Posted on by Michael Habib

Let’s Put a Sailboat on Titan

An illustration showing how a sailboat mission to Titan might land and become operational. Copyright: Estevan Guzman for Universe Today.

The large moons orbiting the gas giants in our solar system have been getting increasing attention in recent years. Titan, Saturn’s largest moon, is the only natural satellite known to house a thick atmosphere. It’s surface, revealed in part by the Cassini probe, is sculpted by lakes and rivers. There is interest in exploring Titan further, but this is tricky from orbit because seeing through the thick atmosphere is difficult. Flying on Titan has been discussed around the web (sometimes glibly), and this was even one of the subjects treated by the immensely popular comic, XKCD.

However, there remains the problem of powering propulsion. The power requirements for flight are quite minimal on Titan, so solar wings might work. But Titan also presents an alternative: sailing.

Images from the Cassini mission show river networks draining into lakes in Titans north polar region. Credit: NASA/JPL/USGS.
Images from the Cassini mission show river networks draining into lakes in Titans north polar region. Credit: NASA/JPL/USGS.

With all those lakes and rivers, exploring Titan with a surface ship might be a great way to see much of the moon. The vehicle wouldn’t be sailing on water, though. The lakes on Titan are composed of liquid methane. The challenge is therefore making the vessel buoyant: liquid methane is only 45% as dense as liquid water. This means we would need a lot of displacement. A deep, hollow hull could do this, however, and it turns out that the liquid methane has an advantage that helps make up for the low density: it is much less viscous than water.

Reynolds number is proportional to the ratio of density to viscosity, and it turns out that friction drag on a hull is inversely proportional to Re. While Titan’s seas and lakes have only 45% the density of water, they also have only 8% of the viscosity. This means that the Titan sailing vessel would only experience about 26% of the friction drag as its Earth equivalent. [Yacht designers have found that the friction drag is about equal to 0.075/(log(Re)-2)^2)]. That leaves us room to make the hull deeper (important to compensate for the density as above), and longer (if we want a longer waterline, which will make the bow waves longer and improve maximum speed).

The sail itself would get less wind, on average, on Titan than Earth. Average wind speeds on Titan seem to be about 3 meters/s, according to Cassini, though it might be higher over the lakes. Average wind speed over Earth oceans is closer to 6.6 meters/s. But, the Titan atmosphere is also about 4x denser than Earth’s, and both lift and drag are proportional to fluid density. All told, this means that the total fluid force on the sail will be about 83% of what you’d get on Earth, all else being equal, which could be sufficient. There would be a premium on sail efficiency and size, and so we might have to take advantage of the low-friction hull to examine shapes with more stability that can house a larger, taller (and presumably high aspect ratio) sail.

This is all quite speculative, of course, but it provides a fun exercise and perhaps provides inspiration as we imagine tall-sailed robotic vessels silently cruising the lakes of Titan.

Titan Mare Explorer. Image credit: NASA/JPL
Titan Mare Explorer. Image credit: NASA/JPL

One concept for a boat on Titan has already been proposed: the Titan Mare Explorer (TiME) would send a floating high-tech buoy to land in a methane sea on this moon of Saturn to study its composition and its interaction with the atmosphere. But this Discovery-class mission concept was nixed in favor of sending the InSight lander to Mars.

But with all the recent discoveries on Titan by the Cassini spacecraft — things like lakes, seas, rivers and weather and climate patterns that create both fog and rain — a mission like this will be given more consideration in the future.

Posted on by Jason Major

Is Saturn Making a New Moon?

A 750-mile (1,200-km) -long feature spotted on Saturn's A ring by Cassini on April 15, 2013

Congratulations! It’s a baby… moon? A bright clump spotted orbiting Saturn at the outermost edge of its A ring may be a brand new moon in the process of being born, according to research recently published in the journal Icarus.

“We have not seen anything like this before,” said Carl Murray of Queen Mary University in London, lead author of the paper. “We may be looking at the act of birth, where this object is just leaving the rings and heading off to be a moon in its own right.”

