The Enigma of Mars and Earth

[dougettinger]

Let's consider a special selection of larger celestial bodies in our solar system that have hard surfaces. I am thinking of the terrestrial planets, the Moon, the Jovian moons, Titan, Triton, Pluto, and other Kuiper Belt bodies as large or larger than Pluto. From this special list two bodies standout that have very unique characteristics. They are Mars and Earth.

Both of these planets demonstrate plate tectonics and hemispheric dichotomies like no other celestial body on my above listing. Why ? The suspected reasons especially for Mars is that a Pluto size body struck its surface some 4 billion years ago. The plate tectonics for Mars has been inactive for almost 4 billion years but its volcanic upland bulge is quite apparent. Recently, Earth is suspected to have received a glancing blow by a Mars size body that then went on to form the Moon. My simple hypothesis is that plate tectonics (active or inactive) and hemispheric dichotomies are a big clue that major (direct hit) collisions have occurred by proportionately large impactors compared to the primary. We are not just talking about meteorite bombardments. Is my hypothesis worthy ?

The subject collisions not only penetrated the already differentiated surfaces but severely cracked the hardened crust globally thereby creating plate tectonics. Materials from the mantle and the impactor oozed from the crustal penetration(s) to form higher uplands or continental crusts on top of the already existing crusts. That is the way I see these events occurring. So why haven't I heard from geophysists and planetary scientists that these much higher granitic continental crusts on Earth were formed by some ominus collision ? Scientists seem to be silent on this matter. What did cause Earth's continents that appear to be very similar to the uplands on Mars ?

Doug Ettinger
Pittsburgh, PA

Some rocks found on Earth are dated to be 4.6 bys old. These few locations could have been uplifted, original oceanic crust that was not re-melted. I thought I would nip this fact in the bud.

Other strong evidence for major collisions on Mars and Earth are the two irregular satellites of Mars and Earth's nearby planetary neighbor, the Moon, that is pock marked by a large meteorite bombardment.

[neufer]

Two bodies standout that have very unique characteristics. They are Mars and Earth.
Both of these planets demonstrate plate tectonics and hemispheric dichotomies like no other celestial body.>>

What about Venus?

<<Venus has several times as many volcanoes as Earth, and it possesses some 167 giant volcanoes that are over 100 km across. The only volcanic complex of this size on Earth is the Big Island of Hawaii. However, this is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years, while Venus's surface is estimated to be about 500 million years old.

About 80% of Venus's surface is covered by smooth volcanic plains. Two highland "continents" make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra, after Ishtar, the Babylonian goddess of love, and is about the size of Australia. Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km above Venus's average surface elevation. The southern continent is called Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland regions at roughly the size of South America. A network of fractures and faults covers much of this area.

Map of Venus, showing the elevated "continents" in yellow:
Ishtar Terra at the top and Aphrodite Terra just below the equator to the right


As well as the impact craters, mountains, and valleys commonly found on rocky planets, Venus has a number of unique surface features. Among these are flat-topped volcanic features called farra, which look somewhat like pancakes and range in size from 20–50 km across, and 100–1,000 m high; radial, star-like fracture systems called novae; features with both radial and concentric fractures resembling spiders' webs, known as arachnoids; and coronae, circular rings of fractures sometimes surrounded by a depression. These features are volcanic in origin.>>

[dougettinger]

Yes, we could possibly add Venus to this special group of Mars and Earth. The high spots are a smaller fraction of the overall surface for Venus compared to Earth and Mars. And tectonic plates, I believe, have not been detected on Venus - possibly due to the difficulty of exploration or maybe due to smaller impactor(s). And its rotation was possibly arrested due to an impactor. So what has caused the high spots, 11 km above the average surface, on Venus ?

Doug Ettinger
Pittsburgh, Pa

[bystander]

Fresh insight into the origins of Planet Earth
Swiss Federal Institute of Technology Zürich - 31 May 2010

For the first time, an international team of researchers has incorporated extensive geochemical data on the formation of Earth into a model – with surprising results: more models can be used for the process of Earth’s accretion than previously assumed.

Earth was formed during the creation of our Solar System when Moon and Mars-sized protoplanets collided, leaving the Earth to gradually “grow”; just how long it took for the Earth to reach its eventual size and what the accretion of the planet was like, however, is much disputed among the experts. “The latest models reveal that an accretion period of around 100 million years is the most consistent with the formation of the Moon and the Earth”, says Bernard Bourdon, a professor from the Institute of Geochemistry and Petrology at ETH Zurich. However, there are also models that clearly suggest the Earth reached 70% of its size in just 10 million years.

