The mystery of how Earth got its Moon is one step closer to being solved.

The European Space Agency's lunar-orbiting craft called SMART-1 has completed the first detailed chemical mapping of the lunar surface. The detected chemicals, such as calcium and magnesium, give a boost to the longstanding theory that the Moon formed from the debris flung into space after a collision between early Earth and a Mars-size planet.

Calcium, in particular, is found deep inside Earth. So if the Moon has a lot of calcium, then perhaps it is made of material that was once inside our planet.

Armed with miniaturized instruments-including an ultra-compact electronic camera, an X-ray telescope the size of a toaster for mapping chemical composition, and high-tech communication gadgets-SMART-1 had lofty goals. It was to pin down out how the Moon came to exist, search for water locked up as ice in the depths of Sun-deprived craters, and map the mineral composition of the Moon's crust.

Apollo science

Prior to the Apollo missions, there was no consensus among planetary scientists regarding the Moon's formation. One theory claimed that the Earth and the Moon formed at the same time from the same disk of swirling dust and gas, while another purported that the Moon is a scoop of Earth that split off in the early stages of our solar system.

Besides sending home awe-inspiring photos, the Apollo missions delivered 842 pounds (382 kilograms) of lunar rocks and soil-the first pieces of chemical evidence to help explain the Moon's formation.

The favored theory now describes a violent collision between the Earth and a planet-size object, which hurled molten rocks and dust from both contenders into space. Over time, the debris congealed into the Moon. 

With most Moon know-how coming from Apollo's six landing sites, scientists saw lots of room for error. To solve the lunar-forming puzzle, a global investigation of the entire surface was needed.

Smart science

Enter SMART-1 (Small Missions for Advanced Research and Technology), a spacecraft equipped with seven high-tech instruments that would give a detailed map of both chemical make-up and topography over the Moon's entire surface.

One of the most important devices, D-CIXS (pronounced dee-kicks) recorded hours of X-ray data. When the Sun's rays hit the Moon, the X-rays caused atoms to fluoresce and emit their own X-rays. The D-CIXS (Demonstration Compact Imaging X-ray Spectrometer) telescope translated the amount of energy released into the type and abundance of different elements.

D-CIXS detected the major components of rocks: aluminum, silicon, magnesium, and calcium. However, elements like calcium are not homogenously mixed throughout the Moon. To paint a three-dimensional picture of the chemical composition, planetary scientists needed both surface and "bulk" data.

Cosmic Coincidence

What the project team is calling a cosmic coincidence helped to land that information. On January 2005, a massive solar flare flooded the Moon with X-rays. Meanwhile, the craft was peering over a region called Mare Crisium-the same location in which Russian Landers had collected soil samples in the 1970s. There, the spectrometer detected calcium in similar amounts to the data collected by the landers.

Plus, calcium showed up in broad areas across the entire lunar surface. This rock-building element lends support to the impact theory.

"From SMART-1 observations of previous landing sites we can compare orbital observations to the ground truth and expand from the local to global views of the Moon," says Bernard Foing, Project Scientist for SMART-1.

More work remains to sort out just how significant the calcium findings are.

"We have good maps of iron across the lunar surface. Now we can look forward to making maps of the other elements," said Manuel Grande of the University of Wales and D-CIXS' Principal Investigator.

The findings will be detailed in the Planetary and Space Science journal.

Dark side

Since the Moon's rotation around its axis is equal to its orbital period, or the time it takes the Moon to travel around Earth, the same side always faces Earth. While scientists have studied samples from the Moon's near side, the far side and its polar regions have remained in the dark.

For instance, the lunar south pole sits in the solar system's largest crater, called the South Pole-Aitken Basin, which is 1,616 miles (2,600 kilometers) across and 7.5 miles (12 kilometers) deep. SMART-1 snapped loads of photos of the crater, while gathering chemical data. With such depths, the scientists hope to get a peek at the Moon's mantle layer, just beneath the crust. Since the Moon accreted material over time, the deeper you go the further back in time you go.

This September, the craft's nearly three-year mission will come to an end with a fiery crash.

As the craft nears shut-eye, its instruments will keep all eyes on the Lake of Excellence, a volcanic plain area surrounded by highlands in the mid-southern latitudes. Such close capture should give scientists insights into the formation of this region.

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