How did MESSENGER revolutionize our understanding of Mercury?

Mercury, one of Earth’s rocky neighbors, is one of our Solar System’s most enigmatic planets. It is notoriously difficult to study due to its close proximity to the Sun. In 1973, NASA launched its Mariner 10 mission, which was a robotic space probe that made the first ever close up observations of Mercury. However, less than half the planet was imaged during Mariner 10’s brief flybys. It wasn’t until over 30 years later when NASA launched its MESSENGER mission in 2004 that we were able to study Mercury in depth. MESSENGER orbited Mercury between 2011 and 2015, studying the planet’s chemical composition, geology, and magnetic field. It gave us detailed images of our mysterious neighbor. We spoke to Dr. Nancy Chabot, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory. She was the Instrument Scientist in charge of the camera team on NASA’s MESSENGER mission.

What have missions such as MESSENGER shown us about the evolution of rocky planets like Mercury?

In 2011, MESSENGER became the first spacecraft to go to Mercury and to stay there, and for the next slightly over four years, the spacecraft took a wide range of measurements. MESSENGER provided the first complete global maps of the planet, discovering vast volcanic lava flows, tall cliffs due to the planet shrinking with time, and a whole new type of geologic landform not seen elsewhere in the Solar System, called “hollows”. MESSENGER’s scientific instruments also mapped out the spectral characteristics of the rocks that make up the surface of Mercury, as well as their chemical compositions. MESSENGER determined the topography of the planet, measuring its highest and lowest elevation points. Mercury has an active magnetic field, which MESSENGER mapped, discovering it was unexpectedly offset to the north rather than centered on the planet. Mercury has a very thin “atmosphere”, called an exosphere because it is so thin. MESSENGER mapped how that exosphere changed in a regular fashion with Mercury’s seasons, and how material from a comet might be an important contributor to that exosphere. MESSENGER also found evidence for water in the form of ice contained on the planet closest to the Sun, in the permanently shadowed interiors of craters that never see direct sunlight.

From all of these entirely new measurements of the planet Mercury and all of the new discoveries that MESSENGER made, we began to understand the planet Mercury in detail for the first time. Mercury has a core of metal at its center and, like Earth, that core of metal is partially molten, generating a magnetic field. But unlike Earth, Mercury’s core is a much larger fraction of the whole planet, and the source of Mercury’s giant core is still not completely clear. Prior to MESSENGER, scientific hypotheses for why Mercury’s core was so large predicted that Mercury’s surface should have very low amounts of “volatile” elements, elements that easily turn into gas when heated, such as potassium and sulfur. Instead, MESSENGER measured unexpectedly high amounts of potassium and sulfur on Mercury’s surface. Consequently, new scientific models and hypotheses are in the process of being developed.

Do all planets that formed close to a star have very large metallic cores, or did Mercury have a unique event, like a large impact with another planet-sized body, that stripped off most of the rock and left the metallic core remaining?

Another great thing about missions like MESSENGER is that along with the new measurements and discoveries, there comes new questions to ask. And all of the MESSENGER datasets are publicly available, enabling many people to investigate these new questions, even long after the spacecraft has finished operating. In 2015, MESSENGER used up the last of its fuel and its orbit decayed such that it impacted Mercury. But the legacy of the MESSENGER data lives on, letting scientists around the world continue to study our Solar System’s innermost planet.

Why is it important to study the evolution of rocky planetary bodies in our Solar System?

Understanding how the rocky planets in our Solar System formed provides us with key insights into our planet Earth and our place on our home world. Humankind has always wondered how the planets came to be, and learning more about the four rocky planets we have in our Solar System (and also studying our rocky Moon!) gives us the clues we need to seek answers to that question. This question, and the scientific pursuits it motivates, are important and valuable for this fundamental reason of us wanting to understand how the world we see around us came to be.

However, we are also discovering more and more planetary systems beyond our own Solar System every day, which is incredibly exciting. This leads us to wonder, do rocky planets in these other systems look like our own rocky planets or do they look different? Is there a planet like Mercury out there? A planet like Earth? Studying the rocky planets in our own Solar System provides the basis to understand not just our planets but also others throughout the universe. The rocky planets in our Solar System are ones we can send spacecraft to and investigate in great detail, like MESSENGER did for Mercury. We can then use that information to try to understand rocky planets around distant stars.

What is the most memorable milestone of your career?

I’ve been fortunate to have a career with a few highly memorable moments, but definitely one of those was when the first images from MESSENGER orbiting Mercury came back to Earth and I saw them for the first time. At that point, in 2011, I was the Instrument Scientist for the Mercury Dual Imaging System, MDIS. MDIS was the imaging instrument on the MESSENGER spacecraft, equipped with two cameras – one to image the surface through a set of color filters and another to image the surface in higher resolution. I led the MDIS team to develop the operational plans to take the first images of Mercury from orbit, and when those first images came back, I remember how perfect and beautiful they were! I was really proud of our team and really happy to be a part of that historic accomplishment. That was an amazing milestone in my career!

That first image from Mercury orbit can be seen here.