Titan's haze-covered limb seen by Cassini on June 6
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Here’s a quick look at one of my favorite cosmic photo subjects – the varying layers of atmosphere that enshroud Saturn’s enormous moon Titan. The image above is a color-composite made from three raw images acquired by Cassini during its latest flyby.
On June 7 Cassini approached Titan within 596 miles (959 km) and imaged portions of the moon’s northwest quadrant with its radar instrument, as well as conducted further investigations of areas near the equator where surface changes were detected in 2010.
The image here was assembled from three raw images captured in red, green and blue visible light channels. It reveals some structure in the upper hydrocarbon haze layers that extend upwards above the moon’s opaque orange clouds — reaching 400-500 km in altitude, Titan’s atmosphere is ten times thicker than Earth’s!
The June 6 flyby was the second in a series of passes that will take Cassini into a more inclined orbit, where it will reside for the next three years as it investigates Saturn’s polar regions and obtains better views of its ring system.
Color-composite image of Titan and Saturn (NASA/JPL/SSI/J. Major)
Here’s a great shot of Titan and Saturn acquired by Cassini on May 6, 2012 just after a pass by the haze-covered moon. It’s a color-composite made from images taken in Cassini’s red, green and blue color channels, and the resulting image was color adjusted a bit to appear more “Saturny”.
UPDATE 7/2/12: The image above is featured in today’s Astronomy Picture of the Day (APOD)… check it out here.
Cassini also made some closer passes of Titan on May 6, taking images within about 710,000 km. After recent passes of Encealdus and Dione, Cassini buzzed past Titan in preparation of a targeted flyby on May 22, after which it will head up and out out of the “moonplane” in order to get a better view of Saturn’s rings and upper latitudes.
After that, Cassini won’t be playing amongst the moons again for three years, so images like this will be a rarity for a while.
Another image of Titan, closer-in and set against Saturn’s rings and clouds, shows the fine, transparent structure of the moon’s upper atmospheric haze layers:
Created by the breakdown of methane in Titan’s opaque atmosphere by UV radiation, the haze is composed of complex hydrocarbons that extend outwards up to ten times the thickness of Earth’s atmosphere!
(The RGB layers weren’t available for this particular view, so there’s no color version of it.)
Check out previous images from Cassini’s flyby of Dione and Enceladus, and follow along with the Cassini mission on the JPL site here.
Top image: Color-composite image of Titan and Saturn (NASA/JPL/SSI/J. Major) Bottom image: Titan in blue wavelength against Saturn (NASA/JPL/SSI)
Titan's Ontario Lacus is found to bear a striking resemblance to Namibia's Etosha Pan. (NASA/JPL/ESA)
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A large lake on Saturn’s cloud-covered Titan seems very similar to the Etosha Pan, a salt-encrusted dry lakebed in northern Namibia that periodically fills with water. As it turns out, Titan’s “great lake” may also be temporary.
Ontario Lacus, so named because of its similarity both in shape and size to Lake Ontario here on Earth, was first discovered near the south pole of Titan by the Cassini spacecraft in 2009. Its smooth, dark appearance in radar images indicated a uniform and reflective surface, implying a large — although likely shallow — body of liquid.
Of course, on Titan the liquid isn’t water — it’s methane, which is the main ingredient of the hydrologic cycle found on the giant moon. That far from the Sun the temperatures at Titan’s poles fall to a frigid -300ºF (-185ºC), much too cold for water to exist as a liquid and so, on this world, methane has taken its place.
A research team led by Thomas Cornet of the Université de Nantes, France has taken a closer look at Cassini’s radar data of Ontario Lacus and found evidence of channels carved into the southern portion. According to the team, this likely indicates that the lakebed surface is exposed.
Cassini image of Ontario Lacus. (NASA/JPL/SSI)
“We conclude that the solid floor of Ontario Lacus is most probably exposed in those areas,” said Cornet.
In addition, sediment layers surrounding the lake suggest that the liquid level has varied.
