Guess the Planet 32: Polygons

Here is this week's guess the planet! What does this image show, and what solar system body does it come from?

Check back on Friday for the answer!

The Rivers of Titan

This week’s image comes from Titan, Saturn’s largest moon, and shows a river flowing across the surface of that distant moon. It was captured by NASA’s Cassini spacecraft in orbit around Saturn. Unlike terrestrial rivers the material flowing through this stream is not water, but rather a mix of liquid hydrocarbons, such as ethane and methane, which would be gaseous were they on Earth. The solid ground into which this river has carved it channel is also not rock, but solid ice of the sort which covers much of Titan’s surface. Despite the vastly different composition there is a striking similarity between the morphology of this river system and its terrestrial counterparts. 

This river system has been dubbed Titan’s “River Nile” due to the similarities between the branching river and the morphology of one of Earth’s longest rivers however this river system isn’t nearly as long, or complex as that of the Nile. The full image shown to the right is approximately 400 km long, and shows the entire catchment of the river, from the sources of its various tributaries to the estuary where it flows into Ligeia Mare one of Titans large polar seas. Earth’s Nile is over 6000 km long, but then Earth is a much larger planet than Titan is!

Various characteristic features of terrestrial rivers can clearly be seen here. The river has several tributaries which converge to form a wider stream. The river starts off as a number of relatively narrow streams, which get progressively wider the further downstream you go. They end in quite a wide channel, which rapidly opens out into the lake. The river system also features prominent meanders. This indicates that like a terrestrial river the liquid is predominantly following topographic lows, carving a gently winding path as it does. The description of this image over at the NASA website notes that the river probably follows a pre-existing fault in the surface of the moon, as it does not meander as much as would otherwise be expected, so is probably constrained by the shape of the fault.  

NASA also notes that the dark colour of the river in this radar image indicates that this feature is filled with liquid. This indicates that this isn’t the channel left by a past event. It is flowing today, and has clearly done so for some time. This is very exciting as it shows that the hydrology of Titan is active. Other worlds like Mars have relic channels but these are usually attributed to sudden outflows of water rather than persistently flowing streams. Titan clearly has a hydrological cycle, and it is believe that hydrocarbon rain feeds this river valley just as water rain does on Earth. Cassini has observed areas which repeatedly get darker, suggesting that they are wetted by rainfall. This puts Titan and Earth in a very exclusive club, and makes comparing the similarities and differences between the two worlds a very useful field of study.

Image Credit:

Titan Rivers (NASA / JPL–Caltech / ASI) https://saturn.jpl.nasa.gov/resources/5700/

Further Reading:

https://saturn.jpl.nasa.gov/news/2493/cassini-spots-mini-nile-river-on-saturn-moon/

Guess the Planet 31: Veins

Here is this week's guess the planet. What sort of feature are we looking at here, and what world does it come from? 

Check back on Friday for the answer.

Canyons on Charon

This week’s image shows a canyon system on Charon, the largest moon of Pluto. This is a very interesting solar system body, as it is only recently that we have gotten a good look at its surface. This image comes from the New Horizons probe which visited the Pluto system in 2015. New Horizons has provided us with a huge amount of data about these small worlds, which will keep planetary scientists busy for years to come, however it had relatively little time to observe the Pluto system, as it never went into orbit around either Pluto or Charon. All of the images were recorded during a brief flyby, and so the observations had to be carefully planned in order to maximise the amount of information acquired. The distance to the Pluto system means that follow up observations aren’t likely any time soon. New Horizons is still operational, and will go on to examine other objects in the Kuiper Belt, however it will be decades before another spacecraft can reach Pluto, even if one were sent in the next few years. 

Charon is one of several small objects to orbit Pluto, but stands out due to its large size. It is almost half the size of Pluto, making it one of the largest moons relative to the planet it orbits, it is likely that it evolved independently of Pluto and was later captured by it. The full image shows how large this canyon is relative to the size of the moon. It is one of several canyons which scar the surface of this moon, another canyon can be seen in teh image below, but this one was imaged at an oblique angle, showing how deeply it cuts into the surface of Charon. NASA’s description of this image notes that this canyon seems to be 9 km deep in places, and that its cliffs might be the tallest in the solar system. 

