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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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Claudia Alexander
Project Scientist, Jet Propulsion Laboratory
L-R) Dr. John A. Simpson and Dr. James Van Allen shown during a Pioneer 10/11 news conference held on 27 Jan. 1973.

What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?

1974: Pioneer 11's exploration of the environment of Jupiter. Pioneer 10 was the first spacecraft to fly by Jupiter, but Pioneer 11 was the spacecraft to find that Jupiter's vast magnetosphere changes as it is buffeted by the solar wind. Pioneer 11 did not fly with sophisticated cameras and spectrometers, but flew with a suite of plasma and magnetic field instruments. This type of mission is now counter to what would be normal on a planetary mission today!

Artist rendition of the Pioneer 10 spacecraft shown flying by the solar system's largest planet Jupiter.

This came on the heels of a large radio telescope's discovery (1955, read about it here) of profound radio signals from the region of Jupiter (indicative of a highly energetic radiation environment) and James Van Allen's discovery of the radiation belts of Earth (1958). The significance of these discoveries is that planetary scientists were starting to realize that a planet's influence did not stop at the edge of its atmosphere, but that there is a whole region of space surrounding it known as the magnetosphere.

Simulated Van Allen Belts generated by plasma thruster in tank #5 Electric Propulsion Laboratory at the Lewis Research Center, Cleveland Ohio (now the John H. Glenn Research Center at Lewis Field).

Before the Pioneer spacecraft, scientists and engineers did not know if a spacecraft could survive space exploration of the planet Jupiter. The Pioneers taught scientists about the radiation environment and what kind of hard electronics would be required for a successful space mission to the outer solar system.

Also, scientists started mapping -- for the first time -- the extreme environment inside Jupiter's moon Io, which was critical to Voyager's success in studying Io.

This is an artist rendition of Pioneer 10 shown looking back at the sun from the position of Neptune's orbit. The plucky little spacecraft was the first spacecraft to leave the solar system.

1977 - 1989: Voyager 1 and 2's grand tour of the solar system. The Voyager's success was in part due to a quirk of nature: the planets were aligned in a once in a many-lifetimes opportunity. The navigators were then able to take advantage of this opportunity for a quick visit to each of the outer planets. This enormous endeavor was achieved by spacecraft built essentially with 1960s to early-1970s technology, which nonetheless gave us a priceless first look of our outer solar system. Similar to how the explorers of the so-called "New World" (1492) opened a whole new way of thinking about the Earth and its inhabitants, Voyager expanded our knowledge -- and our imagination -- about our solar system.

This montage of images of the planets visited by Voyager 2 was prepared from an assemblage of images taken by the two Voyager spacecraft.

1979: The discovery of active volcanism on Jupiter's moon Io by Voyager 1. The importance of this discovery was that for the first time a moon (not a planet) was found to have active volcanism. This discovery led us to postulate new means for a moon to be warm enough for volcanism; new paradigms for moons. It would be later, with further detailed study by the Galileo mission, that Io could be considered, with its voluminous volcanism, as a possible example of an early Earth.

One of the most surprising discoveries of the Voyager 1 mission was the violent volcanoes of Jupiter's moon Io.

1990s: I find the Galileo mission's proof that Jupiter's moon Europa has a liquid ocean layer to be significant. At the time of the Voyager flybys, we were pretty convinced that Europa was frozen solid and substantially old (not as old as Ganymede, but still billions of years old). With Galileo, it took ten years to confirm (consolidate all the evidence) in order to prove to ourselves that Europa is currently active; that there is an ocean underneath the crust and that the conditions may allow for a biotic environment. The latter point requires more direct and detailed measurements and observations, but to identify -- for the first time -- a potential environment in the solar system besides Earth where biology might be possible was a huge accomplishment of the planetary exploration program.

False color has been used here to enhance the visibility of certain features in this composite of three images of the Minos Linea region on Jupiter's moon Europa taken by the solid state imaging camera on NASA's Galileo spacecraft.

1996: The discovery of Ganymede's (a moon of Jupiter) magnetic field is important because this discovery differed from conventional wisdom about icy moons and their evolution. The finding of Ganymede's magnetic field also raised several questions that are likely to be addressed by future missions such as InSight, and the European Space Agency's "JUICE" (or LaPlace) mission. Insight will investigate the formation of a planetary body. JUICE will investigate the role of resonances.

