Dale P. Cruikshank is an astronomer and planetary scientist in the Astrophysics Branch at Ames Research Center. His research specialties are spectroscopy and radiometry of planets, asteroids, satellites, and other bodies in the solar system. In his most recent studies with the Cassini spacecraft, he and his colleagues found hydrocarbons on several of Saturn’s satellites.
NASA Tech Briefs: Your research specialties are spectroscopy and radiometry of planets and small bodies in the solar system. I’m interested in your work with NASA ESA-Cassini mission. Can you tell our readers what that mission was and what it told you about Saturn’s satellites?
Dr. Dale P. Cruikshank: The Cassini mission to the Saturn system has been a tremendously successful space mission conducted both jointly by NASA and ESA, and it’s still in progress, and, in fact, it’s expected to be in progress until the middle of 2017. The mission is a flagship-class mission in that it’s a very large spacecraft with numerous instruments aboard. The ESA component was the Huygens probe, which roughly a year after we arrived at Saturn was released from the main spacecraft bus, and then made a parachute descent through the atmosphere of Saturn’s largest moon Titan and eventually landed on the surface. And, in fact, it conducted scientific measurements for two and a half hours after it landed, even though it wasn’t technically designed to be a lander, per se. The remaining part of the mission, which essentially continues, includes an investigation of the planet Saturn itself, its atmosphere, the aurora that occur in the atmosphere, Saturn’s magnetic field, and the trapped particles that are in it, as well as Saturn’s rings, of course, and Saturn’s satellites, too. Saturn has a very large number of satellites, something like 62, I think, of which the largest ones are close into the planet, and those are the ones that we’ve been studying with several of the instruments on board the spacecraft.
NTB: You found many kinds of ice on several small planetary bodies. Where have you found the ice, and what kinds of conclusions are you able to draw from these findings?
Dr. Cruikshank: Ices are kind of a natural thing to suspect might be present in the cold reaches of the planetary system, and what we have found over the years, both from telescopic observations made from the ground, and now more recently with spacecraft observations, is that there’s a wide variety of ices, not just frozen water, but frozen carbon dioxide, which we find on Mars, in the polar caps, for example, dry ice. When we move farther out into the solar system, in addition to frozen water, we find frozen methane, natural gas, and other hydrocarbons, such as ethane and possibly still more that haven’t been fully identified. If we go even farther out into the solar system, as far as out as Neptune and Pluto, at which the temperatures are something like 35 to 40 degrees absolute, we find frozen nitrogen. And, of course, nitrogen is the same thing that’s in the Earth’s atmosphere, but at those extremely low temperatures, nitrogen freezes into a nice shiny ice on the surface of these bodies.
So, a good bit of what I’ve been interested in over the years, is the exploration of the solar system in terms of its ices – and also minerals by the way – but in particular, the ices. Together, with a couple of colleagues in 1976, which seems like a long time ago, we discovered the methane ice on the surface of Pluto, and we also found, a few years later, methane ice on Triton, which is Neptune’s largest moon, and since then, nitrogen and frozen methane have been found on a number of objects out beyond Neptune, in what’s called the trans-Neptunian object region of the solar system.
NTB: You mentioned minerals. What have been some of your interesting observations when it comes to minerals on other planetary bodies?
Dr. Cruikshank: We know a lot about the minerals on the moon, of course, partly from telescopic work and the fact that we have 850 lbs of the moon back here in the laboratory. The minerals that we find on the moon and other celestial bodies or planetary bodies include the so-called igneous minerals, the ones that are formed in pockets of melted rock, usually in the interior of a planet. We find these igneous minerals, which include olivine, pyroxene, and a few others, but the details of their composition and their distribution on either the moon or Mars are of special interest. We find these minerals also on a large number of asteroids, most of which have orbits between the orbits of Mars and Jupiter. And we go farther out in the solar system, and we still see vestiges of these very same minerals. What we’ve learned in the meantime, while we’ve been searching for all this, is that these very same minerals occur in meteorites, which are fragments of asteroids that fall on the Earth. And they also appear in the dusty regions that surround stars elsewhere in the galaxy. So certain of these minerals, in particular olivine and pyroxene, and a few of their relatives, are more or less ubiquitous throughout our galaxy, and we have every expectation that they occur widely in other galaxies as well.
And the importance of these minerals is that they tell us of the composition of the native materials from which the whole galaxy has formed, as well as the processing that that material has undergone to produce the rocks and minerals, and ultimately the ices that we see at the present time.
NTB: What kinds of work are you doing now with spectroscopic analysis of asteroids and comets?
