Pshyced

There are a lot of upcoming space missions to be excited about; BepiColombo’s trip to Mercury, ExoMars, the Europa Clipper and New Horizons second flyby to name a few. One that has me really excited is Psyche.

Psyche is a NASA mission to explore the asteroid  Psyche 16. This is an M-Class asteroid, which means that it is made dominantly of metals, mostly Nickle and Iron, compared most others types of asteroids which contain much more silicates (rock) within them. This unusual composition suggests that it is a relic left over from the formation of the planets.

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Shape of 16 Psyche (Astronomical Insitute of Charles Univesity, Josef Durech & Vojtech SIdorin)

Early in the life of the solar system, a protoplanetary disk of dust began to form clumps of rock as they collided, gradually these lumps of rocks grew into objects known as planetesimals.

When these planetesimals became large enough, the internal heat, generated by a mixture of gravitational energy, radioactive decay, and heat generated by collisions allowed those larger than about 30 km to internally separate out like oil and water; the denser materials sank to the bottom whilst the lighter material floated to the top. Iron, nickel, and a range of other metals sank to the bottom, it is this same process which formed the Earth’s core.

We have never sampled the Earth’s core, only being able to use seismic, magnetic, gravity and density data to infer information about it, as well as studying the limited number of meteorites which may be remnants of a core exposed by collisions with other planetesimals.

Psyche is the only known exposed planetesimal core in the solar system. This makes it a truly unique and exciting place to visit as it will enable us to look directly at and take measurements on the same kind of material which makes the core of the rocky planets. This will be the first time we have visited a metal body and is likely to look unlike any other body is so far seen in the solar system, possibly with impact craters showing metallic jagged edges.

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Psyche above Psych SSL/Peter Rubin

The Psyche mission is scheduled to launch in 2022 and reach the asteroid in 2026, it will be exciting to see what new insights about the formation of the solar system and our own core.

Put a ring on it

Some of the most spectacular images from within our Solar System come from Nasa’s Cassini spacecraft as it orbits Saturn. One of the reasons for this is that many of the vistas are dominated by the planet’s rings.

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Saturn, its rings (and their shadow), and the moon Tethys (NASA/JPL/SSI)

 

The rings of Saturn are made up mainly of lumps of ice (with a small amount of dust), which while individually are only millimeters up to about a kilometer in size, add up to whole rings structures which whilst only a kilometer deep extend laterally for hundreds of kilometers.

There is still a lot of debate about how they formed and how long they will last. Cassini’s  final orbits are designed to answer some of these questions by flying between the planet and the rings and we should get more answers in the months and years after the end of the mission once the data is analyzed. Current theories suggest that the rings formed around the same time as Saturn did; either from the same planetary nebula which formed the planet or from a moon which was torn apart by Saturn’s gravity. Other researchers have suggested that they are only 100 million years old and transient features which will eventually fall into the planet (over very long time periods). whilst the majority of the evidence points to them being very long lived so far. The exception to this is the outer E rings, these are known to have been generated by the ice volcanoes of Enceladus throwing out icy material into the orbit of Saturn and form a ring.

Rings are dynamic with transient features such as “spokes” of dust caught in the magnetic field chasing around jus above the surface of the rings, disturbances within them like propellers formed by the gravitational distortion of moonlets, or the ripples generated by Daphnis as it travels within a gap in the rings.

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Daphnis a small moon causing ripples within the rings of Saturn (NASA/JPL/SSI)

Pan, a moonlet within a gap in the rings has made the gap by collecting up the ice particles, forming a band around its middle.

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Pan, which orbits within the rings, has built up ice within its middle (NASA/JPL/SSI)

It is not just Saturn which has rings, Neptune, Uranus, and Jupiter all have rings and even the 250 km diameter small planetoid 10199 Chariklo has a ring system around it. Temporary rings probably occur on all planets as comets and asteroids get too close are torn apart by the gravitational pull. forming a ring of debris which would eventually fall to the surface, Mars may develop a transient ring system, when Phobos is torn apart by tidal forces as it slowly spirals towards the surface. This ring will be transient as the material itself falls onto the surface of Mars.

 

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Infrared image of the rings of Jupiter

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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.

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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?

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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).

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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.

 

Global coverage

Volcanoes on Earth have fascinated and terrified people for millennia, extinct volcanoes are seen all over the solar system; on Mars & Mercury and the Moon to name just a few places. Venus and Earth both have active volcanoes on their surface. However, the most volcanically active place in the solar system is Jupiter’s moon Io.

Io is kept volcanically active by the tidal forces of Jupiter and its moons pulling at it, the changes in gravity cause it to flex and bulge which generates heat inside it and causing parts of its to melt. If it was not for this tidal heating then it would no longer be volcanically active, being small and so cooling quickly.