In their description of the first image the new Horizons team note that this canyon lies on the boundary between the highlands to the top of the image and a large plain to the south. Both terrains are heavily cratered, and this highlights something very interesting about this small moon, which makes it very different to the dwarf planet it orbits; Charon is heavily cratered, whereas Pluto is not. 

Pluto has large regions with very few impact craters. Charon on the other hand is pitted with craters, and looks much more like Mercury or Earth’s Moon. This striking contrast is quite surprising. Since both objects are in close proximity they would be expected to be hit by the same number of meteorites and have the same density of craters. The fact that this is not the case indicates that Pluto has a surprisingly young surface. The icy surface of Pluto has clearly been resurfaced regularly, and this suggests that the planet could be far more active than expected given that it is so small, and so wouldn’t be expected to retain much heat. Precisely why Pluto is so active remains to be determined, but it makes these small planets quite an intriguing target for future missions. New Horizons’ observations definitely raised as many questions as they answered. 

Image Credits

 NASA/JHUAPL/SwRI

https://solarsystem.nasa.gov/galleries/a-super-grand-canyon-on-plutos-moon-charon

NASA/JHUAPL/SwRI

https://solarsystem.nasa.gov/galleries/charons-terrain

Further reading

Differences between Pluto and Charon

https://www.forbes.com/sites/startswithabang/2016/01/05/why-are-pluto-and-charon-so-different/#6dd79343440a

Guess the Planet 30) Trough

Here is this week's image. Where in the Solar System might this be?

 

Check back on Friday for more information about the features in this location... 

There will be a slight delay in this week's answer. 

Lunar Landing Sites

This week’s image comes from the Moon, and was captured by the Lunar Reconnaissance Orbiter. It shows the landing site of Apollo 14, the dark trails in this image are the tracks taken by the astronauts while they were exploring the Moon’s surface. 

This annotated image from NASA shows a slightly larger area than the one I posted on Monday. The positions of both the descent stage, and the Apollo Lunar Surface Experiments Package (ALSEP) can be seen. ALSEP was a suit of instruments left behind on the moon to study its environment, and includes seismic sensors and magnetometers, as well as instruments to monitor heat flow and the solar wind. 

Being able to see the tracks the astronauts took gives you a real sense of how they went about exploring this small area of the lunar surface, it’s not often that we see the impact of human activity on a world other than Earth. Since the moon has no appreciable atmosphere the environment around the lunar landing sites is essentially unchanged since the 1960s. We can see precisely what route the astronauts took. This can be very useful for lunar science. as it shows us which areas they looked at, and we can work out where photographs were taken and where samples were collected. It gives us an overview image and allows us to see how the astronaut’s observations fit into the larger context of the site. 

The lunar landings provide us with rare “ground truth” information for this handful of places on the Moon. The observations and samples which the astronauts brought back essentially allow us to calibrate our remote sensing observations, seeing what is there at the small scale, which would be invisible from a satellite image. We can examine samples to find out about the geology of the site, and this has various implications. We are able to confirm that the rocks collected from the Moon match the composition of some of the meteorites in our collections, proving that they originated on the Moon. We can also get precise dates for the age of lunar material using radioisotope dating, of lunar samples and meteorites. This allows us to constrain the history of the moon, and provides a baseline for estimating dates from crater counting. 

On the down side our sample size for these ground truth observations is quite limited. You can see from these images and the others at the links below that the astronauts only covered relatively small areas in their exploration of the moon. The later Apollo missions had Lunar Roving Vehicles (LRVs) which allowed them to cover more ground than the early astronauts who ventured away from their landers on foot. These vehicles gave the astronauts substantially more mobility and meant that samples were collected over a wider area. However, they were only on the moon for a short period of time, so couldn’t go very far. The Apollo 17 astronauts travelled the furthest covered a total distance of 35.74 km. The Apollo 17 landing site and the final position of their rover can be seen in the image below.

