NASA JPL-Caltech | NEOWISE | 2011 Sept 19
Observations from NASA's Wide-field Infrared Survey Explorer (WISE) mission indicate the family of asteroids some believed was responsible for the demise of the dinosaurs is not likely the culprit, keeping open the case on one of Earth's greatest mysteries.
While scientists are confident a large asteroid crashed into Earth approximately 65 million years ago, leading to the extinction of dinosaurs and some other life forms on our planet, they do not know exactly where the asteroid came from or how it made its way to Earth. A 2007 study using visible-light data from ground-based telescopes first suggested the remnant of a huge asteroid, known as Baptistina, as a possible suspect.
According to that theory, Baptistina crashed into another asteroid in the main belt between Mars and Jupiter about 160 million years ago. The collision sent shattered pieces as big as mountains flying. One of those pieces was believed to have impacted Earth, causing the dinosaurs' extinction.
Since this scenario was first proposed, evidence developed that the so-called Baptistina family of asteroids was not the responsible party. With the new infrared observations from WISE, astronomers say Baptistina may finally be ruled out.
"As a result of the WISE science team's investigation, the demise of the dinosaurs remains in the cold case files," said Lindley Johnson, program executive for the Near Earth Object (NEO) Observation Program at NASA Headquarters in Washington. "The original calculations with visible light estimated the size and reflectivity of the Baptistina family members, leading to estimates of their age, but we now know those estimates were off. With infrared light, WISE was able to get a more accurate estimate, which throws the timing of the Baptistina theory into question."
WISE surveyed the entire celestial sky twice in infrared light from January 2010 to February 2011. The asteroid-hunting portion of the mission, called NEOWISE, used the data to catalogue more than 157,000 asteroids in the main belt and discovered more than 33,000 new ones.
Visible light reflects off an asteroid. Without knowing how reflective the surface of the asteroid is, it's hard to accurately establish size. Infrared observations allow a more accurate size estimate. They detect infrared light coming from the asteroid itself, which is related to the body's temperature and size. Once the size is known, the object's reflectivity can be re-calculated by combining infrared with visible-light data.
The NEOWISE team measured the reflectivity and the size of about 120,000 asteroids in the main belt, including 1,056 members of the Baptistina family. The scientists calculated the original parent Baptistina asteroid actually broke up closer to 80 million years ago, half as long as originally proposed.
This calculation was possible because the size and reflectivity of the asteroid family members indicate how much time would have been required to reach their current locations -- larger asteroids would not disperse in their orbits as fast as smaller ones. The results revealed a chunk of the original Baptistina asteroid needed to hit Earth in less time than previously believed, in just about 15 million years, to cause the extinction of the dinosaurs.
"This doesn't give the remnants from the collision very much time to move into a resonance spot, and get flung down to Earth 65 million years ago," said Amy Mainzer, a co-author of a new study appearing in the Astrophysical Journal and the principal investigator of NEOWISE at NASA's Jet Propulsion Laboratory (JPL) in Pasadena. Calif. "This process is thought to normally take many tens of millions of years." Resonances are areas in the main belt where gravity nudges from Jupiter and Saturn can act like a pinball machine to fling asteroids out of the main belt and into the region near Earth.
The asteroid family that produced the dinosaur-killing asteroid remains at large. Evidence that a 10-kilometer (about 6.2-mile) asteroid impacted Earth 65 million years ago includes a huge, crater-shaped structure in the Gulf of Mexico and rare minerals in the fossil record, which are common in meteorites but seldom found in Earth's crust. In addition to the Baptistina results, the NEOWISE study shows various main belt asteroid families have similar reflective properties. The team hopes to use NEOWISE data to disentangle families that overlap and trace their histories.
"We are working on creating an asteroid family tree of sorts," said Joseph Masiero, the lead author of the study. "We are starting to refine our picture of how the asteroids in the main belt smashed together and mixed up."
Was the 'Dinosaur Killer' Unfairly Charged?
