The standard core accretion model of planet formation proceeds from condensation through aggregation of dust particles to formation of km-scale, or larger, “planetesimals”. Once planetesimals form, collisional accretion is aided by the gravity of the planetesimals and simulations consistently produce planets from this starting point. The formation of planetesimals has been problematic, however. Formation by gravitational instability is hampered by turbulence in the disk, and formation by collisional accretion has to occur quickly enough to avoid a fiery fate of the proto-planetesimals in the central star due to aerodynamic drag in the disk. I’ll present the results of experiments designed to study the conditions under which accretion may occur. The experiments also have implications for the collisional evolution of planetary ring systems, and for regolith evolution on planetary satellites and asteroids.
February 7, 2013: Neutral Trans-Neptunian Objects through Rotational Lightcurves
Trans-Neptunian Objects (TNOs) are small bodies with orbits beyond Neptune. Roughly 1/3 of TNOs possess peculiarly neutral colors indicative of fresh surfaces, though these objects are thought to be isolated and inactive. To discern between explanations for these neutral colors, we conducted two surveys to search for homogeneity on the surfaces of neutral TNOs – a brightness variation survey in which we sparsely sampled lightcurves of 38 neutral TNOs to select follow-up targets, and a color variation survey of the 9 follow-up targets to densely sample their rotational lightcurves and obtain rotationally phased colors. Our data showcase a surprising number of rotational lightcurves that can only be explained as binaries (some from within the Haumea collisional family). We also found that one of our objects has the fastest measured rotation period of any outer solar system object at 2.4 or 2.6 hours. I will present these results along with updated amplitude and spin distributions for TNOs and comment on what these data suggest about how the dynamical history of the outer solar system.
Exploring Your Universe delights science enthusiasts of all ages
UCLA held the fifth annual Exploring Your Universe science outreach day on November 17, 2013. This free public event was attended by 4000-4500 participants and featured hands-on activities and demos sponsored by science departments and student groups across campus. Science enthusiasts of all ages spent the day observing the Sun through telescopes, seeing themselves in infrared, making their own fossils, launching rockets, and sampling liquid nitrogen ice cream.
View some photos of the event below. For more photos, click here.
November 22, 2013: Kinetic Physics of the Solar Wind
Understanding kinetic dissipative processes in the solar wind is an important endeavor required to improve our global solar wind models. The candidate dissipative processes must be collisionless in nature and should produce the observed signature of perpendicular anisotropy for protons. Many processes have been proposed e.g. wave-particle interactions, energization at intermittent coherent structures and stochastic heating. We will discuss two efforts related to this line of research:
1) Turbulent Dissipation Challenge: A community driven effort that aims to better quantify the relative contributions of various dissipative processes in the solar wind.
2) Effect of electron equation of state on the kinetics of protons.
Researchers use oldest minerals on Earth to study solar system history
The answer to one of the great mysteries of our solar system’s history may lie within a grain no wider than a single strand of human hair. Scientists have long known that the mineral zircon is very hardy. “Zircon tends to stick around for a long time,” said Beth Ann Bell, a fifth-year UCLA graduate student who studies these tiny grains. And she’s not kidding about zircon’s longevity – the samples she studies are 3.8 to 3.9 billion years old. The Earth itself is 4.5 billion years old.
With their advisor, UCLA Professor Mark Harrison, Bell and her colleagues study individual zircon grains to better understand a critical and highly controversial event in our solar system’s history known as the Late Heavy Bombardment (LHB). During the LHB, which occurred between 3.8 and 4.1 billion years ago, a very large number of craters formed on the surface of the Moon. Analysis of the craters and lunar samples have led some scientists to suggest that the objects that crashed into the Moon were numerous and came from far away, possibly beyond the orbit of Jupiter. “The whole inner solar system should have been impacted and evidence of the LHB should be detectable anywhere, even on Earth“ said Matthew Wielicki, also a fifth-year graduate student. But scientists are still uncertain if the LHB actually happened at all. “There is much debate among planetary scientists as to whether the lunar samples from NASA’s Apollo mission are giving us the full picture of what was happening at that time,” said Wielicki.
To better understand the LHB, Wielicki and Bell analyze zircons on Earth in an attempt to determine whether any of the objects that formed the Moon’s craters also impacted our planet. Like tiny little clocks, zircons can record the timing of an impact event by the heat signatures it leaves behind. Some recorded features, known as shock features, are diagnostic of an impact and can cause a grain to appear as though it was shattered. However, such telltale signs do not always develop, and scientists must instead investigate subtler signs, like the ratios of radioactive elements inside the zircon.
To study element ratios within their zircon grains, Bell and Wielicki use a unique device called a Secondary Ion Mass Spectrometer (SIMS), located at UCLA. “With many techniques you must pulverize your sample, essentially destroying it, in order to study it,” said Bell. With the SIMS, samples are left intact and shot with a beam of energized atoms, or ions, and analyzed in tiny patches. The SIMS can peer into a grain “one atomic layer at a time,” allowing them to study multiple heating events in a single zircon, said Wielicki.
