MESSENGER was inserted into Mercury's orbit in March 2011 and since then, data from the spacecraft's X-Ray Spectrometer (XRS) and Gamma-Ray Spectrometer (GRS) have provided information on the concentrations of potassium, thorium, uranium, sodium, chlorine and silicon, as well as ratios relative to silicon of magnesium, aluminum, sulfur, calcium and iron, according to a press release from Johns Hopkins University Applied Physics Laboratory.
So far, the geochemical maps have been limited to one hemisphere and have had poor spatial resolution, but the authors of the paper, "Evidence for Geochemical Terranes on Mercury: Global Mapping of Major Elements with MESSENGER's X-Ray Spectrometer," published in Earth and Planetary Science Letters, used a new method to produce global maps of the magnesium/silicon and aluminum/silicon abundance ratios across Mercury's surface from information obtained by MESSENGER's XRS.
These maps are the first global maps, not just of Mercury, but of any planetary body obtained by X-ray fluorescence - a process in which X-rays emitted from the sun's atmosphere allow the planet's surface composition to be examined.
One of Mercury's largest features is a terrane that covers more than 5 million square kilometers and "exhibits the highest observed magnesium/silicon, sulfur/silicon, and calcium/silicon ratios, as well as some of the lowest aluminum/silicon ratios on the planet's surface," wrote Shoshana Weider, a planetary geologist and visiting scientist at the Carnegie Institution of Washington, according to the press release. This data has been interpreted to mean that much of the mantle was formed by materials from a large impact event.
The first maps of the absorption of low-energy ("thermal") neutrons across Mercury's surface were created by the authors of a second paper published in Icarus, "Geochemical Terranes of Mercury's Northern Hemisphere as Revealed by MESSENGER Neutron Measurements." The information used in these maps was obtained by the GRS anti-coincidence shield, which is sensitive to neutron emissions from the surface of Mercury.
"From these maps we may infer the distribution of thermal-neutron-absorbing elements across the planet, including iron, chlorine and sodium," wrote lead author Patrick Peplowski of the Johns Hopkins University Applied Physics Laboratory, according to the press release. "This information has been combined with other MESSENGER geochemical measurements, including the new XRS measurements, to identify and map four distinct geochemical terranes on Mercury."
"Earlier MESSENGER data have shown that Mercury's surface was pervasively shaped by volcanic activity," continued Peplowski. "The magmas erupted long ago were derived from the partial melting of Mercury's mantle. The differences in composition that we are observing among geochemical terranes indicate that Mercury has a chemically heterogeneous mantle."
"The consistency of the new XRS and GRS maps provides a new dimension to our view of Mercury's surface," Weider added, according to the press release. "The terranes we observe had not previously been identified on the basis of spectral reflectance or geological mapping."
"The crust we see on Mercury was largely formed more than three billion years ago," said Carnegie's Larry Nittler, deputy principal investigator of the mission and co-author of both studies, according to the press release. "The remarkable chemical variability revealed by MESSENGER observations will provide critical constraints on future efforts to model and understand Mercury's bulk composition and the ancient geological processes that shaped the planet's mantle and crust."
"MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft was launched on August 3, 2004, and entered orbit about Mercury on March 18, 2011 (UTC), to begin its primary mission -- a yearlong study of its target planet. MESSENGER's first extended mission began on March 18, 2012, and ended one year later. MESSENGER is now in a second extended mission, which is scheduled to conclude this spring. Sean C. Solomon, the director of Columbia University's Lamont-Doherty Earth Observatory, leads the mission as principal investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA."