All in all, this reveals a striking resemblance between Ontario Lacus and Namibia’s Etosha Pan — an “ephemeral lake” that is dry for much of the year, occasionally filling with a shallow layer of water which evaporates, leaving salty rings of sediment.
The inherent otherworldly nature of Etosha Pan is further underlined — and perhaps foreshadowed! — by its use as a backdrop in the 1968 sci-fi film 2001: A Space Odyssey.
Although Ontario Lacus was initially thought to be permanently filled with liquid hydrocarbons, the team’s findings draw a strong correlation with this well-known Earthly environment, suggesting a much more temporary nature and showing the value of comparative research.
Satellite image of Etosha Pan, acquired on April 28, 2012. (Chelys/EOSnap)
“These results emphasise the importance of comparative planetology in modern planetary sciences,” said Nicolas Altobelli, Cassini project scientist for ESA.”Finding familiar geological features on alien worlds like Titan allows us to test the theories explaining their formation.”
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Even though there are lakes and rivers of liquid hydrocarbons on the surface of Saturn’s moon Titan, the rains that feed them may come few and far between. According to data gathered by NASA’s Cassini mission, parts of Titan might not see rain for more than 1,000 years.
And according to Dr. Ralph Lorenz, from the John Hopkins Applied Physics Laboratory (JHUALP), a new mission to Titan is exactly what’s needed to get to the bottom of this.
Rain on Titan?! It sounds bizarre, but scientists have observed a complex cycle of liquid on Titan, with lakes and rivers, clouds, and the rain that must feed them. But on Titan, where surface temperatures plunge to -179C, we’re not talking about water. The whole hydrological cycle runs with methane: methane lakes, methane rivers, and methane rain.
And it appears that the rain on Titan can be extreme, with deep river channels that must have had enormous flows for brief periods. But this rain must also be rare. In all of its observations of Titan, Cassini only spotted two instances of darkened regions that might have indicated rainfall.
In a recent talk at the Lunar and Planetary Science Conference (LPSC), Dr. Lorenz presented his estimates of the Titan rainfall, and the need for a new mission that could study it. Titan Mare Explorer. Image credit: NASA/JPL Titan Mare Explorer (TiME)
Dr. Lorenz is one of the scientists involved with the proposed Titan Mare Explorer (TiME) mission; one of three shortlisted missions that might be turned into NASA Discovery missions.
If selected, TiME would travel to the Saturn system, descend through Titan’s thick atmosphere, and land in Ligeia Mara, a large lake on the surface of the moon. It would search for rainstorms on the descent – an extremely unlikely event – and then watch the skies for evidence of rainfall. It would be able to “hear” rain falling directly onto it, and in the liquid around it. TiME would also be equipped with instruments that would let it see cloud formation, rain shafts, and even methane rainbows.
Assuming the rain shafts are 10 km wide, and would be observable at distances of 20 km, the lander should be able to detect rainstorms within a 1200 km2 area. According to Dr. Lorenz:
We might expect a 50% chance for a lander to be rained on directly in a 2500hr mission, but that its camera could observe nearby rainfall an expected ~5 times.
Once in 1,000 years?
While the weather system on Titan is similar to Earth, it probably has some significant differences, which Cassini observations have hinted at. Although there were possible storms seen in 2004, there was a huge gap until 2010. After the “storm”, the surface of Titan was changed with a large darkened area that could indicate saturation of liquid on the surface. These ponds seemed to dry up in future observations.
Estimates indicate that regions near Titan’s poles see rainfall for 10-100 hours every Titan year (30 Earth years). But the drier parts of the moon might not see more than a single rainfall every 1,000 years.
Color-composite image from Cassini raw data acquired on March 12, 2012. (NASA/JPL/SSI/J. Major)
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Little Enceladus and enormous Titan are seen on either side of Saturn’s rings in this image, a color-composite made from raw images acquired by Cassini on March 12, 2012. The original images were taken in red, green and blue color channels, and with a little Photoshop editing I combined them into a roughly true-color view of what Cassini saw as it passed within 1,045,591 km of Enceladus.