Cosmic impacts aren’t the only events in Earth’s history that could produce heat signatures in zircon grains. Using the SIMS, Bell and Wielicki hope to be able to distinguish between zircon grains that have been affected by a meteor impact and those that have been heated by “some other event, like mountain building or volcanism, all which were occurring on Earth during the LHB timeframe.”
Because of efficient weathering and erosion processes, there are no impact craters on Earth which date back to the LHB, so Wielicki works to develop the tools necessary to understand impact-heated zircon grains using zircons from more recent impact events. Bell then tests the validity of those tools on LHB-age zircons whose history is unknown. “The rocks where we find ancient samples are sedimentary, which means they were once older rocks that eroded, and then turned into the sandstone we see today,” said Bell, “we don’t know what types of rock they originally grew in.”
“We are cornering two parts of a three-fold approach to pin down the LHB,” said Wielicki. The third piece of their approach involves studying zircons from other inner solar system objects. “The real excitement comes when we apply our analytical tools to samples from objects like Vesta,” said Wielicki. Vesta, the target of NASA’s Dawn mission, is a large asteroid located in the inner solar system that has been cold for a very long time. Wielicki said, “If we see any heating signal in Vesta’s zircons, we know it must be from an impact.”
For Bell and Wielicki, the picture is far from complete. The LHB, which occurred just before the onset of life on Earth, could have ties to the origins of life. It is unclear, however, if impacts would have acted as “life frustrators,” slowing life’s development, or if they actually delivered the “building blocks” for life, said Wielicki. “Understanding the timing of the LHB may help answer some of the questions about life on Earth, but first we need a better understanding of the impact history for the inner solar system,” he said.
Watch a video profile of Matt Wielicki here. Learn more about his research here.
Watch a video profile of Beth Ann Bell here. Learn more about her research here.
New asteroid discovered with six comet-like tails
Asteroids do not typically sport the long tails found coming from comets. Yet observations of newly-discovered asteroid P/2013 P5 show the object has not only one, but six dusty tails streaming outward from the central body. In a paper published on November 7, 2013, UCLA Professor David Jewitt suggests these uncharacteristic plumes could be made of dust that was ejected from the surface of the rapidly spinning asteroid. Read more about this discovery at the LA Times and the UCLA Newsroom.
Grad student Chris Snead receives prestigious teaching award
Graduate student Chris Snead was honored on October 15, 2013 in a ceremony held by UCLA’s Academic Senate Committee on Teaching. Only five teaching assistants are selected each year for this prestigious teaching award, and Chris is the first from the Earth, Planetary, and Space Sciences department.
Watch the video below to hear how Chris challenged his undergraduate students with activities such as building their own telescopes, visiting Griffith Observatory, and charting the phases of the Moon.
http://www.youtube.com/watch?v=GAY77NKuFfs
Researchers use radar to track near-Earth asteroids and predict hazards
Every year, UCLA graduate student Shantanu Naidu makes a pilgrimage to Arecibo Observatory, a uniquely constructed 300-meter radio telescope on the island of Puerto Rico. His goal: to determine the shape, spin, orbit, and other physical properties of Near Earth Asteroids (NEAs). These large chunks of rock left over from the formation of the solar system orbit around the Sun while remaining relatively close to Earth.
Observing asteroids with radio waves is a far cry from the traditional picture of nocturnal astronomers and mountaintop telescope domes housing fragile mirrors and lenses. Since the wavelengths they employ are far outside the visible light spectrum, radar observations can take place as easily during the day as they can at night. Likewise, the measurements are not affected by weather because the long wavelength radio waves can easily penetrate cloud layers in Earth’s atmosphere.
Naidu bounces radio waves off his targets and examines the reflected signal to reveal the shape of asteroids that would normally appear as “unresolved points of light” through optical telescopes. Radio telescopes can both resolve and track these elusive objects. Radar observations taken from Arecibo over the course of a few hours contain hundreds to thousands of pixels with surface resolutions as fine as 7.5 meters.
One of Naidu’s primary goals is to determine a precise orbit for each NEA he studies. At Arecibo, Naidu can pinpoint the position of an asteroid with an uncertainty of only a few tens of meters, a remarkable feat given that the majority of these objects are more than ten million kilometers from Earth. The precision of an asteroid’s orbit is important because NEAs occasionally come close to Earth as they orbit around the Sun. Scientists want to be able to identify any asteroid that could be a potential hazard decades or centuries before impact. “NASA wants to catalog the orbits of as many Near Earth Asteroids as possible so we can predict if any asteroid is going to collide with the Earth and take countermeasures,” said Naidu. Radar measurements of NEAs enable Naidu and his colleagues to derive orbits for the objects far more accurately than any other method. With a single additional observation, the time interval for reliable trajectory predictions can be improved by a factor of 5 to 10, allowing scientists to chart the position of asteroids over the course of hundreds of years rather than decades.
Radar measurements help provide advanced warning for incoming asteroids, but only a tiny fraction of NEAs are currently being studied. Naidu and his colleagues have observed roughly four hundred of these nearby asteroids, but scientists estimate that 20,000 NEAs with diameters greater than 100 meters exist in the solar system.
