Stardust Mysteries

ImpactTopics from Martian meteorites to planet formation were discussed at a recent international science conclave organized by two Kingsborough Community College geologists, whose research keeps their eyes on the sky and on Earth.

By Neill S. Rosenfeld

They flash across the sky, shooting stars that have fired the imagination ever since there were people. Meteorites, rocks that fall from the heavens -- messengers of the gods, portents of good fortune or cataclysms to come, depending on the culture. But in reality, they're so much more -- the very stuff the solar system is made of.

"You can't have life until you have a planet, so to hold a meteorite -- something that was around before there was a planet -- is totally awesome," says Harold C. Connolly Jr., one of two petrologists, or geologists who specialize in rocks, at Kingsborough Community College's Department of Physical Sciences.

Colleague Michael K. Weisberg says cradling the most primitive type of meteorite, a chondrite, "is like holding the sun, minus the gasses, and they also have organics, which are the building blocks of life."

Each meteorite tells an extraterrestrial story, and many emerged in July at the 73rd annual meeting of the Meteoritical Society in Manhattan, which Connolly and Weisberg organized under the auspices of the City University of New York and the American Museum of Natural History. The meeting drew some 500 scientists from around the world.

Presentations delved into Martian meteorites, planet formation, the origin of organic molecules on meteoroids, the structure of craters and the relation between asteroids and meteors, among other topics.

Sean Solomon of the Carnegie Institution of Washington delivered the keynote lecture. As principal investigator of Messenger, NASA's current mission to Mercury, he described how the first craft to visit the innermost planet since the 1970s whipped by Mercury three times since its launch in August 2004; it goes into orbit in March 2011. Messenger has already detected ion emissions from Mercury's atmosphere, expanded knowledge about the planet's magnetic field and proved that, at least in the past, Mercury had volcanic activity.

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Connelly WeisbergConnolly and Weisberg, colleagues for 30 years, work not only with meteorites found on Earth, but also with materials plucked from the cosmos.

Weisberg was on the international team that analyzed dust from NASA's Stardust Mission; launched in 1999, Stardust returned with samples of the comet Wild 2 in 2006. "We've had particles to study in our laboratories for four years. It turns out that a lot of the particles in the comet are similar to what we find in chondrites, including the chondrules and calcium-aluminum-rich inclusions," Weisberg says.

In other words, the dust and rock formed near our sun, traveled to the deep freeze beyond Neptune, and then mated with ice to become comets. This was a stunning finding, since scientists had thought that the dust and rocks of comets came from other stars and predated our own solar system. (Stardust did retrieve some mineral grains from other stars, which were identified by their unusual isotopes.)

Also, looking beyond Earth, Connolly joined a science team that is competing with two other groups for a $650 million prize -- a NASA mission. If selected, their OSIRIS-REx probe would visit asteroid 1999 RQ36, which NASA's Goddard Space Flight Center calls "a chunk of rock and dust about 1,900 feet in diameter." It would orbit RQ36 for a year, test ways of deflecting it from a possible impact with Earth in 2170, then extend a robotic arm and scoop up a pristine sample of its surface to return to Earth in 2022.

For extraterrestrial petrologists like Connolly and Weisberg, that's the real prize.



At the Meteoritical Conference

The 73rd annual meeting of the Meteoritical Society, held July 26-30 in New York City, included these findings:

  • Audrey Bouvier and Meenakshi Wadhwa of Arizona State University in Tempe and colleagues homed in on the solar system's age by examining calcium-aluminum-rich inclusions in the chondrite Vigarano. Their estimate: 4,568.7 million years, plus or minus 3 million years.
  • When asteroid 2008 TC3 exploded over Sudan's Nubian Desert in a blast that would have devastated a city, it spewed 1-to-10 cm fragments known collectively as the Almahata Sitta meteorite over a 5-by-28 km area. This is one of four known ureilite meteorites, which contain submicrometer-sized diamonds. Fourteen presentations considered TC3's formation, its age and its carbon, metals and diamonds, as well as nitrogen and noble (nonreactive) gases trapped in the fragments.
  • Weisberg and Denton S. Ebel of the American Museum of Natural History described the 7.31 kg NWA 5717 chondrite found in 2006. Its mineral textures and compositions show little thermal alteration, so it retains much of its original structure, including chondrules rimmed with dust-sized mineral grains. The rims formed when the chondrules floated freely in the early solar nebula.
  • Connolly was on a team led by Chi Ma of the California Institute of Technology that discovered two new minerals not found on Earth, chlorine-bearing mayenite and a calcium-mono-aluminate, in a chondrite called NWA 1934.
  • The University of Chicago's Fred Ciesla offered a new technique for calculating the transport and chemical evolution of water ice in pre-planetary disks like the solar nebula. Water is needed for a planet to be habitable.
  • CUNY doctoral student John Wolbeck, working with Connolly, offered an original "icy impactor model" (illustration left and story page 25) to explain both the moon's origin and the source of Earth's water. Given similar rocks on the Earth and the moon, the most common hypothesis is that the moon sheared off when a Mars-sized "planetesimal" hit the young Earth. If it was mostly ice, they reason, it would have delivered enough water to fill the oceans and humidify the atmosphere.

