Ralph Lorenz Talks About Exploring Titan

Special Topic of Planetary Exploration Interview, July 2011

Ralph D. Lorenz Our Special Topics analysis on Planetary Exploration over the past decade shows that the work of Dr. Ralph Lorenz ranks at #3 by numbers of papers and #10 by total cites, based on 145 papers cited a total of 1,866 times during the analysis period. One of these papers ranks among the top 20 over the past two years.

Two of his papers are Highly Cited Papers in Essential Science IndicatorsSM from Thomson Reuters. In the Web of Science®, his record for the period of January 1, 2001 to June 4, 2011 includes 274 original articles, reviews, and proceedings papers cited a total of 2,246 times.

Lorenz is a Research Scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

Below, he talks with ScienceWatch.com about his highly cited research regarding planetary exploration.

SW: Please tell us about your educational background and research experiences.

Although I wanted to work on planetary exploration since my teens, I actually started out with a degree in Aerospace Systems Engineering from Southampton in the UK, and worked for a year at the European Space Agency on the Huygens project before pursuing a Ph.D. back in the UK. I spent 12 years as a postdoc and research faculty at the University of Arizona, before coming to the JHU Applied Physics Laboratory in 2006.

Over the years I've become more of a planetary scientist, but I enjoy being conversant in both the science and engineering fields, which is of course useful in implementing missions to other planets (and is a major activity at APL).

SW: What first drew you to planetary research? Is there a specific area within this field on which you focus, or do you maintain a wide variety of interests?

I was always interested in space, but astronomy seemed rather abstract. Planetary science had much more immediate, experiential feel to it—real exploration and a sense of adventure. It's a very fresh and dynamic science, with the textbooks literally having to be continually rewritten.

One of the most stimulating aspects is that it is so multi- and inter-disciplinary. This is particularly the case in the study of Saturn's moon Titan, since it has a rather Earth-like landscape but with an exotic organic chemistry, a thick, cloudy atmosphere, and a hydrological cycle involving methane. So there's geophysics, meteorology, astrobiology, geomorphology, and even oceanography, and it's really fun to dabble in all these fields.

The downside to having broad interests, of course, is that you get asked to review a lot of things…

SW: Your most-cited original article in our analysis is the January 2007 Nature paper you coauthored, "The lakes of Titan," (Stofan ER, et al., 445[7123]: 61-4, 4 January 2007). Would you tell us a bit about this paper—your expectations going in, your findings, where this work has gone since this publication?

Ralph D. Lorenz
At the lava lake of the Erta Ale volcano in northern Ethiopia. This remarkable location (one of only four active lava lakes on Earth) can inform us on how best to remotely measure volcanic activity on Jupiter's moon lo. View movie.

Ralph D. Lorenz
Sulphurous hydrothermal pools near Dallol, in the Danikil Depression in Ethiopia. Visiting such bizzarre places is a good antidote to the idealized view one is often tempted to adopt about planetary environments.

There were some reasons to think, based on the results of the Voyager mission in 1980, that some or all of Titan's surface might be covered in liquid methane and ethane. Yet when Cassini arrived in 2004, there appeared very little evidence of extant liquids. It was only in late 2006, when we got our first look at Titan's north pole, that we at last found these lakes and seas using radar images (at the time Titan's arctic was in winter darkness). So in many ways, the discovery was a relief—it made Titan a really interesting place. You can read a lot more about all that in my book Titan Unveiled (Princeton University Press, 2008).

I expect Titan's lakes and seas to be probably the most exciting area of research in the next five years or so: as we move into northern spring on Titan, these seas will become better illuminated and perhaps waves will be whipped up by spring winds. Understanding how wave generation works in other planetary environments is something I played with in a NASA wind tunnel some years ago, and I expect that work, and the lakes discovery paper in Nature, to be cited widely in the future.

SW: Actually, quite a bit of your work in our analysis deals with explorations of Titan. Can you give us some of the highlights of what you have learned about Titan over the years? Why this interest in Titan specifically (either by you or just researchers in general)?

Titan is an amazing place—unique in our solar system—in being an icy moon with a thick atmosphere and a hydrological cycle. So while its interior and bulk composition shares many features with other icy moons, its atmosphere and landscape has very familiar aspects, more common to the terrestrial planets and Earth in particular.

Probably the biggest discoveries have been the richness of the upper atmospheric chemistry, the abundance of sand dunes, and the presence of lakes and seas. Because it has an atmosphere that shows weather (we see storm clouds come and go, and even see the ground beneath get dark after it gets rained on…) as well as seasonal change, there's always something new, and always something to be learned with each different method of investigation—whether it's a modeling approach or a new telescope or whatever.