Around 100 million years in the making: the Moon and the Earth (daninho_ibk, flickr)

Redistribution of the elements

The models for a rapid accretion of the Earth have to proceed from the basic premise that the colliding protoplanets blended fully with one other when the Earth was formed and achieved a chemical equilibrium between the elements in the Earth’s metallic core and the silicate mantle, explains Bourdon. This would have occurred when the iron-loving elements sank to the Earth’s center and the “silicate-loving” elements remained in the Earth’s mantle.

The scientist harbored doubts that the Earth could have evolved in only 10 million years; he teamed up with John Rudge, a guest scientist from Cambridge, and Thorsten Kleine, at the time one of Bourdon’s colleagues and currently a professor at the University of Münster, in an attempt to test the theory by fully considering all the known parameters in a model for the first time: on the one hand, the hafnium-tungsten and uranium-lead radioactive clocks; on the other hand, the distribution of the chemical elements between the core and the mantle. This distribution depends on the pressure and temperature conditions during the core formation, which probably varied during the accretion. The conditions of core formation can thus be extrapolated from the element distribution in the Earth’s interior.

Several models fit the bill

In their study published in Nature Geoscience, Bourdon and his team now demonstrate that there are several models that are compatible with the chemical observations. The notable thing is that there is no need for a full equilibrium between metals and silicates: “Up to now, it was always assumed that you could only explain the distribution of the elements through equilibrium; we show, however, that the distribution is just as easy to explain in disequilibrium”, says Thorsten Kleine.

The observations are also compatible with a state of equilibrium of only about 40 percent; this means the cores of the colliding protoplanets could have reached the Earth’s core directly without a major equilibration with the Earth’s mantle. For Bourdon, the crucial thing is that the results show that the notion of a full equilibrium might be wrong since a disequilibrium would require more time for Earth’s full accretion, thus being more consistent with the time when the Moon was formed. “If we assume that the Earth’s mantle preserved signatures of the protoplanets, the end of Earth’s accretion and the Moon’s age coincide”, says Kleine.

Logical picture

The age difference had always puzzled the scientists; after all, the termination of the Earth’s accretion should actually coincide with the Moon’s age as it ended due to the impact of a Mars-sized protoplanet that formed the Moon.The “radioactive clocks” are supposed to have been partially reset by this catastrophic collision. According to the new study, a large part of the Earth probably formed rapidly; however, it took at least 100 million years in all to reach its completion. The rapid accretion at the beginning and a slow completion are consistent with the time of the Moon’s formation, says Bourdon.

Broad bounds on Earth’s accretion and core formation constrained by geochemical models

• Nature Geoscience 3, 439 - 443, (23 May 2010), doi: 10.1038/ngeo872

[bystander]

Earth and Moon Formed Later Than Previously Thought
Science News - 07 June 2010

Astronomers have theorized that the planet Earth and the Moon were created as the result of a giant collision between two planets the size of Mars and Venus. Until now, the collision was thought to have happened when the solar system was 30 million years old, or approximately 4,537 million years ago. But new research shows that Earth and the Moon must have formed much later -- perhaps up to 150 million years after the formation of the solar system.
...
Turbulent collisions

The planets in the solar system are thought to have been created by collisions between small dwarf planets orbiting the newborn Sun. In the collisions, the small planets melted together and formed larger and larger planets. Earth and the Moon are believed to be the result of a gigantic collision between two planets the size of Mars and Venus. The two planets collided at a time when both had a core of metal (iron) and a surrounding mantle of silicates (rock). But when did it happen and how did it happen? The collision took place in less than 24 hours and the temperature of the Earth was so high (7000º C), that both rock and metal must have melted in the turbulent collision. But were the stone mass and iron mass also mixed together?

Until recently it was believed that the rock and iron mixed completely during the planet formation and so the conclusion was that the Moon was formed when the solar system was 30 million years old or approximately 4,537 million years ago. But new research shows something completely different.

Dating with radioactive elements

The age of Earth and the Moon can be dated by examining the presence of certain elements in Earth's mantle. Hafnium-182 is a radioactive substance, which decays and is converted into the isotope tungsten-182. The two elements have markedly different chemical properties and while the tungsten isotopes prefer to bond with metal, hafnium prefers to bond to silicates, i.e. rock.

