Surprises from Planet Mercury

By Margaret Harris in Rome

The European Planetary Science Congress is taking place this year just a stone’s throw from Rome’s Imperial Forum, so it’s appropriate that the scientific programme is speckled with Roman gods and goddesses – specifically Mercury, Venus, Mars, Titan and Saturn. With missions to all these heavenly bodies dominating the agenda, picking a session to attend wasn’t easy, but in the end I plumped for Mercury, hoping to learn more about the smallest, hottest planet in our solar system.

I wasn’t disappointed. It turns out that NASA’s Messenger mission is already reshaping our understanding of Mercury, even though the spacecraft isn’t due to enter Mercury’s orbit until 18 March 2011. Prior to that momentous date, however, the spacecraft performed three flybys, and the data collected during those intentional near-misses have revealed – among other things – a planet that was far more volcanically active, for far longer, than scientists had previously thought.

An earlier mission, Mariner 10, had helped define the image of Mercury as a dead, cratered lump of rock, more like the Earth’s Moon than a “proper” planet. Now that the Messenger flybys have mapped over 90% of its surface (compared to Mariner 10’s 50%), a more complex picture is emerging. Mercury did, in fact, have active volcanoes early in its history; indeed, volcanic activity was so extensive that the top 5 km of the planet’s crust is mostly the remains of pyroclastic flows, with some impact ejecta (stuff kicked up when meteors and so on hit) thrown in. And some of these flows are quite recent, at least by Mercury’s standards – less than 1 bn years old, which makes it younger than some rock formations on Earth.

Later talks in the same session added to the impression of a surprisingly complex planet, and it’ll be interesting to see whether the surprises keep coming once Messenger gets into its stride next year.