What would happen?

[onejayhawk]

One post apocalypse story begins with a comet crashing in the Pacific ocean. Because of the water forced into the atmosphere, its rains day and night for weeks, with unbroken cloud cover for months thereafter.

Comets are snowballs, mostly various kinds of ice. The tail we see is steam escaping, which is important. Comets evaporate. Unless there is a big enough core to produce measurable gravity, the comet will boil away. So even a small comet is pretty big, say 2 km in diameter or 4 km^3 at a minimum.

That's a lot of water, 4 Billion tons, but the real issue is the kinetic energy. Earth's orbital velocity is 30 km/s. The Comet could range from 20-50 km/s. The combination is complicated by a lot of unknowns. Say 50 km/s for convenience. Doing the math that's 5 * 10^21 joules, roughly 1 million megatons explosive yield.

Assuming this occurs over deep ocean water, what really happens, short, intermediate and long term (days, months, decades)?

J

 

[Perfection]

[Wiki]Impact event[/WIKI]

 

[Cutlass]

I was trying to remember what this was called and where it is. This version might not even be the one I saw before.

Here's a few other versions. You can plug in variables to play with events and outcomes.

http://www.killerasteroids.org/interactives/impact/impactCalc1024.html

http://down2earth.eu/impact_calculator/input.html?lang=English

 

[onejayhawk]

I was trying to remember what this was called and where it is. This version might not even be the one I saw before.

Here's a few other versions. You can plug in variables to play with events and outcomes.

http://www.killerasteroids.org/interactives/impact/impactCalc1024.html

http://down2earth.eu/impact_calculator/input.html?lang=English

Interesting. I had not seen these. According to the 1st, I underestimated the blast by an order of magnitude. Seismic effect is a Richter 9 earthquake. Given 1500 m deep water, the crater is 60 km across and complex (whatever that means). The sound will be heard at 70 dB 5000 km away. At that distance a 9-18 m wave is expected.

A mid south Pacific strike would probably wipe out every southern hemisphere city on the Pacific coast. Exact placement might spare Sydney and Melbourne, but heavy population areas in Indonesia , the Thai peninsula, and Chinese coast would be largely swamped in the massive wave. Further north, Philippines, Korea, southern Japan and coastal California would take significant damage. Hawaii and the Island states would be gone. Long term damage, due to the 100+km^3 of vaporized water, is unclear, but weather will almost certainly be effected globally.

In many ways, this is the best case scenario--small comet, least damaging position.

J

 

[Borachio]

http://www.purdue.edu/impactearth/

The Earth is completely disrupted by the impact and its debris forms a new asteroid belt orbiting the sun between Venus and Mars.
100 percent of the Earth is melted
The impact does not make a noticeable change in the tilt of Earth's axis (< 5 hundredths of a degree).
Depending on the direction and location of impact, the collision may cause a change in the length of the day of up to 4.66 hours.
The impact does not shift the Earth's orbit noticeably.

Transient Crater Diameter: 17200 km ( = 10700 miles )
Transient Crater Depth: 6080 km ( = 3780 miles )
Final Crater Diameter: 61400 km ( = 38200 miles )
Final Crater Depth: 8.18 km ( = 5.08 miles )
The final crater is replaced by a large, circular melt province.
The Melt volume is 1.65 times the crater volume
At this size, the crater forms in its own melt pool.

I think I over-did it a tad.

 

[timtofly]

Did you shoot for the moon?

 

[Borachio]

Yup. Just slam-bang the old Moon into the Earth.

I'm surprised how shallow the final crater is. Perhaps because there's so little of the Earth left.

But the diameter of the crater is wider than the Earth itself. Very strange.

Yet the Earth stays in the same orbit and at the same angle despite it all.

 

[onejayhawk]

Yup. Just slam-bang the old Moon into the Earth.

I'm surprised how shallow the final crater is. Perhaps because there's so little of the Earth left.

But the diameter of the crater is wider than the Earth itself. Very strange.

Yet the Earth stays in the same orbit and at the same angle despite it all.

You overloaded the parameters, so you get nonsense results. This is the system's version of smashed to rubble. Also, really large craters are elastic for a few seconds. You get bounce back while the rock is molten. That is why the final crater is so shallow.

This is from minimalist scenario:

The air blast will arrive approximately 50.5 minutes after impact.
Peak Overpressure: 30700 Pa = 0.307 bars = 4.35 psi
Max wind velocity: 64.3 m/s = 144 mph
Sound Intensity: 90 dB (Loud as heavy traffic)

Damage Description:
Wood frame buildings will almost completely collapse.
Glass windows will shatter.
Up to 90 percent of trees blown down; remainder stripped of branches and leaves.

The impact-generated tsunami wave arrives approximately 2.81 hours after impact.
Tsunami wave amplitude is between: 29.6 meters ( = 97.1 feet) and 59.2 meters ( = 194 feet).


That is at a distance of 1000 kn (700 miles). Tornado force wind. The sound will literally travel around the world. The question that comes to me is whether the crust is penetrated. Earthquake and volcanic activity could be triggered far from the impact.

Note that one of the recorded impact event yielded a crater 180 km across. This occurred in shallow water off Mexico. Here is the blast damage from 1000 km:

Time for maximum radiation: 11.5 seconds after impact
Visible fireball radius: 151 km ( = 94 miles )
The fireball appears 34.4 times larger than the sun
Thermal Exposure: 4.15 x 10^8 Joules/m^2
Duration of Irradiation: 49.7 minutes
Radiant flux (relative to the sun): 139


Effects of Thermal Radiation:
Clothing ignites.
Much of the body suffers third degree burns.
Newspaper ignites.
Plywood flames.
Deciduous trees ignite.
Grass ignites.

This crater is one of the extinction events. It was discovered during a mapping expedition in the Yucatan jungle. When water springs were marked on the map, they appeared to be in a line. Closer examination found it was the arc of a circle, with a radius of close to 100 km. Much of the circle is in the Caribbean Sea. The ground water was percolating around the buried glass bowl of the crater bottom.

j