Recorded during the Astrotour to Costa Rica, Fraser talks to Dr. Paul Matt Sutter about the nature of dust and BICEP 2’s claim of discovering primordial gravitational waves.
According to current estimates, there could be as many as 100 billion planets in the Milky Way Galaxy alone. Unfortunately, finding evidence of these planets is tough, time-consuming work. For the most part, astronomers are forced to rely on indirect methods that measure dips in a star’s brightness (the Transit Method) of Doppler measurements of the star’s own motion (the Radial Velocity Method).
Direct imaging is very difficult because of the cancelling effect stars have, where their brightness makes it difficult to spot planets orbiting them. Luckily a new study led by the Infrared Processing and Analysis Center (IPAC) at Caltech has determined that there may be a shortcut to finding exoplanets using direct imaging. The solution, they claim, is to look for systems with a circumstellar debris disk, for they are sure to have at least one giant planet.
The study, titled “A Direct Imaging Survey of Spitzer Detected Debris Disks: Occurrence of Giant Planets in Dusty Systems“, recently appeared in The Astronomical Journal. Tiffany Meshkat, an assistant research scientist at IPAC/Caltech, was the lead author on the study, which she performed while working at NASA’s Jet Propulsion Laboratory as a postdoctoral researcher.
For the sake of this study, Dr. Meshkat and her colleagues examined data on 130 different single-star systems with debris disks, which they then compared to 277 stars that do not appear to host disks. These stars were all observed by NASA’s Spitzer Space Telescope and were all relatively young in age (less than 1 billion years). Of these 130 systems, 100 had previously been studied for the sake of finding exoplanets.
Dr. Meshkat and her team then followed up on the remaining 30 systems using data from the W.M. Keck Observatory in Hawaii and the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile. While they did not detect any new planets in these systems, their examinations helped characterize the abundance of planets in systems that had disks.
What they found was that young stars with debris disks are more likely to also have giant exoplanets with wide orbits than those that do not. These planets were also likely to have five times the mass of Jupiter, thus making them “Super-Jupiters”. As Dr. Meshkat explained in a recent NASA press release, this study will be of assistance when it comes time for exoplanet-hunters to select their targets:
“Our research is important for how future missions will plan which stars to observe. Many planets that have been found through direct imaging have been in systems that had debris disks, and now we know the dust could be indicators of undiscovered worlds.”
This study, which was the largest examination of stars with dusty debris disks, also provided the best evidence to date that giant planets are responsible for keeping debris disks in check. While the research did not directly resolve why the presence of a giant planet would cause debris disks to form, the authors indicate that their results are consistent with predictions that debris disks are the products of giant planets stirring up and causing dust collisions.
In other words, they believe that the gravity of a giant planet would cause planestimals to collide, thus preventing them from forming additional planets. As study co-author Dimitri Mawet, who is also a JPL senior research scientist, explained:
“It’s possible we don’t find small planets in these systems because, early on, these massive bodies destroyed the building blocks of rocky planets, sending them smashing into each other at high speeds instead of gently combining.”
Within the Solar System, the giant planets create debris belts of sorts. For example, between Mars and Jupiter, you have the Main Asteroid Belt, while beyond Neptune lies the Kuiper Belt. Many of the systems examined in this study also have two belts, though they are significantly younger than the Solar System’s own belts – roughly 1 billion years old compared to 4.5 billion years old.
One of the systems examined in the study was Beta Pictoris, a system that has a debris disk, comets, and one confirmed exoplanet. This planet, designated Beta Pictoris b, which has 7 Jupiter masses and orbits the star at a distance of 9 AUs – i.e. nine times the distance between the Earth and the Sun. This system has been directly imaged by astronomers in the past using ground-based telescopes.
Interestingly enough, astronomers predicted the existence of this exoplanet well before it was confirmed, based on the presence and structure of the system’s debris disk. Another system that was studied was HR8799, a system with a debris disk that has two prominent dust belts. In these sorts of systems, the presence of more giant planets is inferred based on the need for these dust belts to be maintained.
This is believed to be case for our own Solar System, where 4 billion years ago, the giant planets diverted passing comets towards the Sun. This resulted in the Late Heavy Bombardment, where the inner planets were subject to countless impacts that are still visible today. Scientists also believe that it was during this period that the migrations of Jupiter, Saturn, Uranus and Neptune deflected dust and small bodies to form the Kuiper Belt and Asteroid Belt.