It takes 50-60 million years for all hafnium to decay and be converted into tungsten, and during the Moon forming collision nearly all the metal sank into Earth's core. But did all the tungsten go into the core?
...
The new studies imply that the moon forming collision occurred after all of the hafnium had decayed completely into tungsten.
...
The result of the research means that Earth and the Moon must have been formed much later than previously thought -- that is to say not 30 million years after the formation of the solar system 4,567 million years ago but perhaps up to 150 million years after the formation of the solar system.

Turbulent mixing of metal and silicate during planet accretion -- and interpretation of the Hf-W chronometer

• Earth and Planetary Science Letters, Vol 295, No 1-2, pp 177-186, 15 June 2010, DOI: 10.1016/j.epsl.2010.03.038

[dougettinger]

This isotopic study is very interesting and dates the Earth and Moon system completion at a much later date than the beginning formation of the solar system. I am not sure how this study makes clear that the Earth and Moon are of similar ages.

These explanations still do not answer my main questions. I will simplify one of the questions. What causes the irregular high spots on the surfaces of Venus, Earth, and Mars ?

[Chris Peterson]

This isotopic study is very interesting and dates the Earth and Moon system completion at a much later date than the beginning formation of the solar system. I am not sure how this study makes clear that the Earth and Moon are of similar ages.

The isotopic age of the two is the same, suggesting they formed at the same time. The error bars remains large; I wouldn't say "much later" age of formation. 30 million years or 150 million years into a 4500 year old system is pretty darn early in either case.

These explanations still do not answer my main questions. I will simplify one of the questions. What causes the irregular high spots on the surfaces of Venus, Earth, and Mars ?

I'd answer with another question: given that all these planets were subject to early impacts, and all are or were tectonically active, why would you expect any of them to not show this kind of surface structure?

[dougettinger]

These explanations still do not answer my main questions. I will simplify one of the questions. What causes the irregular high spots on the surfaces of Venus, Earth, and Mars ?

I'd answer with another question: given that all these planets were subject to early impacts, and all are or were tectonically active, why would you expect any of them to not show this kind of surface structure?

Tectonically active and hemispheric high spots do not necessarily go together. Cracks in the crust, a convective mantle, and large volcanic eruptions do not cause huge continental-size high spots on a planet's surface. How hemispheric high spots may cause plate tectonics. Are you saying that the Moon causing collision created a significant break in the crust and a huge upwelling of mantle material to produce the original continent(s) on Earth ?

Doug Ettinger
Pittsburgh, PA

[bystander]

Deeper impact: Did mega-meteors rattle our planet?
New Scientist | Space | 09 June 2010

ON THE west coast of India, near the city of Mumbai, lies a tortured landscape. Faults score the ground, earthquakes are rife, and boiling water oozes up from below forming countless hot springs.

These are testaments to a traumatic history. Further inland, stark mountains of volcanic basalt provide compelling evidence that this entire region - an area of some 500,000 square kilometres known as the Deccan traps - underwent bouts of volcanic activity between 68 and 64 million years ago.

We don't know why. The Deccan traps lie far away from any tectonic plate boundaries, those fractures in Earth's crust through which lava usually forces its way up from the planet's interior. No volcanism on the scale implied by the Deccan traps occurs on Earth now. However, smaller, equally mysterious "hotspots" dot the globe away from plate boundaries - the smoking volcanoes of the Hawaiian islands, for example, or the bubbling geysers of Yellowstone National Park in Wyoming.

Geologists have generally thought that the history of such features can be traced through the slow churnings and contortions of rock under pressure in Earth's mantle. But it seems there is more to it than that. Sometimes volcanic activity needs - and gets - a helping hand from above.
...

[dougettinger]

Deeper impact: Did mega-meteors rattle our planet?
New Scientist | Space | 09 June 2010

ON THE west coast of India, near the city of Mumbai, lies a tortured landscape. Faults score the ground, earthquakes are rife, and boiling water oozes up from below forming countless hot springs.

These are testaments to a traumatic history. Further inland, stark mountains of volcanic basalt provide compelling evidence that this entire region - an area of some 500,000 square kilometres known as the Deccan traps - underwent bouts of volcanic activity between 68 and 64 million years ago.