In images acquired with Cassini’s narrow-angle camera in 2013, a 1,200-kilometer-long, 10-kilometer-wide arc of icy material was observed traveling along the edge of the A ring. The arc is thought to be the result of gravitational perturbations caused by an as-yet unseen embedded object about a kilometer wide — possibly a miniature moon in the process of formation.

Cassini image of the 179-km-wide Janus from April 2010. Janus' gravity may have helped spur the formation of Peggy. (NASA/JPL-Caltech/SSI)
Cassini image of 179-km-wide Janus from April 2010. Janus’ gravity may have helped spur the formation of Peggy. (NASA/JPL-Caltech/SSI)

The half-mile-wide object has been unofficially named “Peggy,” after lead author Murray’s mother-in-law (whose 80th birthday it was on the day he was studying the Cassini NAC images.) Murray first announced the findings on Dec. 10, 2013 at the AGU 13 meeting in San Francisco.

According to the team’s paper, Peggy’s effects on the A ring has been visible to Cassini since May 2012.

Eventually Peggy may coalesce into a slightly larger moon and move outward, establishing its own orbital path around Saturn. This is how many of Saturn’s other moons are thought to have formed much further back in the planet’s history. Now, its rings having been depleted of moon-stuff, can only create tiny objects like Peggy.

“Witnessing the possible birth of a tiny moon is an exciting, unexpected event.”
– Linda Spilker, Cassini Project Scientist at JPL

While it is possible that the bright perturbation is the result of an object’s breakup rather than formation, researchers are still looking forward to finding out more about its evolution.

Read more on the NASA/JPL news release here.

To find out more about the Cassini mission visit saturn.jpl.nasa.gov and www.nasa.gov/cassini. The Cassini imaging team’s website is at ciclops.org.

Posted on by Jason Major

Cassini Sees a “Zen Garden” on Titan

Radar image of rows of dunes on Titan. Credit: NASA/JPL-Caltech

Looking like the flowing designs carved by a Zen gardener’s rake, long parallel dunes of hydrocarbon sand stretch across the surface of Saturn’s moon Titan. The image above, acquired by Cassini in July 2013, reveals these intriguing and remarkably Earthlike landforms in unprecedented detail via radar, which can easily pierce through Titan’s thick clouds.

I’m feeling a little more enlightened already.

Although it piles into dunes like sand does here, Titan’s sand is not the same as what you’d find on a beach here on Earth. According to an ESA “Space in Images” article:

While our sand is composed of silicates, the ‘sand’ of these alien dunes is formed from grains of organic materials about the same size as particles of our beach sand. The small size and smoothness of these grains means that the flowing lines carved into the dunes show up as dark to the human eye.

Titan's surface is almost completely hidden from view by its thick orange "smog" (NASA/JPL-Caltech/SSI. Composite by J. Major)
Titan’s surface is almost completely hidden from view by its thick orange “smog” (NASA/JPL-Caltech/SSI. Composite by J. Major)

Radar imaging, although capable of seeing through Titan’s opaque orange atmosphere, doesn’t capture visible-light images. Instead it’s sensitive to the varying textures of a landscape as they reflect microwaves; the smoother an object or an area is the darker it appears to radar, while irregular, rugged terrain shows up radar-bright.

The pixelated “seam” cutting horizontally across the center is the result of image artifacting.

Learn more about Cassini’s RADAR instrument here, and read more about this image on the ESA site here.

Posted on by Jason Major

Surf’s Up on Titan! Cassini May Have Spotted Waves in Titan’s Seas

Cassini VIMS image of specular reflections in one of Titan's lakes from a flyby on July 24, 2012 (NASA/JPL-Caltech/SSI/Jason W. Barnes et al.)

It’s no surprise that Titan’s north polar region is covered with vast lakes and seas of liquid methane — these have been imaged many times by Cassini during its ten years in orbit around Saturn. What is surprising though is just how incredibly smooth the surfaces of these lakes have been found to be.

One would think that such large expanses of surface liquid — some of Titan’s seas are as big the Great Lakes — would exhibit at least a little surface action on a world with an atmosphere as dense as Titan’s. But repeated radar imaging has shown their surfaces to be “as smooth as the paint on a car.” Over the past several years scientists have puzzled over this anomaly but now they may have truly seen the light — that is, reflected light from what could actually be waves on Titan!

Seasonal winds may be finally kicking up waves in Titan's lakes. (Illustration © Ron Miller.)
Seasonal winds may be finally kicking up waves in Titan’s lakes. (Illustration © Ron Miller.)

Using data acquired during flybys of Titan in 2012 and 2013, planetary scientist Jason Barnes from the University of Idaho and a team of researchers from several other institutions including JPL, Cornell, and MIT, have identified what might be waves in the surface of Punga Mare, one of Titan’s biggest lakes.

For a sense of scale, Lake Victoria, the largest lake in Africa, could fit lengthwise across Titan’s 380-km (236-mile) -wide Punga Mare.

Read more: Titan’s North Pole is Loaded with Lakes

Variations in specular highlights in four pixels observed in the surface of Punga Mare by Cassini’s VIMS (Visible and Infrared Mapping Spectrometer) have been interpreted by the team as being the result of waves — or, perhaps more accurately, ripples, seeing as that they are estimated to be a mere 2 centimeters in height.

Still, based on what’s been observed thus far on Titan, that’s downright choppy.

If the Cassini observations interpreted by Barnes et al. are indicative of waves in Punga Mare, they could also explain previous specular variations seen in other bodies of liquid, like the smaller Kivu Lacus (top image).

Map of Titan's northern "Land o' Lakes" made from Cassini radar imaging passes (NASA/JPL/USGS)
Map of Titan’s northern “Land o’ Lakes” made from Cassini high-resolution radar imaging (NASA/JPL/USGS)

“If correct this discovery represents the first sea-surface waves known outside of Earth.”

– Jason W. Barnes et al.

Then again, wave action isn’t the only possible answer. Similar varied specular highlights could also be caused by a wet surface — like a methane mud flat. Further observations will be needed to rule out other possibilities and obtain a more accurate “surf forecast” for Titan.

The findings were presented by Jason Barnes at the 45th Lunar and Planetary Science Conference in Houston on March 17, 2014. Read the team’s abstract here, and read more in this article by Alexandra Witze on Nature News.

Posted on by Jason Major

Watch Two Dark Moons Sneak Into Cassini’s Shots

Raw image of Saturn with two moons acquired by Cassini on March 11, 2014 (NASA/JPL-Caltech/SSI)

On March 11, NASA’s Cassini spacecraft was acquiring some images of Saturn’s back-lit limb when two of its moons decided to make an entrance. Like stage hands in a darkened theatre the moons quickly passed  across the scene, moving between Saturn and the spacecraft and, because of exposure time and spacecraft motion, getting a bit blurred in the process.

In the image above the silhouette of one moon can be seen at bottom right — Mimas, perhaps — while another’s crescent can be made out at upper left… possibly Enceladus. Very cool!

Watch an animation of the moons below:

Two of Saturn's moons drift into the scene on March 11, 2014 (NASA/JPL-Caltech/SSI. Animation by Jason Major.)
Two of Saturn’s moons drift into the scene on March 11, 2014 (NASA/JPL-Caltech/SSI. Animation by Jason Major.)

While I admit I’m not 100% sure which moons these are, based on their apparent shapes, positions, and relative sizes I’d make my guess that these are 318-mile (511-km) -wide Enceladus and the 246-mile (395-km) -wide Mimas.

Possible location of icy spray, if this is Enceladus
Possible location of icy spray, if Enceladus is in fact this moon’s real name

Cassini was 843,762 miles (1,357,903 km) from Saturn when the images were acquired. And, if the larger moon at left is Enceladus, I’m thinking south in these images is up based on the barely-perceptible presence of a lighter area along its top edge that could be icy spray from its southern geysers. (See enlarged detail at right.)

Saturn, of course, is on the right. A small segment of the bright arc of its backlit limb is what’s running diagonally down across the image.

These images have not yet been calibrated or cataloged by NASA or the Cassini team.

See the latest raw images from Cassini on JPL’s mission page here.

*I say “dark moons” but actually Enceladus and Mimas are pretty bright, both being composed of a lot of ice. Enceladus is actually the most reflective world in the Solar System!

Posted on by Shannon Hall

Physicists Reveal the Hidden Interiors of Gas Giants

Looped movie of the hydrogen jet in the sample chamber. Credit: Sven Toleikis/DESY

In astronomy we love focusing on the bigger picture. We’re searching for exoplanets in the vast hope that we may begin to paint a picture of how planetary systems form; We’re using the Hubble Space Telescope to peer into the earliest history of the cosmos; And we’re building gravitational wave detectors in order to better understand the physical laws that dominate our universe.

All the while we continue to learn about our very own neighborhood. Only recently we learned that Europa has geysers, Mars was perhaps once a lush planet, and comets can in fact disintegrate. Discoveries in our solar system alone never cease to amaze.

For the first time researchers are able to probe the hidden interiors of gas giants such as Jupiter and Saturn. With very little experimental knowledge about the hydrogen deep within such planets, we have always had to rely on mathematical models. But now, researchers have simulated the lower atmospheric layers of these planets in the lab.

The team of physicists led by Dr. Ulf Zastrau from the University of Jena heated cold liquid hydrogen to resemble the dense liquid hydrogen deep within a gas giant’s atmospheric layers.

The team used an X-ray laser operated by a national research center in Germany, Deutsches Elektronen-Synchrotron (DESY), to heat the liquid hydrogen, nearly instantaneously, from -253 to +12,000 degrees Celsius. Initially the X-ray heats only the electrons. But because each electron is bound to a proton, they transfer heat to the proton until a thermal equilibrium is reached. The molecular bonds break during this process, and a plasma of electrons and protons is formed.

In just under a trillionth of a second, physicists are able to create a plasma that’s thought to be radically similar to the plasma deep within the atmospheres of our beloved gas giants.

But first the team had to create cold hydrogen. While it’s abundant throughout the universe, it’s hard to get our hands on the stuff here on Earth. Instead researchers cooled gaseous hydrogen to -253 degrees Celsius using liquid helium. This was a very temperamental process, requiring precise temperature control. If the hydrogen got too cold it would freeze and the researchers would have to use a small heater to re-liquefy it. At the end of the day a jet of cold liquid hydrogen with a diameter no greater than 20 micrometers would flow into a vacuum.

Physicists would then shoot intense pulses of the X-ray laser at the cold hydrogen. They could control the precise timing of the X-ray laser’s “flash” in order to study the properties of liquid hydrogen. The first half of the flash heats up the hydrogen, but the second half of the flash is delayed by varying lengths, which allows the team to understand exactly how thermal equilibrium is established between the electrons and the protons.

The experimental results provide information on the liquid hydrogen’s thermal conductivity and its internal energy exchange, which are both crucial to better understanding gas giants. The experiments will have to be repeated at other temperatures and pressures in order to create a detailed picture of the entire planetary atmosphere.

“Hopefully the results will provide us among others with an experimentally based answer to the question, why the planets discovered outside our solar system do not exist in all imaginable combinations of properties as age, mass, size or elemental composition, but may be allocated to certain groups,” said Dr. Thomas Tschentscher, scientific director of the European XFEL X-ray laser in a press release.

The paper has been accepted in the scientific journal Physical Review Letters and is available for download here.

Posted on by Jason Major

Watch Saturn Slip Behind the Moon

Occultation of Saturn on Feb. 22, 2014 by Colin Legg

Or, more accurately, watch the Moon pass in front of Saturn. Either way you get the same result: a beautiful video of planetary motion in action!

On the morning of Saturday, Feb. 22, the Moon drifted in front of the planet Saturn from the point of view of certain locations on Earth. Luckily one of those locations was Perth, Australia, where astrophotographer Colin Legg happens to be, and thus we all get to enjoy the fantastic results of his photographic and astronomical acumen.

Check out the video below:

The occultation — as such events are called whenever one celestial object passes in front of, or “hides,” another (the root of the word means “to conceal”) — may make it look like a tiny Saturn is getting absorbed by a giant Moon. But (obviously) they are separated by a vast distance: at the time of the occultation, 9.658 AU, or about 1,444,816,000 kilometers (897.7 million miles).

These sort of events will become a bit more common as the Moon is “headed towards a ‘shallow’ year in 2015 relative to the ecliptic; it will then begin to slowly open back up and ride high around 2025,” according to a recent Universe Today article by David Dickinson.

For those of you interested, Colin lists his equipment as a Celestron C8, f/10, prime focus. His camera is a Canon 5D2, running Magic Lantern RAW video firmware in 3x crop mode @ 1880 x 1056 resolution. Footage was taken at 1/60 sec exposure, ISO 200, 10 fps.

See more of Colin’s work on his Facebook page here.

Video/image credit: Colin Legg. All rights reserved. Used with permission.

Posted on by Fraser Cain

Can Moons Have Moons?

Can Moons Have Moons?

The Earth has a single moon, while Saturn has more than 60, with new moons being discovered all the time. But here’s a question, can a moon have a moon? Can that moon’s moon have its own moon? Can it be moons all the way down?

First, consider that we have a completely subjective idea of what a moon is. The Moon orbits the Earth, and the Earth orbits the Sun, and the Sun orbits the center of the Milky Way, which orbits within the Local Group, which is a part of the Virgo Supercluster. The motions of objects in the cosmos act like a set of Russian nesting dolls, with things orbiting things, which orbit other things. So maybe a better question is: could any of the moons in the Solar System have moons of their own? Well actually, one does.

Right now, NASA’s Lunar Reconnaissance Orbiter is happily orbiting around the Moon, photographing the place in high resolution. But humans sent it to the Moon, and just like all the artificial satellites sent there in the past, it’s doomed. No satellite we’ve sent to the Moon has ever orbited for longer than a few years before crashing down into the lunar surface. In theory, you could probably get a satellite to last a few hundred years around the Moon.

But why? How come we can’t make moons for our moon to have a moon of it’s own for all time? It all comes down to gravity and tidal forces. Every object in the Universe is surrounded by an invisible sphere of gravity. Anything within this volume, which astronomers call the “Hill Sphere”, will tend to orbit the object.

So, if you had the Moon out in the middle of space, without any interactions, it could easily have multiple moons orbiting around it. But you get problems when you have these overlapping spheres of influence. The strength of gravity from the Earth tangles with the force of gravity from the Moon.

How many moons are there in the Solar System? Image credit: NASA
How many moons are there in the Solar System? Image credit: NASA

Although a spacecraft can orbit the Moon for a while, it’s just not stable. The tidal forces will cause the spacecraft’s orbit to decay until it crashes. But further out in the Solar System, there are tiny asteroids with even tinier moons. This is possible because they’re so far away from the Sun. Bring these asteroids closer to the Sun, and someone’s losing a moon.

The object with the largest Hill Sphere in the Solar System is Neptune. Because it’s so far away from the Sun, and it’s so massive, it can truly influence its environment. You could imagine a massive moon distantly orbiting Neptune, and around that moon, there could be a moon of its own. But this doesn’t appear to be the case.

NASA is considering a mission to capture an asteroid and put it into orbit around the Moon. This would be safer than having it orbit the Earth, but still keep it close enough to extract resources. But without any kind of orbital boost, those tidal forces will eventually crash it onto the Moon. So no, in our Solar System, we don’t know of any moons with moons of their own. In fact, we don’t even have a name for them. What would you suggest?