It may be one of the best images from Cassini yet this year! Cloud-covered Titan and tiny Prometheus (can you see it just above the rings on the right?) are literally dwarfed by their parent Saturn in an image captured on Jan. 5, 2012.
Prometheus’ pinpoint shadow can also be seen on Saturn’s cloud tops, just inside the thin, outermost F ring shadow at bottom left.
The two moons themselves couldn’t be more different; Titan, 3,200 miles (5,150 km) wide, is wrapped in a nitrogen and methane atmosphere ten times thicker than Earth’s and is covered with vast plains of dark hydrocarbon dunes and crisscrossed by rivers of liquid methane.
Prometheus imaged by Cassini in Dec. 2009.
Prometheus, on the other hand, is a potato-shaped shepherd moon 92 miles long and 53 miles wide (148 x 53 km) that orbits Saturn just inside the narrow, ropy F ring. While it doesn’t have an atmosphere, it does create some impressive effects on the icy material in the ring!
Another moon, Pandora, casts its shadow onto Saturn just outside the F ring shadow at bottom center. 50 miles (80 km) wide, Pandora shepherds the outer edge of the F ring but is itself not visible in this image. Watch an animation here.
This image was featured on the Cassini Imaging Central Laboratory for Operations (CICLOPS) website on Feb. 28, 2012. The view looks toward the southern, unilluminated side of the rings from about 1 degree below the ringplane.
Image credit: NASA / JPL / Space Science Institute.
Special thanks to Ninian Boyle astronomyknowhow.com for information in parts of this guide
This month, the Solar System gives us a lot to observe and we’ll even start to see the ‘spring’ constellations appear later in the evenings. But February still has the grand constellations of winter, with mighty Orion as a centrepiece to long winter nights.
The Sun has finally started to perform as it should as it approaches “Solar Maximum.” This means we get a chance to see the northern lights (Aurora), especially if you live in such places as Scotland, Canada, Scandinavia, or Alaska or the southern light (Aurora Australis) if you live in the southern latitudes of South America, New Zealand and Australia. Over the past few weeks we have seen some fine aurora displays and will we hope to seesome in February!
We have a bit of a treat in store with a comet being this month’s favourite object with binoculars as well, so please read on to find out more about February’s night sky wonders.
You will only need your eyes to see most of the things in this simple guide, but some objects are best seen through binoculars or a small telescope.
So what sights are there in the February night sky and when and where can we see them?
Aurora Looking north from the science operations center at Poker Fla,Alaska. Credit: Jason Ahrns.
The Aurora or Northern Lights (Aurora Borealis) have been seen from parts of Northern Europe and North America these last few weeks. This is because the Sun has been sending out huge flares of material, some of which have travelled towards us slamming into our magnetic field. The energetic particles then follow the Earth’s magnetic field lines towards the poles and meet the atoms of our atmosphere causing them to fluoresce, similar to what happens in a neon tube or strip light.
The colours of the aurora depend on the type of atom the charged particles strike. Oxygen atoms for example usually glow with a green colour, with some reds, pinks and blues. So the more active the Sun gets, the more likely we are to see the Northern (or Southern) Lights.
All you need to see aurora is your eyes, with no other equipment is needed. Many people image the aurora with exposures of just a few seconds and get fantastic results. Unfortunately auroras are “space weather” and are almost as difficult to predict as normal terrestrial weather, but thankfully we can be given the heads up of potential geomagnetic storms by satellites monitoring the Sun such as “STEREO” (Solar TErrestrial RElations Observatory).
Spaceweather.com is a great resource for aurora and other space weather phenomenon and the site has real-time information on current aurora conditions and other phenomenon.
Planets
Mercury is too close to the Sun to be seen at the beginning of the month, but will be visible very low in the south west from the 17th onwards. At the end of February Mercury will be quite bright at around mag -0.8 and will be quite a challenge. It can be seen for about 30 minutes after sunset.
