ANN ARBOR, Mich.--By measuring the charged particles in the planet Mercury's magnetic field, a University of Michigan sensor enabled the first observations about the surface and atmospheric composition of the closest world to the sun.

"We now know more about what Mercury's made of than ever before," said Thomas Zurbuchen, a professor in the departments of Atmospheric, Oceanic and Space Sciences and Aerospace Engineering. "Holy cow, we found way more than we expected!"

Zurbuchen is project leader of the Fast Imaging Plasma Spectrometer (FIPS), a soda can-sized sensor on board the MESSENGER spacecraft, which performed the first of three scheduled Mercury flybys in January. A paper on FIPS' results from this flyby is published in the July 4 edition of Science.

Since the Mariner 10 spacecraft's 1975 discovery of Mercury's magnetic field, scientists have speculated about how this magnetic field and the solar wind interact with the planet's surface and exosphere, or thin atmosphere.

FIPS detected silicon, sodium, sulfur and even water ions around Mercury. Ions are atoms or molecules that have lost electrons and therefore have an electric charge.

Because of the quantities of these molecules that scientists detected in Mercury's space environment, they surmise that they were blasted from the surface or exosphere by the solar wind. The solar wind is a stream of charged particles emanating from the sun. It buffets Mercury, which is 2/3 closer to the sun than the Earth, and it causes particles from Mercury's surface and atmosphere to sputter into space. FIPS measured these sputtered particles.

"It's like we did a forensic analysis of Mercury," Zurbuchen said. "This flyby got the first-ever look at surface composition.

"The Mercury magnetosphere is full of many ionic species, both atomic and molecular, and in a variety of charge states. What is in some sense a Mercury plasma nebula is far richer in complexity and makeup than the Io plasma torus in the Jupiter system."

Io is a volcanically active moon of Jupiter that is often considered one of the most exciting space environments, Zurbuchen said. Images and other measurements made by MESSENGER suggest that Mercury's surface composition was determined at least in part by volcanic processes.

FIPS was built at the University of Michigan by more than 10 U-M engineers and technicians with help from more than 50 students.

The paper is called "MESSENGER Observations of the Composition of Mercury's Ionized Exosphere and Plasma Environment."