SYDNEY: Mars grew out of gas and dust of a very different composition from the material that became the Earth, says a new study.

The research is another piece in the puzzle towards understanding how the planets came together from the disc of matter that was the early Solar System.

The study, which analysed the chemical composition of the Red Planet from data collected by space probes and meteorites, also sheds light onto how water affected Mars rocks.

Distinct recipes

Jeffrey Taylor a planetary geologist at the Hawaii Institute of Geophysics and Planetology in Manoa, Honolulu, will present the finds today at the Australian Earth Sciences Convention 2008 in Perth.

His analysis comes from new data on the planet retrieved by NASA's orbiting spacecraft Mars Odyssey as well as the Mars rovers and Martian meteorites on Earth.

In particular, global chemical analysis of Mars' surface from Mars Odyssey's gamma ray spectrometer shows that Mars has almost double the amount of oxidised iron compared to Earth.

Mars also has almost double the ratio of the radioactive elements potassium to thorium (K–Th). The ratio of K–Th is an effective chemical fingerprint for temperature, and shows that on average Mars formed at a lower temperature than Earth did.

"The reason [the K–Th ratio] is important is it shows there were chemically distinct gradients in the [disc-like] solar nebula from which the planets formed," said Taylor. "Theories on the Solar System formation currently predict a series of planetary embryos interacting and smashing together. If that's true, it should iron out the compositional ingredients, but Mars shows that hasn't happened."

Vast ocean of magma

Mars' ancient surface formed just 10 to 15 million years after the oldest objects known from the early Solar System – these are meteorite components known as CAIs, or Calcium-Aluminium Inclusions. At this early stage in its history the surface of Mars would have been a vast ocean of magma, which then solidified to form most of the planet's 50-kilometre-thick, basaltic crust.

Further bombardment of the planet by meteorites obliterated the shape of ancient lava flows and created the dusty surface we see today, Taylor said. While continued cycling of the Earth's crust has destroyed most of its ancient surface, relatively quiescent Mars still has surfaces which have remained virtually unaltered in 4.4 billion years.

The results also show that while water was present on Mars in the past in large quantities, it was there only briefly and intermittently. Water on Mars was more acidic than rainwater on Earth, and rocks would have weathered – altered from exposure to rain and wind – more quickly than they do on Earth.

However, chemical analysis of the rocks show little trace of alteration by water, nor is there any sign of large expanses of chlorine that would have been left by a hypothetical northern sea.

Water everywhere, but only briefly

Huge drainage channels leading from the southern hemisphere of Mars into the northern plains indicate there was a rapid outflow of water, leading scientists to postulate the presence of the sea. However, Taylor says such events must have been short, intense outbursts.

"The water story is very mysterious and something we don't know enough about," said Taylor. "Even if water was extensive, it didn't leave a big geochemical signal at the surface."

Andrew Prentice, a mathematician from the Centre for Stellar and Planetary Astrophysics at Monash University in Melbourne, who is presenting his own theory of Solar System formation at the conference, said Taylor's research sounds "very reasonable." He added that it fits with his theory that the Solar System formed from a nebula with a compositional gradient rather than planetary collisions.

"Planets formed where we see them today and retained the chemistry appropriate to their distance from the Sun," said Prentice. "This result is in sharp variance with the predictions of the accepted disc model of Solar System formation, where widespread radial mixing [and collisions] destroyed the compositional gradient and made all planets the same bulk composition."