Venus will improve throughout the month in the south west and will pass within half a degree of Uranus on the 9th of February. You can see this through binoculars or a small telescope. On the 25th Venus and the slender crescent Moon can be seen together a fabulous sight. At the end of month Venus closes in on Jupiter for a spectacular encounter in March. Venus
Mars can easily be spotted with the naked eye as a salmon pink coloured “star” and starts off the month in the constellation of Virgo and moves into Leo on the 4th. Mars is at opposition on March 3rd but is also at its furthest from the Sun on the 15th February making this opposition a poor one with respect to observing due to its small apparent size. The planet will still be visually stunning throughout the month.
Mars
Jupiter starts off the month high in the south as darkness falls and is still an incredibly bright star-like object. Through good binoculars or a small telescope you can see its four Galilean moons – a fantastic sight. On the 8th at around 19:50 UT, Europa will transit Jupiter and through a telescope you will see the tiny moons shadow move across its surface. Throughout February, Jupiter moves further west for its close encounter with Venus in March.
Jupiter
Saturn rises around midnight in the constellation of Virgo and appears to be a bright yellowish star. Through a small telescope you will see the moon Titan and Saturn’s rings as well. Saturn
Uranus is now a binocular or telescope object in the constellation of Pisces. On the 9th Uranus and the planet Venus will be within half a degree of each other.
Uranus
Neptune is not visible this month.
Comets Comet Garradd Credit: astronomy.com
Comet Garradd is still on show early in the month — if you have binoculars — and as the month progresses the viewing should improve. You can find the comet in the constellation of Hercules not far from the globular cluster M92. It is about a half a degree away or around the same width as the full Moon. The comet is around magnitude 7 or a little fainter than the more famous globular cluster M13 also to be found in Hercules, so you will definitely need binoculars to see it. The comet is heading north over the course of the month which should mean that it will become a little easier to see. At the beginning of the month you will have to get up early to see it, the best time being around 5:30 to 6:30 GMT. By the end of the month though, it should be visible all night long.
Moon phases
Full Moon – 7th February
Last Quarter – 14th February
New Moon – 21st February
Constellations
In February, Orion still dominates the sky but has many interesting constellations surrounding it.
Above and to the left of Orion you will find the constellation of Gemini, dominated by the stars Castor and Pollux, representing the heads of the twins with their bodies moving down in parallel lines of stars with each other.
Legend has it that Castor and Pollux were twins conceived on the same night by the princess Leda. On the night she married the king of Sparta, wicked Zeus (disguised as a swan) invaded the bridal suite, fathering Pollux who was immortal and twin of Castor who was fathered by the king so was mortal.
Castor and Pollux were devoted to each other and Zeus decided to grant Castor immortality and placed Castor with his brother Pollux in the stars.
Gemini has a few deep sky objects such as the famous Eskimo nebula and some are a challenge to see. Get yourself a good map, Planisphere or star atlas and see what other objects you can track down. Credit: Adrian West
Radar image of sand dunes on Titan. Credit: NASA/JPL–Caltech/ASI/ESA and USGS/ESA
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Titan is a world that is amazingly Earth-like in some ways, with rain, rivers, lakes and seas. Mind you, the liquid in this case is methane/ethane instead of water, at the bitterly cold conditions on the surface. Also like Earth, Titan has vast sand dune fields, covering about 10 million square kilometres (39 million square miles), or 13% of Titan’s surface. The Cassini spacecraft has been studying these dunes with its radar (in order to see through the perpetually smog-like atmosphere), with interesting results.
Titan’s dunes show regional differences, although they are only found in equatorial areas, between 30°S and 30°N. They are found in both highlands and lowlands, but primarily in lower elevations. The ones at higher altitudes are thinner and more widely spaced, and the gaps between them are brighter in the radar images, which means that there is probably less sand available than at lower altitudes. The dunes also become narrower and more widely spaced at northern latitudes.
Comparison of dunes on Titan (left) with those on Earth (right). Credit: NASA/JPL–Caltech/ASI/ESA and USGS/ESA
Because Titan’s southern hemisphere has shorter but more intense summers, due to Saturn’s elliptical orbit around the Sun, there is less moisture in the soil in those regions, making them more ideal for dune-forming. There is more moisture in the northern regions, where most of the lakes and seas are found.
