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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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Life Elsewhere?
Cassie Conley
Planetary Protection Officer, NASA Headquarters
Viking in the Oven
In this image we see one of the Viking landers being prepared for dry heat sterilization.

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

Viking was such an ambitious mission for planetary protection. I think that Viking is the most significant mission that NASA has sent and is possibly the most significant space mission ever sent by humanity. Why, you might ask? The engineering for Viking was more challenging than any successful mission before or since. The entire spacecraft was prepared in such a way that it could be baked in a dry oven. This was done in order to kill off any Earth organisms that were on the spacecraft. This way, the spacecraft would not detect Earth life with the instruments it carried and it would not contaminate the Red Planet with Earth life.

Viking on the surface of Mars (in color)
The boulder-strewn field of red rocks reaches to the horizon nearly two miles from Viking 2 on Mars' Utopia Plain. The salmon color of the sky is caused by dust particles suspended in the atmosphere.

Viking 2 landed 3 September 1976 -- about 4,600 miles from its twin, Viking 1, which touched down on 20 July.

Viking, however, did not find life on Mars and because of this the mission did in some ways discourage the U.S. from trying to do another life-detection mission. In terms of the technical capabilities, we have not replicated anything like Viking before or since that time. (So far, the European Space Agency (ESA) is the only other space organization to have plans for a life-detection mission, but they haven't finished it yet.) The Soviet Union was also discouraged by the results from Viking. Supposedly, when asked why the Soviet Union had canceled its Mars program after Viking, the response from someone in the program was "It wasn't interesting anymore." Viking had not found life on Mars. It was a negative result, but that doesn't mean that we did not find anything. There were very many quite important results from Viking about Mars.

Looking at the Viking mission now in retrospect, and from the perspective of a planetary protection officer and biological scientist, there were some very important results in terms of the subsequent influence on biology here on Earth and the search for life in space. Viking was presented to the public as a definitive search-for-life mission. We now understand that it is not possible to do a single definitive mission. It is too difficult. It is hard to do even here on Earth. Also, we did not really understand Earth before or during the Viking mission. At the time of Viking, we were only aware of a fraction of 1% of the total organisms on Earth. A fraction of 1% were the ones that we knew existed -- we assumed that there wasn't anything else. This assumption was just scientific naiveté. We simply didn't understand the range of life on Earth back then.

Methane-Ice Worm
Although this is not one of the worms found back in the 1970s, this is an example of a type of worm found living in extreme conditions.

Scientists discovered this new kind of centipede-like worm in 1997 living on and within mounds of methane ice on the floor of the Gulf of Mexico, about 150 miles south of New Orleans. Methane ice, a gas hydrate, forms naturally at the high pressures and low temperatures of the deep sea, but is usually buried deep in marine sediment. The Gulf of Mexico is one of the few places where hydrate can be found exposed on the ocean bottom. Occasionally this seeping, solid methane bursts through in mounds, often six to eight feet across.

For example, we were not aware of the ability of some organisms to survive in extreme environments. The tubeworms that were discovered living within the hydrothermal vents of the mid ocean ridges back in the 1970s were discovered by a ship of ocean geologists. (These worms were discovered as the Viking mission was flying to Mars). There was not a single biologist on board because everybody "knew" that there wouldn't be any life down there. So to think that we could be so successful in our first attempt to detect life on another planet when we did not completely understand it here was very ambitious, albeit somewhat misguided.

Jupiter Family Portrait
A comparison portrait of Jupiter's four Galilean moons Io, Europa, Ganymede, and Callisto, each with different characteristics. (In this image composite, Jupiter is not at the same scale as the satellites.)

Certainly in terms of planetary protection, the Galileo mission takes second place in my mind as a significant mission. Galileo was really the first time that we sent a mission to explore another system of worlds. We had sent missions to other worlds before of course: Earth's Moon, Mars, Mercury and other planetary flybys. However, Galileo was really the first big mission going to orbit another planet that had so many moons. This meant that we had to have a totally different strategy in how we were going to do the exploration. We had to figure out how we were going to encounter all of the different targets. We were successful in this endeavor even with all of the difficulties involved with the failure of one of the spacecraft's antennae.

Galileo also provided strong evidence that Jupiter's moon Europa has liquid water oceans; making Europa the most probable place after Earth to host life. In order to protect Europa's oceans, the spacecraft was deliberately destroyed by sending it into Jupiter's crushing atmosphere.

Icy Europa
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.

Unfortunately, the Galileo spacecraft had not been sterilized in the way that the Vikings had been and therefore was carrying live organisms from Earth. It may sound strange to throw away a functional spacecraft, but it was done in order to protect the potential habitat for life on Europa. The reason we did not sterilize this spacecraft in the first place was because we did not think that there could be any potential life habitats around Jupiter.

Jet Blue
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.