Ongoing Volcanic Eruption at Tvashtar Catena, Io

Volcanism on Io, (NASA/JPL/UoA)

Whilst there are some other geological features on Io, such as mountains, there are few impact craters observed, which shows that it has a relatively young surface. It does have many volcanic craters. The volcanoes are dominated by basalt lavas (like those on Hawaii) these can form lava flows which flow over hundreds of kilometres. In addition to this, the volcanoes erupt lava flows of sulphur and sulphur dioxide,  The crater-like depressions which are seen on the surface, look a lot like calderas. On Earth, which form when a magma chamber is emptied and a volcano collapse in on itself (it is not known if this same mechanism occurs on Io).

The first evidence of volcanic activity was plumes of material spotted rising above the surface like a fountain. These plumes create a huge amount of material mainly sulphur dioxide which rains back onto the surface.

The highly volcanically active nature of Io means that each year the equivalent of around 1-1.5 cm of material over the whole moon is produced, a staggeringly fast rate of depositing material over a large area in geological terms. This leads to craters on the surface being hidden giving it one of the youngest surfaces of all the bodies in the solar system.

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Volcanic plume on Io as seen from Voyager (Nasa/JPL/USGS)

Volcanoes dominate much of the landscape of Io, and perhaps provide some insights into what was happing other bodies in the solar system before the cooled down and became volcanically inactive. It shows how the appearance bodies can be dominated by the external gravitation influence and interaction with of other bodies.

 

 

Giant ticks on Venus

Venus is probably the only other planet still volcanically active other than Earth, it has lava flows. Venus express has shown sulphur dioxide changes and hot spots on the surface which suggest that it is still volcanically active.

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Heat map of Idunn mons, showing a recent lava flow (ESA/Nasa/JPL)

The volcanoes on Venus should be similar to hotspot volcanoes on Earth like Hawaii. On Venus, they are mostly similar but on much larger scale.

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Pancake domes on venus (Nasa/JPL)

Other volcanoes seen on the planet are called pancake volcanoes. 15 km diameter but only 1 km high these are an artefacts of higher viscosity lavas, which on Earth would build up to steep-walled stratovolcanos  atmospheric pressures (90 times that of Earth’s atmosphere) prevents more viscous lavas from building up vertically and the high surface temperature of Venus which allows the lava to flow for longer before setting forming pancake shapes.

Sometimes these volcanic structures can undergo collapse along the edges of them with radiating valleys around the outside, creating the appearance on an insect with legs. these are known as scalloped margin domes or Tick – Like structures.

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The Tick – a scalloped dome volcano on venus (NASA/JPL)

The volcanoes of Venus show what a difference that the conditions on a planet can make to the shape and style of surface features even if the processes seem similar processes to those on Earth.

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.

 

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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.

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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.

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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.

The marks on Mars

Looking at Earth, Venus, and Mercury so far has shown different systems of tectonics. The last terrestrial planet Mars has some familiar features but all is not as simple as it seems.

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Mars Magnetic crust as seen by Mars Global surveyor (Nasa/JPL)

Magnetic images of Mars show band like repetitions of positive and negative anomalies at its southern hemisphere. These bands look similar to magnetic bands seen in Earth’s oceanic crust.

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Bands of magnetic anomalies in the crust of the Atlantic ocean

The bands on Earth’s ocean floors represent the gradual formation of crust over time; as the plates move apart and magma rises up to form new crust it records the magnetic field as it forms, over time the periodic flipping of the Earth’s magnetic field leads to an alternating pattern.

Mars doesn’t currently have an active magnetic field, and crater dating shows that the south is older than the northern 1/3 of the planet which is topographically 3-6 km lower than the southern portion of the planet. This northern lowland does not show magnetic banding. Theories suggested for this lowland include proto-tectonics like that on Earth but which did not occur until after the planet’s magnetic field stopped and/or large impacts causing the substantial topographic difference between the two regions.

Another feature which provides clues about the nature of Martian tectonics is Tharsis Plateau, this is a vast volcanic system close to the equator and includes the 22 km high Olympus Mons, the highest volcano in the solar system. This volcano was able to grow much bigger than the hotspot volcanoes seen on Earth due to a lack of movement, if the plates had been moving then the volcano would not have developed to the same extent. This shows that Mars the plates were not moving unlike on Earth.

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Olympus Mons (JPL)

The huge weight of the plateau put a lot of stress on the crust around it and led to the formation of  Valles Marineris. The extra load on the crust mean that large valleys formed close to the edge of Tharsis Plateau as the weight caused the crust to buckle and break and shear.

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Valles Mariners, a crack caused by the weight of nearby volcanic plateau (NASA)

The magnetic bands suggest that early in Martian history it may have had some form of spreading ridge generating crust in a magnetic field. Both of these processes stopped as Mars cooled. These bands and the north-south topographic divide hint at an early active tectonic history, however, there are few signs of large-scale subduction which would allow recycling of plates so it seems unlikely that Mars ever developed a fully functioning multiple plate tectonic system, though an understanding of its history is far from complete.