This is an artist's concept depicting the magnetic field of Ganymede.

2006: The discovery of the variable rotation of the planet Saturn by the Cassini mission. This measurement has helped illuminate features of the giant planets that were not appreciated prior to the Cassini mission. (Once again, we think we've learned everything with a single mission, only to have new phenomena be discovered that casts everything we've learned before into a different light.)

Saturn sits nested in its rings of ice in this image by Cassini of the graceful giant. This natural color mosaic was acquired by the Cassini spacecraft as it soared 39 degrees above the unilluminated side of the rings. The images in this view were obtained on 9 May 2007, at a distance of approximately 1.1 million km (700,000 miles) from Saturn. Image scale is about 62 km (39 miles) per pixel.

2009: The confirmation of Titan's methane/ethane cycle and its curious chemistry by Cassini-Huygens. The ethane/methane components of the Titan environment have been well known for decades from ground based observations, but it is the insights into this cycle provided by the Cassini measurements (circa 2009) that I find most illuminating.

These new insights show that methane might actually be "missing" -- allowing new paradigms to be forged as to how the chemistry works; whether there is an underground ocean that allows the lakes to disappear in winter and re-appear in the summer hemisphere, and whether some of the bi-products might be consumed in new chemical pathways unrealized until now.

While we've looked at Io for examples of early Earth volcanism, we are now looking at Titan for examples of early Earth chemistry. We think Earth was a so-called "reducing" environment, where hydrogen and its bi-products (including methane) were more important than oxygen and its bi-products (like water). Now we have a space laboratory in the form of the moon Titan, providing concrete examples of the exotic chemistry that might take place.

The existence of oceans or lakes of liquid methane on Saturn's moon Titan was predicted more than 20 years ago. But with a dense haze preventing a closer look it was not possible to confirm their presence; that is until the Cassini flyby on 22 July 2006.

2010: The discovery that the dynamics of the rings of Saturn resemble the dynamics of the formation of planetary discs around other stars. The Cassini mission at Saturn is making the detailed measurements that make these comparisons and extrapolations possible. This is helping us understand the arcane physics that is possible in such an environment.

Saturn's most prominent feature, its dazzling ring system, takes center stage in this stunning natural color mosaic which reveals the color and diversity present in this wonder of the solar system. Gaps, gravitational resonances and wave patterns are all present, and the delicate color variations across the system are clearly visible.

Here is an example: the so-called "propellers." These are features in a ring where a relatively large ring particle seems to be stirring up its environment -- in the same way my ice-cream maker does at home: the blades create a pile-up as the container rotates. We see these pile-ups in the rings, and this causes us to wonder if there's a similar sort of condensation taking place -- a condensation that leads to the formation of planetesimals. It's fascinating.

A propeller-shaped structure created by an unseen moon is brightly illuminated on the sunlit side of Saturn's rings in this image obtained by NASA's Cassini spacecraft.

2011: The discovery of comet Hartley 2's Earth-like deuterium to hydrogen ratio, which turns contemporary notions of solar system formation on its head!

This image shows part of Hartley 2's nucleus. The sun is illuminating the nucleus from the right. A distinct cloud of individual particles is visible. This image was obtained on 4 November 2010 by the Deep Impact-EPOXI mission.

I was definitely one of those who believed that comets were not the source of the Earth's oceans since the "flavor" of water in the Earth's oceans bears no resemblance at all to that of comets. However, there is a similarity in flavor found in the isotopic signatures -- deuterium to hydrogen ratios, etc.

To have Earth-like water in a solar system object that is believed to have been formed far out in the original planetary disc implies that the solar system was much better mixed up (more homogenous) during formation, without the so-called "snow-line" as previously thought.

This finding changes the way we think about the planets and why they are so different from one another. It's a very significant measurement, one that's causing us to re-think everything we thought we knew about how the solar system formed -- what sort of "cooking" process took place in those early days.

In your field of work, what are some examples of the great achievements and discoveries in planetary science and robotic exploration throughout the past 50 years?