Dr. Cruikshank: We continue to do ground-based telescopic work on comets and asteroids using spectrographs, and some of
this work is done with the largest telescopes in the world, including the Keck 10-meter telescopes in Mauna Kea in Hawaii. A lot of the objects in the outer part of the solar system are so faint and distant because they’re so small, that it requires the largest telescopes in the world really to detect their light and analyze it in a way that we’d like to, to get the information about the composition of these things, and ultimately, something about their origin and history.
We’re continuing to study comets and small asteroids. A particular class of asteroid that has just kind of emerged in the last few years, in fact, has kind of blurred the distinction between comets and asteroids. Asteroids typically are rocky bodies. Some of them are metallic and are at very stable orbits around the Sun, and we figured that they’ve been in those stable orbits for most of the age of the solar system, in excess of 4 billion years. During that time, they should’ve pretty well cooked out any gaseous material or icy material that might’ve been inside. What we’ve found, to our great surprise, just in the last couple years, is that some of these things considered to be asteroids are actually behaving like comets. So somehow they sequestered a supply of ice and gaseous material in their interiors for all this time, and even now, things happen to cause this stuff to leak out, and to give what was formerly a well-behaved asteroid the characteristics of a comet, namely a cloud of gas and dust surrounding it, and in some cases a tail.
NTB: With these kinds of observations, what other tools are you using to make them?
Dr. Cruikshank: One of the other important tools that we use is a technique called radiometry, which means to measure the intensity of the heat radiation that’s given off. Now, objects that are extremely cold and far from the Sun, don’t give up very much heat, but they do give off some. And with the techniques we have on the ground, both with big telescopes and in space, with infrared telescopes, we can measure that heat. Now it isn’t just a matter of trying to measure their temperatures and let it go at that, but it’s intended, in fact, to try to give us information about the structure of their interiors and surfaces, because the rate at which heat leaks out is a measure of the nature of the surfaces of these objects, and we can combine that information with the compositional data we get from spectroscopy to get a much clearer view and more complete view of the way these little bodies in space are packed together, and the way they respond to the heat and light of the Sun that shines on them.
NTB: How can these techniques in spectroscopic analysis and radiometry be used in commercial applications? Are there any partnerships with industry that resulted from this work?
Dr. Cruikshank: Not directly from this work. Industry, of course, builds the spacecraft that we use to make these measurements in space, when we go to a planet and get into orbit around it, as we are with Saturn and the Cassini mission right now, and with other spacecraft that simply fly by a target and make the measurements on the fly and then radio the information back, so that’s a partnership.
Principally, the spectroscopy technique has been long used in chemistry and in physics for well over 100 years to probe the inner workings of molecules and atoms, by the light that they either emit or absorb. So spectroscopy is an enormously powerful tool for studies of a composition, for materials in a laboratory, whether it be for pure research or for industrial purposes, and that same technique with just a few modifications is what we used either at a tail-end of a big telescope to look out into space, or on a spacecraft which is flying by a planet, or a spacecraft such as the Spitzer Space Telescope, which is still in orbit and working, to look at other galaxies and star systems in other planets elsewhere in our universe.
NTB: Dale, I was hoping to look a bit into your biography here, too. You have an asteroid named after you, is that right? In 1988, Asteroid 3531 was named Cruickshank by the International Astronomical Union. Is that a good one?
Dr. Cruikshank: It’s a nice honor; it’s certainly not a unique honor. The people who discover the asteroids -- and there are some specialists, some of whom have found hundreds and hundreds of them -- according to the International Astronomical Union, have the privilege of recommending a formal name rather than just a number for an asteroid. So a lot of these folks, some of whom I work with, honor their friends and sometimes their pets, and their favorite rock group with names that originally, when only a few asteroids were known, were named after Greek Gods and other notables and mythological characters. And here we are now, with one named after me, and several other people I work with, in the company of Greek Gods. It’s an ego boost. It’s not by any means a unique honor, as I said, and by the way, there are at least a quarter of a million more asteroids unnamed, so there’s potential for naming a good fraction of a population, an asteroid for each.
NTB: You spent time in the USSR as a National Academy of Sciences scientist. What kinds of work did you do there?
Dr. Cruikshank: That was a long time ago. In the end, on three different visits, I spent a total of about two years there. When I finished my PhD work at the University of Arizona, in the late ‘60s, it was clear that there was a Russian scientist that was working on the same types of problems that I was, and so I applied for this program that the National Academy had as an exchange arrangement with the Soviet Academy of Sciences, and was able to spend a year there in 1968-69, working with the guy who was doing very similar kind of work to mine. At that time, he and my boss back in the US were the only people in the world doing this kind of work because it was pushing the technology of the time to make infrared, spectroscopic measurements, so it was an opportunity to see first-hand what he and his people were doing, to learn from him, to work with him, and establish what turned out to be a life-long friendship and collaboration. So we had many years of collaborating and discussing and mutual visits, and that turned out to be a very positive thing in the development of my career.