“As one goes to the north, the soil moisture probably increases, making the sand particles less mobile and, as a consequence, the development of dunes more difficult,” said Dr. Le Gall of LATMOS-UVSQ in Paris.
The characteristics of Titan’s dunes also provide clues to the moon’s climate and geological history.
According to Nicolas Altobelli, Cassini–Huygens project scientist, “Understanding how the dunes form as well as explaining their shape, size and distribution on Titan’s surface is of great importance to understanding Titan’s climate and geology. As their material is made out of frozen atmospheric hydrocarbons, the dunes might provide us with important clues on the still puzzling methane/ethane cycle on Titan, comparable in many aspects with the water cycle on Earth.”
It should also be noted that the sand on Titan is composed of solid hydrocarbons instead of silicates like sand on Earth. Similar in appearance, but like the rest of Titan, very different in composition. They are reminiscent of the dune fields in Namibia or southern Arabia, but are much larger – they average about 1-2 kilometres (0.6-1.2 miles) wide, 100 metres (328 feet) tall and extend for hundreds of kilometres/miles!
It would be interesting to see a Titanian version of Lawrence of Arabia…
Titan's thick, smog-like upper atmosphere obscures our view of the lower atmosphere and surface. The much smaller moon Enceladus is also seen in this image. Credit: NASA/JPL
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Titan, the largest moon of Saturn, is in some ways the most Earth-like world in the solar system, with a thick nitrogen atmosphere, rain, rivers, lakes and seas. Albeit it is much colder, and liquid methane/ethane takes the place of water, but the hydrological processes are quite similar to those here. There may, however, also be a liquid water-ammonia ocean below the surface. Now, new research suggests that Titan is Earth-like in another way as well, with a layered lower atmosphere similar to ours.
It’s been long known that Titan has a dense atmosphere; you can’t even see the surface due to a thick smog-like upper haze composed of hydrocarbons. As it turns out, the lower atmosphere has two distinct layers; the lowest layer, like on Earth, is known as the boundary layer, which has the most influence on climate and weather.
There has been a lot of uncertainty about the nature of Titan’s lower atmosphere, so scientists developed a 3-D climate model to try to answer those questions – previous data from Voyager 1, Cassini and Huygens had led to conflicting results. This was largely due to the fact that the lower atmosphere can’t be observed directly because of the opaque upper atmosphere. The new climate model shows that there are two lower layers which are distinct from each other as well as from the upper atmosphere. The lowest boundary layer is about 800 metres (2,600 feet) thick while the next layer is about 2 kilometers (1.2 miles) deep.
According to Paulo Penteado from the Institute of Astronomy, Geophysics and Atmospheric Science at the University of São Paulo in Brazil, “The most interesting point is that their model shows the presence of two different boundaries, the lower one caused by the daily heating and cooling of the surface – and varying in height during the day – and the higher one caused by the seasonal change in global air circulation.”
Benjamin Charnay from the French National Centre for Scientific Research (CNRS) in Paris and lead author of the study, adds: “This unprecedented organisation of the boundary layer has several consequences. It controls the atmospheric circulation and wind patterns in the lower atmosphere; it controls the size and spacing of dunes on Titan; it could imply the formation of boundary layer clouds (of methane on Titan). Such clouds seem to have been observed but not explained.”
These differences are surprising, since Titan receives far less solar energy from the Sun than Earth does. This solar insulation, which determines temperature variations in the atmosphere, is 1,000 times weaker on Titan than on Earth. Such a dynamic atmosphere on Titan was unexpected, but it may hold clues as to the formation of our own atmosphere. This could also be extrapolated to exoplanets; if a smaller world so far from the Sun can have unanticipated Earth-like conditions, how many exoplanets, now being discovered by the thousands, could as well?
The findings were published in the January 15, 2012 issue of Nature Geoscience.