So there's something for everyone, and so the Titan literature has always been rich compared with other moons. The literature has of course exploded given the avalanche of data from the Cassini-Huygens mission, which has been at Saturn since 2004—papers are coming out at over 100 a year.

SW: Another of your papers, the 2001 Geophysical Research Letters article, "Titan, Mars and Earth: Entropy production by latitudinal heat transport" (Lorenz RD, et al., 28[3]: 415-8, 1 February 2001), deals with predicting the circulation of temperature on other planets. Please tell us about this paper and its potential for practical applications.

That work started out very innocently, wondering what the equator-pole temperature gradient should be on Titan, and at the time I was also working on a couple of space missions to the Martian polar regions. When those missions failed on arrival I suddenly found myself with time to look at the problem a bit more and found that approaches previous workers had used for Mars paleoclimate seemed to fail for Titan.

I then got led into all kinds of interesting areas related to nonequilibrium thermodynamics. The idea is that advective heat flows across a planet may statistically favor a combination that maximizes the amount of mechanical work that can be produced (or equivalently, the rate of entropy production). This is a very different way of looking at the problem of climate—in some ways a bit heretical.

In principle, it may serve as a useful tool in assessing or improving numerical models, and in making a zeroth-order estimate of climate on planets about which we know very little. But there are still some important details to work out. I could imagine this may ultimately prove to be the most fundamentally important area that I have worked on, but the way funding works, it's very much a sideline to my Titan work.

SW: How has the field as a whole changed over the years?

Ralph D. Lorenz
Titan Unveiled gives a personal account of the arrival of the Huygens probe at Titan in 2005, carrying instrumentation Ralph built 11 years before, and the subsequent discoveries of dunes and lakes on Titan and the efforts to develop new missions to Titan.

Planetary science has grown substantially over the 20 years I've been involved. Spacecraft missions back then were few and far between, but now we are in an amazing era when there are new findings by the week from spacecraft orbiting (and roving on) Mars, at Saturn, Mercury, Venus, flying past comets and so on. And that's just our solar system—the only planets we knew about 20 years ago—now we know about hundreds of planets around other stars.

SW: You've published a few papers on phenomena at Racetrack Playa in Death Valley. Am I right in thinking these discuss the playa landscapes as analogues to settings on other planets, and if so, how? Are there other places on Earth that can be viewed as analogues?

One of the fun sides of planetary geomorphology and meteorology is that we can really best understand data from the planets by investigating analogue environments on Earth. Titan is of course particularly replete with terrain and phenomena that have similarities with Earth.

Especially since the discovery of dunes on Titan, I'm a big fan of deserts and have learned a lot comparing what you see with your boots on the ground with what we can observe in optical or radar images from space—these insights help calibrate what we see in such data from other worlds. I also do some field research on dust devils—whirlwinds that we also see on Mars. So I'm lucky to have some excuses to get away from the office and computer and see the real world in action.

I've been studying the conditions at Racetrack with some of the same instrumentation I use to study dust devils elsewhere. Unconnected with its famous moving rocks, Racetrack Playa has many morphological similarities with Ontario Lacus, a large lake on Titan that is apparently drying up.

I'm sometimes asked by TV producers where best to film—"Where on Earth looks like Titan?" My response is usually unhelpful—"Well, where on Earth looks like Earth?" Titan is a diverse place—if you want somewhere that looks like Titan's vast dune fields, the Namib desert or the Sahara is good. If you want some of its lakes, look at Florida: some other lakes and seas look more like Lake Mead. And so on.

SW: Where do you hope to see Titan research go in the next decade?

What Cassini-Huygens has shown us about Titan has opened a whole new arena of richer and deeper scientific questions, which need new missions to Titan to answer. Given its low gravity and thick atmosphere and lakes, there are all kinds of possibilities for exploration which can really excite the public too—airplanes or balloons as well as more conventional landers and orbiters. I've spent a lot of time in the last four years studying various options, which is fun because it engages my engineering interests—I even published a paper on the performance of hot air balloons (Lorenz RD, "Linear theory of optimal hot air balloon performance—application to Titan," Aeronautical Journal 112[1132]: 353-5, June 2008).

Recently (May 2011), NASA announced the selection for further study of a mission concept I've been working on called TiME ("Titan Mare Explorer")—basically a radioisotope-powered capsule that will float for months in one of Titan's seas ("Ligeia Mare") and measure its composition and study its weather and how air-sea exchanges happen in a completely different environment from Earth. This sort of scientific exploration is really evocative and has great potential to excite the public.End

Ralph D. Lorenz, Ph.D.
Johns Hopkins University Applied Physics Laboratory
Laurel, MD, USA


Stofan ER, et al., "The lakes of Titan," Nature 445(7123): 61-4, 4 January 2007 with 103 cites. Source: Essential Science Indicators from Clarivate Analytics.



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