Guess the Planet 44) Orb

Check out this image! 
Which planet do you think is shown here? And what does this image tell us about it?

Check back later in the week for the answer.

What can we learn from an eclipse?

The eclipse in this week’s guess the planet image is quite similar to the one which was visible from some parts of the world on Monday.. The main difference is that in this image, it is the Earth which is blocking out the sun. The observer is not on the moon, but rather in orbit around it. This image was captured by the Kaguya spacecraft of the Japanese Space Agency’s (JAXA), when the Earth passed between it and the sun. A film of this eclipse, including a spectacular diamond ring effect can be found on the JAXA website. 

Eclipses have been observed since antiquity and our ability to predict them has always been very good. We know the movements of the planets with great precision, and can easily project those movements into the future. There are few sources of uncertainty which can affect these motions, so phenomena such as eclipses, the recurrence of comets, and the movements of the planets across the night sky can all be predicted quite reliably. This is also true of periodic meteor showers such as the Perseids and Leonids which occur when the orbit of the earth intersects fields of debris, generated by the passage of comets. As a comet moves along its orbit it leaves debris behind. This means that whenever the earth crosses the comet’s path it hits some of this material, even if the comet itself is a very long way away at the time. This debris burns up in the atmosphere producing shooting stars, which can always be seen in the same parts of the sky at certain times of year. 

We have quite a few images of eclipses from other planets. The image below was captured by NASA’s surveyor 3, from the surface of the moon in the 1960s. The image isn’t as high quality as the more recent observation from Kaguya, but is still an impressive example of how our home planet can block out the sun. 

The image below was captured by the New Horizons spacecraft as it passed Pluto. Here the dwarf planet is silhouetted against the sun. At first glance this might not seem like a particularly useful image, after all none of the surface of the small world is visible from this angle. However, it wasn’t the surface which the New Horizons team were interested in when they took this image. The blue glow around Pluto is its atmosphere, and from this angle the light of the sun passes directly through the “limb” of the atmosphere, allowing measurements of its properties to be captured. 

You can learn a lot about the chemical composition of a material from the way light interacts with it. Different elements preferentially absorb and reflect light of different wavelengths. This is why materials appear to be different colours, and looking at the colours, or regions of the spectrum, that interact with materials is a good way to determine their chemical makeup. This process is called spectroscopy and can be used at a range of scales. On the very small scale tiny amounts of material can be superheated to see what wavelengths of light they emit. On the very large scale the wavelengths emitted by stars and galaxies can be analysed to look for peaks in certain parts of the spectrum which indicate their chemical composition. Other materials won’t emit light, but will reflect or scatter it, and again some colours of light will be reflected more strongly than others. This is the case in this image of Pluto’s atmosphere.

Examining the interaction of light with the atmosphere revealed a lot of information and allowed the New Horizon’s scientists to work out how Pluto’s haze probably formed. NASA’s description of this image states that: “…the haze is a photochemical smog resulting from the action of sunlight on methane and other molecules in Pluto's atmosphere”. One interesting thing about this image is the blue colour. On Earth we are used to seeing a blue sky, and it appears that colour because our atmosphere preferentially scatters light with a blue wavelength. The new Horizons team note that the same is true of the particles that form the haze on Pluto leading to this characteristic colour.  

Image Credits:

Japanese Space Agency (JAXA) (C) JAXA/NHK, http://space.jaxa.jp/movie/20090218_kaguya_movie01_j.html

NASA https://science.nasa.gov/science-news/science-at-nasa/2009/25feb_kaguyaeclipse

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute https://photojournal.jpl.nasa.gov/catalog/PIA21590

Guess the Planet 43: Eclipse

Check out this image of an eclipse somewhere in the solar system. This isn't our moon, so which solar system body is blocking out the sun? And where was this image taken?

Check back later in the week for the answer!

Life, the Universe, and Everything…

At first glance this week’s image seems to show a red plain which would not look out of place on Mars. However, we can tell that this is a view of Earth The large white structure is clearly artificial, and the wiggling line connecting it to the edge of the image has the characteristic shape of a road. The red surroundings are not what they first appear, as this is actually a false colour view image from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument aboard the Terra Spacecraft in orbit around Earth. This instrument captures images of the Earth’s surface, but unlike a conventional camera it doesn’t just rely on the visible wavelengths which we are used to seeing. ASTER’s camera also captures information in the infrared region of the spectrum, which can tell us a lot about the temperature of the areas it surveys.

