NASA's MESSENGER spacecraft data reveals presence of chromium in Mercury's core

The spacecraft acquired measurements that revealed the planet varies chemically from Earth — despite both having metallic cores. 

Arizona State University scientists have now used data from the probe as well as theoretical models to quantify and map the amount of chromium on Mercury's surface. 

Presence of chromium 

In general, chromium imparts color to gemstones such as ruby and emerald. Furthermore, chromium protects metalwork against corrosion. 

Apart from that, the element may be found in a variety of chemical states. 

According to the study, its abundance can reveal "chemical conditions under which it was incorporated into rocks." 

This knowledge is critical for understanding Mercury's origin and geological history. Mercury is an oxygen-deficient planet that may have evolved from different building components in the early solar system. 

The researchers conducted laboratory tests to investigate the reactivity of chromium to various levels of oxygen in the system. They also created a computer model to study the distribution of chromium in Mercury's various layers (crust, mantle, and core). 

According to researchers, the majority of chromium is held within the planet's huge metallic core, and its abundance presumably increased as the planet’s oxygen levels dropped. 

“Our model, based on laboratory experiments, confirms that the majority of chromium in Mercury is concentrated within its core. Due to the unique composition and formation conditions of Mercury, we cannot directly compare its surface composition with data obtained from terrestrial rocks. Therefore, it is essential to conduct experiments that simulate the specific oxygen-deficient environment in which the planet was formed, distinct from Earth or Mars,” said Asmaa Boujibar from Western Washington University, and co-author of this study, in an official release. 

This study is important to estimate the elemental composition as well as the geological processes at work. 

The study was published in the Journal of Geophysical Research Planets. 

Study abstract:

Mercury, the innermost planet, formed under highly reduced conditions, based mainly on surface Fe, S, and Si abundances determined from MESSENGER mission data. The minor element Cr may serve as an independent oxybarometer but only very limited Cr data have been previously reported for Mercury. We report Cr/Si abundances across Mercury's surface based on MESSENGER X-Ray Spectrometer data throughout the spacecraft's orbital mission. The heterogeneous Cr/Si ratio ranges from 3.6 × 10−5 in the Caloris Basin to 0.0012 within the high-magnesium region, with an average southern hemisphere value of 0.0008 (corresponding to about 200 ppm Cr). Absolute Cr/Si values have systematic uncertainty of at least 30%, but relative variations are more robust. By combining experimental Cr partitioning data along with planetary differentiation modeling, we find that if Mercury formed with bulk chondritic Cr/Al, Cr must be present in the planet's core and differentiation must have occurred at log fO2 in the range of IW-6.5 to IW-2.5 in the absence of sulfides in its interior and a range of IW-5.5 to IW-2 with an FeS layer at the core-mantle boundary. Models with large fractions of Mg-Ca-rich sulfides in Mercury's interior are more compatible with moderately reducing conditions (IW-5.5 to IW-4) owing to the instability of Mg-Ca-rich sulfides at elevated fO2. These results indicate that if Mercury differentiated at a log fO2 lower than IW-5.5, the presence of sulfides whether in the form of a FeS layer at the top of the core or Mg-Ca-rich sulfides within the mantle would be unlikely.