Tomorrow, on March 18, NASA’s Messenger spacecraft will mark the end of its fourth year in orbit around Mercury, and in a few weeks will mark the end of its mission altogether. So, what have we learned?

If you asked people to pick their favorite planet, Mercury wouldn’t top many lists. It is the smallest planet in our solar system and the closest one to the Sun. It’s temperature ranges, from -280 degrees Fahrenheit at night to 800 degrees Fahrenheit during the day mean that life, as we know it, couldn’t possibly exist there and never will.

It is not a prime candidate for a manned mission, or for a colony or a rover mission. At first glance it isn’t much to look at however there are some interesting features, one of which is ice. It was just last year that Messenger confirmed the existence of ice on the planet closest to the sun.

There are also other unexpected finds. A recent study used the X-Ray Spectrometer (XRS) on Messenger to produce geochemical maps of Mercury. By using X-rays from the sun, the researchers were able to examine the geochemical composition of Mercury’s terrain.

“The consistency of the new XRS and GRS [Gamma-Ray Spectrometer] maps provides a new dimension to our view of Mercury’s surface. The terranes we observed had not been previously identified on the basis of spectral reflectance or geographical mapping,” Shoshana Weider, of the Carnegie Institution of Washington, according to Space.com.

Weiner is lead author of a paper that appears in Earth and Planetary Science Letters this week. Among the terrane features found by the researchers is a 3 million square mile area of the planets surface that shows the highest levels of silicon compared to elements such as magnesium, sulfur and calcium.

One of the mysteries of Mercury is how it became the “Iron Planet”. Mercury has a large core, with heavy concentrations of iron and a thin silicate exterior. While Weider’s research and other data sent back by Messenger might provide some clues, there is still no definitive answer according to geochemist Larry Nittler, deputy principle investigator of the mission.

“I would argue that we still don’t know the answer to that question a whole lot better than we did before we went into orbit,” said Nittler according to Sky and Telescope.

A second new study focuses on a map that used GRS to trace the absorbing of thermal neutrons on Mercury’s surface. The map shows the distribution of the elements that absorb those neutrons. By combining the new data with previous data the authors identified four distinct terrains.

The planet’s most well-preserved impact crater, the Caloris basin, has smooth interior plains with a composition different from other terrains on the planet. According to the researchers, this is caused by a partial melting of Mercury’s mantle in that area.

“Earlier MESSENGER data have shown that Mercury’s surface was pervasively shaped by volcanic activity. The magmas erupted long ago [and] 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,” said Patrick Peplowski.

Peplowski of the Johns Hopkins University Applied Physics Laboratory is lead author of a paper in the journal Icarus.

“The crust we see on Mercury was largely formed more than 3 billion years ago. 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,” said Nittler an author on both papers.

Messengers mission is nearly over. The spacecraft is nearly out of fuel and it will soon crash into the planet and be lost. NASA project scientist Sean Solomon expects the craft to survive for about another six weeks before being lost in the northern hemisphere of Mercury.

However, it will be many years before researchers finish sifting through all of the data returned by the spacecraft.

News, photographs, multimedia and other data on the Mercury mission can be found at nasa.gov/mission_pages/messenger

In 2017, the European Space Agency’s BepiColombo will launch, arriving at Mercury to gather additional information in 2024.