False colour images like this one are very common in remote sensing. By comparing how the surface looks in different spectral bands we can learn a lot about its properties which would literally be invisible in a visual image. Different parts of the spectrum can be used to categorise land use, determine the health of the environment, and assess the impact of human activity. 

The man made features such as roads and buildings can give us a sense of scale for this image, which according to the NASA’s photojournal “was acquired April 14, 2013, covers an area of 6.2 by 8.2 km, and is located at 25.7 degrees north, 106.9 degrees east”. This puts it in the Guizhou Province of China. The object in the centre of the image is the dish of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). This observatory has only recently been completed and is now one of the largest radio telescopes in the world. Google Earth gives us a closer look at the structure under construction, and shows the scale of what has been built here.

 

Radio telescopes are one of our best tools for studying the universe beyond our solar system. They consist of massive receivers, which pick up radio signals from distant stars and galaxies. This means that they are also using electromagnetic radiation to acquire an image, but are using a section of the spectrum much further from the visible than even an infrared camera. The information which they can acquire is also very different. Radio astronomy is a good compliment to that achieved using visible light, in the same way that using different spectral bands can compliment visible wavelength images of the Earth.

Radio telescopes take the form of large antennae, which receive and interpret the signals produced by distant stars and galaxies. However these signals tend to be very weak, as they have travelled across vast distances to reach the earth. This means that ever larger telescopes are constantly being built to pick up ever fainter signals from the distant reaches of the universe.  

The FAST telescope takes the form of a vast, circular dish, built into a natural depression in the Chinese countryside. This google search contains numerous images of the array under various stages of construction, as well as highlighting the real colour of the terrain around it. The FAST array will make detailed observations of Pulsars, as well as surveying the interstellar molecules to tell us more about the distribution of elements across the universe.

Image credits: 

• NASA/METI/AIST/Japan Space Systems, and U.S./Japan ASTER Science Team https://photojournal.jpl.nasa.gov/catalog/PIA20986
• Google Earth

Guess the Planet 42) Red

Check out this image. Where do you think this landscape is? and more importantly what can we see here? 

Check back later in the week for the answer!

Landslides of Serenity Chasma

This week’s guess the planet image comes from Charon, Pluto’s largest moon. It shows a series of landslides in a valley which has been informally named Serenity Chasma. Charon is quite an unusual object. Most moons are much smaller than the body which they orbit, but Charon has a diameter of more than half that of Pluto. Charon and Pluto are gravitationally locked, so that the same faces of the two worlds are always facing each other. This means that Pluto doesn’t move in the Charonian sky. As with Pluto, it was only fairly recently that we got our first good look at Charon. This week’s image comes from the New Horizons spacecraft, which visited the system in 2015 and credit goes to their team.

The version of the image below was annotated by the New Horizons team. The arrows indicate where several landslides have separated from the scarp, and produced small fans of debris further down the hill slope. These are quite subtle features, especially given that the images we have of Charon are much lower resolution than those which are available for Earth and Mars. However NASA have also provided a 3D view, annotated to show one of the features. As you can see from the scale information in the images, this scene covers a very wide area. These landslide features are by no means small features, but extend for kilometres down the slope. They are significant as they are the first such features to be found on Charon, or any body in the Kuiper Belt. 

Landslides occur when a hill slope becomes unstable. This instability can gradually increase over time, as the rocks become weaker, or the slope steeper.  At some point a “trigger” will occur, causing the slope to collapse and a “mass movement” to occur. A huge volume of material will detach from the slope and travel downhill until it comes to rest on flatter ground below. In the case of large landslides, like these ones, the material can be deposited over a very large area, often producing a wide fan of deposited material. The events which trigger a landslide can vary. Some are triggered by earthquakes, which provide a large amount of energy to dislodge unstable slopes. Volcanic processes can also shake the ground and trigger collapse. 