This article shows the furthest distances that vehicles have travelled on other planet (and moons), as of 2013 http://www.space.com/79-distances-driven-on-other-worlds.html. Both the Oppourtunity and Curiosity rovers on Mars have advanced substantially in that time, however another rover, manned or otherwise, has not been sent back to the moon. Although the majority of the moon’s surface remains unexplored by humans, we can use the observations made by the Apollo astronauts to shape how we interpret remote sensing data, and send robotic probes to cover more ground. these landmark missions remain some of the most important steps in the history of human space exploration, and it will be a long time before we go further. 

It remains to be seen when humans will next visit our nearest neighbour, but in the meantime robotic observations such as those from the LRO continue to provide useful data about the surface of our moon, pushing our knowledge of planetary science forwards as well as showing us the places we visited in the 20^th century. 

Image Credits:

Apollo 14 Landing Site NASA LRO team

https://www.nasa.gov/mission_pages/LRO/news/apollo-sites.html

Apollo 17 Landing site, NASA LRO team.

https://www.nasa.gov/mission_pages/LRO/news/apollo-sites.html

Further Reading:

Descriptions and images from the Apollo 15 landing site:

http://lroc.sese.asu.edu/posts/491

Guess the Planet 29: Dark streaks

Here is this week's guess the planet image. What is being shown here, and which world does it come from? 

Check back on Friday for the answer!

Cantaloupe Terrain

This week’s image comes from Triton, the largest satellite of Neptune. It was captured by the voyager two spacecraft and shows a region of “cantaloupe terrain”. Credit for this image goes to NASA’s Voyager team. Cantaloupe terrain is a good example of typical astro-geological naming conventions. When the voyager spacecraft imaged this region it was not immediately clear what was going on or how this distinctive terrain might have formed. It was thus named based on what it looked like, in this case the distinctive skin of a cantaloupe melon. This terrain is only found on this one moon, and consists of small depressions interspersed with ridges as illustrated above. Another view of this sort of terrain can be seen below. 

 

These depressions are 30-40 km across, and are believed to result from the overturning of Triton’s surface, which is largely icy in composition. The reason for the unusual surface texture is the way this overturn took place. Tidal heating from Neptune resulted in the formation of numerous diapirs. These are regions where ductile material rises up and forces its way through an overlying brittle layer. The term is derived from the Greek word for piercing, as the underlying material squeezes its way through the overlying layers. On Earth diapirs of magma tend to intrude into overlying rocky strata. On Triton ices of various different sorts make up the layers which are being deformed by the same processes. The result is a pitted and churned up surface, deformed by the rise of this material.   

Triton is one of only a few moons in the solar system to be geologically active, and it is believed that its interior is melted by tidal heating from Neptune. This moon is also unusual because it has a retrograde orbit. This means that it orbits in the opposite direction to the other neptunian satellites, and the planet itself. This anomalous orbit suggests that the moon didn’t form in its current position, but was rather captured by the gravity of the larger planet. 

Triton is believed to have originally been Kuiper Belt object, a dwarf planet like Pluto, which it is similar to in many ways. In fact Pluto’s orbit is elliptical, and actually crosses the path of Neptune bringing it closer to the sun for 20 years in every 248. Pluto’s orbit will never bring it into contact with the larger planet, as they are never in the same parts of their orbits when they cross. However Triton wasn’t as lucky. At some point its orbit brought it too close to Neptune and it was swept up by the ice giant planet, becoming a moon. This means that in some ways Triton is a warmed up Pluto, it is likely made of similar materials, but the tidal heating from its large neighbour keeps the interior much warmer, giving it a geothermal gradient which is probably lacking on Pluto. The diapirs that caused the cantaloupe terrain are one product of this warmer state, and cryovolcanism is believed to take place on the moon as well. 

Image Credits:

NASA/JPL/Universities Space Research Association/Lunar & Planetary Institute

 https://photojournal.jpl.nasa.gov/catalog/PIA12186

https://pds.jpl.nasa.gov/planets/captions/neptune/tritncnt.htm

Further Reading: http://gizmodo.com/this-is-the-most-detailed-map-of-neptunes-moon-triton-1625889935