In this image of Saturn taken by Voyager 1 we also see Saturn's moons Tethys and Dione.

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

"The finding of volcanism on Io really changed our idea of what a habitable zone means in the solar system. We thought that the outer solar system was cold, icy and dead."

I would have to put the Voyagers first on my list. The Voyagers completely changed the way we view the outer solar system. Through Voyager we were able to see what Jupiter's moons looked like, what Saturn looked like from the other side and so much more. No longer were the outer planets and their moons just fuzzy little blobs in the sky, but actual places we could imagine being.

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

The most surprising moon visited by Voyager was Jupiter's moon Io. We had no idea that we could have a moon similar in size to our own Moon that had two to three hundred active volcanoes going off on it all the time. We only flew past Io a few times during the Voyager encounters, but every time we did fly by there were hundreds of volcanoes erupting on this moon.

The finding of volcanism on Io really changed our idea of what a habitable zone means in the solar system. We thought that the outer solar system was cold, icy and dead. We also thought that a planet or moon could only have liquid water or sufficient warmth if it was "snuggled up" close to a star. However, we found moons that run on tidal energy rather than sunlight in the outer solar system. In the case of Europa, this could mean that there may be life just underneath the ice.

The dark spots are called "lenticulae," the Latin term for freckles. Their similar sizes and spacing suggest that Europa's icy shell may be churning away like a lava lamp, with warmer ice moving upward from the bottom of the ice shell while colder ice near the surface sinks downward. Other evidence has shown that Europa likely has a deep melted ocean under its icy shell. Ruddy ice erupting onto the surface to form the lenticulae may hold clues to the composition of the ocean and whether if it could support life.

The same is true of Saturn's moon Enceladus. Unlike Europa, there is not a doubt that there is water on Enceladus. We know that each day, as the tides go by, the cracks on Enceladus open and shut and water gushes out. This evidence, and the evidence at Europa, supports the theory of possible life habitats in the outer solar system. No one had expected this before the missions to the outer planets.

Cassini imaging scientists used views like this one to help them identify the source locations for individual jets spurting ice particles, water vapor and trace organic compounds from the surface of Saturn's moon Enceladus.

Titan, with its thick atmosphere, is a place where it is literally raining organic molecules all the time -- imagine being knee deep in organics. Titan is really so organic-rich, we have to wonder: "Could there be a life system based on methane rather than water?" "Or, in fact, is there water underneath the methane?"

I am sometimes disappointed by how so few of the public know that we landed on Titan. We actually parachuted through the atmosphere of Titan and landed on the surface near a giant methane lake. We saw methane rivers, and it was raining as the probe fell down. It is absolutely incredible that there is a little spacecraft on Titan right now!

This mosaic of three frames provides unprecedented detail of the high ridge area including the flow down into a major river channel from different sources on Saturn's moon Titan.

When I was a child, life somewhere else in our solar system seemed really unlikely, at least no one was talking about it seriously. Now we can ask the question: "Which of the places shall we explore first?" There may be life in half a dozen places in our solar system.

In regards to evidence for life in our Universe: Recently, I was speaking with Jason Dworkin (head of astrobiology at Goddard). He showed me some of the aerogel from the Stardust mission that had picked up samples from comet Wild 2. From this comet dust they are isolating little bits of our DNA -- amino acids. Scientists are finding the building blocks of life in these comet samples. It was strange to be sitting in a room with something on the table that had flown through a comet's tail and returned a sample to Earth. It is just mind blowing to think that we have that sort of a connection to our local Universe.

This image shows the tracks left by two comet particles after they impacted the Stardust spacecraft's comet dust collector. The collector is made up of a low-density glass material called aerogel.

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?

Being an astronomer, certainly the idea of adaptive optics comes to mind. A lot of people wonder why NASA does not have any plans to build another visible light telescope in space. (The James Webb Telescope is all infrared.) The answer is we don't need to anymore.

We can use ground-based telescopes instead. What we do is shine a laser up through the atmosphere to see how the laser dances around in the air. We then take that data, feed it into the computer and cancel out the effect of the atmosphere. This advance has opened up so many possibilities. We are now able to see fainter and fainter galaxies from the ground.

We are also now able to measure not only the expansion, but the acceleration of the Universe. I am friends with Brian Schmidt, one of the co-winners of a Nobel Prize in Physics (2011). (We were undergrads together at the same college. I remember going on observing runs when he was around doing this work.) No one expected the Universe to be accelerating. People thought that the measurements would show that the Universe is decelerating -- slowing down over time -- or at least that it was at a stable rate. Everybody would laugh and joke saying: "What if it is accelerating?" However, Brian and his colleagues have found evidence for the acceleration of the expansion of the Universe.

NASA's Hubble Space Telescope took this iconic image of the Eagle nebula, dubbed the "Pillars of Creation," highlighting its finger-like pillars where new stars are thought to be forming.

Other:

In this image by Mars Reconnaissance Orbiter's HiRISE instrument we see seasonal polar caps on Mars, which are made of carbon dioxide (a.k.a. dry ice). When springtime on Mars occurs, this dry ice evaporates and causes some erosion of the surface. This erosion gives us "araneiform" terrain (various formations on the surface, such as "spiders," "caterpillars" and "starbursts").

I very often look at the pictures of Mars that are coming back from the Mars Reconnaissance Orbiter (MRO) and from the rovers. I enjoy watching the little movie of the dust devils -- the little tornadoes turning around on the deserts of Mars. When I look at that great little video I think: "That is Mars! That is Mars where those tornadoes are and here I am watching them!"

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• Michelle Thaller's People Profile

• Voyager 1 Mission Page
• Voyager 2 Mission Page
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• Stardust Mission Page
• Hubble Space Telescope Mission Page
• Mars Exploration Rovers Mission Page

• Saturn's Planet Page
• Io's Moon Page
• Europa's Moon Page
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Last Updated: 23 August 2012