When Naidu observes an asteroid for the first time using radar, he hopes to hit the jackpot and see not just one object, but two or three. What originally appears to be a single asteroid could instead be an asteroid binary, two asteroids that orbit each other like moons orbiting a planet. “When we observe, we see that one in every six asteroids larger than 200 meters has a moon around it, so we know that binaries form a significant portion of the NEAs,” said Naidu. “Fifteen years ago, we didn’t even know that binaries existed.”
Fourth-year graduate student Julia Fang works to model the “orbital architecture” of these complex multi-asteroid systems. She creates computational models to predict how radiation from the Sun or a close encounter with the gravitational field of a planet could change the orbital paths of a multi-asteroid system. She hopes to recreate the history of these complicated systems in order to understand what processes might be responsible for producing their current orbits. “Asteroids provide clues about the orbital history of the planets and how they evolved,” said Fang.
Both Naidu and Fang are advised by UCLA Professor Jean-Luc Margot, one of the world’s foremost experts in high-precision radar observations of asteroids. Additional information about the UCLA radar program is available at: http://radarastronomy.org.
Learn more about Jean-Luc Margot’s research here.
Researchers analyze extrasolar asteroids using light from distant stars
When a Sun-like star reaches the end of its lifetime, it blows off its outer layers in a sustained stellar windstorm, leaving behind an Earth-sized, ultra-dense “white dwarf” star. Astronomers thought they knew what to expect from these celestial leftovers, but were puzzled over a decade ago when they found that a large fraction of observed white dwarfs emit more infrared light than predicted. Most white dwarf stars are composed of hydrogen and helium, but spectral measurements of some stars revealed puzzling signals from heavier elements such as calcium.
To UCLA Professor Michael Jura, the presence of additional elements indicated that the stellar atmospheres of these white dwarf stars were contaminated from an outside source. Many scientists hypothesized that the interstellar medium, a cosmic soup of stray particles inhabiting the space between stars, was responsible for this stellar pollution. Jura thought the answer might instead lie with extrasolar asteroids. “It was a mystery. A number of these stars had been known for quite a few years, but nobody knew quite why they were polluted,” he said. Jura believes that the stellar contamination occurs when an asteroid perturbed out of its normal orbit plummets towards its parent star and is violently ripped to shreds by gravitational forces. Starlight from the white dwarf is consequently absorbed by the newly created disk of dust and debris left over from the shattered asteroid. The dusty ring encircling the star re-radiates the starlight as infrared light that is invisible to the human eye but can be measured by specialized telescopes on Earth.
The swirling cloud made from atomized asteroids does more than absorb light; it eventually becomes part of the star itself. “What is particularly important is that this disk doesn’t just orbit the star, but that it slowly accretes onto the star,” said Jura. Bits of asteroid falling into the white dwarf star contaminate the stellar atmosphere with heavier elements that wouldn’t ordinarily be present. “Because we have these dust disks which are broken-up asteroids, we have a tool for measuring the elemental composition of extrasolar asteroids,” he said.
To figure out what these asteroids were made of before they were destroyed, Jura and his graduate student Siyi Xu use data taken from the Hubble and Spitzer space telescopes. They also observe using the Keck telescopes on the big island of Hawaii a few nights every year. So far, they have detected 19 different elements heavier than helium in their white dwarf measurements.
“We find the compositions of extrasolar asteroids are quite similar to meteorites in our own solar system. For one particular star, GD 362, the best match is mesosiderite, a type of stony-iron meteorite,” said Xu. Oddly enough, Xu is able to measure traces of certain elements in meteorites vaporized by distant stars more easily than scientists studying intact meteorites in their labs. “It is very hard to measure the bulk composition of a meteorite in a lab without destroying it completely,” Xu said. “Since the asteroid is already broken up for us, we can measure all of the abundances and make a comparison.”
Determining the composition of extrasolar asteroids may help scientists understand how Earth-like exoplanets around stars are formed. “We picture that when rocky planets form, they build up from nearby chunks of orbiting rock and debris,” Jura said. “In our own solar system, that process was somewhat inefficient, so we have asteroids left over.” Our solar system is one of many planetary systems with surplus building blocks left behind from planet formation; scientists estimate that nearly 30% of white dwarf star systems have extrasolar asteroid populations.
Previously, astronomers have only been able to guess the composition of asteroids in other star systems based on what they have learned about asteroids closer to home. “We think they are probably the original suite of asteroids that formed when the star was forming planets,” Jura said. “It’s just plain fun to think that you can actually figure out what these other planetary systems are made out of.”
Watch a video profile of Mike Jura here. Learn more about his research here.
Watch a video profile of Siyi Xu here.
‘Gravity’ movie gets the science mostly right
This week, Prof. Jean-Luc Margot discussed the scientific accuracy of the movie ‘Gravity’ with CNN. The movie, which premiered on October 4, 2013, stars George Clooney and Sandra Bullock as astronauts on a routine space shuttle mission that goes disastrously wrong. Watch the interview here or below.