From the abstract:
“We conclude that Titan’s troposphere is well structured, featuring two boundary layers that control wind patterns, dune spacing and cloud formation at low altitudes.”
The abstract and article are here. The full article is available for $18.00 US or by subscription to Nature Geoscience.
An artist's imagination of hydrocarbon pools, icy and rocky terrain on the surface of Saturn's largest moon Titan. Image credit: Steven Hobbs (Brisbane, Queensland, Australia).
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Thanks to the Cassini mission and the Huygens probe, we’ve glimpsed a wet world when science took a look at Saturn’s moon, Titan. Its atmosphere is rich in methane and its average temperature is about -300 degrees Fahrenheit (about 90 kelvins). Although the chemical composition is different than ours, Titan still has similar features such as clouds, fog, rain and even lakes. However, the origin of these features haven’t really been well explained until now.
Researchers at the California Institute of Technology (Caltech) have been hard at work creating a computer program based on observations made by Cassini imaging and radar that could help explain Titan’s weather patterns and liquid surface deposits. One major oddity was discovered in 2009 when Oded Aharonson, Caltech professor of planetary science, and his team confirmed Titan’s lakes appeared to be gathered around its poles – more predominately in the northern hemisphere than compared to the south – yet that’s not the only curiosity. The areas around the equator were suspected to be dry, but the Huygens probe revealed areas of run-off and four years later researchers observed a storm system delivering moisture. Need more? Then check out the clouds observed by ground-based telescopes… They gather around southern middle and high latitudes during Titan’s southern hemisphere summer season.
“We can watch for years and see almost nothing happen. This is bad news for people trying to understand Titan’s meteorological cycle, as not only do things happen infrequently, but we tend to miss them when they DO happen, because nobody wants to waste time on big telescopes—which you need to study where the clouds are and what is happening to them—looking at things that don’t happen,” explains Mike Brown of the California Institute of Technology (Caltech).
Sure. The researchers have worked hard at creating models that could explain these exotic weather features, but such explanations involve way out theories, such as cryogenic volcanoes that blast out methane vapor to cause clouds. However, the latest computer renderings are much more basic – the principles of atmospheric circulation. “We have a unified explanation for many of the observed features,” says Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering. “It doesn’t require cryovolcanoes or anything esoteric.” Schneider, along with Caltech graduate student Sonja Graves, former Caltech graduate student Emily Schaller (PhD ’08), and Mike Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy, have published their findings in the January 5 issue of the journal Nature.
Why is this data set different than its predecessors? According the Schneider, these new simulations were able to reproduce cloud patterns which match factual observations – right down to the distribution of lakes. “Methane tends to collect in lakes around the poles because the sunlight there is weaker on average,” he explains. “Energy from the sun normally evaporates liquid methane on the surface, but since there’s generally less sunlight at the poles, it’s easier for liquid methane there to accumulate into lakes.” Because Titan has an elongated orbit, it’s a bit further away during the northern hemisphere summer allowing for a longer rainy season and thus a stronger accumulation of lakes.
So what about storms? Near the equator, Titan isn’t very exciting – or is it? Originally it was theorized the area was almost desert-like. That’s why when the Huygens probe discovered evidence of run-off, it became apparent that existing models could be wrong. Imagine the surprise when Schaller, Brown, Schneider, and then-postdoctoral scholar Henry Roe discovered storms in this supposedly arid region in 2009! No one could figure it out and the programs did little more than predict a drizzle. With the new model, heavy rains became a possibility. “It rains very rarely at low latitudes,” Schneider says. “But when it rains, it pours.”
So what else makes the new Titan weather computer model even more unique? This time it runs for 135 Titan years and links the methane lakes – and how methane is distributed – to its atmosphere. According to the research, this matches current Titan weather observations and will help to predict what could be seen in coming years. Making testable predictions is “a rare and beautiful opportunity in the planetary sciences,” Schneider says. “In a few years, we’ll know how right or wrong they are.”
“This is just the beginning,” he adds. “We now have a tool to do new science with, and there’s a lot we can do and will do.”
