Universe Update, April 2014
By Ryan Wyatt
The third Thursday of every month, the Morrison Planetarium hosts “Universe Update” at the 6:30 planetarium show during NightLife. I select my favorite astronomy stories from the past month, and I give a brief run-down of current discoveries while taking audiences on a guided tour of the Universe. As you may or may not know, the planetarium sports a three-dimensional atlas of the Universe, so we can take you places virtually while talking about the latest astronomy news.
I always start at Earth and work my way out to cosmological distances, so I’ll list the news stories in the same order—from closest to farthest from home.
Believe it or not, our closest neighbor in space, the Moon, still harbors many mysteries… Including something as basic as its age! For the most part, we have used radiometric dating of lunar samples (i.e., moon rocks) to estimate how long ago the Moon formed (which likely took place when a giant impact nearly wiped out Earth and created sufficient debris to form the Moon). But an announcement from the Southwest Research Institute describes a new process for making the same estimate—based on details of that catastrophic formation event! A team of researchers started by creating oodles of (well, 259, to be exact) computer simulations of how the terrestrial planets (Mercury, Venus, Earth, and Mars) grew over time, as more and more meteorites impacted the inner planets and added mass. This process transforms the composition of the planets’ outer layers, so if we think of the lunar-forming impact as a “reset” for Earth’s composition, then the simulations provide a “geologic clock” that counts off the millennia since that catastrophe. Pretty spiffy.
The aforementioned lunar-forming impact certainly qualifies as a major celestial blow to our planet, and you may have heard of other, smaller catastrophic events, too—such as the one that probably did away with the dinosaurs. In between those two events, both in terms of time and in terms of size, Earth experienced a major blow, as somewhat breathlessly described in this recent press release about the work of Stanford’s own Donald Lowe:
Picture this: A massive asteroid almost as wide as Rhode Island and about three to five times larger than the rock thought to have wiped out the dinosaurs slams into Earth. The collision punches a crater into the planet’s crust that’s nearly 500 kilometers (about 300 miles) across: greater than the distance from Washington, D.C. to New York City, and up to two and a half times larger in diameter than the hole formed by the dinosaur-killing asteroid. Seismic waves bigger than any recorded earthquakes shake the planet for about half an hour at any one location—about six times longer than the huge earthquake that struck Japan three years ago. The impact also sets off tsunamis many times deeper than the one that followed the Japanese quake.
This cataclysmic event took place some 3.26 billion years ago—long after the Moon formed and well before the dinos went extinct. The ginormous impact may have also created the South African geological features known as the Barberton greenstone belt. Ouch!
But the era of impacts hasn’t completely come to a close… This kind of thing happens all the time on Earth, as you can see in last week’s video from the B612 Foundation that depicts dozens of asteroids exploding in Earth’s atmosphere between 2000 and 2013—each one releasing between one and 600 kilotons! Some of those could have destroyed an entire city had they hit Earth in the right spot. No surprise, then, that the data identifying these atmospheric ka-booms came from the Nuclear Test Ban Treaty Organization, which operates a network of infrasound detectors that pinpointed the blasts.
Aside from Earth and the Moon, I’ll confine our solar system peregrinations to a single stop at Saturn…
More than just a visit to a planet, however, Saturn has a lot to offer: its rings, of course, as well as a healthy population of moons, and an intrepid spacecraft that’s been exploring the system for nearly a decade. The Cassini mission will enter its final phases over the next few years, before spiralling into the ringed planet, and now you have an opportunity to help name the last phase of the Cassini mission. (NASA crowdsources all kinds of tasks these days, so lazy…) Each phase of the mission has received its own moniker (the current phase is the “Solstice” mission), and this final set of orbits will take the spacecraft into the narrow gap between the planet and its innermost moons! A website allows you to vote on proposed names—e.g., Grand Finale, Close Shave, and The Plunge—or to suggest one of your own.
Cassini’s visit comes about thirty years after the Voyager spacecraft swung by Saturn, which coincides with the planet’s orbital period of about 29 years. That means that we’re seeing the planet in the same season as the Voyagers saw it during their visit—summer in the northern hemisphere, when Saturn’s north pole is lit most effectively by the Sun. And the Voyagers discovered a rather bizarre feature around the north pole, a hexagonal cloud pattern that has persisted till today. Cassini has given scientists an opportunity to examine the hexagon for an extended period of time, and a recent announcement summarizes many of their findings.
But of course, Cassini hasn’t just followed up on earlier discoveries: it has also made many of its own. Including perhaps a new moon forming in Saturn’s rings! Cassini has observed a “disturbance” about 750 miles (1,200 kilometers) long and 6 miles (10 kilometers) wide, the form of which suggests the gravitational influence of a small, hitherto unobserved moon. We wrote this up in a recent Science Today article, so you can read more there.
