38
Some background:
We are able to determine the age of certain rocks and minerals using measurements of radioactive and radiogenic isotopes of certain elements. The most common are U-Th-Pb, Rb-Sr and Sm-Nd. Simply put, the resulting date is the time that has passed from the crystallisation of that mineral. Obviously there are complexities, but there are not ...
19
The answers that have been provided are correct but they're omitting the fundamental issue that explains why they are correct:
When you date a rock you get the point that it solidified, not the point the material came into being. Most rocks on Earth have melted time and again and thus are useless for figuring out how old the Earth is.
17
The only elements that were formed on Earth are those produced by radioactive decay. There are four natural decay chains that start with transuranic elements and none terminate in iron; neither do the decay chains that are artificial or those that result from cosmic radiation. So all of our iron is from the Earth's formation or meteor impacts since then.
14
Let me first correct a small misconception. Where you are talking about 'the magma ocean', you are implying that one exists. This is in fact false. There is no 'magma ocean' in the Earth at the moment (and it has been like that for several billions years). The lavas you see erupting in volcanoes are not coming from a magma ocean. They are coming from either ...
13
tl;dr: Compared to Earth, the atmosphere on Mars is very thin; in addition, it contains much less of oxygen and water (i.e., is very dry). It is much colder there. These conditions may slow down oxidation to an irrelevant rate.
For an object of pure iron, note that according to the English Wikipedia about Mars:
"The atmosphere of Mars consists of about ...
12
Love & Brownlee (1991) put together numerical models for the experiences of micrometeorites entering the atmosphere. They say:
The peak temperatures experienced by submillimeter micrometeoroids rarely exceed 1700°C. Maximum temperature and mass loss rate generally occur at altitudes between 85 and 90 km during ∼1 sec of peak heating. A typical melted ...
12
If water on Earth came from meteorites, why doesn't Mars have substantial water?
First off, that's a conjecture regarding the origin of the Earth's water rather than a known fact. A few times a year or so, a new journal article will appear that argues that the Earth's water is primordial, then another arguing that it came from comets, then yet another ...
11
All the material that eventually formed our solar system is essentially recycled star dust. All iron on Earth was produced by large stars that existed before our Sun formed: the iron was created during nuclear fusion and later released when the parent star(s) exploded, presumably supernova. After our solar nebula had formed and material had been ...
10
why wouldn't that water evaporate on impact, and wouldn't the atmosphere at that time allow the vapours to escape Earth?
The water would very likely evaporate on impact.
However, gravity would prevent the gas phase water molecules from leaving Earth.
The speed of a water molecule must be compared to the escape velocity of Earth (11 km/s) to determine ...
8
Craters actually can be identified by formation of high-pressure materials such as diamonds or stishovites and coesites (varieties of shocked quartz). A good example of this is the Popigai crater in Russia.
Pressures and temperatures at subduction zones are usually too low to form big diamonds but microdiamonds (10-80 microns) were discovered within ...
6
Therriault et al. (1993) suggest an initial diameter of 192-300km. Turtle & Piarazzo (1998) suggest a smaller initial diameter but make no estimates of the depth; they do, however, suggest an impactor of 10-14km diameter. Dietz (1961) describes the impact crater as "40km across and 16km deep" - i.e. does not take into account the extensive erosion that ...
6
I believe the element iron (Fe) is formed by stellar nucleosynthesis.
stellar nucleosynthesis:
it's a process of continuous fusion of the star element to reach heavier and heavier elements. starting from hydrogen (H) till iron (Fe) (the heaviest element the process could reach).
Each 2 atoms of light elements fuse together to form heavier element atom.
...
6
Simple enough, see the meteorites formed alongside the planet, however, since the planet was pretty much a molten soup you can't date it properly, because we can only date it after it cooled down. Meaning that the meteorites hold a much more accurate time-frame since they were not melted down to form our crust! which means their dates (I am using the term ...
6
No, there are nowhere near this many craters on earth.
I think you're asking if there are lots of craters on earth too, but they're hard to see because of vegetation and the oceans. The answer is 'no'. The main reason is that — thanks to plate tectonics — most of the crust is substantially younger (hundreds of millions of years) than the surface of, say, ...
