Category Archives: Climate

The Martian: Could a Martian Dust Storm Maroon Mark Watney?

This is an analysis of the technical, scientific and engineering aspects of Mark Watney’s Mars survival story in The Martian by Andy Weir. The primary motivation for criticizing the technical aspects of The Martian is to share the level of scrutiny PHYSIC has been through. I could share early drafts of PHYSIC to demonstrate this, I suppose, but I’m not brave enough to show how thoroughly inadequate my early drafts are. So poor Andy gets another jerk picking on his very popular book instead.

The first technical glitch in The Martian that I’m going to address is at the very beginning of the story. It isn’t an obvious problem. There is quite a bit of room for debate. What really bothers me is the lost opportunity. Weir needs a reason for Mr. Watney to be abandoned on Mars, left for dead by the rest of the crew and without means to communicate. Mars is well known for massive dust storms. Spirit and Opportunity were in danger of shutting down due to insufficient power because their solar panels were substantially blocked by dust from a massive dust storm. Fortunately, dust devils ended up clearing them off and missions continued in full health.

Weir inflicts the mother of all dust storms on our protagonist, winds gusting to about 175KPH/105MPH. That sounds really scary. Winds of that speed would be rated as a category two hurricane. Weathering such conditions in what amounts to a tent in an environment where a hole could end a mission and maybe lead to someone’s death is treacherous. One would think that anything with a large surface area or a flimsy connection would be in danger of being yanked off of the artificial habitat, a comms tower, or the return vessel. Weir reasonably states that the maximum wind speed that the mission is rated for is 150KPH/90MPH, winds that would still be rated as a category one hurricane.

The problem with this entire line of reasoning is that we aren’t on Earth anymore. A windstorm on Mars is nothing like a wind storm on Earth. Atmospheric pressure on Mars is roughly 1% of that on Earth. High speed winds can’t deliver the same quantity of energy on Mars as they do on Earth. To understand how wind is different on Mars, we need to find the appropriate data and equations to give us a sense of scale. First, we need to understand the physics, so we know what is important. The equation for drag works quite nicely.

Force of drag is proportional to fluid density times velocity squared
F ~ p*v*v

I removed the constants and the cross-sectional area from the equation, because we merely want to get a relationship between the force of wind on Earth versus the force of wind on Mars. From this equation, we find out that we need to find out the difference in atmospheric density(p) between Mars and Earth. By referencing the NASA Mars Fact Sheet and Wikipedia, we find out that on Mars p=0.02 and on Earth p=1.2. So, for the same velocity, wind is approximately 60 times as powerful on Earth as on Mars.

If one wants to find out what a 105MPH wind on Mars would feel like, we still have to do some calculating:

Air density on Earth times velocity squared equals air density on Mars times velocity squared
Pe*Ve*Ve = Pm*Vm*Vm
Ve*Ve = Pm/Pe*Vm*Vm = 1/60*105*105 = 183.75(mph)^2
Ve = 13.6mph

Well, that was disappointing. Less than 14MPH. Unless that satellite dish was unsecured, it isn’t going anywhere. If it was sufficiently heavy, it isn’t going anywhere.

It would be disingenuous for me to leave the analysis at this point. The process of a wind storm building up, called saltation, absorbs energy from the atmosphere. Moving the mass of the sand and dust around robs the air of momentum. Therefore, our equation doesn’t accurately represent the force delivered in a 105MPH dust storm on Mars. We really need to understand what is going on with those sand particles before we make a definitive statement that 105MPH Mars wind is not a big deal. The best that I can say with certainty is that there doesn’t seem to be enough energy in the Mars atmosphere to deliver punishing storms and NASA is inclined to agree with me.

This information is an impediment to the story. I’m not certain I’m correct, either. Mostly, I’m bothered by something alien in nature being written off as similar to home. Weir spends no time on the dust storm, as though it isn’t different. How surreal would it be to walk in a massive dust storm with hundred mile per hour winds and not have it completely knock you over? Certainly, a 13MPH gust would not be insignificant when gravity is only 38% of Earth, but explaining these kinds of experiences is the crux of the experience of being a Martian. When these details, the only ones that really matter, aren’t attended to, I feel cheated. What was the point of trying to write a technically accurate account?

What still needs to be addressed is how to maroon Mark Watney under these new conditions. My alternate solution isn’t perfect either. Mark went outside during the storm to attend to the structure holding the communications array to make sure all was secure. As he is working, there is a lightning strike. The dish is destroyed, Mark’s suit and personal communications are damaged and he is dazed. He walks off in the wrong direction in a Martian electrical storm. The rest of the crew looks for him, wrongly concludes he was killed and end up deciding to evacuate.

There are still problems to second guess. I’m not sure how lightning works on Mars. It’s mentioned in the saltation literature as a possible driver of the enormous dust storms. There is also question as to how concerned the crew would be about a lightning strike on the habitat or the Mars ascent vehicle. I still like it better than the notion that there are delicate parts on the MAV. I would definitely think about this more over the long term to come up with a better story.