Comet Bigger Than Expected
Two weeks ago, we reported on the close flybys of two comets, both predicted to occur during the past week. One of them—Comet P/2016 BA14 (Pan-STARRS), thought to have broken off from a larger body (the other comet)—passed 2.2 milion miles (3.5 million kilometers) from Earth on March 22, giving astronomers an opportunity to observe the third closest-known encounter with a comet.
Calculations based on its observed brightness had initially led astronomers to estimate that BA14’s diameter was “no more than 200 meters across” (less than 700 feet). However, it is now known that the comet’s surface is darker than was expected, reflecting only 3 percent of the light hitting it (or about as dark as fresh asphalt). That, in turn, means that the comet must have been larger than was calculated in order to reflect the amount of light that was detected. Radar observations made with NASA’s Deep Space Network at Goldstone, California, were capable of resolving features as small as eight meters (26 feet) on the comet’s surface, and scientists used them to determine that BA14 was about a kilometer (3,000 feet) across, making astronomers breathe a sigh of relief that it passed as far away as it did.
What if it hadn’t?
The Chelyabinsk fireball—a burning meteoroid that fell over Russia on February 15, 2013—is estimated to have been about 20 meters (65 feet) in diameter. It exploded in the atmosphere at an altitude of 29.7 kilometers (18.4 miles), releasing about 30 times more energy than the atomic bomb dropped on Hiroshima. The most powerful impact event recorded on Earth took place in 1908, when an asteroid measuring up to 190 meters (620 feet) across exploded over Tunguska, Siberia about five to ten kilometers (three to six miles) above the ground, releasing 1,000 times the energy of the Hiroshima bomb and flattening a forest of 80 million trees across an area of 2,000 square kilometers (770 square miles). The Chelyabinsk fireball is one of the topics discussed in Incoming!, the new Planetarium show now playing at the Academy’s Morrison Planetarium. –Bing Quock
Saturn’s Rings (and Some Moons) May be Younger Than Dinosaurs
If dinosaurs had had telescopes (a fantastic mental picture), they might have caught a very different view of the planet Saturn than we observe today—they may not have seen the planet’s stunning rings.
According to a new study published in the Astrophysical Journal, some of Saturn’s inner moons, along with its iconic rings, might be much younger than previously thought—potentially only 100 million years old—and the evidence lies with the orbits of moons themselves.
In addition to impressive rings, Saturn has a lot of moons (62 at last count), and although many of them are small, they are relatively close to together, so their orbits are influenced not just by the gravity of the giant planet, but also by each other. All of their orbits slowly change, but at different rates. Some moons end up in orbital resonances (for example, a moon completing one orbit for every two times another moon does) which amplifies the effect, elongating and tilting them out of their original orbital plane.
In 2012, French astronomers found the gravitational interaction (tidal forces) between the inner moons and fluids deep in Saturn’s interior are causing them to spiral into larger orbits relatively quickly compared to moons farther out. This could mean that, given their present positions, these moons—and presumably the rings—are much younger than previously assumed and that they may have not formed along with the planet 4.5 billion years ago.
New research done at the SETI Institute backs up the idea, using computer modeling of the orbits of Saturn’s moons based on Cassini data. When looking at the current positions of three of Saturn's inner moons, Tethys, Dione, and Rhea, and comparing them to computer models that predicted how their orbits should have altered over time, the team did not find evidence of changes in orbital tilts typical of older moons. Basically, Saturn’s closer small and mid-size moons don't appear to have moved very far from where they were first formed.
“Moons are always changing their orbits. That's inevitable,” said Matija Cuk of the SETI Institute. “But that fact allows us to use computer simulations to tease out the history of Saturn’s inner moons. Doing so, we find that they were most likely born during the most recent two percent of the planet’s history.”
To figure out when that might have been, the team used Cassini’s observations of ice geysers on Saturn’s sixth-largest moon, Enceladus. Assuming that the geysers are powered by geothermal energy generated by tidal interactions with Saturn, and that the activity levels remained constant, they inferred the strength of the tidal forces from Saturn. From there, computer simulations indicated that Enceladus would have moved the small amount it appears to have in only 100 million years.
More distant moons, such as Titan and Iapetus, are still thought to have formed much earlier, potentially along with Saturn. So what could have caused these moons to form so much later? “Our best guess is that Saturn had a similar collection of moons before, but their orbits were disturbed by a special kind of orbital resonance involving Saturn's motion around the Sun. Eventually, the orbits of neighboring moons crossed, and these objects collided. From this rubble, the present set of moons and rings formed.”
Ring systems, such as those found around all four gas giant planets, are composed of small chunks of ice and rock and are thought to form as asteroids, comets, moons, or other objects pass too close to the planet and are torn apart by the planet's gravity. (Mars might temporarily be added to the list of ringed planets in another 20-40 million years when one of its moons, Phobos, is eventually torn apart by the red planet’s tidal forces.)
But such a youthful age for Saturn’s inner system could have some interesting implications. It would mean that we are glimpsing the planet at a privileged time—they would not have existed for most of the planet's 4.5 billion years and since small moons are still forming within Saturn’s rings, gathering material, it is possible that the ring material will eventually be used up or fall into the planet. It would also reinforce that our solar system is not static—that we live in a dynamic, and occasionally chaotic, neighborhood that changes on timescales far beyond our short human lives.
