Effect of high frequency of impact events on geography,climate, and environment

Question

Earth sized planet in the goldilocks zone. Similar mineral composition and surface water. Photosynthetic life evolves and thrives on land and sea. Atmosphere and climate comparable to the beginning of the Pleistocene glaciation period on Earth. Over a period of a half million years the planet begins to experience a higher rate of impact events from meteorites.

What effect would this have on the planet's geography, climate, and environment?

Upper atmosphere detonation of “stones” (stony asteroids) and “irons” (iron asteroids), <3% which penetrate to surface, with yield <10 megatonnes occurs daily.

Meteorites less than and equal to 75m with 10-100MT yield impact annually. Irons make craters up to 1.5km in diameter (Barringer Crater); Stones produce air-bursts (Tunguska). Land impacts could destroy areas the size of a city (Washington, London, Moscow.)

Irons and stones up to 160m in diameter produce ground-bursts every 10 years. 100-1,000MT yield, 3km crater. Ocean impacts produce significant tsunamis. Land impacts destroy areas the size of a large urban area (New York, Tokyo).

Objects up to 350m with 1,000-10,000MT yield impact every 100 years. Ocean-wide tsunamis are produced by ocean impacts. Land impacts leave 6km craters and destroy areas the size of a small state (Delaware, Estonia).

Answer 1

Upper atmosphere detonation of “stones” (stony asteroids) and “irons” (iron asteroids), <3% which penetrate to surface, with yield <10 megatonnes occurs daily.

Iron rich meteorites exploding in the atmosphere won't cause massive amount of aerosols - the density is too high for the particles to stay airborne even in atomic sizes (CO2 molar mass - 44, iron molar mass - 56). They actually may contribute to an algal bloom by fertilisation. Nope, little to no effect, even if daily.

Meteorites less than and equal to 75m with 10-100MT yield impact annually. Irons make craters up to 1.5km in diameter (Barringer Crater); Stones produce air-bursts (Tunguska). Land impacts could destroy areas the size of a city (Washington, London, Moscow.)

Meh, unpleasant if you live nearby, but the impact has limited effects on geography or ecology. The affected vegetation will regrow in 20y tops and 500000 visible craters may appear cute from airplane but won't change the geography.

Irons and stones up to 160m in diameter produce ground-bursts every 10 years. 100-1,000MT yield, 3km crater. Ocean impacts produce significant tsunamis. Land impacts destroy areas the size of a large urban area (New York, Tokyo).

We are in the Krakatoa eruption energy range (200 MT). About the same ecological impact as the previous category. May create areas improper for vegetation on 100y scale, but still no dice for climate change, much less for an extinction event.

Geographic changes? Well, Earth has about 1500 volcanoes that erupted in the last 10000 or so years, another 48000 craters in 500,000 years won't make a too much of a difference. We're also in the range of 9 Richter scale earthquakes. which can cause permanent changes in ground topography, but not the geography.

Objects up to 350m with 1,000-10,000MT yield impact every 100 years. Ocean-wide tsunamis are produced by ocean impacts. Land impacts leave 6km craters and destroy areas the size of a small state (Delaware, Estonia).

Moving up the scale, the things get a bit unclear:

• The energy Earth receives from Sun every day is 430e18J.

• 1Mt TNT is 4.18e15J. 1000 - 10,000Mt TNT is 4.18e18J - 41.8e18J. Or 1-10% of daily Earth energy intake. In one go, concentrated - at the max end of the range, non-trivial percentage of the energy will go into high speed ejecta and IR radiation, some probably leaving Earth, but the rest will stay with us.

• that Chicxulub was simply ginormous at 11-80km diameter - well over the 350m in the question. Didn't cause major geographical changes (the crater itself is only 200km in diameter) but the impact winter would have been horrendous. Glad to see it excluded from the question.

• 99942 Apophis - estimated at 370m with an approx energy impact of 1,200 megatons. Hitting a sedimentary formation, estimated to create a 5.1-kilometre impact crater; or an incoherent short-range tsunami with a potential destructive radius of roughly 1,000 kilometres. The impact is estimated as survivable for the humanity, without triggering an impact winter

• Torino scale lists some other potential impactors, unfortunately not many of them have estimations on the effect they'll have (e.g. (415029)_2011_UL21 - 2.5km, (242450)_2004_QY2 - 5.5km)

So, at the lower end of the scale, no impact winter, significant chance in the local topology extending to 1000km, low probability of modifying the tectonic plate dynamics. Due to the "1 in a century frequency", the vegetation is going to survive and recover.

At the higher end of the scale? Well, I don't know, nasty things may happen given enough such events, I doubt it will lead to total extinction though.

Answer 2

This gives me a very simple answer to your question of 'what will be the effect on the geography, climate, and environment'? and that is 'What geography, climate', and environment'? Naturally, since the impacts are at random, a mere 510 million square klicks of destruction won't guarantee destruction of all the Earth, but there's a few things we know.

First, there will be over a trillion square kilometers of destruction on the Earth before anything started showing up on the land.[Calculations below.] Naturally, not all of that will be on land, but considering that the earth is a mere 0.05% of that area, the land shall not be spared. Not to mention that the sea effects caused by this will more or less destroy the land and wipe it barren. The land will be, more or less, uninhabitable, especially since all these meteors rain iron over the land they hit which organic life cannot use. So the geography will be kind of like ours, except there will be craters all over the place and also everything fragile and most thing non-fragile will be destroyed.

The climate isn't going to be spectacular either - these impacts will drive things into the atmosphere that we don't want in the atmosphere if they hit mineral deposits. See, the Chicxulub meteor/comet which destroyed the dinosaurs did so in part because it hit some mineral deposits and spread some nasty stuff in the atmosphere. In other words, if one of these hits a sulfur deposit - there goes all the sulfur in the atmosphere. Not to mention that there's an above-average chance that this frequent bombardment might trigger more volcanoes. And the atmosphere will be taking heat from all these projectiles. Among other things. Projected forecast: periods of intense heat with one atmosphere composition followed by periods of intense cold when the atmosphere gets clogged with a difference one, and both filled with bad chemicals.

And, with hostile geography and climate, there won't be an environment. Well. Not on the surface, anyway. Or the surface area of the water. But, even with all this going on, deep sea life forms as well as microscopic sea life will still be able to survive.

Calculations:

• Surface Area of the Earth: 510.1 million square kilometers.

• Age of the Earth: Estimate 4.5 billion years.

• Life first showed up on Earth: Estimate 3.5 billion years ago

• Plant Life on the surface of Earth: Estimate 470 million years ago

• Area Destroyed In A Year: 1.77 square kilometers. (Based on a blast with 1.5 kilometer diameter blast.)

• Area Destroyed In Ten Years: 24.77 square kilometers. (Based on 10 years + a 3.0 kilometer diameter blast.)

• Area Destroyed in One Hundred Years: 275.97 square kilometers. (Based on 10 10 years + a 6.0 kilometer diameter blast.)

• Area Destroyed in 470 Million Years: 1.297 billion square kilometers

• Area Destroyed in 3.5 Billion Years: 965 billion square kilometers

• Area Destroyed in 4.5 Billion Years: 1.242 trillion square kilometers