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ISS007-E-10807 (21 July 2003) --- This view of Earth's horizon as the sunsets over the Pacific Ocean was taken by an Expedition 7 crewmember onboard the International Space Station (ISS). Anvil tops of thunderclouds are also visible. Credit: Earth Science and Remote Sensing Unit, NASA Johnson Space Center

Image Credit: ISS007-E-10807 (21 July 2003) – Earth Science and Remote Sensing Unit, NASA Johnson Space Center

The Year in Planetary Science

Richard Leis

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Published.

Humanity improves the vision it turns on the universe in two ways: seeing farther than before and resolving greater detail. 2005 was a year of much more detail, of blurry bodies resolving into dynamic worlds and undiscovered objects in our own backyard suddenly coming into view. Augmented by robotic surrogates, adaptive optics, new remote sensing capabilities, and intelligent data-mining agents, here are the discoveries made or announced in 2005 that transformed our view of our solar system.

Spirit and Opportunity on Mars

Never before have robots on the surface of another world traveled so far or functioned for so long. Despite signs of old age, the Mars Exploration Rovers Spirit and Opportunity continue to explore Mars.

Spirit climbed a mountain, observed dust devils, and returned panoramas of Gusev Crater from its high vantage point. Opportunity spotted a meteorite, survived getting stuck in a dune, and returned images of a variety of outcrops on its way to new craters for exploration.

The 10th Planet

It was not a hoax, a conspiracy theory, or pseudoscience, but a soap opera of events that led to the announcement earlier this year of the tenth planet in our solar system, 2003 UB313. After being scooped by another team on an transneptunian object slightly smaller than Pluto, Michael E. Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University were forced to confirm that discovery and then announce one of their own: Planet X.

The planetary body larger than Pluto and much further away from the sun was actually discovered during a reanalysis of data from October 21, 2003. Dr. Brown and his team then went back through even older observations to see if the object had shown up before but simply been missed. Sure enough, they came across an observation of 2003 UB313 made in 1989.

The story did not end there. Allegations of fraud were made against the other team, who may have used Dr. Brown’s own work as their own. Meanwhile, the debate over the definition of “planet” was reignited, with Dr. Brown strongly defending the use of that label for his discovery. For many planetary scientists, Pluto is not a planet, but simply a large member of the Kuiper Belt, a region of small objects outside the orbit of Neptune. Dr. Brown and others argue that Pluto should retain its classification as a planet simply for cultural reasons. And if Pluto remains a planet, then any object discovered to be larger than Pluto and orbiting the Sun should also be classified as a planet. Thus, Dr. Brown concluded, 2003 UB313 must be considered the 10th planet.

2003 UB313 has a surface of methane ice, just like Pluto. With the recent discovery of a companion moon scientists hope to nail down the mass and size of both objects. A decision over the classification of 2003 UB313 is still forthcoming.

The Plumes of Enceladus

A tiny moon orbiting Saturn in the frigid outer solar system should be silent and long dead. Most of Saturn’s moons are in fact just that. A series of discoveries by the Cassini spacecraft in 2005 limited this to a generalization forever by returning spectacular images of ice plumes erupting from the surface of Enceladus. The mystery, of course, is where the energy comes from to drive this activity. There is heat inside of Enceladus, heat that makes the tiger stripes near its south pole warmer than the rest of the moon, heat that causes material to vaporize or erupt from this region, resulting in kilometers-high plumes that help support a tentative but oxygen-rich local atmosphere and provide the material to constantly replenish one of Saturn’s rings.

Pulling Back the Shroud of Titan

Cassini provided humans their first glimpse of the surface of Titan late last year. The view left scientists scratching their heads. Then, on January 14, 2005 ESA’s Huygens probe descended through the thick orange smog of Titan’s atmosphere to reveal terrains that were surprisingly Earth-like, with river channels and shore lines suggesting large volumes of liquid at work. When Huygens landed it continued capturing images from the surface, including an orange-hued view of its surroundings.

Not only does the surface of Titan show the signs of active reworking by liquid, but the atmosphere is full of methane, a relatively unstable gas that would not show up in the atmosphere if it were not constantly replenished. What Huygens did not provide was images of standing liquid on the surface, long suspected as the methane reservoir. After Huygens landed it began to settle into the soil and recorded a rise of methane, presumably liquid methane that was vaporized by the heat of entry. The pebbles surrounding the landing site were well-rounded, a sign of fluvial processes here on the Earth. The highlands, where the channels start, were light, while the channel beds and sea-like lowlands were stained dark. This comes from hydrocarbons that snow from the atmosphere and are carried downstream.

Where, then, is the liquid? Huygens had stopped broadcasting from the surface of Titan, but Cassini continues to encounter Titan, with the capability to pry beneath the thick atmosphere by using various remote sensing wavelengths including radar. During one flyby, Cassini captured an image of what appears to be a lake. During another, a volcano. Persistent methane clouds have been detected.

Scientists hypothesize that Huygens landed during a dry season, or perhaps during low tide. Titan might experience monsoonal seasons with periodic torrential liquid methane rains followed by little activity. The surface is obviously quite young, but many more observations by Cassini will be necessary before scientists feel confident in their understanding of the processes at work.

All but one of Cassini’s moon flybys over the next two and a half years will be of Titan. These flybys will be at altitudes of 2000 kilometers (1300 miles) or lower to provide even more detailed data about the surface of Titan.

Deep Impact

We bombed the hell out of a comet and learned that what we thought we knew about these objects was wrong.

On July 03, 2005 Deep Impact encountered Comet Tempel 1. When the event was over, Comet Tempel 1 had a new crater and a rising plume of debris from colliding with the Deep Impact impactor. The parent probe captured images and other data of the impact that are still be analyzed.

What we learned:

  • Comets vary greatly among each other in their surface terrain.
  • These surface terrains can be quite complex.
  • Some comets are loosely packed, held together by gravity.
  • Comets may be compositionally quite complex.

Deep Impact detected the presence of water vapor and carbon dioxide gas after impact, while the Spitzer Space Telescope detected “clays; iron-containing compounds; carbonates, the minerals in seashells; crystallized silicates, such as the green olivine minerals found on beaches and in the gemstone peridot; and polycyclic aromatic hydrocarbons, carbon-containing compounds found in car exhaust and on burnt toast” according to September 07, 2005 press release from NASA and the Jet Propulsion Laboratory.

The parent probe continues to function and was placed into a new orbit that will allow mission scientists to return to Comet Tempel 1 or encounter a different comet in a few years.

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