Guess the Planet 49) False Colour

Here is this week's guess the planet image. What does this false colour map depict, and which planet will we be visiting this time around?

Check back later in the week for the answer!

Valhalla Multi Ring Impact Structure

This week’s image comes from Callisto, one of the moons of Jupiter. It shows the Valhalla impact structure, a large crater with several concentric rings. Valhalla is the largest multi ring crater in the solar system. The entire structure is about 3800 km in diameter. This image was captured by the Voyager One spacecraft, when it flew past this small moon in 1979. The crater is located in the Jupiter facing quadrant of the moon’s northern hemisphere. Several smaller impact craters are superimposed on the larger basin, suggesting that it is fairly old, and that other impacts have occurred since. Valhalla is not the only multi ring structure on Callisto. Another four are located on the moon, including the nearby Asgard and Utgard impacts. Asgard is the larger of the two, and the Utgard basin is superimposed upon it, indicating that it must have formed later. Craters are the main surface feature on Callisto, and the presence of numerous multi ring basins suggests that numerous very large impacts have occurred.  

The multi ring basins also result from the properties of Callisto’s lithosphere, the solid outer layer of the moon. This is likely to be quite thin and brittle, with a large proportion of ice within its composition. When a giant impact hit, the lithosphere is punctured. This imparts stress to the softer layer below, and causes concentric fractures to form around the impact site. These have the form of heavily degraded “Grabens”. These are rift valleys which generally form through tectonic processes. Graben are named after the German word for trench, as they consist of troughs where a block of a planet’s crust has been forced downwards relative to the surrounding blocks. They are bordered by two normal faults, running parallel to one another, in this case around the impact structure. These faults form steep scarps, discontinuous cliffs and ridges that give the structure its distinctive form. The result looks like a series of ripples, and this is a reasonable analogy. The stress caused by the impact has propagated outwards from the impact site in all directions. This stress has caused a series of failures of the brittle crust, resulting in concentric faults and scarps. Although the process that formed the outer rings is tectonic, it was triggered by the force of the massive impact. 

Image credit: NASA/ JPL https://en.wikipedia.org/wiki/Valhalla_(crater)#/media/File:Valhalla_crater_on_Callisto.jpg

Guess the Planet 48: Ripples

Check out this image. What do you think causes the ripple like structures seen here, and which solar system body is this?

 Check back later in the week for the answer!

Shadows on Mercury

This image comes from the innermost planet of the solar system, and shows a map of illumination around the South Pole of Mercury. NASA’s description of the image states that it is coloured “on the basis of the percentage of time that a given area is sunlit. Areas appearing black in the map are regions of permanent shadow.” 

This is an interesting image because darkness and light are very important to our understanding of Mercury. This is because the cycle of day and night there isn’t like that on the Earth. We are used to a year being much longer than a day, as our planet rotates on its axis hundreds of times for each revolution around the sun. However this is not the case on all solar system bodies. 

For a long time it was thought that the day on mercury was the same length as its year. This would mean that, much like the relationship between the Moon and the Earth, one side would always face the sun, while the other would be in perpetual darkness. This process is called tidal locking, as the tidal forces exerted by the two objects gradually shift the orbital and rotational speeds until they form a resonance. In the case of the Earth-moon system this resonance is 1:1. The planet rotates once for every trip around the sun, and thus the same face of the moon is always visible from the Earth. 

This seemed to be the case on Mercury. Numerous telescopic observations confirmed that the same face of the planet always seemed to be pointed away from the sun. It appeared to be tidally locked. If this were also true on mercury then it would make it a very unusual environment. Since mercury has very little atmosphere to speak of, the redistribution of heat from the sun facing day side to the colder night side would be expected to be quite slow. Temperatures vary massively ranging from around -170 ^oC at night, to over 400 ^oc in the day time. If water exists on mercury, it would thus be expected to only be found on the night side, as any that made its way to the day side would quickly evaporate and be lost to space. 

This model for day and night on Mercury informed our understanding of the planet’s environment right up until the 20^th century. However in the mid 1960s it became apparent that, like many early models, it didn’t quite describe the reality. Radar observations were conducted to measure the rotation of the planet, and concluded that mercury was rotating on its axis substantially faster than previously thought. A year didn’t seem to be one day long, but rather three. This was a very big change, and initially it seemed to contradict vast amounts of observational data.  Mercury had been known about since antiquity, and so has been studied extensively for as long as telescopic observations have been possible. The same face had always seemed to point towards the sun. Astronomers had examined this wealth of data, and seen a clear pattern, which fit well with a certain model of how mercury’s orbit could work, namely a 1:1 resonance. But how could this be the case if it was rotating at the speed these new observations suggested?

As is often the case in science it turned out that a “confounding variable” was influencing our previous observations. The incorrect model assumed that the reason we always saw the same face of mercury was because of its orbital resonance. In fact there was another reason which hadn’t been considered. Mercury and Earth are both in orbit around the sun, but it takes them different lengths of time to complete a trip around the sun. The result of this is that they are not always close together. There are times when Earth and Mercury pass close to one another, and other periods when they are much further apart. Close approaches make detailed observations more practical, while at other times of year it is much harder to see one planet from the other. This meant that the times at which all of the historical observations had been made conformed to a certain pattern, relative to the orbital properties of the two planets. 

Mercury’s orbital resonance meant that, like clockwork, whenever it passed close to the Earth, the same face was always pointed towards the sun. It wasn’t until new observational techniques could be used, that it became clear that this property of Mercury’s orbit was confounding the previous observations, and had led to an inaccurate model of the system. 

We now know that Mercury is actually in a 3:2 resonance with the sun. This means that it rotates around its axis three times, for every two orbits around the sun. This doesn’t seem very good for our chances of finding water on Mercury. All of the planet’s surface will become staggeringly hot during the day when it is directly illuminated by the sun, before dropping massively at night. Not only can there not be an entire night side covered with glaciers and ice, but will there be any water at all? 

Fortunately for anyone who might want to visit Mercury in the future, the image we started with shows that there are places which are always in the dark. Because of the angle of the sun, the polar craters shown in the image we started with are always in shadow. Chao Meng-Fu, a 180 km impact crater near the South Pole is considered very likely to contain water ice, and so would be a prime site for a future mission to the closest planet to the sun. 

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington https://photojournal.jpl.nasa.gov/catalog/PIA19416

https://photojournal.jpl.nasa.gov/catalog/PIA15533