Guess the Planet 7) Dark Smudge

Here is this week's guess the planet post. What is the dark smudge in the lower part of this image, and what solar system body is this? 

Apologies that this post is slightly delayed, this week is a bit busier than usual. The answer will be posted on Saturday rather than Friday, so that you get the usual length of time to think about the answer.

Geysers on Enceladus

This week’s image comes from Enceladus, an icy moon of Saturn. Credit goes to the Cassini team. The Cassini spacecraft orbits Saturn and performs frequent flybys of the various moons that orbit the gas giant.

This image shows plumes, or geysers erupting from the surface of the moon. They originate near the south pole of Enceladus and the material they eject seems to feed into Saturn’s ring system. These eruptions are very interesting as they are strong evidence for a process called Cryovolcanism. We usually think of volcanoes as being very hot, erupting molten rock in the form of Lava. On cold icy worlds like Enceladus similar processes are believed to occur. However the materials involved are very different.

 The term cryo comes from a greek word meaning “icy cold” and this is very apt when discussing the moons of Saturn. Instead of solid rock the surface of Enceladus is covered with ice. At the surface this doesn’t often thaw, as temperatures never get high enough. However it appears that the heat from the planets interior is sufficient to melt ice in the subsurface. This liquid water likely behaves much as magma does on Earth, including forming volcanoes in some areas. Cryolava erupts from the moon’s surface forming the geysers pictured here.

Cryovolcanism had been posited on a variety of cold planets and moons, including Europa, Titan and Pluto. However most of the evidence for cryovolcanism on these bodies came from the identification of features which appear similar to terrestrial volcanoes. Without studying them on the ground, or catching an eruption it is difficult to be certain whether they are really cryovolcanic features or not.  It wasn’t until 2005 that this image, and others from Enceladus caught a cryovolcanic process in the act. The existence of these plumes indicates that this moon is geologically active and provides strong evidence that cryovolcanism is possible across the colder regions of the solar system.

Guess the Planet 6: Jets

Here is this week's guess the planet, an image which some of you might be familiar with. I figured we'd go for a slightly different angle this time around. Which solar system body do you think is shown here, and what are the bright jets coming from its surface?

Check back on Friday for the answer!

Dune field

This week’s guess the planet picture comes from much closer to home than some of our previous images. This is a scene of the Taklamakan Desert on Earth. This sandy region in western china is quite sparsely inhabited. It is the world’s second largest “erg” or area of continuous sand. Many other deserts are larger, but only the interior of the Arabian peninsula has a larger expanse of shifting sand.  

Since this image comes from Earth we can get some good colour pictures for a change. Here is an alternate view, of an area a little to the west of the section I posted on Monday.

 Here you can see a road crossing the desert on the left hand side of the image. Credit for this picture goes to the Landsat team, and Google Earth. The striking dune patterns we see here are the result of aeolian processes, which means that they are shaped by the wind. This means that dune fields can only occur on planets which have a significant atmosphere. Most of the larger terrestrial planets have an atmosphere of some sort. Earth and Venus have the thickest atmospheres, while that of Mars is much thinner. Mercury and many of the solar system’s moons are largely airless, they do not have sufficient gravity to retain an atmosphere so gases which form there are quickly lost to space. 

Whether a planet has an atmosphere or not has huge implications for the geomorphology, because the movement of air is a very important process for eroding and weathering a planet’s surface. 

Erosion is the process which breaks down the surface of the planet, shattering stone and grinding down mountain ranges. It can occur through a variety of different processes including wind, water, chemistry and life. On bodies which lack most of the above the most significant erosive force can be the impact of meteorites. Chemical weathering can theoretically occur anywhere, but generally requires water to get the process started.

Dune fields and ergs are found across the solar system. Mars has small dune fields in many of its impact craters. Massive ergs surround the polar caps. Dune fields have been observed on Venus and on Titan. 

Studying the dune patterns can tell you a lot about the prevailing winds and the material which is being moved around by them. I’ll be featuring a few different types of dunes in future posts. The ones we’re looking at today are called linear dunes. They consist of parallel ridges of sand, generally waving. The direction of sand movement is along the long axis of the dunes. These often form in bidirectional wind regimes, where there are two dominant wind directions. This is very useful information if we are looking at another planet, and can’t put sensors ont he ground to measure wind speeds.

Guess the Planet 5: Ripples

In this weeks guess the planet we'll look at these ripple like features. Where in the solar system can they be found? Check back on Friday for the answer in a more detailed post about these features.

Buried impact crater on Mercury

This week’s guess the planet image comes from Mercury, and was imaged by the messenger spacecraft in 2011. Unfortunately the messenger team haven’t provided scale information to go with this image in their gallery, although we can infer that it covers quite a large area.

The most obvious feature of this image is the heavily cratered plane, however there are some other features of note as well. Craters are depressions, so they have what we call “negative relief”, they go down into the surface of the plane. However there are also some positive relief features in this image, narrow ridges which stick up out of the surface of the plane. The main thing I want to talk about this week is how to distinguish between the two types of feature in images like this. 

When you first looked at this picture, many of you may have seen the craters as domes, sticking up out of the image. This is quite a common optical illusion. Because we are looking down on the area, all we have to tell us which are positive features and which are a negative are the shadows. But it can be hard to figure out where the sun is coming from. Luckily we know what craters are and can easily identify them. Since they have to be negative relief features we can work out that the sun is coming from the bottom left corner of the image, and casting shadows over the crater floors. Knowing this doesn’t necessarily make it easier to see them as depressions, if you have got stuck seeing them as domes. One good way to trick you mind into seeing the image correctly is actually to turn it upside down. 

Once we can distinguish between negative and positive relief features we can interpret the image. We can see that this cratered plane is crisscrossed by ridges, and that they form a large circular feature on the left hand side of the image. These are called “wrinkle ridges”, which form as a result of the contraction of cooling lava. These features are not unique to Mercury, but are also common on the moon, and have been observed in some locations on Mars as well.   These ridges are interesting as they appear to be following the rim of a large, buried impact crater.  

This crater would once have been an obvious feature of the landscape. However it has been completely buried when this region, part of Mercury’s volcanic planes, was inundated with lava. As this lava cooled contraction occurred, causing the development of the wrinkle ridges. The presence of the buried crater had a controlling effect on the development of these ridges causing them to form in a circle. The result is that we can still see where the crater once was. It’s amazing what you can figure out from a few shadows!

Image Credit:

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington