Cloud Cover’s Role in Exoplanet Studies

I confess it had never occurred to me to consider cloud cover on exoplanets in quite the same light that a new study does. But two Spanish astronomers from the Astrophysical Institute of the Canary Islands (IAC) are taking a look at how clouds operate over different kinds of surfaces, in the process figuring out what our Earth would have looked like from space in different eras. It’s an interesting thought: Given the movement of Earth’s continents in the past 500 million years, what would cloud patterns have been like over land and sea as landforms changed?

The researchers chose several times to study, from 90, 230, 340 to 500 million years ago, pondering how changes in light reflected from the Sun would have operated here and, by extension, how they might operate on distant exoplanets. We’ll need to keep these things in mind when we get the capability of studying the atmospheres of terrestrial planets around other stars. And it turns out that, according to the researchers, cloud cover is anything but random. Deserts tend to stay clear of clouds, while rainforests are frequently and densely clouded over.

As described in this article in Astrobiology Magazine, Enric Pallé and Esther Sanromá worked with 23 years worth of data from the International Satellite Cloud Climatology Project to study how landforms and clouds correlate. They then plugged in differing continental distributions over time, going back to roughly the period when the atmosphere’s temperature and composition varied too greatly from our own to make adequate projections. It turns out that daily variations were scarce through much of the studied period, but 500 million years ago, the changes in light were clearly marked. The paper on this work explains the situation:

The re?ected light of the Earth at 500 Ma ago… presents much more variability in contrast to the light curves obtained for the other epochs. That is again related to the continental distribution. In this epoch, most of the continents were clustered in one hemisphere, but it also has several big islands that cause strong variability in the re?ected light. Moreover, this epoch presents a signi?cantly higher mean albedo value. That can be related to the fact that in this epoch continents were covered by deserts, thus involving a higher re?ectivity.

A light curve is about the best we’re going to get for a terrestrial exoplanet once we actually do gain the ability to study exoplanets through space-based planet-finder observatories of the future, at least in the beginning. Pallé and Sanromá’s work implies that a terrestrial world with small variations could be one with vegetation on the surface. Rocky planets with no atmosphere and planets with global cloud cover would show no variations whatsoever, but as Pallé says, “if it has an atmosphere with broken clouds, we will see it in variability.” The scattered clouds, then, might be flagging the presence of oceans and implying the existence of living ecosystems.

In the paper, Pallé and Sanromá go on to comment:

We ?nd that our model reproduces well the major features of the cloud distribution and the photometric light curve of the Earth at present. When applied to past epochs of the Earth, we ?nd that both the mean albedo value and the diurnal light curve variability remain stable as long as desert area are con?ned to the tropical regions. When this condition is not met, as during the Late Cambrian about 500 Ma ago, both the mean albedo value and the photometric variability are greatly increased. This increased variability could help in the determination of the rotational period of the planet from an astronomical distance. Due to the large compositional and chemical changes of Earth’s atmosphere, we have not attempted to reconstruct cloud cover maps for epochs prior to the Late Cambrian. However, it is likely that the conditions for this period, i.e., higher albedo values and photometric variability, hold for much of the previous epochs of the Earth, not considering possible albedo variations due to atmospheric changes coming from clouds, aerosols, and hazes.

Image: Figure 1 from the Sanromá and Pallé paper, showing global views of the Earth’s continental distribution during the Late Cretaceous (90 Ma ago, top left), the Late Triassic (230 Ma ago, top right), the Mississippian (340 Ma ago, bottom left), and the Late Cambrian (500 Ma ago, bottom right). Credit: Ron Blakey, Colorado Plateau Geosystems.

We’re obviously only beginning the study of exoplanet atmospheres, but it’s certainly worth noting that we’ve had useful results in characterizing the atmospheres of some ‘hot Jupiters,’ work which is producing various models to explain what astronomers are seeing. We’ve also studied Earth’s own light through ‘Earthshine’ (reflected off the Moon) analyzed at various wavelengths, and have examined our planet’s light curve from space. Moreover, an interesting body of work has arisen to analyze the characteristics of light as absorbed by vegetation.

It also seems obvious that when we do observe a planet similar to the Earth, it will not be at the same stage of evolution as the Earth today, making modeling of our own planet’s earlier eras a useful exercise as we piece together an observing strategy. The paper is Sanromá and Pallé, “Reconstructing the Photometric Light Curves of Earth as a Planet Along its History,” accepted by the Astrophysical Journal (preprint).


Toward a New ‘Prime Directive’

The Italian contribution to the interstellar effort has been substantial, and I’m pleased to know three of its principal practitioners: Claudio Maccone, Giancarlo Genta, and Giovanni Vulpetti. It was with great pleasure, then, that I took Roberto Flaibani up on his offer of appearing in his excellent blog Il Tredicesimo Cavaliere (The Thirteenth Knight). Roberto had translated several Centauri Dreams articles into Italian in the previous year and was now looking for comments on the ramifications of human contact with extraterrestrials as we push into interstellar space. This article on Star Trek’s Prime Directive grew out of our talks and became part of a broader discussion of related articles on Roberto’s site. I thank him for continuing to translate my work into Italian, and now offer the original essay to Centauri Dreams readers.

I should probably throw in a qualifier — I’ve always enjoyed Star Trek but am hardly a rabid fan, getting most of my science fiction not from film or TV but novels and short stories. So this is a bit of a jeu d’esprit, one that acknowledges that the show has indeed spun out provocative and often controversial scenarios.

The Prime Directive embodies a flawed but useful ethical principle that should remain in place, though not without extensive revision. To understand why we need to re-think some aspects of the Prime Directive, let’s consider the context in which it operates. Because it grew out of ‘Star Trek,’ we have to posit a universe much like that one to illuminate the regulation’s strictures. Let’s assume, then, that humans become a spacefaring civilization on not just an interplanetary but an interstellar scale. That means that through whatever means, we have acquired ways of getting to the stars in short time frames, and that an organization has emerged within which this exploration continues, an analogue to the Federation behind the directive.

Why would the Prime Directive emerge in the first place? Here it is important to remember that in the ‘Star Trek’ universe, the directive is actually a regulation that applies only to Starfleet. Indeed, the series shows us that if a citizen of the Federation has decided on his or her own volition to interfere with another civilization, Starfleet is powerless to prevent such actions. The regulation doubtless was called for because the outermost wave of human expansion would be the exploring arm embodied in Star Fleet itself. What happens after a given region of space is first charted and explored is up to individual action, but the people most likely to be involved in first contact with an alien culture are those operating under Federation regulations.

All of this seems like logical extrapolation, and it is a tribute to the ‘Star Trek’ universe that despite the number of television episodes in various configurations and movies using many of the same characters, the storyline has been kept relatively consistent. If we ever do develop a way of sending human crews to other worlds, we will ponder the question of how we interact with any intelligent species we find there. And it’s likely we’ll consider a principle something like “the right of each sentient species to live in accordance with its normal cultural evolution,” a phrase pulled from the text of the Prime Directive. What we are looking at is the development of ‘metalaw,’ a term devised by attorney Andrew Haley in 1956 to specify a system of laws that apply not just to human beings but to all relationships between intelligent species.

Evolving Metalaw and Human Culture

Why not simply resolve to treat alien cultures using the principles of the Golden Rule — treat aliens as we would wish to be treated by them? Haley went on to point out the problem with this approach in a paper called “Space Law and Metalaw – A Synoptic View,” recognizing that aliens are different from ourselves in ways we may not begin to understand. Treating them as we would like to be treated might cause them injury or even destroy them. Haley revised the Golden Rule this way: “Do unto others as they would have you do unto them.” Robert Freitas, who has written thoughtfully on the subject of metalaw, notes that this ‘Great Rule’ has its own problems: “…in practice the Great Rule would be as difficult to apply as the concepts of noninterference and physical security. If we are to ascertain the desires of the other party, we must interact with them to a certain degree – and this may cause sociocultural damage. We still are left with the problem of developing nonconflicting, serviceable metalegal rules.”

We are in the earliest stages of developing ‘metalaw’ today, but a widening sphere of human activity among the stars will eventually force us to move the concept forward. Aerospace engineer Giancarlo Genta (Politecnico di Torino) has examined these questions in his book Lonely Minds in the Universe (Copernicus, 2007), studying the entire question of whether an alien being in today’s world could be considered a ‘person.’ An interesting legal issue would arise if we suddenly found ourselves face to face with a being from another world. We may believe in extending equal rights to all humans no matter their origin, but an extraterrestrial who walked out of a spaceship after landing on Earth would not necessarily be recognized by law as a ‘person.’ Would this creature be considered an animal? If so, would an alien animal have rights under existing law?

The Prime Directive can be assumed to have grown out of discussions exactly like these, and it hinges on the extension of the idea of personhood to alien intelligences. In his paper “Metalaw and Interstellar Relations” (cited above) Robert Freitas sees two routes to personhood:

  • The use of a clear morality; i.e., the ability of the beings in question to make moral or ethical judgments, even if those judgments do not necessarily coincide with our own
  • The presence of self-awareness, of being separate from one’s surroundings.

The presence of either morality or self-awareness is seen as the key to personhood. Here we seem to be splitting hairs in legalistic fashion, but the issue is important because human law revolves around the concept of the person. Metalaw, in other words, leads us invariably to the thinking that leads to the Prime Directive, which we can now quote in more extended form:

As the right of each sentient species to live in accordance with its normal cultural evolution is considered sacred, no Star Fleet personnel may interfere with the normal and healthy development of alien life and culture. Such interference includes introducing superior knowledge, strength, or technology to a world whose society is incapable of handling such advantages wisely. Star Fleet personnel may not violate this Prime Directive, even to save their lives and/or their ship, unless they are acting to right an earlier violation or an accidental contamination of said culture. This directive takes precedence over any and all other considerations, and carries with it the highest moral obligation.

A Silence Between Civilizations?

We now find ourselves in a quandary, for the Prime Directive must be interpreted, just as any body of law or regulation must be understood and acted upon by those affected by it. If we read the text closely, we have no choice but to conclude that the only contact possible between two alien civilizations would happen when the two civilizations are at precisely the same point of development, or as Giancarlo Genta phrases it, the same cultural level. A right is assumed in the Prime Directive for each species to proceed through a ‘normal’ cultural evolution, one which cannot be interfered with by introducing superior knowledge or technologies.

A civilization less advanced than our own in terms of technology, then, would be one we could not contact directly. A civilization more advanced than our own would, if acting under the principles of a similar Prime Directive, be unable to contact us. If the Prime Directive were a universal principle, no species would be able to contact another unless it encountered one so similar to itself that the contact would be considered harmless. Given the wide variation in stellar ages around us in the Milky Way, it seems spectacularly unlikely that we would find a species this similar to ourselves, and all studies of alien cultures would have to be conducted with maximum secrecy to avoid contaminating the alien civilization in question.

This seems an unwise outcome on various levels, but there’s more. What do we mean by the ‘same cultural level?’ Culture is what we recognize around us in the form of the familiar accouterments of society and the technologies we use to provide and service them. But the very idea of an alien culture presupposes a development unlike our own. We could not assume that a culture that seemed similar to our own at the level it was at in the time of ancient Greece would necessarily undergo a similar period of imperial expansion on its planet, a gradual awakening to other cultures across its oceans, a period when learning was lost and an eventual renaissance. Nor could we assume that the beings who live within this culture operate with the same set of principles that we do, or that they would develop technologies comparable to our own.

Note the other flaw in the Prime Directive statement above. We are told that interference consists of introducing superior knowledge, strength, or technology to a world ‘whose society is incapable of handling such advantages wisely.’ The statement implies that if a society is deemed capable of handling these advantages with wisdom, the strictures of the Prime Directive to not apply. But who is to make the decision as to the wisdom of an alien civilization, and how accurate can such an assessment be given the short periods involved in a first contact scenario? And what does the Prime Directive mean by not interfering with the ‘normal and healthy development’ of another culture? To understand what is normal and healthy for an alien civilization may be an impossible accomplishment, and certainly not one we could achieve without extensive study. No, the Prime Directive binds us too severely and limits any contact.

More Supple Rules of Contact

Where to go from here? We need a revised Prime Directive based on an evolving metalaw, one that recognizes that each encounter with an extraterrestrial civilization is going to be different from any other. Genta cites the eleven rules of metalaw compiled by the Austrian lawyer `Ernst Fasan, drawing on the earlier work of Andrew Haley. These are drawn from Fasan’s book Relations with Alien Intelligences: The Scientific Basis of Metalaw (1970), and contain the seeds of a future Prime Directive that should prove more flexible:

1. No partner of metalaw may demand an impossibility.

2. No rules of metalaw must be complied with when compliance would result in the practical suicide of the obliged race.

3. All intelligent races of the Universe have in principle equal rights and values.

4. Every partner of metalaw has the right to self-determination.

5. Any act that causes harm to another race must be avoided.

6. Every race is entitled to its own living space.

7. Every race has the right to defend itself against any harmful act performed by another race.

8. The principle of preserving one race has priority over the development of another race.

9. In case of damage, the damager must restore the integrity of the damaged party.

10. Metalegal agreements and treaties must be kept.

11. To help the other race by one’s own activity is not a legal but a basic ethical principle.

Here we have a set of guiding principles that do not exclude contact between civilizations of different levels of development and complexity. Instead, Fasan’s ideas form a framework that a future commander of an interstellar mission could consult to make decisions about the level of contact appropriate for that situation. Fasan cannot answer all our questions — in particular, we have the conundrum that the ethical concepts embedded here may be profoundly anthropocentric, and we certainly have no idea whether other intelligent races would agree or abide by such ideas. But the scant work that has thus far been done on metalaw points us to the need for an enhanced, more carefully thought-out Prime Directive, one that does not entangle an exploring party far from home with legalities that could compromise a beneficial first contact.

Here it is time to quote Genta directly:

Such rules are without doubt a good starting point on which to build the laws and ethics or relationships between species, but they have been elaborated by one of the sides only — and it could not be otherwise, since it is not even certain that the other sides exist. Moreover, if it is true that the other species with which we could come in contact are much more ancient than ourselves, it is likely that they already faced this problem and established rules for relationships among species.

The reality is that as we expand to nearby stars and beyond, we will learn from our early contacts with other civilizations — if indeed they exist — and shape our metalaw flexibly from each of these encounters. Metalaw cannot be other than an evolving set of principles, incapable of setting down as a Prime Directive that does not grow with our knowledge over time. The Prime Directive offers us a wonderful way to consider the issues. But we need to be aware that it is a template only, and that what we find among the stars will help us shape its future direction. We must also be thinking about these matters long before we actually get to the stars. As Robert Freitas reminds us, “When intelligent extraterrestrial life is discovered, mankind must be prepared, for in all of human history there will be but one first contact.”


Fasan, E, Relations with Alien Intelligences: The Scientific Basis of Metalaw, Berlin Verlag, Berlin, 1970. See also his paper “Discovery of ETI: Terrestrial and Extraterrestrial Legal Implications,” Acta Astronautica 21 (2) (1990), pp. 131-135.

Freitas, R, “Metalaw and Interstellar Relations,” Mercury 6 (March-April, 1977), pp. 15-17 (available online)

Genta, G. Lonely Minds in the Universe. New York: Copernicus, 2007.

Haley, A “Space law and Metalaw – A Synoptic View,” Harvard Law Record 23 (November 8, 1956).


New Multiple Planet Systems Verified

Confirming Kepler’s planet candidates is a crucial part of the process, because no matter how tantalizing a candidate appears to be, its existence needs to be verified. We have more than 60 confirmed Kepler planets and over 2300 candidates, many of which will eventually get confirmed, but it’s interesting to see that the mission’s latest announcements relate to multiple planet systems and how their presence can itself speed up the verification process.

In today’s focus are the eleven new planetary systems just announced, 26 confirmed planets in all, which actually triples the number of stars known to have more than one transiting planet. One of the systems, Kepler-33, has been demonstrated to have five planets. We also have five systems (Kepler-25, Kepler-27, Kepler-30, Kepler-31 and Kepler-33) showing a 1:2 orbital resonance — the outer planet orbits the star once for every two orbits of the inner planet — and four systems with a 2:3 resonance, with the outer planet orbiting twice for every three times the inner planet completes its orbit.

Image (click to enlarge): The artist’s rendering depicts the multiple planet systems discovered by NASA’s Kepler mission. Out of hundreds of candidate planetary systems, scientists had previously verified six systems with multiple transiting planets (denoted here in red). Now, Kepler observations have verified planets (shown here in green) in 11 new planetary systems. Many of these systems contain additional planet candidates that are yet to be verified (shown here in dark purple). For reference, the eight planets of the solar system are shown in blue. Credit: NASA Ames/Jason Steffen, Fermilab Center for Particle Astrophysics

Usefully for verification purposes, these are systems in which the planets are relatively close to their host stars, with orbital periods between six and 143 days, the tight configuration creating a clear Transit Timing Variation (TTV) signature as the planets tug and pull on each other. TTV makes verification much simpler and eases the need for backup observations from ground-based telescopes. We’ve looked at Transit Timing before in its application for possible detection of exomoons, but it’s also useful for analyzing planetary systems around fainter, more distant stars.

Eric Ford (University of Florida) and colleagues discuss the utility of TTVs in the paper on their work on Kepler-23 and Kepler-24:

For systems with MTPCs [multiple transiting planet candidates], correlated TTVs provide strong evidence that both transiting objects are in the same system. Dynamical stability provides an upper limit on the masses of the transiting bodies. For closely-spaced pairs, the mass upper limit is often in the planetary regime, allowing planets to be confirmed by the combination of correlated TTVs and the constraint of dynamical stability.

And Jack Lissauer (NASA Ames) and team discuss the validity of Kepler’s multiple planet candidates in a separate paper. The italics are mine:

Roughly one-third of Kepler’s planet candidates announced by Borucki et al. (2011) are associated with targets that have more than one candidate planet. False positives (FPs) plague ground-based transit searches, but the exquisite quality of Kepler photometry, combined with the ability to measure small deviations in center of light during transits (Jenkins et al. 2010; Batalha et al. 2010), have been used to cleanse the sample prior to presentation in Borucki et al. (2011). Accounting for candidates on each one’s individual merit, Morton & Johnson (2011) estimated the fidelity of Kepler’s planet candidates (fraction of the candidates expected to be actual planets) to be above 90%. Yet the fidelity of multiple planet candidates is likely to be higher than that for singles (Latham et al. 2011; Lissauer et al. 2011a). We show herein that the vast majority of Kepler’s multiple planet candidates are true multiple planet systems.

Find multiple planets in the same system, then, and the odds on their being verified are excellent, what Lissauer calls ‘validation by multiplicity,’ based on our knowledge of the properties of the host star and examination of planetary transits that show similar signatures around the same star. Thus the gravitational dance of multiple planets leads to faster verifications as the orbital period of each planet changes through the slightest of variations in its transit timing. Now we have yet another crop of exoplanets, fifteen of them between Earth and Neptune in size. But whether these smaller planets are rocky worlds or gaseous ‘Neptunes’ will have to be the subject of further study.

The papers are Lissauer et al., “Almost All of Kepler’s Multiple Planet Candidates are Planets” (preprint); Ford et al., “Transit Timing Observations from Kepler: II. Confirmation of Two Multiplanet Systems via a Non-parametric Correlation Analysis,” accepted at the Astrophysical Journal (preprint); Steffen et al., “Transit Timing Observations from Kepler: III. Confirmation of 4 Multiple Planet Systems by a Fourier-Domain Study of Anti-correlated Transit Timing Variations,” accepted by MNRAS (preprint); and Febrycky et al., “Transit Timing Observations from Kepler: IV. Confirmation of 4 Multiple Planet Systems by Simple Physical Models,” in press at the Astrophysical Journal (preprint).


Project Bifrost: Return to Nuclear Rocketry

Back in the days when I was studying Old Icelandic (this was a long time ago, well before Centauri Dreams), I took a bus out of Reykjavik for the short journey to Þingvellir, where the Icelandic parliament was established in the 10th Century. It was an unusually sunny day but that afternoon the storms rolled in, and just before sunset I remember looking out from the small hotel where I was staying to a rainbow that had formed over the lava-ridden landscape. It inevitably brought to mind Bifröst, the multi-colored bridge that in Norse mythology connected our world with Asgard, where the gods lived. The idea may have been inspired by the Milky Way.

In the world of rocketry, a new Bifröst has emerged, one designed to link the nuclear rocket technologies that were brought to a high level of development in the NERVA program with our present-day propulsion needs. For despite a serious interest that resulted in a total of $1.4 billion in research and the testing of a nuclear engine, NERVA (Nuclear Engine for Rocket Vehicle Application) was cancelled at the end of 1972. The work that went into the concept dated back to studies at Los Alamos starting in 1952 and extended through the 1950s with Project Rover.

The design in question is a nuclear thermal rocket (NTR), which uses nuclear fission instead of chemical combustion to heat a hydrogen propellant, making for both high exhaust velocity and high thrust. As Tabitha Smith notes in an article on Project Bifrost, Rover/NERVA technologies were once regarded as a natural follow-on to the Apollo missions, cited by President Kennedy in the same speech in which he challenged the United States to land a man on the Moon and bring him home by the end of the decade. In fact, some believed NERVA could get us to Mars before 1980. Its advantages were clear: If used as an upper stage for the Saturn rocket (Saturn S-N), the nuclear technology would have allowed payloads as large as 340,000 pounds to reach low Earth orbit, up from the 280,000 pounds that the all-chemical Saturn V could achieve.

Image: NERVA nuclear rocket under test. (Smithsonian Institution Photo No. 75-13750).

Anti-nuclear sentiment was part of NERVA’s downfall, but so too was the post-Apollo retreat from space and the expenditures it involved. It was probably NERVA’s link with a possible Mars mission that caused many politicians to put on the brakes, unwilling to see NASA commit itself to an even more intractable and expensive goal than the original Moon missions. Whatever the case, nuclear propulsion has been in the doldrums ever since, which is why Project Bifrost has sprung into existence. The abovementioned Tabitha Smith is research lead for Bifrost as well as being chief strategic officer for Washington DC-based General Propulsion Sciences. Along with colleague Brad Appel at GPS, Smith initiated the project in collaboration with Icarus Interstellar.

Appel sees nuclear technologies as a major step toward next-generation space travel, drivers for the manned mission to Mars we have been anticipating since the dawn of the space era and the concept studies of Wernher von Braun. Smith quotes Appel on the advantages of a nuclear thermal rocket:

“…imagine you are planning a road trip from New York to Los Angeles and back. Except, there are no gas stations along the way — you need to pack all of the fuel along with you. Using a chemical rocket to send humans to Mars would be like making the road trip in a cement truck. You might barely make it, but it would be one enormous, inefficient, and expensive voyage. Using an NTR, however, would be more akin to taking a Prius. It’ll make it there comfortably, and it can go a lot further too.”

Project Bifrost makes sense, given that while commercial space companies like SpaceX are moving to become cargo carriers to LEO, there is little work within the US government to further develop concepts like NERVA. Smith was recently in Moscow to pursue the idea of international collaboration in nuclear thermal rocketry, invited there as part of President Dmitry Medvedev’s initiatives to spur entrepreneurship and international collaborations in Russia. Whether or not the journey bears fruit, General Propulsion Sciences and Icarus Interstellar intend to bring NTR technologies up to date. A nuclear alternative to chemical methods would spur renewed interest in a Mars mission once thought to be all but inevitable before the end of the 20th Century.


The Dunes of Titan

The methane/ethane cycle we see on Titan is reminiscent of the water cycle on Earth, which is what people are really talking about when they refer to this frigid place as vaguely ‘Earth-like’ — this is not exactly a temperate climate! But we have a long way to go in understanding just how the cycle operates on the distant moon, which is why new work on Titan’s sand dunes is drawing interest. By studying the dune fields, we can learn about the climatic and geological history they depict and perhaps get clues about other issues, such as why Titan’s lakes of liquid ethane and methane are found mostly in the northern hemisphere.

What Cassini is showing us are regional variations among Titan’s dunes, a landscape feature that covers some 13 percent of the surface in an area roughly equivalent to that of Canada. But every time we run into an Earth analogue on Titan, we’re confronted with major differences. Titan’s dunes are made not of silicates but of solid hydrocarbons that wind up as tiny grains after precipitating out of the atmosphere. They’re also much larger than sand dunes on Earth, averaging 1-2 kilometers in width, hundreds of kilometers in length, and 100 meters in height.

Have a look at Cassini’s view of Titan’s dune fields as compared to what we see on Earth:

Image: Two different dune fields on Titan: Belet and Fensal, as imaged by Cassini’s radar. The image also shows two similar dune fields on Earth in Rub Al Khali, Saudi Arabia. Fensal is at higher latitude and elevation than Belet and clearly shows thinner dunes with brighter and wider areas in between, suggesting less abundant dune material in this region. Credit: NASA/JPL-Caltech/ASI/ESA and USGS/ESA.

This ESA news release points out that radar data from Cassini have allowed researchers to see clear correlations between the size of Titan’s dunes and their altitude and latitude. The major dune fields are all found in lowland areas, with dunes at higher elevations much more widely separated and, judging from the bright radar echo Cassini detects from them, covered by thinner layers of sand. Titan’s dunes are also largely confined to its equatorial region, in a band between 30°S and 30°N. As we move north, the dunes become narrower and more widely spaced.

The key may be Titan’s weather. Seasons here are just over seven Earth years long, and the elliptical nature of Saturn’s orbit results in shorter, warmer summers for the southern hemisphere. The result: The wetness of the surface in the southern areas from ethane and methane vapor in the soil is reduced. Drier sand makes for easier dune formation. “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,” says Alice Le Gall (LATMOS-UVSQ, Paris).

If Le Gall’s thinking is correct, the climate variation also could explain the location of Titan’s lakes and seas, found in the northern hemisphere largely because the soil would be moister there. “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,” adds Nicolas Altobelli, ESA’s Cassini-Huygens project scientist. Thus the dunes of Titan, made from frozen atmospheric hydrocarbons, may help us in the continuing effort to piece together the moon’s methane/ethane cycle, a work in progress that is still a long way from completion.