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On Monday I asked you to identify this “evening star”. This is a term that is generally associated with the planet Venus, and has been since Roman times. However this particular object isn’t Venus, although it looks very similar to it when seen in this way. I also mentioned that there was another, smaller, body in this image. This other solar system object is just below the main planet, although it is hard to see without enhancing the image. These two objects appear close together in the sky, which doesn’t necessarily mean that they are close together in space. However in this case they actually are. This is Earth and its Moon as seen from Mars. The picture was taken by NASA’s Curiosity Rover. The annotated image below was released by the Curiosity Rover team, and shows the positions of Earth and the Moon. 

It is no surprise that Earth appears as a bright planet in the martian sky. It is the largest of the terrestrial planets, and at closest approach Earth and Mars are only 54.6 million kilometres apart. None of the planets emit their own light, so how easy they are to see when we look up at the night sky is entirely down to how much sunlight they reflect. This is controlled by a number of factors. First it is fairly intuitive that larger planets will reflect more light, and that the closer you are to the sun the more light there will be to reflect. If Jupiter and Saturn were the size of Earth they would be much harder to see, but as they are vastly larger they more than make up for the reduced sunlight in their distant part of the solar system. Venus is very bright because it is relatively large, close to the sun and close to the Earth. However Mercury, which is closer to the Sun, is much darker. This tells us that there is another factor in play, and that it must be to do with how reflective Mercury and Venus are. 

Venus

Mercury

If we look at images of these two planets we can see that they are very different. Venus is covered with bright clouds, while the surface of mercury is much darker, covered with solidified lava and potted by meteorites. The reflectivity of the surface is called its “albedo” and there is a sharp contrast between that of Mercury and Venus. Every type of surface has an Albedo, so a planet’s overall reflectivity is the average of the different terrain types that cover its surface. The clouds of Venus reflect most of the sunlight which hits them, 75 percent in total. Mercury on the other hand reflects only 6 percent of incident light. 

The albedos of Earth and Mars are actually very similar, although for different reasons. Mars has a lot of bare, redish rock, but also the bright polar caps, and wisps of cloud in its thin atmosphere. Earth has much more cloud and ice, but is also covered with a large ocean. The seas are dark, and so reduce the overall albedo of our planet. Even so Earth comes out slightly brighter, with an albedo of around 30 percent, compared to 29 percent for Mars. 

From this information we can infer that Earth will look somewhat brighter in the martian sky than Mars does to us. Their albedos are similar, and they are obviously the same distance apart. However the earth is closer to the sun, and substantially larger.  

Image Credit:

Earth from Mars: NASA/JPL-Caltech/MSSS/TAMU

https://photojournal.jpl.nasa.gov/catalog/PIA17936

Mercury: https://upload.wikimedia.org/wikipedia/commons/d/d9/Mercury_in_color_-_Prockter07-edit1.jpg

Venus: https://upload.wikimedia.org/wikipedia/commons/e/e5/Venus-real_color.jpg

Further reading:

More about this image here: https://www.space.com/24593-mars-rover-curiosity-sees-earth-photos.html

Brightness of the planets: http://magicvalley.com/lifestyles/recreation/column-a-planet-s-brightness-depends-on-several-factors/article_4bce554f-c310-5071-88d0-e39e68a54da4.html

Albedo of solar system bodies: http://sciencing.com/albedo-planets-5203.html

Guess the Planet 36: Evening Star

See what you make of this week's image. There are clearly two planets in this picture. The photographer is standing on one of them, and the other can be seen as a bright star in the evening sky not far from the centre of the image. So what planet do you think it might be and can you spot a third object in this image?

Check back on Friday for the answer!

The Oceans of Europa

Here is this week's answer article, sorry its been a bit delayed! 

This week’s image comes from the icy moon of Europa. This satellite of Jupiter has long captured the imagination of both scientists and the public due to the fact that it most likely has an ocean of liquid water deep below its icy shell. This image, which was captured by NASA’s Gallileo spacecraft, covers a large swathe of the small moon’s surface. In the full image below you can see variations in colour across the area around the grid-like cracks. The whiter and bluer regions are predominantly covered by purer ice, while the reddish and brownish areas are likely to be a mixture of other materials such as salts which have been brought to the surface by activity on this small world.  