Coincidentally, another recent Science Today article described the discovery of a possible ocean under the icy surface of Saturn’s moon Enceladus. We loooooove finding evidence for liquid water on other worlds, mostly because life on Earth depends on liquid water. (Kinda self-centered, but when you only have a single example of life in the Universe, you have to extrapolate from what you know…) The evidence for a subsurface ocean seems fairly compelling, so perhaps it’s time to fund the Enceladus Explorer project to send a mission to visit there!
(All right. I can’t resist one additional stop inside the Solar System… Albeit on its outer limits. Observations of the asteroid Chariklo, which spends most of its time farther from the Sun than Saturn, suggest that the tiny planet-wannabe actually has rings! As the press release notes, this discovery makes it the only asteroid known to have rings—and indeed, “the smallest object by far found to have rings.” Emphasis mine because it really is the smallest object by far found to have rings: other ring-bearing worlds include the giant, gassy planets Jupiter, Saturn, Uranus, and Neptune. A far cry from this tiny place, a mere 250 kilometers, or a little more than 150 miles, in diameter. As a separate press release was quick to point out, the likeliest explanation for the asteroid’s rings is that they formed in a violent collision—cf. that first story about how Earth’s moon formed. The Solar System can be a scary place.)
Traveling far beyond our home collection of planets and asteroids (ringed or otherwise), we encounter other stars… Don’t get me wrong, it’s quite a hike to the nearest star: light takes more than four years to make the trip, whereas light crosses the orbit of, say, Neptune in a matter of hours. But in the planetarium (or the text of an article), we can make the virtual trip quite rapidly.
The biggest news from this part of the Universe is the continuing discovery of planets around many of these nearby stars… Including one described in yet another Science Today article from the past month! NASA’s Kepler mission announced the discovery of an Earth-sized planet in the habitable zone of its parent star. Kepler has identified literally thousands of potentially interesting places near us, but this one in particular represents a very encouraging step in finding a world much like our own home planet.
BTW, those planets don’t just find themselves! To tease out evidence for these extrasolar planets, astronomers need to make careful observations, of course, but they also sift through their data with specialized computer code. Fun-loving programmer Stefano Meschiari adapted a powerful software package to create the game Super Planet Crash, which allows players to rack up points by adding planets (or brown dwarfs or dwarf stars) in orbits around a Sun-like star, then speeding up time to watch the system evolve. Since the planets and other objects interact gravitationally, the system can end up flying apart—or presumably (assuming the game’s name has any validity) a planet can end up crashing into its neighbor. According to the announcement from UC Santa Cruz (where Meschiari studied as a graduate student and developed the game), Super Planet Crash lies at the entertaining end of a spectrum of tools for students and researchers alike. And once you play the game, you’ll have an opportunity to donate to educational causes.
From the outset, I’ve been describing objects in terms of their distance from Earth, but we shouldn’t take that for granted. It takes a lot of painstaking work to determine the distances to these faraway objects. Astronomers use a variety of tools to accomplish this task: tools that build on one another in a fashion often referred to as the cosmic distance ladder. In the past month, three different announcements have described advances in our ability to estimate distances in astronomy, effectively strengthening the rungs of the distance ladder.
At the base of the ladder, parallax provides the most direct measurements of distance to an object, making basically no assumptions about the object itself. Unfortunately, the farther an object, the smaller its parallax and the harder to measure its distance. Researchers working with the Hubble Space Telescope recently announced that they had stretched its “stellar tape measure” ten times farther into space. In other words, improved precision now allows the Hubble to measure the parallax of objects ten times farther away than previously! A spiffy diagram accompanying the press release shows the radically larger volume of space this opens up to exploration using this technique.
Astronomers may use parallax to gauge the distances to nearby objects, but that’s just the first (reliable) rung on the aforementioned distance ladder. Knowing objects’ distances allows us to calculate their intrinsic brightnesses, and as long as we know that we’re seeing a similar object elsewhere in space, we can establish its brightness (or luminosity) and calculate its distance, even if we can’t measure its parallax! It’s like looking at two 100-watt light bulbs—if one appears dimmer than the other, it must lie farther away. And furthermore, if you make a careful enough measurement and do the math, you can figure out exactly how far away both the bulbs are. Astronomers do this all the time to establish distances to faraway objects.
In just this past month, a group of researchers announced new ways of calculating these distances. It turns out that the apparent color of an object helps astronomers figure out what kind of object it is—and then estimate how far away it must lie. But figuring out a star’s color can prove challenging, so astronomers developed a map of how color changes over the sky. And they’ve been using the same map since 1989! The new results allow for a much better estimate of objects’ color and thus of their distances. Bread and butter astronomy!
That’s all for now. Check back for next month’s update! Or come to NightLife on Thursday, May 15th, and check out “Universe Update” live in the Morrison Planetarium, with Josh Roberts guest hosting.
Ryan Wyatt is the Director of Morrison Planetarium and Science Visualization at the California Academy of Sciences.