Guess the Planet 40) Dome

Here is this week's guess the planet image: 
What is this dome like structure and on which planet can it be found?

Tune in later in the week for the answer.

Stair-Step Faulting

This week’s image shows stair-step scarps on Ganymede, one of the Jovian moons. This image was captured by the Galileo satellite. NASA’s archive page for this image describes its dimensions thus: “North is to the right of the picture and the Sun illuminates the surface from the west (top). The image is centered at -14 degrees latitude and 320 degrees longitude, and covers an area approximately 16 by 15 kilometers (10 by 9 miles).” So these cliffs are on quite a large scale! The fault has pulled the surface apart, dividing the crust into several blocks, which have become rotated producing these steep scarps. 

This suggests that tectonic processes have played an important role in shaping the surface of Ganymede, which shouldn’t be surprising given the forces exerted on the Jovian moons by the gravity of Jupiter and the resonances between the various Galilean Satellites. As we saw on Io a few weeks back these effects can be quite dramatic, but unlike its neighbours Ganymede is not undergoing tidal heating. Ganymede is not being resurfaced to the same extent as Io, but rifting is still occurring across the moon, leading to the formation of younger, terrain. Grooved terrains like this are common across the young, bright regions of Ganymede. 

Ganymede is a very interesting body. It is not only the largest moon of Jupiter, but the largest moon in all of the solar system. It might look small in comparison to the gas giant it orbits, but Ganymede is larger than Mercury, and if it orbited the sun directly would be a substantial planet in its own right. Its large size means that is has a spherical shape and a differentiated interior. Its internal structure is divided into a core, likely composed of metallic materials, a silicate mantle much like that of the earth, and a crust. The crust of Ganymede is believed to mainly consist of water, and there appears to be a liquid ocean present beneath the surface. Under the cold conditions in this part of the solar system the icy crust will behave much more like terrestrial rock than the ices we are familiar with on Earth. This means that we can look to geology to interpret features on the moon’s surface.

The striking landscape shown in this week’s image formed as a result of faulting of the crust. The fault has pulled the surface apart, dividing the crust into several blocks, which have become rotated producing these steep scarps. This sort of landscape shows that tectonic processes have played an important role in shaping the surface of Ganymede. As we saw on Io a few weeks back the Galilean satellites are under a lot of stresses, both from the gravity of Jupiter and the resonances between the moons.  However, unlike its neighbours Ganymede is not undergoing tidal heating. Consequently, Ganymede is not being resurfaced to the same extent as Io, but rifting is still occurring across the moon, leading to the formation of younger terrain.

Extensional faulting is a tectonic process which occurs across the solar system whenever sections of a planets crust move apart, allowing new material to rise from below. On Earth the formation of extensional faults is largely due to the pulling apart of tectonic plates. However this is quite an unusual arrangement. On most other planets there are no true plates and faulting occurs due to other sources of tectonic stress, such as the bulging or contraction of the crust. Compared to most solar system bodies Ganymede and Europa actually behave a lot like Earth. Their icy surfaces fracture into plates, which a likely floating on the liquid oceans below. Clearly the materials are very different, but under these conditions they behave a lot like tectonic plates producing faults like those seen in this image. 

Image Credit: NASA/JPL/Brown University https://photojournal.jpl.nasa.gov/catalog/PIA02582

Guess the Planet 39: Steps

Here is this week's image. What are these step like structures, and which solar system body can they be found on? 

Check back later in the week for the answer!

Peak Ring Craters

Craters are very common across the solar system, with a large number of planetary bodies being liberally covered by them. I always feel as though I’m cheating when I use a crater on the blog, as it is definitely one of the harder landform types to match to a specific solar system body. Nonetheless this particular structure stands out as particularly impressive. This is the Aksakov Crater on Mercury, and is a very striking example of a peak ring crater. This image was captured by the messenger spacecraft in 2015, so credit goes to their team. The concentric and eccentric circles seem in this impact basin are due to two unrelated processes. Firstly the large crater is a peak ring crater, and the concentric circles are the result of the forces released during its formation. The other basins, off centre from the peak ring structure are the result of a later separate impact events, and it is just coincidence that they hit in roughly the same part of mercury. 

As we’ve discussed in previous crater focused blogs, large impact structures can exhibit a large amount of variety compared to their smaller cousins.  In the case of a smaller impact the ground around the crash site will soon settle into the familiar bowl shape of a simple crater. However large impacts produce a wider range of morphologies as the material which has been displaced by the impact returns to equilibrium. 

Complex craters occur when a large meteorite impacts the surface; this hypervelocity impact imparts a vast amount of energy to the surface into which it impacts. Shock waves are sent out from the impact site in all directions and have a devastating effect on the surroundings.  The initial “contact and compression” phase sends waves of compression rippling through the ground around the impact, and through the impacting meteorite itself. This leads to the “excavation phase” where this compressed material is forcibly ejected from the area, being scattered around the vicinity. A lot of this displaced material will be completely ejected from the area where the impact occurred and this leaves behind a large cavity. This “ejecta” can often be seen blanketing the terrain in the vicinity of the impact site. 

