Mercury, Venus, Earth, and Mars are terrestrial (rocky) planets. Among these, Mercury is an extreme: the smallest, the densest (after correcting for self-compression), the one with the oldest surface, the one with the largest daily variations in surface temperature, and the least explored. Understanding this "end member" among the terrestrial planets is crucial to developing a better understanding of how the planets in our Solar System formed and evolved.
To develop this understanding, the MESSENGER mission, spacecraft, and science instruments are focused on answering six key outstanding questions that will allow us to understand Mercury as a planet:
- Why is Mercury so dense?
- What is the geologic history of Mercury?
- What is the nature of Mercury's magnetic field?
- What is the structure of Mercury's core?
- What are the unusual materials at Mercury's poles?
- What volatiles are important at Mercury?
To become the first spacecraft to orbit Mercury, MESSENGER followed a path through the inner solar system, including one flyby of Earth, two flybys of Venus, and three flybys of Mercury. This impressive journey yielded the first return of new spacecraft data from Mercury since the Mariner 10 mission more than 30 years ago.
Following the three Mercury flybys and the successful orbit insertion of the MESSENGER spacecraft, the spacecraft ended its mission by crashing onto the surface of Mercury on April 30, 2015. Members of the MESSENGER Science Team continue to examine the wealth of new data now available on the Solar System's innermost planet. Team members are sharing the new results through published articles, presentations at scientific conferences, and at regular Science Team meetings.