Our Solar System: Mercury

As a kid I was always picking up rocks and wondering at the diversity of them all. Where did they come from? And family camping trips gave me a sampling of the varied landscapes of California and the West. When I found out I could combine my love of the outdoors with the study of rocks and landscapes, I knew I had found my career. My graduate work led me into the use of remote sensing – using satellites to study the Earth and by extension, other planets as well. I became interested in using what I learned from applying remote sensing techniques on Earth to the other planets. 

These are worlds that are not like our own. Pockmarked by billions of years of impacts, some large enough to fragment a planet. Poison atmospheres and sulphuric acid rain. Rivers and lakes of liquefied natural gas. Gas balls so massive that hydrogen is squeezed into a metal in their cores. Cold so intense that nitrogen freezes and forms flowing glaciers. Frozen moons that harbor oceans beneath their icy surfaces. Places where a jump will send you into orbit. 

Science fiction? No, it’s our neighborhood, the worlds of our own solar system. 

Orbiting an average yellow star toward the outer fringes of the Milky Way galaxy are eight planets and some leftovers. One of those planets we call home. In order to learn more about our neighborhood, we’ve launched spacecraft to visit all the planets as well as some of those leftovers. We found the inner planets, near the Earth, were somewhat like the Earth – they had solid rocky surfaces and were of a similar size. The outer four planets, however, were completely different: huge and made up of gas and ice. The many moons of the outer planets held interesting surprises, though. In the next few months, we’ll visit all the planets as well as those leftovers (asteroids, comets, Pluto, and beyond). We’ll see how different conditions in the solar system led to very different results and we’ll learn more about the Earth and our place within the solar system. And we’ll consider other solar systems beyond our own; we now think that there are more planets in the Milky Way galaxy than there are stars…

But the visit today is to the innermost planet, Mercury. 

In the mid 1800s just after Neptune, the last of the planets to be discovered was sighted, calculations by Urbain Joseph Le Verrier, the most famous astronomer of his day, predicted a new planet circling the sun inside the orbit of Mercury due to strange wobbles it experienced. He called it Vulcan. For the next 60 years people set up telescopes searching for the elusive planet. Some claimed to see it. Then Albert Einstein came along with his new theory of general relativity and explained the odd motions of Mercury through the bending of its image by the sun’s gravity. That day Vulcan vanished, leaving Mercury as the closest planet to the Sun. 

You’d be forgiven if you looked at a satellite photo of Mercury and assumed it was our moon. It’s only slightly larger and pockmarked by impact craters just like our satellite. There are differences, but it took many years to fully understand them as the first spacecraft to visit Mercury, Mariner 10, merely flew by several times as it orbited the sun and, as luck and orbital mechanics would have it, was only able to photograph the same side of Mercury each time it passed by. Only half of the gigantic Caloris impact basin was tantalizingly imaged. Finally, in 2008, the MESSENGER mission flew by and then entered orbit around Mercury for an extended visit, allowing complete photography and inspection with many state of the art instruments. Mercury was revealed. 

Besides being closest to the sun, Mercury has other special attributes. As it’s much smaller, it wasn’t able to keep any atmosphere it might have had in the beginning, so meteorites were able to impact at will, covering the surface with craters. However, extensive lava flows have poured forth, much like the Moon’s dark lava plains, and covered some of the craters. In addition to the impact craters and lava flows, huge faults stretch across Mercury. These all have large vertical offsets and show that Mercury has actually shrunk by about 10 km (6 miles) since it formed. 

Mercury was once thought to show only one face toward the Sun, but now we know that its ‘day’ lasts 59 Earth days (its year is 88 Earth days long). Because it’s so close to the sun and it has no atmosphere to even out the temperature differences, the sunlit side roasts at about 450°C (840°F) while the night side freezes at -170°C (-275°F). Mercury’s rotation axis doesn’t tilt like the Earth’s so there are no ‘seasons’ and this led to an interesting discovery. Powerful radar antennas on Earth were able to image Mercury and they found a unique signature in deep craters at the poles. Comparing those signatures to known targets, the investigators found that water ice was the only substance that made sense. On the hottest planet in the solar system! When Messenger arrived at Mercury some years later, it made a complete 3-D topographic map of the planet and found that the craters at the poles were forever shaded, causing them to act like non-stop freezers, trapping any water that appeared at Mercury, such as from comets or outgassing from the interior. 

MESSENGER ran out of fuel and was crashed into Mercury in 2015. But another mission is on its way to continue study of the planet: BepiColombo is a joint project between the European and Japanese space agencies and should arrive at Mercury in 2021, settling into orbit in 2025. It will concentrate on the geology and internal structure of the planet as well as its very thin atmosphere and surprising magnetic field. 

Mercury is periodically visible from Earth, although it’s always close to the Sun. It’s now disappearing into the rising sun and will next appear in late January 2021, just after sunset. 

Next time: the veiled planet Venus.