meteorites that are enriched in components, such as water, coming from an enriched, deep mantle, or have they been overprinted by interaction with the Martian crust.

"The hydrogen isotopic composition of the water in the enriched meteorite clearly indicates that they have been overprinted, so this meteorite tells scientists more about the Martian crust than about the Martian mantle," Alexander said. "Conversely, the other meteorite yields more information about the Martian interior."

The water concentrations in the meteorites are very different as well. The non-enriched meteorite has a rather low concentration, meaning that the interior of Mars is rather dry. The enriched meteorite, however, has 10 times more water than the other one, suggesting that the surface of Mars could have been very wet at one time.

"To understand the geologic history of Mars, more information about both of these environments is needed," Carnegie's Conel Alexander said.

The team's findings will be published in the December issue of Earth and Planetary Science Letters.