I want to talk about five key experiments (thought experiments in the form of computer simulations) followed by eight key and related measurements by robotic exploration. These 13 achievements (a baker's dozen) in history combined together have illuminated the physics of the outer solar system (and maybe the inner solar system too).

Harold Urey: a pioneer in so many ways. Urey and Stanley Miller conducted a famous laboratory experiment in 1953 demonstrating that amino acids (a key biological suite of organics) can be formed from a "soup" of water, carbon dioxide and ultraviolet light. This paved the way for our contemporary notions of the sorts of chemistry, coupled with electro-magnetic input, critical not only for astrobiology, but also for terrestrial life.

From their discovery, it is now easy to speculate about different combinations of soup-material that might lead to different flavors of amino acids, and from there to speculate about different biotic environments (or abiotic ones, which are nonetheless interesting to study). All of these different soup-like combinations might be possible in the deep regions of outer space.

Moreover, strange, long, carbon-chains are seen in deep space:polycyclic aromatic hydrocarbons (PAH),diaminomaleonitrile, polyoxymethylene (POM), etc. Urey's experiment provided a critical way to postulate how the simple building blocks can be altered in order to lead to more complex planetary building blocks.

Fred Whipple: Whipple postulated the "dirty-snowball" comet model, dispensing with the old notion of comets as a flying collection of sand. With Whipple's dirty snow-ball theory there was plenty of opportunity for crusty material to form, on which some Harold Urey-style chemistry might take place. Still, we really didn't know what a comet was (even today we are not certain). Many theories were predicated upon notions of how the solar nebula collapsed, and how planetesimals formed after that. New theories would come, only to be shot down, but Whipple's dirty snow-ball idea has endured.

Jan Oort's model (another key accomplishment) of the dynamics of the solar system: Oort postulated that comets exist in a cloud around the solar system -- a shell -- that can be found between five and 100 thousand AU (a huge distance, even for astronomical units) away. This cloud is now known as the Oort Cloud.

An illustration of the Kuiper Belt and Oort Cloud in relation to our solar system.

Bob Johnson's "sputtering:" When Bob Johnson conducted sputtering experiments in his lab in the early 1970s, he (like all of his colleagues in planetary science) thought of them more as science fiction, than a key process in space -- no one really expected to find this process in a planetary environment, let alone that it is a critical process.

When the first sputtering measurements were taken (in the lunar environment) no one was more surprised than Dr. Johnson. Dr. Johnson has investigated this process (now termed with the fancy label "Radiolysis") on every major mission ever since.

Sputtering accounts for how energy is deposited from an atmosphere or magnetosphere onto a surface, with an energy cascade into the regolith (whether rocky or icy), or the atmosphere, which may ultimately be responsible for providing key energy to any abiotic, or potential biotic, process below.

The YORP Effect: (The Yarkovsky-O'Keefe-Radzievskii-Paddack effect.) Among the many useful and ground breaking theories developed for the dynamics of how comets and asteroids behave in space, one of the more exotic was the so-called YORP effect.

The YORP Effect describes an asteroid or a comet as a loosely consolidated group of (heavy) particles -- rubble -- that spin together to form a larger body in a distinctive shape (a diamond, or bean shape). In 2008, the Rosetta mission observed asteroid Steins, which was the first confirmed YORP effect object in space.

Giotto's picture of comet Halley in 1986: For thousands of years, humans have lived in fear of comets. In 1986, humankind got its first up-close and personal view of these mysterious objects. Just as Whipple had postulated (and confirmed with many ground based observations of Halley's spectra from the coma and tail), the nucleus itself spewed jets from discrete areas.

In 1986, the European spacecraft Giotto became one of the first spacecraft ever to encounter and photograph the nucleus of a comet, passing and imaging Halley's nucleus as it receded from the sun.

Nonetheless, nothing can take away the mystery quite like a picture, and this one showed a generic "bean" shape with "mountains" and valleys and dark regions with gases emanating out of them. The darkness of the nucleus was perhaps the biggest surprise. It was extremely dark and there is no terrestrial analog for a material that is composed mostly of water ice, and is at the same time also completely black -- as dark as asphalt. Could a process like radiolysis play a key role in the creation of this black material? Is it a crust? Or is this truly what the nucleus is made out of?

