Guess the Planet 22: Rings

Here is this weeks guess the planet. It is fairly clear that these aren't a surface feature, but which planet do they orbit? 

Check back on Friday for the answer.

Valles Marineris

This week’s guess the planet comes from Mars, and is an image from the HiRISE camera on the Mars Reconnaissance Orbiter. Credit goes to NASA/JPL/University of Arizona. The HiRISE team have a great write up about the geology of this site, which explains what is going on in the image. In essence it shows an impact crater on the very edge of a large canyon. This is the edge of one of the largest and most recognisable features on the planet. The Valles Marineris. This immense canyon system is 4000 km long and seven kilometres deep. The valley system is obvious from space, as this mosaic of images from the Viking spacecraft shows. More information about this immense valley system can be found at this NASA link.

They compare its dimensions to those of the Grand Canyon on Earth, which is dwarfed by the martian valley system. The two features are often compared, however they actually have very little in common. The Grand Canyon is tiny because it was carved by the action of water. It formed over the course of several million years, as the Colorado river cut through the soft rock of the Arizona desert. This is very different to its martian counterpart. The modern consensus among planetary scientists is that Valles Marineris formed as a tensional fracture in the martian crust. It is likely that erosion by water caused later changes to the valley, but was not the dominant force in its formation.

This means that a better analogue for its formation are the rift valleys of Earth. These continent spanning structures are formed by tectonic processes. The East African rift valley is pictured here. It spans thousands of kilometres and formed as the tectonic plates pulled apart, causing new crust to form in between through volcanism.

There is just one problem. Mars does not appear to exhibit plate tectonics. On Earth, we see lots of evidence of the movement of the plates, from the interlocking shapes of continents to chains of volcanoes, and the earthquakes caused by the movement of faults. On Mars this is does not seem to be the case. We’ve discussed in a previous blog http://outer-reaches.blogspot.co.uk/2017/01/the-tallest-mountains.html that the largest martian volcanoes tower above their terrestrial counterparts, suggesting that the crust on which they sit isn’t moving over a volcanic hotspot. We don’t know for certain yet whether marsquakes occur.

This means that while Valles Marineris formed through rifting it wasn’t as a result of two tectonic plates pulling apart. Instead it was probably due to the presence of those massive volcanoes. Olympus Mons is just one of several huge shield volcanoes which occupy a region of Mars called Tharsis. This can be seen just to the north west of Valles Marineris on the image above, where two large, circular features are visible on the edge of the view. These are Ascreus and Pavonis Mons, two of the largest volcanoes on the planet. Along with Olympus and Arsia Mons (which are just off the edge of this image) they form a massive triangle of volcanic activity, and a substantial bulge in the surface of Mars. The thickening of the martian crust in this area would have produced substantial tensional forces, and it is believed that this is what caused the rifting along the Valles Marineris.

Image credits

HiRISE image of the edge of Valles Marineris; NASA/JPL/Universityfo Arizona

Viking Mosaic of the Valles Marineris Hemisphere of Mars; NASA/JPL-Caltech

Satellite image of the east African rift; NASA

 

Guess the Planet 21: Slopes

Here is this week's guess the planet. One of these depressions in a crater, but what is the other and which planet (or other solar system body) does it come from?

Check back on Friday for the answer article!

Why are Craters Useful?

This week’s guess the planet picture is of Herschel Crater on Mimas, one of Saturn’s moons. This image was captured by the Cassini spacecraft. Mimas is a small and heavily cratered moon, with an icy composition. The leading hemisphere of Mimas is dominated by the Herschel Crater. This large impact basin displays many of the features which are common in impact structures across the solar system. In Monday’s post, I drew attention to the prominent central peak. This is very obvious in this glancing view of Mimas. You can clearly see the topographic profile across the crater, with steep walls on both the inside and outside of the rim, and then an irregular central uplift. The bowl-shaped interior of the crater is not entirely flat, but is covered with hummocks and irregularities.

NASA have a much more detailed description of Herschel crater and what it tells us about Mimas, which can be read at this link. They draw attention to the thick blanket of “ejecta” which was thrown out onto the surrounding plains by the impact. They also notes that some of the smaller craters surrounding the Herschel Basin are “secondary craters” produced by ejected material impacting the surface further away. This image shows a head on view of the crater, and gives us a better look at its surrounding terrain. Mimas has been humorously described as looking like the Death Star from Star Wars, and in this view you can see why.

Craters are found on numerous bodies across the solar system, they can be very useful for interpreting the surfaces into which they impact. This is because the number of craters on a surface can tell us approximately how old it is. Any new surface, such as a recent lava flow will clearly be uncratered. As time goes by more and more impacts will mar that pristine surface. Resulting in the heavily cratered terrains common on worlds like Mercury and the Moon. Many planetary scientists spend a lot of time counting craters in order to estimate the age of a geological unit.

This method relies on the fairly basic principle that if one geological feature overlies another then it must have formed later. The surface into which the craters impact had to have formed before the impact occurred. This allows us to determine what is called a relative date for the surface. Determining which terrains are the oldest, and what order the events occurred in.

Relative dates are useful, but frequently we need more information. It we want to compare several regions which don’t overlap then we need absolute dates, which give us a lot more information to work with than just establishing the order of events for a specific site. However, in order to get a we would need to calibrate our relative dates, by figuring out how many craters impacted into surfaces of known age. On Earth, we can get absolute dates quite easily here, using the decay of radioactive elements to work out how recently a material formed. Unfortunately, Earth doesn’t have many surviving impact craters. Our planet is “resurfaced” much more rapidly than those of many other solar system bodies, wiping away the signs of meteorite impacts. We don’t have the heavily cratered surfaces we would need to calibrate our crater counts.

