Bad Astronomy

Mercury’s hot and cold south pole

The MESSENGER spacecraft, orbiting Mercury for nearly a year now, took this pretty nifty shot of the tiniest planet’s south polar region, showing deep, dark craters in the Goethe basin:

This region is about 300 km (180 miles) from the true south pole of the planet. On Earth that might be a cold spot, but on Mercury, cold spots are hard to come by.

… however, see how dark those craters are? Since they’re near the pole, the Sun never gets far above the horizon for them, and the crater floors are shrouded in perpetual darkness. That does make them cold! Well below the freezing point of water, it’s thought. Interestingly, radar observations of Mercury have indicated something in the crater floors is highly reflective, and water ice fits that bill. It’s not at all confirmed, but it’s entirely possible Mercury – a planet hot enough in the open Sun where zinc can exist as liquid lakes on the surface – might have frozen lakes of ice locked in crater bottoms near its poles!

While gazing idly at this picture, another thought popped into my head. Mercury is small, and has lower gravity than the Earth. The impact speed of an asteroid depends partly on the gravity of a planet, since the asteroid will accelerate as it falls to the surface. Earth has more gravity, so you might expect impact speeds to be higher here than on Mercury. The impact speed of an object falling to Earth would be 11 km/sec, but only 4.3 km/sec for Mercury.

However, that’s not the entire case. Remember, everything is in motion in our solar system. Mercury orbits nearer the Sun, so it moves a lot faster in its orbit than Earth does; about 48 km/sec as opposed to Earth’s 30 km/sec. So in reality the impact speeds of objects can be much higher on Mercury: 48 km/ sec + 4.3 km/sec = 52 km/sec, compared to 30 km/sec + 11 km/sec = 41 km/sec for Earth.

Moreover, the energy released on impact – which is what carves out the crater and causes widespread devastation – increases as the impact velocity squared. So for a given size/mass of an impactor, hitting Mercury yields an explosion 1.6 times greater than it would on Earth!

That’s on average; the direction the asteroid comes in makes a difference (it might “catch up” with a planet, making a slower impact speed, versus coming in from head on and doubling (or more) the speed), but still, it’s interesting to me. I guess real estate agents are right: it’s not the size that matters, it’s the location.

Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Related posts:

- Jaw-dropping mosaic of Mercury’s battered, beautiful face
- More Mercury
- MESSENGER’s family portrait
- Watermelon planet (a personal favorite of mine)