“You don’t drown by falling in the water; you drown by staying there.” -Edwin Louis Cole
Our Solar System is — at least from our perspective — the most well-studied system of planets, moons, asteroids and comets in the entire Universe.
And in this system, the closest planet to our Sun, Mercury, was also one of the most poorly understood planets until very recently. Because Mercury is so close to the Sun, it’s very difficult to view it under good conditions with a telescope; the risk of ruining your optics by exposing them to direct sunlight is tremendous! (Hubble has never imaged Mercury for exactly this reason.) You have to wait until after sunset, and even then, Mercury is so small and distant that ground-based telescopes can barely resolve features on it.
It was only in the early 1970s that we got our first good picture of the planet Mercury, thanks to the Mariner 10 spacecraft, which was the first NASA mission to visit the innermost world in our Solar System.
As we anticipated, Mercury bears a strong resemblance to our Moon: it’s a rocky, heavily cratered, atmosphere-less object in the inner Solar System. But one thing you may not realize about Mercury is that it also has another feature in common with the Earth’s Moon: unlike the Earth, it’s hardly tilted at all with respect to the Sun!
In fact, of all the planets in the Solar System, Mercury has the smallest axial tilt of every one! On Earth, our orbit is a nearly perfect circle around the Sun. Yes, it’s technically an ellipse, but when the Earth is farthest from the Sun, it’s really only 3.5% farther away than when it’s closest to the Sun. This is why our seasons are determined by our axial tilt — a significant 23° — rather than the eccentricity of our orbit. (In fact, Earth’s winter in the Northern Hemisphere corresponds to our closest approach to the Sun; the effect of axial tilt dominates by far!)
But the opposite is true on Mercury; it’s actually the most eccentric planet in the Solar System, a full 52% farther away at aphelion than at perihelion! Because its axial tilt is virtually negligible — at 0.1° — it’s the ellipticity of Mercury’s orbit that determines the seasons on that world. So unlike the Earth, which has each of its poles see six months of daylight followed by six months of night, Mercury’s poles always have the Sun appear just at the horizon, on the border between night and day.
You might be wondering about those shadows on the pole itself; this is actually a very similar story to the Moon. The Earth’s Moon is tilted at just 1.4° with respect to the Sun, meaning that the Sun’s rays are always grazing the poles of the Moon.
Because the Moon is heavily cratered, including at the poles, that means that the polar regions that have deep enough craters close enough to the pole itself will never be exposed to direct sunlight!
These regions of the Moon that live permanently in shadow are worse than the Thunderdome: two men enter, no man leaves. The reason the Moon has no atmosphere is because direct sunlight on the surface of the Moon is energetic enough to impart any atom or molecule it strikes with enough velocity to escape the Moon’s gravity. So the entire surface of the Moon is devoid of water molecules, hydrogen gas, oxygen, methane, ammonia, or nitrogen. It’s only the heavy rocky particles of the Moon’s outer surface that remain behind.
But inside a permanently shadowed crater, the temperature is somewhere around a cool 50 Kelvin, and once you fall in, you’re never coming out.
That includes materials from comets, asteroids, and anything else that lands on the surface of that world. And of course, comets and asteroids are loaded up with our favorite compounds: water, methane, and ammonia among them!
So it’s no surprise that we’ve found both water and organics (any molecule with a carbon bond in them) inside these permanently shadowed craters on the Moon. And it should come as no surprise that — with the advent of dual imaging from Messenger and the Arecibo Radio Observatory on Earth — both water and organics are found in the pits of these permanently shadowed craters on Mercury, too!
Don’t get me wrong; it’s a great discovery! But it’s also a completely expected discovery; bigger news would have been if they hadn’t discovered water and organics in these permanently shadowed regions. As scientist Sean Solomon stated:
“For more than 20 years the jury has been deliberating on whether the planet closest to the Sun hosts abundant water ice in its permanently shadowed polar regions. MESSENGER has now supplied a unanimous affirmative verdict.”
This wouldn’t work on Venus or Jupiter, whose atmospheres would prevent permanently shadowed craters from harboring material indefinitely, but it does confirm what we thought, and allow us to make a convincing leap to other Solar Systems with some certainty!
Around any star, any rocky planet with no atmosphere and a sufficiently small axial tilt should have permanently shadowed craters at its poles, which will contain ices and other frozen materials common to that Solar System. Finding this water and these organics in the permanently shadowed craters of Mercury isn’t a surprise; it’s exactly what this picture predicts. Unless you’re super-close to your parent star (and remember, folks, that Mercury is still over 50 million km from the Sun on average), that sunlight won’t be able to get in there and kick those particles out!
And that’s why there’s not only water on Mercury, there’s probably water on every Mercury- and Moon-like world out there!