Luckily our nearest neighbour has very little resurfacing, and is largely covered in cratered terrain. We can get absolute dates for surfaces on the moon in the locations where samples were collected by the Apollo astronauts, and compare these to crater counts to connect the two dating systems.

The result is an estimate of the cratering rates throughout the moon’s history, which can be applied to surfaces which have never been visited or sampled. Statistical methods allow us to apply these cratering rates to other solar system bodies as well, although there is some debate as to how accurate the results are. In particular, secondary craters can throw off the crater count, since they formed as a result of an existing impact, not a new meteorite. Determining which craters are due to direct meteorite impacts and which are secondary can be a difficult exercise.

As with many methods in planetary science, crater counting is very valuable, but has to be used carefully. We have to understand the uncertainty in the observations and measurements we make. By doing so we can use techniques like this, and get useful information out of them, even if they do not give us a perfect result. We will never know the exact date of a surface unless we go there to take samples. Even then laboratory dating methods only give a range of probable ages. However by combining all of this information we are able to develop a strong theory for how the surface of a body evolved, and can tell a lot about the processes at work there.

 Image Credits:

Mondays image: NASA/JPL-Caltech/Space Science Institute https://saturn.jpl.nasa.gov/raw_images/400528/

Larger image of Mimas: NASA/JPL-Caltech/Space Science Institute http://photojournal.jpl.nasa.gov/catalog/PIA12568

Guess the Planet 20: Central Peak

Here is this week's guess the planet. This impact basin is quite a substantial feature of the solar system body on which it is found, it also has quite a large central peak. So where can it be found? 

Pancake Domes on Venus

This week’s guess the planet image comes from Venus. The Large circular features are volcanic in origin, and are called “Pancake Domes”. They are very wide and flat with steep sides and a shallow profile. These domes are found in Tinatin Planitia, and were imaged by the Magellan spacecraft using radar. The NASA’s description of this image tells us that “The largest dome is 62 km in diameter. North is up.” Giving a sense of scale to this site.

Venus is a very volcanic planet. Most of its surface is covered by lava flows, and there are numerous clusters of shield volcanoes. These tend not to be as tall as their terrestrial counterparts, but are generally much wider. Some can be as much as 700 km wide, and so contain far more erupted material than taller, but steeper volcanoes like Hawaii on Earth. Lava domes, like the ones in our image also have terrestrial analogues. However, the pancake domes are 100 times larger than typical lava domes on Earth. Many pancake domes have cracks in their surfaces. Small central pits are also common.

These domes seem to have formed by the eruption of very viscous lava. The composition of lava has a massive effect on how it behaves. Different materials produce different eruption types and result in differently shaped volcanoes. Viscosity determines how far, and fast lava will flow. The viscosity of a lava is largely determined by the temperature of eruption, which in turn is determined by the chemical composition of the rock which has melted to produce it.

There are several broad categories of lava composition, which depend on the amounts of iron, magnesium, aluminium and silica they contain. These control the behaviour of the volcanic processes and the morphology of the lava flows they leave behind.

 Mafic lavas have the most Iron and Magnesium, making them basaltic in nature. Mafic lava flows have the highest eruption temperatures and thus are not very viscous. They can flow for very long distances, forming shield volcanoes and flood basalts. The dark coloured Luna Maria are basaltic plains erupted early in the moon’s history. Large shield volcanoes on Venus, Earth and Mars were also the result of this sort of lava.

 Felsic lavas are generally the most viscous. Their composition is dominated by silica and aluminium, with much less Iron and magnesium. This gives them the lowest eruption temperatures and they tend to form blocky, fragmented lava fields. Andesitic or intermediate lavas are somewhere in between. They often form steeper volcanoes, and many of the iconic cones that we see on earth are of this type. “Ultramafic lavas” can also be found in some areas.

These categories cover the range of types that are possible, but the exact composition of a lava flow will depend on the precise blend of rocks from which it formed. This results in a lot of variation, from one volcanic area to another. We can infer a lot about the properties of a volcanic site from its morphology, but we have to be careful that we have multiple lines of evidence to back up these theories, as other factors can also control the shape of a lava flow, and weathering can change things dramatically in the years since the volcano last erupted.

Lava domes like our pancake domes require viscous flows, but this doesn’t necessarily mean that they had to have been formed by felsic lavas. Other factors can affect the viscosity of a lava flow, and on Earth we find lava domes with a range of compositions. Before we can make inferences about composition we need to look at the surrounding environment, and see which other forms of volcanism occur in the vicinity of our domes. We can then see whether patterns emerge in the types of feature which are generally found together. This ultimately allows us to infer the formation process from the landscape, and ultimately speculate on things like rock composition and lava type.

By exploring multiple lines of evidence we can develop a better understanding of how these features formed, and what this means for the environment of Venus.

Happy Pancake Day!

Image Credit:

NASA Magellan team

http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/mgn_c115n009_1.html

Further Reading:

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

http://volcano.oregonstate.edu/oldroot/volcanoes/planet_volcano/venus/unusual.html

http://geology.com/stories/13/venus-volcanoes/

 

 

 

Guess the Planet 19) Round Structures

Here is this week's guess the planet. What are these round structures? why are they so regular? And which planet can they be found on?

Check back on Friday for the answer.