Additionally, if the moon Enceladus—currently considered a promising location to look for signs of extraterrestrial life—is so much younger than previously estimated that its potentially habitable conditions may not be as stable or long lived as previously thought. At the very least, life may not yet have had time to develop.
Either way, ancient or newly formed, Saturn’s rings and inner moons remain fascinating focuses of study (or even backyard viewing) and reveal a wealth of information about not just Saturn, but our solar system as a whole. The Cassini mission will continue to monitor and collect data on Saturn and its rings, and moons until its mission end September 2017. –Elise Ricard
Has Something Hit Jupiter… Again?
In 1992, Comet Shoemaker-Levy 9 passed so close to the planet Jupiter that tidal forces caused by the giant world’s powerful gravity ripped it to pieces. These comet fragments continued to orbit until 1994, when they finally slammed into Jupiter, exploding in its atmosphere and leaving a chain of dark, Earth-sized scars that persisted for several months afterward. Astronomers were able to observe the impacts from Earth, using both ground- and space-based telescopes, including NASA’s Galileo spacecraft, which was en route to Jupiter at the time. The comet’s demise offered the first opportunity to observe such an event, and the impacts thrilled observers… especially since they happened on Jupiter and not on Earth!
During the morning hours of March 17 local time in Europe, amateur astronomers in Austria and Ireland observed and recorded a flash of light that briefly appeared on the edge of Jupiter’s disk. Judging from the size of the very brief flash, astronomer Phil Plait suggests the impacting object was on the order of tens of meters across, or about the size of a house. Maybe two.
It has been long held that Jupiter, with its powerful gravitational field, plays the role of a “planetary shield,” protecting the inner planets from incoming asteroids, and some scientists suggest that without that protection, conditions amenable to life might not have developed on Earth. Not that our planet, and the other rocky worlds haven’t been hit—all still have plenty of impact craters to show for it, although Earth has the fewest due to the effects of erosion by water and the atmosphere. On the other hand, as reported here by Elise Ricard in February, recent theories challenge the planetary shield idea, suggesting that Jupiter’s presence may actually increase the likelihood of collisions rather than reduce it, pelting the inner planets with water and organics.
Whichever way it went, it was the right way for us, and you can find out more about the interplanetary transport of materials by asteroids and comets in Incoming! –Bing Quock
Peeling Away at the Secrets of Planet Janssen
Astronomers using NASA’s Spitzer Space Telescope have made the first temperature map of a so-called “super-Earth.” The world in question is Janssen, also known as 55 Cancri e (which is a more functional name, since it includes the constellation to which it belongs, but as an alternative, in December 2015, the International Astronomical Union adopted the new name, as reported here). Super-Earths are planets that have from one to ten times the mass of Earth and are thought to be the most common type of planet in the galaxy. The term “super-Earth” refers only to the planet’s mass and no other characteristics, but scientists writing in the March 30 issue of the journal Nature use Spitzer’s observations to make a few interesting inferences.
Janssen (a.k.a. 55 Cancri e) is located about 44 light years away in the constellation Cancer the Crab and is the smallest of five planets circling the star Copernicus, or 55 Cancri. At nearly twice the Earth’s diameter and at least eight times the mass, it orbits its star every 18 hours at a distance of about 2.3 million kilometers (1.4 million miles). Because of its closeness to the star, Janssen is also known to be tidally locked, keeping the same side perpetually turned toward the planet, just as our own Moon keeps the same side facing Earth.
In February, scientists reported they had made a spectroscopic analysis of Janssen’s atmosphere—the first-ever of a super-Earth. Using instruments on the Hubble Space Telescope, astronomers detected hydrogen, helium, and a hint of hydrogen cyanide, but no water.
Previously, the Spitzer Space Telescope produced a heat map of the planet Upsilon Andromedae b, located 44 light years away in the constellation Andromeda the Princess. Now officially known as Saffar, Upsilon Andromedae b is a “hot-Jupiter” (or a Jupiter-like gas giant very close to its star). Spitzer detected day and night temperatures that differ by 1,370°C (or 2,500°F).
Turning Spitzer’s heat-seeking eye toward the smaller, more Earth-like Janssen in Cancer, astronomers found a similar spread between opposite sides of the planet, with daytime readings of nearly 2,400°C (4,400°F) and a nightside temperature 1,100°C (2,060°F). For comparison, the molten magma gushing from Hawaii’s Mount Kilauea has a temperature of 1,160°C (2,120°F), leading to some speculation that the sunward side of Janssen is covered by an ocean of lava. The enormous temperature differential from one side of the planet to the other indicates very inefficient transport of heat. Lead author Brice Olivier Demory of the University of Cambridge writes that “this could be explained by an atmosphere that would exist only on the day side of the planet, or by lava flows at the planet surface." According to the latter thought, those lava flows would harden on the nighttime side, further inhibiting any heat-transfer across the planet.
The Spitzer Space Telescope was launched in 2003 as the last of NASA’s Great Observatories. Originally designed for a two-and-a-half year long mission, the telescope ran out of liquid helium in 2009. This was needed to supercool the instruments so that their own heat didn’t interfere with observations. However, although Spitzer’s far-infrared detectors are no longer functioning, the Infrared Array Camera is continuing to operate in what is called the “Spitzer Warm Mission,” and has been recalibrated so it can perform high-precision observations that it wasn’t originally designed for. –Bing Quock