6
Without water there would be no multi-billion year history of plate tectonics, no glacial action, and no fluvial erosion, so the Earth would look unrecognizably primitive. But we have an atmosphere, so all the impact crater rims would be subject to aeolian erosion whilst low points, including crater bowls, would have aeolian sedimentary infill. Volcanic ...
6
The story told in the movie about the meteorite is fictional. Here is a quote:
The Kartenhoff, the oldest in human possession. The very meteorite
which made this crater.
The Kartenhoff is a fictional meteorite (I found few sources about the movie saying so, this is the most comprehensive one). It is not the oldest known to humans, and it didn't make ...
5
Somewhat more from volcanic activity than meteor impacts, but both are important.
The Earth has an iron-nickel ($\ce{Fe}$-$\ce{Ni}$) core that originates from Earth's formation out of the collision of planetesimals which themselves contained iron, rock and ice. The Earth was very hot at that time, and the iron along with some siderophilic elements sunk to ...
5
Impact diamonds
Yes, diamonds can form in meteorite impacts. For this several things need to happen:
A meteorite of the correct size and velocity,
The stuff it hits needs to contain carbon.
If you have a meteorite hitting granite or ocean you're not going to get any diamonds. It usually has to hit something that has biogenic carbon (let's say peat, coal, ...
5
Erosion (especially), viscous relaxation, uplift, crust recycling (in the long term), volcanic activity, filling of the crater with deposits, and distortion by crust deformation (eg earthquakes) are all more important than vegetation, except for very small craters. Other planets and bodies have few or none of these processes.
5
Well the oceanic crust get recycled (through subduction), the oldest being only ~200 million year old, and the average, much younger. And on continents it is not just vegetation but dynamic processes such as collision, rifting, erosion etc. that quickly (in geologic time scales) modify the landscape. Even Chicxulub is not obvious if you just look at the ...
5
So (naturally occurring) elements on earth can only come to be here in three ways.
Either they are formed here via radioactive decay.
They came via meteor.
They were already here.
As mistermarko stated above iron isn't normally formed via radioactive decay, so we're left with the last two choices.
However if we go back far enough, earth itself was a bunch ...
5
Being in a submarine in the ocean is not a good idea because if a large asteroid hits the ocean the shock wave created, and its energy, would be very large. If the submarine survives intact its occupants may not. The occupants could be thrown about so much they liquidize & turn into people puree.
Similarly being airborne in a blimp or airplane would be ...
4
The surface of 'iron' meteorites certainly gets hot enough to exceed the curie temperature, hence the characteristic ablation texture that you see on meteorites in museums. However, that's a surface feature, and wouldn't affect the interior of all but the smallest of meteorites. The tumbling passage through the atmosphere for just a few seconds, coupled with ...
4
You don't need much of a depth to be able to stop a meteorite. Less than 100 m water depth should be enough to stop such a meteorite as shown in the provided link without any particular crater on the sea bed.
The Physics part of the story is provided here;
The rest is mathematics.
Short. At collision all the kinetic energy is transferred to Pressure, ...
4
Gravity anomalies surveys are used to understand the nature of the crustal material. Positive anomalies, or higher gravity than surroundings suggests higher density (heavier) material, (iron, metallic composition) where lower some lighter alternatives.
A good example of this is the Chicxulub impact. The map of gravity anomalies show the crater quite ...
4
It can be problematic to use an earthquake magnitude scale for such purposes. To explain why, you have to look at the different scales how they were produced. The magnitude scales are normally used related to earthquakes on earth, resulting from elastic rebound effects from fault activity. The Richter scale (local magnitude, ML) is, easily said, measured by ...
4
There are craters on the Earth, Arizona's Meteor Crater being perhaps the best-known example. The reason that there aren't a lot more (obvious) ones is that the Earth has lots of dynamic processes, ranging from weather to plate tectonics, that gradually erase them. The Moon doesn't have these things, so craters last for billions of years.
Only top voted, non community-wiki answers of a minimum length are eligible
