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Is it possible for life to evolve— and survive— on a planet that is regularly hit by [relatively small— usually no more than a few feet across when they make an impact] meteorites? If so, what effects would this have on the creatures/civilizations living there?

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    $\begingroup$ define "regularly", the earth is hit by hundreds of meteorites a day. $\endgroup$ – John Mar 23 '17 at 1:54
  • $\begingroup$ Larger ones (roughly two or three feet across) would hit every few days or so, and smaller ones would hit more often. $\endgroup$ – Harshmellow Mar 23 '17 at 1:59
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    $\begingroup$ Is this impact frequency per unit area or over the whole of the planet? $\endgroup$ – Joe Kissling Mar 23 '17 at 3:29
  • $\begingroup$ Also what is the composition of the impactors? $\endgroup$ – Joe Kissling Mar 23 '17 at 6:06
  • $\begingroup$ @Harshmellow Still sounds like earth. space.com/… $\endgroup$ – John Mar 24 '17 at 16:00
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The hypothetical planet sounds remarkably like planet Earth.

Small objects frequently collide with Earth. There is an inverse relationship between the size of the object and the frequency of such events. The lunar cratering record shows that the frequency of impacts decreases as approximately the cube of the resulting crater's diameter, which is on average proportional to the diameter of the impactor.[5] Asteroids with a 1 km (0.62 mi) diameter strike Earth every 500,000 years on average.[6] Large collisions – with 5 km (3 mi) objects – happen approximately once every twenty million years.

Source: Impact Event

Remember three-quarters of the Earth's surface is ocean and the land surface area is mostly uninhabited, so meteorites two or three feet across on impact could be happening all the time and we wouldn't notice a thing.

This means life could evolve and survive without too much trouble. Open a window, look outside and appreciate this is the sort of planet you are living on, and Earth didn't have too much trouble allowing life to evolve, survive and thrive here.

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Besides a4android's discussion on the similarities of your planet to Earth...

A sphere has an enclosed volume of $ \frac{4}{3} \pi r^3 $. For a 1 m (a little over three feet) diameter sphere this becomes 0.52 m$^3$. (You can see intuitively that this is the right order of magnitude as the enclosed volume of a sphere is a fair bit smaller than the enclosed volume of the smallest cube that can fully contain the sphere.) Meteorites of course aren't perfect spheres, but to a first order approximation, this works.

Basalt is pretty typical as far as space rocks go, and has an average density of about 2.99 Mg/m$^3$. Thus our sphere has a mass of $ 2.99 \times 0.52 $ Mg or 1,566 kg. Heavy in human terms, but still fairly lightweight as far as above-pebble-sized space rocks go.

Ignoring the atmosphere, an incoming mass will be accelerated roughly to the planet's escape velocity. Earth's escape velocity is about 11.2 km/s. Because the appreciable atmosphere is only a few tens of kilometers thick, the meteorite won't have time to be slowed down appreciably, particularly if on a direct, head-on impact trajectory. (A gracing impact would be different, but the head-on impact is the worst case scenario as far as collision energy is concerned. The absolute worst case would be a head-on retrograde impact, which would effectively add the orbital velocity kinetic energy of the planet and the kinetic energy of the impactor, as velocities are always relative each other. For a retrograde impact, however, you have to figure out how the rock ended up in a retrograde orbit in the first place.)

Ignoring relativistic effects, the kinetic energy of a rigid object is equal to $ \frac{1}{2} mv^2 $. When mass ($m$) is measured in kg and speed (velocity, $v$) in m/s, the resultant value is in joules. The kinetic energy of our impactor is about 98 GJ if it hits Earth in such a way that we can ignore Earth's orbital velocity; say, from straight above one of the poles. Your planet will probably have a different mass and thus a different escape velocity, so you should adjust accordingly.

To put the number 98 GJ in perspective, Wolfram Alpha provides some nice order-of-magnitude comparisons. For example, it is about 27,300 kWh (on the order of what you need to heat two houses for a year in a northernly climate) or the energy obtained by total fission of about 1.2 grams of uranium-235. In other words, a perhaps surprisingly small amount of energy.