The result of the excavation phase is a bowl shaped crater. the centre, where the impact occurred has been forced down by the force of the impact. This displaces material all around it, forcing the crater rim to become raised relative to the original ground surface. Combined with the material that has been ejected this leaves behind a large depression called the “transient cavity”. 

It is referred to as transient because it is very unlikely to stay unmodified for very long. The final phase of crater formation is the “modification and collapse phase”. The initial shocks of the impact are over, and gravity begins to act to return the crater to an “equilibrium state”. The energy imparted by the impact has moved material very rapidly against gravity, and the result is that a lot of the craters structure is unstable. Sections of the rim will often collapse, creating shallower slopes which are more stable. The centre of a large crater will also often rebound, forming a central peak. If the crater is large enough then a complex interaction develops between the forces driving the rebound, the collapse of the resulting central peak, and the collapse of the unstable rim material. the result is a ring of central peaks like that seen at Aksakov crater.

Studying craters can be challenging, as we don’t often witness them happening. Fortunately Earth gets hit by relatively few meteorites which would be large enough to form a peak ring crater. The only peak ring impact structure we have found on Earth is the Chicxulub Crater, in Mexico’s Yucatán Peninsula. This crater is believed to result from a massive impact event which likely caused the mass extinction at the end of the Cretaceous period. Consequently we are not too keen to have any other impactors of a similar size hit the Earth any time soon. Complex computer models are thus required to work out how these forces interact, and determine what processes resulted in the complex crater morphologies we see throughout the solar system.

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington https://photojournal.jpl.nasa.gov/catalog/PIA19212

Guess the Planet 38: Circles

Here is this weeks image. What is going on here, and which planet, or other solar system body is this spectacular landscape located on?

Check back later in the week for the answer!

Tohil Mons

This week’s image comes from Io, jupiters volcanic moon. This image shows a mountain named Tohil Mons and was captured by the NASA Galileo Spacecraft. Credit goes to the Galileo team. There is a link to NASA’s description of this image below. They report that the mountain is 5.4 kilometres in height. “North is to the top and the Sun illuminates the surface from the upper right.” Tohil Mons is located on the Antijovian hemisphere of Io. The sharp shadows in this image are one way to estimate the height of this mountain. But the Galileo spacecraft also performed stereo observations where images were taken from multiple angles and combined to form a 3D model. This allows us to get a good look at this mountain in 3D.

The surface of Io is covered with volcanic features, including many large volcanic craters, called “calderas”. There are several such craters located around this peak, including one just to the east of the mountain however the mountain in the centre of this image is not itself a volcano. Very few of Io’s volcanic features have strong positive relief, most consist of low lying cones and craters. 

Instead most of the highest mountains are believed to form through tectonic processes, although the precise mechanism is still being debated. This is an area where research is still very much ongoing so I don’t have a definitive explanation to share. Even on Earth where we have, or can get all the data we could possibly want it can take a lot of hard work to reconstruct the formation history of geological features. On moons like Io, which have been visited quite infrequently, and where limited data is available it becomes even harder. 

Tohil Mons appears to be a complex “massif” with quite rugged terrain and a straight ridge running to the southwest. A massif is a coherent block of the crust which has been uplifted or displaced by tectonic forces to form one or more mountains. It may exhibit layering, as do several other mountains on Io. It is also surrounded by a basal scarp. This is a steep cliff, or scarp at the bottom (or base) of the mountain which separates the upland area from the surrounding volcanic plains. These features are consistent with a tectonic origin. 

Io lacks plate tectonics of the sort seen on Earth, instead it is undergoing extreme volcanism due to its proximity to Jupiter and the resonance between its orbit and those of the other Galilean satellites. This volcanic activity is likely what has caused these tectonic features. As resurfacing by lava flows takes place the thick blocks of crust between volcanic sites are buried or displaced as they are forced into a smaller area by the formation of new terrain. Thrust faulting occurs resulting in uplifted areas such as these massifs.  

This means that while Tohil Mons is not a volcano in its own right its history is deeply tied to the volcanism which shapes every aspect of Io’s surface. 

Further Reading:

https://en.wikipedia.org/wiki/Mountains_of_Io

https://en.wikipedia.org/wiki/Tohil_Mons

Image Credits: NASA/JPL/University of Arizona

https://photojournal.jpl.nasa.gov/catalog/PIA03600

https://en.wikipedia.org/wiki/Tohil_Mons#/media/File:Tohil_Monsstructure.jpg

Guess the Planet 37: Rough Terrain

Here is this week's image. What do you think this area of rough terrain might be, and which world can it be found on? 

Check back on Friday for the answer!