"Given similar rocks on the Earth and the moon, the most common hypothesis is that the moon sheared off when a Mars-sized 'planetesimal' hit the young Earth."



Inside Jupiter's Largest Moon,Ganymede

Based on observations by the Galileo spacecraft, NASA's Jet Propulsion Laboratory estimates that Jupiter's moon Ganymede is 46 to 50 percent ice, 12 percent iron and 36 percent silicates, about the same as its moon Callisto.That supports CUNY graduate student John Wolbeck's icy impactor model and solves most remaining problems associated with the widely accepted Giant Impact Hypothesis about the formation of Earth's moon:

  • Lunar samples are identical to Earth's mantle
  • Moon is iron deficient
  • Low eccentricity of lunar orbit
  • Quenching allows larger impact energy
  • Allows volatile substances to persist
  • These icy planetesimals already exist
  • Deuterium/hydrogen water ratio similar to Jovian region
  • And it could be the source of Earth's water.


Icy Solution for Two Puzzles?

WolbeckThe first mystery is where our moon came from. The second is how Earth got its water. Those mysteries may well share a single solution, according to CUNY doctoral candidate John Wolbeck.

Working with associate professor Harold C. Connolly Jr., of Kingsborough Community College and the Graduate Center, Wolbeck advanced his hypothesis at this summer's Meteoritical Society meeting.

But first, some background: When Earth was a mere 45 or 50 million years old, more than 4.5 billion years ago, it collided with a Mars-sized object about half the size of today's Earth and twice the size of the moon.

This generally accepted scenario was proposed in 1975 by William K. Hartmann and Donald R. Davis of the Planetary Science Institute in Tucson. Their Giant Impact Theory says the collision ejected massive amounts of rock into space. This debris eventually clumped together to form the moon. This explains why the rocks that Apollo astronauts brought back from the moon some 40 years ago so closely match those on Earth.

But a conundrum troubled Wolbeck, who is a licensed professional engineer and an associate professor and interim chair of the Department of Science, Engineering and Architecture at SUNY's Orange County Community College. "An object the size of Mars would have had its own unique signature, so lunar rocks should not be identical to Earth's," he says.

So here's his hypothesis: If what hit Earth was half ice, then heat from the collision would have vaporized the water into superhot steam. The solar wind would have blown away most of the vapor, removing the impactor's unique signature. The collision also would have liquefied the rock and iron at the impactor's core, with the rock becoming part of Earth's geology and the heavier iron sinking to become part of Earth's iron core.

"This hypothesis allows the impact theory to work and explains why the moon is so similar," Wolbeck says. It also explains the source of Earth's water. Before, scientists thought that comets -- which are dirty ice balls -- had rained water on our planet. But, Wolbeck points out, all of the comets that space probes have sampled so far have ice made of heavy water, which contains a one-proton, one-neutron isotope of hydrogen called deuterium. Most of Earth's water has ordinary hydrogen, with just a proton. So, since comets like Halley, Hale-Bopp and Hyakutaki had deuterium at twice Earth's level, it appears unlikely that our water came from comets.

Wolbeck estimates that his icy impactor carried 300 times the amount of water now in our oceans. The water actually quenched the impact, he theorizes, keeping the rock-melting temperatures lower than they otherwise would have been. Most of the water vanished into space, but because of the lower temperatures, enough was left to form our oceans, lakes, rivers and atmosphere, even after 4.5 billion years of solar burnoff.

Are there small planets that are half ice? Look no further than Jupiter's two largest moons, Callisto and Ganymede. "So these objects exist. In fact, they are common," Wolbeck says.

Since he is still finishing his coursework and has not yet begun to write a dissertation that would back his hypothesis with hard calculations, Wolbeck has a long way to go. But scientists at the Meteorical Society meeting took notice. "When scientists hear the idea, they're skeptical at first," Wolbeck says. "But as they walk away, they say, 'It might be right and I hope it's right, because it's so cool.'"



HARD ROCK Definitions

  • Asteroids are big rocks, up to almost 600 miles across, found mainly between Mars and Jupiter
  • Meteoroids are smaller rocks in space
  • Meteors are meteoroids that plunge through Earth's atmosphere, which produces a very thin melt layer on their outside
  • Meteorites are meteors that hit Earth's surface
  • Chondrites are the most primitive meteorites
  • Unless altered by later heating or reaction with fluids, they usually contain a mixture of chondrules (rounded rock droplets that melted and quickly cooled), calcium-aluminum-rich inclusions (among the solar system's earliest solids) and other components, including pre-solar mineral grains produced by other stars in the universe)
  • Chondrules and calcium-aluminum-rich inclusions were the first rocks formed in the protoplanetary disk (also known as the solar nebula), the gas-dust cloud that condensed to form the sun and planets.