NTB: As an astronomer at the Institute for Astronomy, you helped with the development of Mauna Kea, an important observatory site. What role did you play in that development?
Dr. Cruikshank: When I went to the University of Hawaii in the summer of 1970, the decision had already been made to build a telescope on that site, which is at an altitude of about 14,000 feet. It’s a difficult environment to work in, because of the low oxygen and the limited access by a bad road and all that, but nonetheless, the University of Hawaii and NASA had already put a medium-sized telescope up there, and it was just coming into operation in the summer of ’70. So I was one of the first users of that telescope. The conditions to use it were a bit adverse, but I was young and eager, and it was an opportunity unmatched anywhere else. Together, with colleagues who had the similar inclination, we made very good use of that telescope and demonstrated the utility of that particular site, as a world class, maybe the best in the world, site for infrared astronomy from the ground. That’s before we had infrared telescopes in space.
So, on the basis of the work that we did, and the results we got, which were quite unique, other organizations decided to put infrared telescopes up there, too. For example, the United Kingdom put an infrared telescope up that went into operation in ’79. The Canadians and the French teamed up to put a large telescope there that also went into operation in ‘79. And NASA decided to put a dedicated infrared telescope up there as well. It was the third of those three that came into operation in ’79. Since that time, the importance and efficacy of that site have been so well demonstrated and reinforced that the Japanese have a telescope there, the Keck Observatory, with the two largest telescopes in the world, have been established there, and there’s yet another giant telescope and more to come.
So it was the early work that my colleagues and I did at that newly opened site, which was, I repeat, difficult to work at, and still is, because of the altitude, that made it clear that infrared astronomy could best be done there, that infrared astronomy is critical to the understanding of the universe, planets, stars, galaxies, the whole thing, and that that’s a great place to build a telescope. Many countries and many organizations have done so and it remains to this day the premier infrared observatory site in the world.
NTB: Looking at your years of spectroscopy and radiometry experience, what has been the most exciting discovery for you?
Dr. Cruikshank: I think the most exciting discoveries have been the detection of these ices on the objects in the outermost part of the solar system: in particular, Neptune’s largest satellite, Triton, which is an object roughly the size of our moon, but is a long, long way away. We found frozen nitrogen there for the first time. I already mentioned the discovery of frozen methane on Pluto, which was a few years earlier, and eventually nitrogen on Pluto as well.
And the excitement around those is that those discoveries were the first indications that we had of the presence of these frozen ices, other than frozen water and frozen carbon dioxide, in various parts of the outer solar system. In the case of both Pluto and Triton, the nitrogen ice on the surface is actually the source of thin atmospheres that surround those two bodies, so both Pluto and Triton have very, very thin atmospheres that result from the presence of the nitrogen ice slowly evaporating on the surface.
So at the same time that we found the ice on the surface, we found atmospheres surrounding those bodies, and that’s a very exciting thing to experience and to participate in when you reveal a truth about objects that are so remote and thought to be so impossible to observe because of their distance and small size. Nonetheless, the techniques that we have and are able to finesse allow us to find these amazing things, and then that leads them to further understanding of these bodies: how they came to be, what has their evolution been in the intervening 4 billion years or so, and so on.
Now just to follow up on that a bit, since I picked both Triton and Pluto – In 1989, the Voyager spacecraft, a NASA spacecraft, flew by Neptune and Triton and revealed the surface of Triton in a way that we never thought we would see before, with high resolution views of its icy surface, and it also showed that its surface is quite young, geologically speaking, which means that something’s going on inside of Triton that keeps melting the ice from time to time. We also found geysers shooting up out of cracks in the surface of Triton up into this thin atmosphere I mentioned. In that process, Triton was revealed as not just a cold, dead icy body far, far away, but as a dynamic planet-sized object that’s doing something. We don’t know what exactly, but it’s not dead and lifeless in the geological sense.
At the same time, we are now on our way to Pluto. In July of 2015, the New Horizons spacecraft, a NASA spacecraft, will fly by Pluto, in the way we did with Triton in ‘89, and we are expecting tremendous discoveries when we see Pluto at last up close and personal with the new Horizon spacecraft, which by the way, is working very, very well. We have every expectation that it will have a highly successful flyby in July of 2015.
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