Mars Is Going Through a Bit of a Dry Spell. How Do We Know? Rusty Meteorites.

I love meteorites. I have quite a few myself; while I’ve never found one on a hunt I’ve bought them from collectors. There’s a wealth of science in them; many are as old or even older than Earth, having formed in the early solar system. Others are from asteroids that got smacked by other asteroids, the impact sending out shrapnel that eventually impacted Earth.

Of course, other planets get hit by them too. We’ve seen asteroid (or comet) impacts on Jupiter… but we’ve also seen meteorites on Mars. In 2005 the rover Opportunity found a meteorite on Mars, the first time we’d ever seen one on another planet’s surface. Many more have been found since then.

It turns out that finding meteorites tells us more than just about them: They can be used to measure the environment in which they sit, too. A team of scientists used some Martian meteorites to do just that, and discovered something rather interesting: Mars has been very, very dry for the past several million years.

Now, we know Mars is dry. But there’s some evidence for moisture; for example, the Curiosity rover found daily and seasonal moisture exchange between the surface and the atmosphere.

Lots of meteorites contain iron. Some are mostly iron, like a peculiar looking one found by Curiosity just recently. Others are mostly rock (called stony meteorites), but even they are rich in iron. While out in space that iron is pristine, but once it lands on a planet like Mars, weathering can begin, including oxidation. Under water, for example, the iron in meteorites will rust. That can also happen if there’s moisture in the air, too.

Combining all this, the scientists looked at several meteorites found by rovers on Mars, including the chemical analysis done by the rovers (Opportunity has a spectrometer, an instrument that can determine the chemical composition of specimens). They examined the data and found that the meteorites contained oxidized iron — most likely due to Martian weathering.

They also were able to make various assumptions about the ages of the meteorites; for example some are near a crater that has been dated at 50 million years old. That’s an upper limit to their age; they could have been from that impactor, or they may have fallen later. Others were dated using similar methods.

What the scientists found is that the rate of oxidation is at best the same as what it would be at some of the driest places on Earth (like Antarctica), and could be far slower than that, perhaps as much as ten thousands times slower.

What this means is that in recent times, at least at these locations, Mars is dry, dry, dry. Perhaps at other spots the conditions are wetter, but the resting places for the meteorites are positively desiccated. As it happens, Mars may be in an interglacial period, an epoch of time when it’s a tad warmer than usual, with drier conditions at the equator. These meteorites support that idea.

In some sense that’s good news; it means that interpreting the geology at these places is easier, since weathering is low. But it also means that finding extant life on Mars may be harder. Looking at higher latitudes where we know there's ice under the surface is still worth trying, though.

I love all this. One aspect of doing scientific investigation is looking around you (or whatever you’re using to look around) and seeing what there is to see, inventorying it, and then figuring out what you can do with the tools at hand. And in so many cases, nature provides! It’s funny to think that rocks from space can hit a bigger rock from space, and from that we can make all sorts of measurements, even from tens of millions of kilometers away.

But this is why we explore space, why we send rovers and landers and orbiters and flybyers to other worlds. Sometimes we don’t know what we’ll measure until we get there, and we won’t even know how we will measure them until we get there. But once we’re there, entire worlds are open for us to investigate.