 The most striking feature of this image is the presence of a grid of highly regular, rectilinear cracks. The presence this grid is actually a major piece of evidence for the presence of a subsurface ocean. It is believed that these cracks are left over from occasions when the icy crust broke up into several “rafts”, floating on the ocean below. These blocks of ice would have been much larger than the icebergs we are familiar with in terrestrial oceans, and would not have been adrift on an open sea. Instead a better analogy would be the tectonic plates responsible for the distinctive pattern of continents and oceans on the Earth. These are large sections of crust which float on, and indeed merge into the fluid mantle below. However there are no proper gaps between them, or sections of exposed magma. The cracks we are seeing in this image are the boundaries between these plates, and show just how many areas have exhibited movement over geological time. 

These ice rafts seem to have fused into a single icy shell, and it is possible that the ocean below is frozen as well. There isn’t very much heat from the sun that far out in the solar system, and the internal heat from Europa’s core would be expected to have dispersed long ago. However the scientific consensus is that Europa is far from dead and frozen through. We believe that there is still a liquid ocean below the ice, and that this small moon is very active. One major piece of evidence for this is cryovolcanism. Plumes similar to those found on Enceladus have been seen emanating from Europa, and suggest a ready source of liquid water. Another line of evidence can be seen in any image of the moon. The surface of Europa is extremely smooth, with very few craters. This suggests that it has been resurfaced fairly recently (in geological terms at least) this lends weight to the hypothesis that there is liquid water below the ice. 

This subsurface ocean is likely to be kept liquid by the heat from tidal flexing. The gravitational pull of Jupiter has a very strong effect on the small bodies which orbit it, and the flexing of Europa’s crust introduces a lot of energy into the system. This likely counteracts the very cold conditions which would be expected in this part of the solar system and provides a strong geothermal gradient between the core of Europa and the liquid layer. It has been estimated that there is substantially more water on Europa than there is on Earth, even if much of it is frozen that is a substantial sea. It is even possible that this ocean could host the necessary chemical for the development of life. A lot of work related to Europa is dedicated to examining the likely chemical composition based on what limited data we currently have. More missions to the Jupiter system are planned, and a variety of methods for sampling the subsurface ocean have been proposed including having a satellite fly through a plumes or sending a lander to drill into the ice. It is hoped that the Europa Clipper mission, which will launch in the 2020s will include a lander and shed more light on this fascinating moon.  

Image Credit:

https://photojournal.jpl.nasa.gov/catalog/PIA03002

Guess the Planet 35: Grid

Here is this week's guess the planet image. What is causing this grid like pattern, and which world can it be found on?

Check back on Friday for the answer!

Aram Chaos

This week’s image comes from Mars and shows one of the canyons of Aram Chaos. This channel is one of many which riddle the landscape of this region, creating an area of “chaos terrain”. The image I posted on Monday is part of this HiRISE image, and credit goes to the NASA HiRISE team. A Themis image of the whole basin, showing the jumble of blocks which make up the Chaos terrain is shown below. 

Aram Chaos is located where the eastern end of the Valles Marinaris opens out into the Chryse planitia. This places it near the “dichotomy boundary”, the line which separates the northern lowlands of Mars, from the heavily cratered highlands of the southern hemisphere. This region of the planet is very interesting, as it marks the dramatic shift from one terrain type to another. It has been speculated that the northern lowlands were once a vast sea, the dichotomy boundary certainly looks like an ancient shoreline. However there is considerable debate as to whether a sea could have formed in the past. It certainly seems likely that early Mars had substantially more liquid water than it does today, but whether there were once oceans like those of Earth remains uncertain. It is likely that smaller bodies of water occurred in a more ephemeral manner, freezing as the climate shifted over time. 

This brings us back to the Aram Chaos, which has been the site of one of these bodies of water at several points during its history. The site consists of a large impact crater 280 km in diameter. This crater has been heavily modified by later processes, producing a distinctive region of chaotic terrain. The basin is filled with large canyons which separate huge mesas, blocks of stone kilometres to tens of kilometres across. It is the jumble of blocks and mesas which gives areas of chaos terrain their name. In satellite images of the surface they look like a complex labyrinth etched into the surface of Mars. These blocks formed due to the thawing of water at this site. The sedimentary materials here were gradually deposited over time, as an accumulation of water saturated dust, sand and ultimately ice. It all froze into a solid mass, filling the basin. However at a later point in the history of Mars there was a sudden rise in temperature, probably due to the intrusion of magma into the ground below these deposits. This caused massive melting and released the water which had been frozen in the ground in the form of ice. 