In some cases less dramatic processes trigger the landslide. Some slopes are held together by icy material within a soil. When this thaws the slope becomes less stable and collapses. In other cases gradual erosion of a hillside, whether by wind, water or other processes can destabilise a slope. On Earth biological weathering or the activity of humans can be responsible for triggering a landslide, although this probably isn’t the case on Charon. Charon also doesn’t have much of an atmosphere, although it might “share” some gasses with Pluto. This means that wind erosion isn’t going to be a factor on the moon. Add to this the extremely cold conditions that far from the sun and many of the likely sources of erosion are unlikely to be active in the present day. 

Assessing the cause of a landslide from remote sensing images may seem like a daunting prospect, but in some cases there can be clues as to how the event occurred. Wet material will flow downhill very differently to a mass of dry rocks and boulders, and will leave a different deposit behind once the dust settles. We can thus look at the shape of deposits like these.  Where we have good enough data the shape and composition of the material that makes up the fans can be assessed. With a bit of detective work we can sometimes determine how long ago the landslide occurred and what factors led to its occurrence. 

The material which makes up these landslides is expected to be ice, or the sort which makes up most of the crust of the small moon. However we don’t have a high enough resolution image of the terrain here to see whether it is in large blocks or not.  We can’t know for sure with the information we have, but given the lack of other trigger processes on Charon, the most likely explanation is that they were triggered by either an Earthquake of some sort, or by a meteorite impact. This potentially occurred a long time ago, as there wouldn’t have been much geological activity on Charon to erode the resulting deposits. Without getting a closer look at these features, and seeing whether they have any impact craters on them we can’t know for sure. Dating using crater counts is the only way we can estimate ages on planets which we haven’t collected samples from.  

 

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute https://photojournal.jpl.nasa.gov/catalog/PIA21128

https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA21129

Guess the Planet 41: Scarp

Check out this hillside on an undisclosed body in the solar system. The features I'm going to be talking about in this image are quite subtle, but there are five of them in the picture. See if you can figure out what they are, and where in the solar system this location might be.

Check back later in the week for the answer!

Novae and Coronae

This week’s image shows Yavine Corona, a volcanic feature near the equator of Venus. The description of this image on the NASA photojournal describes it as “a 500-km-wide asymmetric feature at latitude 5 degrees S., longitude 248.5 degrees; looking northeast”. This gives us a sense of the scale of the feature, a huge irregularly shaped dome with extensive fractures.  The 3D view was made by combining synthetic aperture radar data with elevation information, all from the Magellan Spacecraft. 

They don’t state to what extent the vertical scale has been exaggerated in this image, but it certainly looks as though it has been, this is particularly the case with the smaller dome to the right of centre, which has the characteristic features of a gentle hill where the “relief” has been exaggerated. This is common practice when presenting topographic features of this scale. It helps to pick out subtle details which would otherwise be too small to detect with the naked eye. We are also used to seeing hills and mountains from below, rather than from the sky. What would look like a steep slope from the ground, can often seem far too shallow when seen from a different angle, especially when the feature is hundreds of kilometres across. Some structures like shield volcanoes are very tall, but don’t actually have the steep slopes we associate with other types of mountain. This means that we often need to exaggerate the image in order to correctly interpret what we are seeing. 

The word Corona means “crown” in Latin, and continues the theme of planetary scientists naming things based on their approximate appearance. Coronae are vaguely “crown like” volcanic features, consisting of a heavily fractured plateau, often surrounded by a deep trench. The whole edifice is riddled with concentric and radial fractures. Coronae are believed to form through the upwelling of plumes of Magma. These partially melt the surface, causing collapse and faulting. However the exact mechanism is still being debated. There are 513 such features on Venus, and similar structures have been observed on Miranda, one of the moons of Uranus. The features on Miranda would have formed through the upwelling of molten ice, but are otherwise quite similar in form.  

Coronae are not the only volcanic feature in this image. Yavine Corona also exhibits two “novae”. These are circular hills around 100-300 km in diameter. They are named for the star like fractures that radiate out from their centre. Novae may be an early stage in the development of the larger coronae. The image below shows a close up of the southern of the two structures. The NASA photojournal notes that this feature is approximately 100 km across. Further data is needed to determine precisely how novae and coronae form, and whether one feature develops into the other. 

Image Credits: NASA/JPL/USGS

https://photojournal.jpl.nasa.gov/catalog/PIA00098

https://photojournal.jpl.nasa.gov/catalog/PIA00150