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Yesterday my sister and I have discussed the recent efforts of SpaceX to create a permanent colony on mars.

When coming to the several dangers of creating a habitat on a different planet, we got stuck on the probability of mars getting hit by an astronomical object thats large enough to wipe out life anywhere on the planet. I assume the probability is higher than on Earth, due to Mars being closer to the Asteroid Belt, its size and thinner atmosphere. There are more potential objects to enter martian atmosphere and can be smaller than on Earth to create the same hazardous impact for life.

I have tried to research the numbers, but the only calculations I found were for specific asteroids and their probability to hit Mars.

On this StackExchange there are some questions concerningthe probability of an individual (on earth) or a specific spot (on the moon) getting hit, but I found none for the hit of an object with a certain amount of impact (I hope thats the right word here).

So my question is:

What is the probability of an astronomical object hitting mars anywhere on its surface and kill all people currently living there?

Any impact destroying life-support systems and thus wiping out life on mars indirectly can also be included.

Assume that there is only one habitat, it is built directly on the surface and the first humans have arrived with the latest flight.

If there are any questions about the scope of this question, don't hesitate to ask.

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  • $\begingroup$ Other than the extremely rare case of an object hitting Mars that would otherwise have continued on to Earth, I don't think the proximity to the Belt makes much difference. $\endgroup$ – Carl Witthoft Jul 6 '18 at 15:02
  • $\begingroup$ An impactor probably has to be larger on Mars to have the same effect - Mars is smaller, so the impactor will probably hit slower. $\endgroup$ – Martin Bonner supports Monica Jul 7 '18 at 15:29
  • $\begingroup$ @MartinBonner you're forgetting terminal velocity in Earth's atmosphere $\endgroup$ – Carl Witthoft Jul 9 '18 at 12:50
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We actually have a very good idea of this because the Mars Reconnaissance Orbiter has been orbiting Mars for over a decade. The MRO is, basically, a spy satellite around Mars and is continually taking high-resolution photos of the surface. It has revisted much of the surface, taking pictures multiple times over the years.

As a consequence, we have a very good idea of the rate of formation of craters on Mars. We have seen numerous craters that have formed between visits to a spot, but none are very large. The key thing is that we now have a very solid measurement of the flux of smaller objects onto Mars and can say with certainty, that that sort of impact is way down on the list of perils. (It's probably less than the odds of being killed by a tornado in the Midwest.)

Larger impacts are a bit harder since we haven't seen any, but we have two lines of evidence. First, space debris follows a distribution in size vs. frequency that we've measured, and it's not much different around Mars' orbit than it is around Earth's.

Second, the extensive photography we have of Mars' surface (which we now have mapped better than Earth's!) allows us to count craters and to make estimates of their age. We know that small craters a few yards to tens of yards across are much more common and we know the rate at which they hit, so astronomers count the number of small craters on the floors of larger craters and can make a very good estimate of the age of the larger crater.

From this we have developed measurements of the rate of cratering at all sizes, and, again, the hazard isn't that great.

I'd note that the risk of what on Earth would be civilization-killers is actually lower on Mars because of its lack of an atmosphere. The Chicxulub impact that killed the dinosaurs killed most of them through the superheated steam and air shockwave that went around the Earth followed by the years of Asteroid Winter that followed. Mars' atmosphere isn't heavy enough to support either, so "all" you'd have to do is hunker down and wait out the fall of rock fragments... (Still, it's an event better watched from orbit than from the ground.)

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    $\begingroup$ This was really surprising "Second, the extensive photography we have of Mars' surface (which we now have mapped better than Earth's!)" How come? $\endgroup$ – csiz Jul 6 '18 at 15:37
  • $\begingroup$ @csiz: just speculating, but probably less weather issues to deal with, and no political aspects either. $\endgroup$ – whatsisname Jul 6 '18 at 16:10
  • $\begingroup$ Definitely having no weather helps. Also no forests obscuring the land, nor any glaciers, and also no lakes and oceans! $\endgroup$ – Mark Olson Jul 6 '18 at 16:12
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    $\begingroup$ We have not mapped Mars' surface better than Earth's. 1. There are far more reconnaissance satellites orbiting Earth than Mars, so there are just more photos getting taken overall. 2. Bandwidth: getting data from a satellite around Earth is much easier than getting is from MRO, and if I remember right this is actually the limiting factor in the data return from MRO. 3. Private business are interested in Earth imagery but not really Mars imagery, so for that reason along there is a lot more money heading towards Earth imaging. $\endgroup$ – NeutronStar Jul 6 '18 at 18:46
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    $\begingroup$ Depending on interpretation, the (solid) surface of Mars is better mapped than the (solid) surface of Earth, just because 75% of Earth surface is covered by water and satellites can't photograph it. With a more straightforward interpretation, that statement is likely to be false. $\endgroup$ – Pere Jul 6 '18 at 21:05
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Further to Mark Olson's answer, while an asteroid impact is popularly thought of as the reason for extinction events on Earth, the reality is more prosaic, with volcanism and glaciation being the most common causes. In all cases, including impact events, the loss of species is primarily the result of a dramatic change to the global environment.

The K–Pg extinctions were mostly plants (from loss of photosynthesis), pure herbivores (from loss of plant food) and pure carnivores (from loss of prey animals especially herbivores). A Mars base would have to find solutions to a temporary reduction in solar energy, e.g. to sustain its hydroponics, maintain indoor warmth, etc - but hopefully it would have alternative energy sources (generation or storage) anyway.

Excluding an impact in the local vicinity, a Mars base is less at risk from the immediate aftermath than if a similar impact occurred on Earth. There won't be a giant tidal wave or fire front to contend with. There won't the same global infrared atmospheric firestorm as that of the Chicxulub impact, since the Mars atmosphere is much thinner and has little free oxygen. There will be no burnt forests and no vaporisation of combustible hydrocarbons and sulphur in undersea carbonate rock, each of which contributed to the "impact winter".

Debris impacts might be an issue, but remember the lower Mars gravity will affect the ejecta differently. Now that would make an interesting question in its own right.

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    $\begingroup$ The impact on humans of any atmospheric effects that do happen would also be lower: Cities on Mars already need to be sealed pressure vessels with their own air supply. $\endgroup$ – Peter Cordes Jul 7 '18 at 5:31
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    $\begingroup$ Yes indeed, but it depends on how evolved the colony is. For self-sufficient sealed cities à la Total Recall, the atmospheric risks would be lower but there would be greater risk from fractured seals due to atmospheric shock waves, seismic events, impact ejecta etc. For small colonies of a few sealed buildings, the impact on energy and food production might be the biggest risk, either from direct damage or from reduced sunlight due to atmospheric particles and gases. Significant battery and food storage would mitigate this. $\endgroup$ – Chappo Says Reinstate Monica Jul 7 '18 at 6:58
  • $\begingroup$ @Chappo "reduced sunlight due to atmospheric particles and gases" Well, there's already dust storms, which would need to be dealt with even in the absence of impact. $\endgroup$ – a CVn Jul 7 '18 at 12:31

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