We don't know why. The Deccan traps lie far away from any tectonic plate boundaries, those fractures in Earth's crust through which lava usually forces its way up from the planet's interior. No volcanism on the scale implied by the Deccan traps occurs on Earth now. However, smaller, equally mysterious "hotspots" dot the globe away from plate boundaries - the smoking volcanoes of the Hawaiian islands, for example, or the bubbling geysers of Yellowstone National Park in Wyoming.

Geologists have generally thought that the history of such features can be traced through the slow churnings and contortions of rock under pressure in Earth's mantle. But it seems there is more to it than that. Sometimes volcanic activity needs - and gets - a helping hand from above.
...

The article on the Deccan traps is very, very interesting. I only knew these traps to be a hot spot, but not a proposed impact location. Thanks for the reference. However, this does not answer my main question as to why large continents, raised surfaces, exist on Earth. The Indian sub-continent as well as all the other raised land masses already existed prior to the dated impact. India was connected to Antarctica and broke loose to move past Africa and slam into Asia causing the Himalayan Mountains.

Was the original supercontinent before it drifted apart and to other locations the result of the Earth's largest collision, the Mars size impactor that supposely created the Moon ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

Tectonically active and hemispheric high spots do not necessarily go together. Cracks in the crust, a convective mantle, and large volcanic eruptions do not cause huge continental-size high spots on a planet's surface.

How do you figure? Such events seem very capable of causing high spots.

Are you saying that the Moon causing collision created a significant break in the crust and a huge upwelling of mantle material to produce the original continent(s) on Earth ?

Not at all. I don't think there is anything about that collision that produced crustal features as we now observe them. The collision essentially melted the entire crust. It has probably never been more uniform in thickness than it was immediately after the formation of the Moon.

[dougettinger]

Tectonic plate movements certainly will cause mountain ranges at subduction zones and crashing of continents to cause mountains. The high spots that I refer to are not mountains, but the actual continents sitting high above the level of the Earth's average sea basins. The extent of these surface differences is not seen on other solid surface bodies in the solar system.

It is possible that enough heat did melt the crust in place. This is not confirmed. But if the crust did entirely melt then of course, it re-solified first as before.

The question remains: What caused the unusual high spots, the continent(s), on Earth's surface ? The answer simply may be that no one knows or has speculated on this topic.

Doug Ettinger 6/10/2010
Pittsburgh, PA

[Chris Peterson]

Tectonic plate movements certainly will cause mountain ranges at subduction zones and crashing of continents to cause mountains. The high spots that I refer to are not mountains, but the actual continents sitting high above the level of the Earth's average sea basins. The extent of these surface differences is not seen on other solid surface bodies in the solar system.

That is not clear. Mars, for instance, has a very wide range from highest to lowest surface height with respect to the average geoid. The Moon appears to have a crust that varies considerably in thickness from one side to the other. Except for the Moon, we don't really know much about the crustal thickness of other bodies; we work with what we infer from surface height only.

The question remains: What caused the unusual high spots, the continent(s), on Earth's surface ? The answer simply may be that no one knows or has speculated on this topic.

Again, I'd emphasize that we have no way of knowing if the Earth's crust is unusual. But the matter is something that geologists are aware of. The high crustal regions unassociated with tectonic uplift (typically, cratons) have lower density than other continental crust, and both have higher density than oceanic crust. Consequently, they float higher on the underlying mantle. To understand this, it would be necessary to explore the geologic history of the Earth in more detail than we are able to manage. But broadly, we think of the crust originating early on as a thin structure that was repeatedly broken up by tectonics and by impacts. It isn't hard to imagine that this resulted in a degree of lateral differentiation, with different crustal zones having very slightly different densities- which is all you need to have high and low spots.

There are other mechanisms, as well. Where I live here in Colorado, the land is quite high- something not associated with local tectonics (since we are in the middle of a large plate). But uplift is generally understood in terms of the expansion rate at the mid-Atlantic ridge exceeding the subduction rate at the edge of the Pacific. The timing of uplift here coincides with the formation of the Atlantic, and the forces involved are theoretically capable of lifting the center of an entire continent.

The way I see it, there are a variety of mechanisms, some primordial and some current, that result in the Earth having an active, somewhat chaotic crust building process, which in general is more active than weathering processes. I would expect a system thus described to produce a surface similar to what we actually have.