Creator of Craters

 

Since Galileo first pointed a telescope at the Moon and saw its marked surface people have been debating how the craters they observed formed, for much of history there were two interpretations; either an impact crater or formed by volcanic eruptions.

Picard_crater_oblique_4191_h3

Impact or Volcano Picard crater -Luna Oribter 4 (NASA)

It was not until the space age when close up photos (and later moon landings) revealed most of the creators on the surface were formed by impacts (volcanoes do exist on the moon but form very different features). But what are the differences and how can you tell remotely without landing?

Barringer_Crater_aerial_photo_by_USGS.jpg

Barringer Crater Arizona, formed by a meteorite impact (USGS)

On first glance, both types of craters can look very similar, both would form round features with rims. Both can have ejecta around them, formed from the material thrown out from the inside (either the material the surface is made from or ash from volcanic eruptions).

Aniakchak-caldera_alaska

Aniakchak, Alaska volcanic caldera (US National Parks service)

However when you think about how they formed you can understand some of the differences when looking at them: volcanoes are explosive, expansions of material often with repeated events close to or in the same place.

Most volcanoes are built upon the landscape and so often the inside of the crater will be at a higher altitude than the surrounding terrain (with the exception of Maars due to interactions with water under the ground and features called calderas which are collapsed volcanoes). Whereas impact craters are formed by a surface beeing excavated by an impactor, whilst the sides of the crater can be built up from material thrown out of the crater floor itself will be lower than the surrounding terrain.

Volcanoes often show evidence of their volcanic nature around them, whether it is lava or ash flows inside and on the sides of the volcano. The volcano itself will be built up of volcanic rock. An impact craters wall will be made up of the same material excavated from the site. there are of course exceptions to this, for example on Mercury some of the impact craters are filled in with lava which broke through the bottom of the crater, however, the wall of the crater is of different material to later lavas which fill in the basin.

Volcanic craters can also be asymmetrically distorted by the wind or the direction of the eruption (or deformed by later eruption events). The overwhelming majority of impact craters are circular. Larger craters also have a central mound formed by the rebounding material which you don’t see in volcanoes (though new volcanic cones can grow in existing craters).

There are lots more features of craters evident when you get a close-up view on the ground but they are hard to observe from satellites where most of our planetary data comes from. As with much of geology, by observing a combination of these features a story of where a crater came from and how it formed can be built up from remote sensing data.

 

Cryotectonics

Previous posts have looked at the nature of tectonics on the terrestrial planets and how they are very different to Earth. Further out in the solar system, it is not the planets which are of geological interest but the moons, many of these are ice worlds. Cores of rock covered in layers of ice. Some of these have pockets or layers of water in them generated by tidal friction generating heat.

Evidence for this layer of water or at least an icy slush is shown by fountains of ice sprayed into space on both Enceladus and Europa showing that these moons are active places however, there are other telltale signs of a system similar to plate tectonics.

 

pia08386-smalleucledius_nasa_jpl

Ice erupting from volcanos on Enceladus (NASA/JPL/Space Science Insitute)

Photos of Europa, an Ice moon of Jupiter, revealed a cracked surface covered in bands. Limited cratering suggests that surface is relatively young (around 40-90 million years). Some of the surface shows repeating bands similar to spreading ridges seen on Earth. Where the surface ice cracks and moves apart and new water rises up and fills the gap with ice.

Jup_Eru_Galialo_PIA19048-medium_NASA_JPL

Europa as seen by Galileo (Nasa/JPL-Caltech/SETI institute)

These same spreading ridges and associated bands are also clearly seen on Miranda, a moon of Uranus, which has alternating patterns around a central rift, just like is seen on the ocean plates of Earth.

Where tectonics must differ is what happens at the other end; ice is very buoyant making it difficult to find a form of subduction which would allow pulling of the crust down into the water/ slush layers like the plates on Earth. As the surfaces of these moons cannot be ever increasing in diameter, something has to be happening to the ice.

On Europa there no regional scale mountain belts where the crust thickens to accumulate this extra crust. Instead, there is some evidence of complex fault systems. Similar to subduction zones, but which behave differently and have been termed subsumption zones – one section of the ice crust is forced under another, where it rapidly becomes incorporated into the underlying water ice… so rather than continuing to sink like the plates on Earth it rapidly becomes part of the underlying layers, enabling tectonics to continue.

pia18185-orig_miranda_nasa_jpl

Miranda as seen my Voyager 2 (Nasa/JPL)

The ice moons of the outer solar system are harsh alien places, but tectonically they may be the most Earth-like of all the bodies in the solar system.

Earth’s plates

No body is perfect; forces on both the inside and outside of them push and pull them around. This leads to cracks and wrinkles on surfaces which represent the passage of time. On planetary bodies, this is referred to as tectonics.

Most bodies have tectonic features spread out over their surface. Earth is different. The surface is divided up into interlocking blocks which move around over time, called tectonic plates. These plates are constantly moving: colliding, separating or moving past each other, this movement causes the majority (but not all) of earthquakes and volcanoes on the Earth. The vast majority of tectonic features are related to either current or historical plate boundaries.

1200px-plates_tect2_en-usgs

The Earth’s tectonic plates, courtesy of the U.S. Geological Survey

The driving force behind plate tectonics is dominated by the gradual overturning of the mantle as it moves heat from the core to the crust in a process called convection. The plates are also pulled by old cold crust sinking down and pulling the plate with it.

These forces linked to the moving of plates transfer stresses through the crust and lead to the rocks deforming. On a large scale, this leads to several features on the Earth’s surface which can be seen from space. In future posts, we’ll look at some of the features on other planets but first, let’s have a look at a few features formed by plate tectonics on the  Earth:

Faults

china-fault

A fault offsets banded rock south of the Tien Shan mountains as seen by Landsat 8 (USGS/NASA)

Faults are breaks in rocks along which movement occurs and are associated with earthquakes. They release strain built up in the crust and are found on all plate margins, including those that slide past each other like the San Andras fault. These are often seen as breaks and offsets in geological patterns.

Rifts

In areas where the crust is under tension it can be pulled apart, the rocks break into a series of faults, which form a staircase-like feature dropping down to form a valley. As the crust gets thinner as it is pulled apart it enables volcanism to start and volcanoes form along the valley (such as in the East African Rift valley).

This rifting can continue and the crust can break into two separate plates. with a volcanic ridge in the middle which generates new crust as the two sides move apart. These features form the seas and the oceans.

iss030-e-035487_lrg

East African rift valley, formed by multiple faults (seen as lines going diagonally across the image) which lead to low lying areas as the plate thins  (NASA/JPL)

Mountains

sts066-124-059-medium

Mt Everest and other mountains as seen from STS-066 (NASA, JSC)

In areas where plates collide with each other, the outcomes vary depending on the type of crust colliding.  If it involves a thinner denser crust called oceanic crust (which is the type generated by mid-ocean ridges) subduction occurs – an oceanic plate will be forced down and sink into the mantle. This activity leads to a trough and then a line of volcanoes behind them. Where two pieces of thicker, less dense continental crust collide the parts crash into each other they crumple rather thank sink and form mountain ranges such as the Himalayas.

Earth is unique with its highly active plate tectonic system defining its surface, other planets have been and are tectonically active but in different ways with features spread out and controlled by different tectonic systems.