The Ocean Floor

Last week’s guess the planet image comes from the Earth. However, it isn’t a view which many of us will be familiar with. This is a section of a map of the ocean floors, showing the central Indian Ocean. The red areas show regions of high ground, while the blue patches indicate lower regions. The full version can be found at this link. This map is one of the most detailed views of the entire ocean floor to have been produced, and was a vital contribution to the science of bathymetry, the study of underwater topography. The map was produced using gravity measurements. These show how the water in the oceans is displaced by variations in the height of the sea floor. This is called a geodesic method, as it relies on the shape of the earth, and the variations in gravity, rather than direct imaging of the sea floor.

This study highlights an important trade off that must be considered when looking at data of this sort. Ideally we want the best quality of data available, the most detailed images and the highest resolutions. However coverage is also vital. If we only have high resolution data of a few key places that can often be less useful than less detailed images which cover a larger region. The latter can give us a better understanding of the context and environment as a whole. This trade off is very important when considering spacecraft images. Many probes only have a short time to record data, so it is important to carefully consider how much high resolution, low coverage data should be acquired, and when it is more important to get larger scale, lower resolution images. 

Mapping the ocean floors is no different. There are many methods for recording bathymetric data, but many rely on ship mounted instruments to image the depths. Techniques like sonar can be used for this but they require a ship to actually sail over the area being studied taking measurements as it does. However, relatively few survey ships are available to chart the sea floor, and so most maps of that sort have been made for major shipping routes, rather than trying to systematically build up a global picture. It is easy to forget how large the world is, or how much of it is covered by water. Surveying the entire ocean floor by this means would produce a very detailed dataset, but would take far too much time and effort to be achievable. 

The gravity based method cannot match the resolution of sonar measurements, but is the best way to get a global picture of the ocean’s depth. The global picture is vitally important when examining the ocean floor, because the topography of much of the sea bed is defined by the global process of plate tectonics. The pattern of highs and lows shown in this map indicates the boundaries of the Earth’s tectonic plates, allowing the way they meet and interact to be studied in unprecedented detail.

The spreading centres where plates pull apart are marked by ocean ridges, while subduction zones are often accompanied by deep trenches like the Marianas Trench, where the lowest point on Earth can be found. I actually chose the Indian Ocean because the mid Atlantic ridge might have been a little too conspicuous; whereas the region shown above has a range of topography, reflecting its complex tectonic history. 

 Image Credit: NASA Earth Observatory

https://www.nasa.gov/image-feature/new-seafloor-map-helps-scientists-find-new-features

https://earthobservatory.nasa.gov/IOTD/view.php?id=87276

https://earthobservatory.nasa.gov/IOTD/view.php?id=87189

 

Apologies for the delay in bringing you this blog. Things have been very hectic, and its taken longer than usual to get this one ready. I will probably wait until December before posting the next guess the planet, so that I have a looming deadline out of the way. 

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

Guess the Planet 47: Pole

Here is this week's guess the planet image. What do you think this false colour image is showing us? and which planet does it depict?

Check back later in the week for the answer!

Frost Covered Dunes and Phase Changes

This week’s image takes us back to Mars, and because it comes from the HiRISE instrument we can have a look at a full colour version in wonderful detail. The addition of colour makes this scene even more spectacular than the black and white version. What we are looking at here is part of the circumpolar dune field, which surrounds the northern icecap of Mars. 

The North Pole itself is covered by a large icecap, made up primarily of water ice. This remains frozen year round, but during the winter a thin layer of Carbon Dioxide ice forms on top of it. This seasonal layer is only around a meter thick. Carbon Dioxide is also responsible for the bright streaks in our image. The glistening patches are covered by CO₂ frost, which accumulated during the northern winter. As the HiRISE team explain in the caption to this image, the satellite captured this scene during the early spring. This means that the seasonal frost layer was disappearing at this time. The feathery patterns which we can see in this image indicate places where the frost is sheltered from the sun, and so has yet to evaporate. 

Martian frost doesn’t melt, as water ice does on Earth, but rather goes straight from the solid state to a gaseous one, in a process called sublimation. Sublimation is very important on Mars, because the temperature and pressure conditions there mean that several of the substances which shape the martian landscape aren’t stable in their liquid phase. This is particularly true of water, which can only exist as a liquid for short periods of time on Mars, or under unusual conditions. It is fairly intuitive that the cold temperatures on Mars will make the liquid water freeze again, but what is equally important is the low pressure. 

Pressure has a large effect on the position of the freezing and boiling points of a substance. These are the temperatures at which it will change phase from a solid to a liquid and from a liquid to a gas, respectively. We are used to seeing these phase changes take place at set temperatures on Earth. With our fairly high atmospheric pressure water almost always freezes at 0^oC, and boils at 100 ^oC. The fact that these are nice round numbers is no coincidence. The Celsius, or centigrade, scale was deliberately calibrated based on these commonly observed properties. 

Although the modern centigrade scale is named after him, it doesn’t work quite the same way as the system which Swedish physicist and astronomer Anders Celsius first proposed in the early 1740’s. Celsius made measurements in the opposite direction, so that zero degrees represented the boiling point of water while 100 was calibrated to fall at the freezing point. The system we use today is actually based on a scale that was first proposed by the French physicist Jean-Pierre Christin. He appears to have come up with the idea independently of Celsius, at around the same time. The ascending scale took off when Celsius’ scale was reversed by another Swedish scientist; Carolus Linaeaus for use in his botanical research. The 0-100 degree system quickly became the most popular and is now a scientific standard. 

If he had lived on Mars, Celsius would likely have had a much harder time calibrating his temperature scale. The low atmospheric pressure means that there is no easily defined interval in which liquid water is stable. It will begin to boil the moment it is exposed to the thin atmosphere. This naturally produces water vapour, but ironically it can also turn the bulk of the boiling water back into ice. Evaporation requires energy, and this is drawn from the boiling liquid. When very hot water boils there is plenty of energy to go around, and the liquid will keep on evaporating until none is left. When cold water boils this energy is in short supply, and so the liquid gets colder and colder until it freezes again. This process is called evaporative cooling, and could result in ice covered rivers, where the exposed water boils away, until a cap of more stable ice forms above it. When we look at outflow channels on Mars, or the debris flows left by martian gullies, we need to take this process into account, as it will have a substantial effect on the appearance of the landscape such flows of water leave behind. 

Image Credit: NASA/HiRISE/University of Arizona https://www.uahirise.org/ESP_050703_2560