Such an impact will pretty obviously have significant local consequences, but the global or even regional consequences should be small especially as this presumably doesn't happen every day. Overall, life should have little difficulty coping with this, but some particular evolutionary strategies are likely to be heavily selected against, such as...

Keep in mind that if your planet is Earth-like, then most of it (in the case of Earth, about 2/3) is covered by water. A water impact near land might make things unpleasant for beings living near the coastline, but if your planet suffers regularly from this, then the resulting selection pressure will pretty quickly cause those beings to move inland.

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  • $\begingroup$ Typical meteors arrive with velocities of 25 to 70 km/s. Escape velocity + earth orbital velocity + speed of earth orbit crossing jupiter orbit kissing. Worst case is escape velocity of earth to edge of solar system. $\endgroup$ – Sherwood Botsford Mar 24 '17 at 19:11
  • $\begingroup$ @SherwoodBotsford Even at 70 km/s, the kinetic energy is only about 39 times larger, $3.837 \times 10^{12}$ J. This can be compared to Little Boy's $63 \times 10^{12}$ J, and look at what the city of Hiroshima, Japan is like now, only about 70 years later. $\endgroup$ – a CVn Mar 27 '17 at 7:40
  • $\begingroup$ Vest pocket nuclear scale bombs make for a tough neighbourhood. Much depends on the density. If they are 1 per square mile per year, I think you have a problem. $\endgroup$ – Sherwood Botsford Mar 27 '17 at 19:11
  • $\begingroup$ @SherwoodBotsford Sure, but we still don't know anything about the impact frequency other than that it is "regular". Which, of course, can mean two different things: "happens according to a regularly repeating schedule" being one (yearly pass through an asteroid thicket?), "happens often" being the other (solar system swarming with small rocks on various orbits?). The former is more correct, the latter is more colloquial, and the OP doesn't seem to offer much in terms of hints as to which meaning is intended. And given that OP hasn't been seen on the site for the last five days... $\endgroup$ – a CVn Mar 28 '17 at 7:19
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Cowboy Bebop Session 09: Jamming With Edward describes an Earth that has become widely dysfunctional due to incompetent governance, culminating in a manmade disaster (partial destruction of Luna) that causes a persistent meteor bombardment of the Earth's surface.

The frequent and repetitive meteor damage is a strain on civilized life everywhere on Earth, and is realistically depicted as the major factor preventing economic recovery.

You should inter alia watch this episode if you plan to write about a bombarded inhabited planet.

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The film "Enemy Mine" features a planet that frequently receives meteor showers, the life that has developed there is very turtle-like.

Assuming your suggested impact size and an extremely high frequency of impacts, the planet would probably have a very rugged surface and a lot of dust in the atmosphere, but other than that it is not too much of a factor if it is constant. Life is generally more dangerous due to risk of direct hits, and evolution would have to adapt to protect life from additional impact debris, but ideally from direct hits as well. I don't believe turtle-like life might be ideal, but rather something closer to insects with hard exoskeletons that are able to dig themselves into the ground for protection, and without the risk of being trapped if their tunnel network collapses. The flora would depend on how competitive it is, it could probably develop to be very earth-like due to low risk of direct hits. I was about to say that I'd expect smaller plants due to the dust in the atmosphere, but the power output of the star might make up for it.

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Much depends on frequency and size. A cubic meter or so of rock takes out a good chunk of town. See Michael Kjörling's answer.

Suppose that a U.S. sized nation got 10 a year. This would likely mean every few years one would hit some place important. It would be a Katrina Hurricane or Los Angeles earthquake sort of event.

If it's one per square mile per year, I don't think it's survivable.

It takes fewer larger rocks.

Near coast ocean strikes are probably more damaging.

As a model consider this:

Pioneer societies had a doubleing time of about 1 generation. E.g. on the average pioneer societies raise 4 kids per couple to maturity.

So you can kill off 50% of the people per generation and probably deal with it.

Assume forest fire type regrowth of impact sites. Within 5 years you will have something densely green on the site.

Assume also that for every square km destroyed outright, that there is a similar area effectively destroyed but spread out. E.g. 1 km2 destroyed and 10 km2 that is 10$ less productive or 3 km2 that is about 30% down.

So you could probably impact 25% of y9our land mass per generation and have it survivable.

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