The Physics of Martian Meteorites: How Escape Velocity Gets a Rock to Earth

Image: NASA. Allan Hill 84001. This Martian meteorite show potential signs of life on Mars.

How many times has there been a presidential address to announce scientific evidence for life on Mars? The answer is once — in 1996 president Clinton said that the above rock (ALH84001) showed potential signs of past life on Mars. Of course it wasn’t conclusive evidence and people pretty much forgot about because of things like Y2K and the World Wide Web, and E-Sports.

But I already know what you are asking. How the heck could a rock from Mars end up on the Earth and how do you know it’s from Mars? OK, let’s just get over the easy part — the Mars origin? The short answer is that the tiny bit of trapped gas inside the rock matches the atmosphere on Mars. That’s cool.

Now for the physics part. How do you get from Mars to Earth? Let’s start with some basic models.

Projectiles and Escape Velocity

Suppose you have a rock on the surface of Mars and you toss it into the air (or is air just what we call the stuff on Earth? I don’t know). If the rock doesn’t go too high, we can say that there is a constant downward gravitational force on it that has a magnitude of m*g. Here m is the mass of the rock and g is the gravitational field on the surface of Mars (a value of 3.73 Newtons per kilogram).