 

Much of this water overflowed the basin, carving an outflow channel which extends away from Aram Chaos into the Ares Vallis. The formation of this outflow channel wasn’t the only dramatic effect which this water had on the landscape. Ice has a much larger volume than liquid water, so the thawing of the ground ice led to massive amounts of collapse across the basin. This is what led to the jumble of blocks and canyons that characterise the region. A bit more erosion over the following millennia resulted in the characteristic landscape of blocks and canyons seen today.

   

Studying the morphology and mineralogy of areas like Aram Chaos can tell us a great deal about their history. We can infer the processes which took place from the landscape, and the presence of sedimentary material overlaying the chaos terrain indicates that it remained flooded for some time, allowing material to accumulate. This fascinating area has been imaged by NASA and ESA spacecraft many times and you can learn more about its history at the links below.

In other news I’ve just had a paper published in the journal Icarus. It is open access so if you would like to find out about the strange boulder patterns of the martian northern plains then check it out. It’s obviously a lot more detailed than the articles I’ve been writing here, but has just as many cool HiRISE images!  http://www.sciencedirect.com/science/article/pii/S0019103517301082

Further Reading:

http://m.esa.int/Our_Activities/Space_Science/Mars_Express/Heavily_eroded_Aram_Chaos

https://themis.asu.edu/feature/50

Image Credits:

HiRISE image of chaos terrain: http://www.uahirise.org/ESP_011792_1795

The Aram Chaos Basin: https://themis.asu.edu/files/features/050_aram_chaos/050aramchaos.png

Guess the Planet 34: Blocks

Here is this week's guess the planet. This trough separates several blocks of higher terrain, but what caused this landscape, and which planet is this?

Check back on Friday for the answer article!

Glaciers

This week’s guess the planet image shows a glacier on the island of Svalbard in the terrestrial arctic. The image I posted on Monday shows crevasses in the surface of the glacier. The larger image below, which is also from Google Earth, shows the “snout” of the glaciers, the end where they breaks up as it encounters the sea. 

Glaciers are very interesting features, because they are a great example of how seemingly solid substances can flow. They form in locations where ice accumulates from year to year, rather than melting away in the warmer months. In general this occurs on shadowed, pole facing slopes in mountainous areas, which remain very cold throughout the year. More and more snow accumulates in such areas and is gradually compacted into ice under its own weight. 

Once a large enough mass of ice forms it will begin to flow, as the weight of accumulating ice forces material down slope. Once a glacier gets moving it will flow through several mechanisms. You can see in the image above that this glacier has numerous fractures in its surface. These are perpendicular to the direction of flow, and indicate that the upper layer of the glacier is brittle. However deeper regions of the ice do not behave in the same way. Once a glacier reaches 30-50 meters deep it begins to deform in a “plastic” manner. This means that it behaves more like a Newtonian fluid.

However the glacier doesn’t stay solid all the way through. A massive slab of ice moving across the uneven surface of the ground inevitably causes friction. Glaciers have a vast amount of erosive potential and can scour out huge valleys as the move down hill. This means that there is often water below the ice, which has thawed as a result of the energy the glacier releases. This lubricates the lower surface of the glacier and lets it slide over the surface more efficiently. One of the main ways glaciers are classified is as “warm based” and “cold based”, depending on whether they have liquid water helping them along. Massive volumes of rock and soil can be eroded by the glacier and displaced to the sides of its path, or carried to its snout where they are deposited as “Moraines”, ridges of clay and boulders mixed up in their journey down the glacier. 

The depositional zone is defined by the overall balance of accumulation and melting across the glacier as a whole. The zone of accumulation is generally in high areas where a large volume of snow and ice is deposited. The zone of ablation at the snout of the glacier is where it is actively melting. In this image you can see that the end of the glaciers is calving into icebergs as they reach the sea. The accumulation zone is off the edge of the image, towards the interior of Svalbard. 

The line between the zones of accumulation and ablation is very dependent on climate. If it is near the snout then there will be lots of accumulation and the glacier will continue to grow and advance. The warmer the climate becomes the further this divide moves back towards the source. The glacier begins to retreat, exposing the u-shaped valley it has spent at the very least thousands of years carving. It leaves behind moraines and deposits of boulder clay, as well as “eratics” huge blocks of rock carried far from the sites where they were eroded. 

At present our warming climate means that the world’s glaciers are retreating at an unprecedented rate. We need to take action in order to check climate change and ensure that these magnificent landforms don’t vanish altogether. This isn’t just an academic concern. The seasonal melting that takes place at the snout of an upland glacier produces a vast amount of fresh water. In upland areas they are an important source of drinking water, relied upon by thousands of communities worldwide. 

Image Credits: Google Earth