The same is true for Enceladus around Saturn. No one thought that there could be liquid water on a moon of Saturn, but the Cassini mission has found evidence. At the end of the Cassini mission this spacecraft will have to be crashed into Saturn in order to protect Enceladus. This is a lesson for the search for life in general: There can be habitats for life in places we never thought possible.

You might ask: "Is it hard to sterilize a spacecraft?" In some ways, yes. It is hard to sterilize a spacecraft from a systems engineering standpoint, in that you have to design it in a way that it can be placed in an oven to bake (or whatever sterilization procedure that you choose to apply) and then be able to function afterwards. With Viking they set out from the beginning to make a spacecraft that was going to be sterilized in an oven. So when they were making the parts, they designed them to withstand the sterilization temperatures and process. This type of planning and follow through is expensive, but if you include it from the beginning you end up with a much better spacecraft. For example, carbon fiber panels are actually lighter than metal ones and are cured at high temperatures -- the same temperatures used to sterilize a spacecraft. So if you pick the right materials you do end up with an advantage. You have to design that; you have to think of that at the very beginning of the planning process.

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?

Allen Hills Meteorite
Alien! Alien? Is this what an ancient Martian looked like? The tube-like form on the above highly magnified image was once believed by many to be a fossil of a simple Martian organism that lived over 3.6 billion years ago.

Several things tie in here: One (not a robotic mission, but highly relevant) was the announcement that the Mars meteorite (ALH84001) found in 1984 in Antarctica's Allen Hills might have indications of fossil life from Mars. Although that conclusion is no longer accepted, it rekindled public interest in searching for life beyond Earth.

The Allen Hills meteorite also furthered our understanding of biology here on Earth. Initially, it was hypothesized that this Martian meteorite might have fossils of life in it. However, they did not look like anything we knew on Earth. The fossils were so tiny. The immediate response and reaction from biologists was that such small organisms couldn't possibly exist. The next reaction was: "Well, maybe we should check whether that's true on Earth." In looking for very, very small bacteria and organisms on Earth we now find them everywhere. We just hadn't looked before. The search for life elsewhere is really informing how we look for life on Earth and how we understand life on Earth -- the more we look, the more we find. I think that this is one of the major impacts of the study of space and the search for life in space -- it has furthered our understanding of our own planet.

NASA has had a major role in stimulating discoveries about life on Earth because the Exobiology (now Astrobiology) program at NASA has funded work for the last 50 years that nobody else would touch. Work that is much too "far out," much too esoteric, much too chancy. But the Exobiology program funded it because it might have bearing on our search for life elsewhere. This is how we discovered the third domain of life on Earth: Carl Woese' very early sequencing experiments discovered that there are not just two different lineages of life on Earth, but three. This was actually pooh-poohed; that is until he got the Exobiology funding and could document it.

Pathfinder Panorama
An enhanced version of the famous Mars Pathfinder photo of the lander and Sojourner rover. This browse version shows only part of the 360-degree panorama. The high-resolution version includes the whole image.

Another event I find significant, and which has to do with an advancement in robotic spacecraft, is the landing of the Pathfinder mission on Mars. Pathfinder used a totally different landing system; one NASA had never tried before. This later enabled the two Mars Exploration Rovers to land on Mars. Spirit and Opportunity have each made amazing discoveries.

Opportunity Tracks
NASA's Mars Exploration Rover Opportunity used its navigation camera for this northward view of tracks the rover left on a drive from one energy-favorable position on the northern end of a sand ripple to another. The rover team calls this strategy "hopping from lily pad to lily pad."

The Mars Exploration Rovers have told us about the geology and science of Mars. And Opportunity continues to tell us more and more about how the climate has changed on Mars. This information comes back and updates our understanding of Earth's climate and the possibility for climate change on Earth. We need this understanding of other planets (comparative planetology) to inform us better about what the possibilities are for our own planet, and what we might do to help Earth continue to be a nice place to live.

Mars Reconnaissance Orbiter Captures Phoenix
Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE) camera acquired this image of Phoenix hanging from its parachute as it descended to the Martian surface.

Another advancement is the landing of the Phoenix spacecraft. Mars Reconnaissance Orbiter observed the landing of Phoenix from its orbital view about the Red Planet. This landing demonstrated the power of having a program of exploration (not just individual missions). Also, the Phoenix observations on the surface of nearly-pure water ice just centimeters beneath the circumpolar surface of Mars further informs us about this planet.

"Dodo-Goldilocks" Ice Trench
NASA's Phoenix Mars Lander's Surface Stereo Imager acquired these color images on the 21st and 25th days of the mission, or Sols 20 (left) and 24 (right) (15 and 19 June 2008).

These images show sublimation of ice in the trench informally called "Dodo-Goldilocks" over the course of four days.

The Mars Science Laboratory spacecraft is the next opportunity we'll have to make discoveries about Mars.


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