Stardust-NExT's new observations of Tempel 1 (2011): The Deep Impact mission (2005) blew a hole in comet Tempel with a missile and buried the old Whipple idea of comets being a loosely consolidated, fluffy snowball. It was only with the NExT mission that we had a chance to actually see what we had done to the comet with the impactor.

These two images show the different views of comet Tempel 1 seen by NASA's Deep Impact spacecraft (left) and NASA's Stardust spacecraft (right). Two craters, about 300 meters (1,000 feet) in diameter, help scientists locate the area hit by the impactor.

However, with NExT's view we saw that the surface of the comet looked nothing like we had expected or had trained ourselves to believe from those early Whipple days. What we found were indications of surface geology with such features as flows across the surface. These findings really changed our ideas about comets: How "old" they are, how processed they are and how much they preserve the material left over from the primordial collapsing clouds out of which the planetary bodies all supposedly formed.

Asteroid Lutetia has been revealed as a battered world of many craters. ESA's Rosetta mission returned the first close-up images of the asteroid showing it is most probably a primitive survivor from the violent birth of the solar system.

Asteroids Vesta and Lutetia's surface mineralogy findings by the Dawn and Rosetta missions in 2010: These findings implied that the two comets had formed in much different locations than where they are currently: an indication of significant planetary migration and asteroid parent body scattering in our solar system.

NASA's Dawn spacecraft obtained this image with its framing camera on 18 July 2011. It was taken from a distance of about 6,500 miles (10,500 km) away from the protoplanet Vesta. The smallest detail visible is about 1.2 miles (2.0 km).

Another key in planetary physics is all the instrument development for experiments to collect dust. For years, the same team in Germany produced dust instruments to fly on mission after mission throughout the solar system. These types of instruments flew on all the early missions, starting with Pioneer, and the investigations culminated in measurements from Saturn's moon Enceladus (2010 from Cassini).

At least four distinct plumes of water ice spew out from the south polar region of Saturn's moon Enceladus in this dramatically illuminated image.

The instruments went from recording the amount of dust, to measuring more about the composition and size distribution. We learned an incredible amount about the amount of "dust" in our solar system, where it travels, the physics of how it gets around, and finally a lot about adsorption. We now speculate about the ability of dust to transport volatiles. This transportation would intimately tie the processes involving exploded volatiles (geysers, and other dynamic surface processes), to deposition and chemistry at other exotic locales. (See this article about Enceladus potentially seeding Titan with volatiles).

Saturn's rings cut across an eerie scene that is ruled by Titan's luminous crescent and globe-encircling haze, broken by the small moon Enceladus, whose icy jets are dimly visible at its south pole. North is up.

The physics of Titan's upper atmosphere: In Titan's upper atmosphere we find a Buckyball influx of dusty, organic tholins deposited at the top of Titan, possibly carrying trapped material migrated from another moon (Enceladus) to the surface (2010 from Cassini). These findings gave us insight into the whole idea of "seeding" and the role played by radiolysis.

This close-up view of comet Hartley 2 was taken by NASA's EPOXI mission during its flyby of the comet. It was captured by the spacecraft's Medium-Resolution instrument.

Hartley 2's topology (EPOXI, 2010): This takes us back to the idea of trying to figure out how a comet works as a machine. For decades we worked with the ideas of Whipple; we created models about how the gases escaped, etc. However, the up-close observations of the EPOXI mission have moved the chains forward significantly, showing us the possible connections between geology and surface topography, which have allowed us to finally understand more about the coupling of the gas kinetics and the creation of lag deposits; how the material itself contributes to the thermal conductivity.

Genesis principal investigator and lead scientist Dr. Don Burnett examines material recovered from Genesis' hard landing site.

Genesis' measurements of the content of dust particles: This showed us that these dust particles more correctly fall along a line that connects meteorites and extra-terrestrial water, and not the substance that formed the Earth and the moon -- making us re-think how the solar system is "organized" chemically.

Artist's view of ESA's Rosetta cometary probe.

Taken together all of these observations provide mounting evidence about the nature of the building blocks of the planets and solar system formation. The Earth ocean-like deuterium ratio from Herschel opened up many new lines of inquiry, which will be solved with critical measurements by Rosetta (my mission).

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