Life on planet regularly hit with meteorites

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.

posted about 7 years ago

Canina‭

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