Science NOW | Richard A. Kerr | 2011 Sept 20
Wanting to bring the master evildoer, not just a henchman, to justice is human enough. So when planetary scientists traced the asteroid that wiped out the dinosaurs 65 million years ago back to a rampaging rock named Baptistina in the asteroid belt there was palpable satisfaction that the ultimate culprit seemed to have been nailed.
But now a group of astronomers is challenging that claim. Baptistina did blast another asteroid to smithereens, sending a devastating shower of debris into the inner solar system, but that cataclysmic collision probably came too late to have sent the dino killer to Earth, they argue.
The original CSI-like case against Baptistina involved an odd link between an asteroid's size and the ability of sunlight to move it across the asteroid belt. In their 2007 Nature paper, planetary scientists William Bottke, David Vokrouhlický, and David Nesvorný of the Southwest Research Institute (SwRI) in Boulder, Colorado, identified asteroids whose similar orbits mean they are members of the "family" of asteroids formed in a collision between asteroids 170 and 60 kilometers in diameter, 40-kilometer-diameter Baptistina being the largest survivor.
By assuming how reflective Baptistina family members are, the SwRI group could estimate the size of each asteroid from the amount of visible light they reflected. Their sizes, in turn, determined how quickly sunlight could ease debris away from the collision. As each fragment absorbs solar energy, it radiates the heat away to give an ever-so-gentle rocketing effect. That nudging could have driven fragments toward a known orbital spot from which Jupiter's gravity could fling them toward Earth. Judging by how far from the collision Baptistina family members have gotten, the group estimated that the collision occurred about 160 million years ago, early enough for the solar-driven rocketing to drive a 5- or 10-kilometer fragment to the jumping-off point to Earth by 65 million years ago.
But now team members on the Wide-Field Infrared Survey Explorer (WISE) mission dispute this line of evidence in a paper just out in The Astrophysical Journal. Joseph Masiero of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, and colleagues report that WISE has returned estimates of the size of 100,000 asteroids, the Baptistina family among them. These estimates are more accurate than those the SwRI group had because WISE detects infrared light, not the visible spectrum. The SwRI group had to assume a reflectivity, but the WISE infrared measurements yielded actual measurements of reflectivity that are four times larger than the SwRI group had assumed. That in turn gave smaller sizes, faster moving fragments, and therefore a younger collision time—about 80 million years ago—than the SwRI group had calculated.
"This doesn't give the remnants from the collision very much time to ... get flung down to Earth 65 million years ago," says Amy Mainzer, a co-author and the principal investigator of the asteroid phase of WISE at JPL. So instead of the Baptistina collision being ultimately responsible, another, as-yet-undated collision may have been responsible. Or the dino killer was a random asteroid that happened to wander out of the asteroid belt then.
Bottke doesn't see a problem. Indeed, the WISE results "if anything ... make our story stronger," he writes in an e-mail. The SwRI group's 2007 calculations show that "lots of things escape from the Baptistina family right away ... decreasing the age of the Baptistina family is not a problem."
Before, the collision was so early that the dino killer would have had to have been among the sparse debris that reached Earth long after the collision, Bottke says; with a more recent collision, far more Baptistina fragments would have been raining toward Earth 65 million years ago. Bottke's argument "provides a good counterpoint to the conclusions reached by the WISE team," writes dynamicist Derek Richardson of the University of Maryland, College Park. Now perhaps the prosecution and the defense can work on a settlement.
NEOWISE: Rethinking the Dinosaur Killer
Centauri Dreams | Paul Gilster | 2011 Sept 21
Finding the Dino Killer
Centauri Dreams | Larry Klaes | 2007 Oct 23
An asteroid breakup 160 Myr ago as the probable source of the K/T impactor - William F. Bottke, David Vokrouhlický, David Nesvorný
- Nature 449 48 (06 Sep 2007) DOI: 10.1038/nature06070
- arXiv.org > astro-ph > arXiv:1109.4096 > 19 Sep 2011