Dr. Meshkat and her team also noted that the systems they examined contained much more dust than our Solar System, which could be attributable to their differences in age. In the case of systems that are around 1 billion years old, the increased presence of dust could be the result of small bodies that have not yet formed larger bodies colliding. From this, it can be inferred that our Solar System was once much dustier as well.
However, the authors note is also possible that the systems they observed – which have one giant planet and a debris disk – may contain more planets that simply have not been discovered yet. In the end, they concede that more data is needed before these results can be considered conclusive. But in the meantime, this study could serve as an guide as to where exoplanets might be found.
“By showing astronomers where future missions such as NASA’s James Webb Space Telescope have their best chance to find giant exoplanets, this research paves the way to future discoveries.”
In addition, this study could help inform our own understanding of how the Solar System evolved over the course of billions of years. For some time, astronomers have been debating whether or not planets like Jupiter migrated to their current positions, and how this affected the Solar System’s evolution. And there continues to be debate about how the Main Belt formed (i.e. empty of full).
Last, but not least, it could inform future surveys, letting astronomers know which star systems are developing along the same lines as our own did, billions of years ago. Wherever star systems have debris disks, they an infer the presence of a particularly massive gas giant. And where they have a disk with two prominent dust belts, they can infer that it too will become a system containing many planets and and two belts.
A comet on a comet? That’s what it looks like, but you’re witnessing the most dramatic outburst ever recorded at 67P/Churyumov-Gerasimenko by the Rosetta spacecraft. The brilliant plume of gas and dust erupted on July 29 just two weeks before perihelion.
In a remarkable display of how quickly conditions on a comet can change, the outburst lasted only about 18 minutes, but its effects reverberated for days.
In a sequence of images taken by Rosetta’s scientific camera OSIRIS, the brilliant, well-defined jet erupts from the side of the comet’s neck in the Anuket region. It was first seen in a photo taken at 8:24 a.m. CDT, but not in one taken 18 minutes earlier, and had faded significantly in an image captured 18 minutes later. The camera team estimates the material in the jet was traveling at a minimum of 22 mph (10 meters/sec), but possibly much faster.
It’s the brightest jet ever seen by Rosetta. Normally, the camera has to be set to overexpose 67P/C-G’s nucleus to reveal the typically faint, wispy jets. Not this one. You can truly appreciate its brilliance because a single exposure captures both nucleus and plume with equal detail.
We all expected fireworks as the comet approached perihelion in its 6.5 year orbit around the Sun. Comets are brightest at and shortly after perihelion, when they literally “feel the heat”. Solar radiation vaporizes both exposed surface ices and ice locked beneath the comet’s coal-black crust. Vaporizing subsurface ice can created pressurized pockets of gas that seek a way out either through an existing vent or hole or by breaking through the porous crust and erupting geyser-like into space.
Jets carry along dust that helps create a comet’s fuzzy coma or temporary atmosphere, which are further modified into tails by the solar wind and the pressure of sunlight. When conditions and circumstances are right, these physical processes can build comets, the sight of which can fill the human heart with both terror and wonder.
This recent show of activity may be just the start of a round of outbursts at 67P/C-G. While perihelion occurs on this Thursday, a boost in a comet’s activity and brightness often occurs shortly after, similar to the way the hottest part of summer lags behind the date of summer solstice.
Rosetta found that the brief and powerful jet did more than make a spectacle — it also pushed away the solar wind’s magnetic field from around the nucleus as observed by the ship’s magnetometer. Normally, the Sun’s wind is slowed to a standstill when it encounters the gas cloud surrounding the nucleus.
“The solar wind magnetic field starts to pile up, like a traffic jam, and eventually stops moving towards the comet nucleus, creating a magnetic field-free region on the Sun-facing side of the comet called a ‘diamagnetic cavity’,” explained Charlotte Götz, magnetometer team member, on the ESA Rosetta website.
Only once before at Halley’s Comet has a magnetically “empty” region like this been observed. But that comet was so much more active than 67P/C-G and up until July 29, Halley’s remained the sole example. But following the outburst on that day, the magnetometer detected a diamagnetic cavity extending out at least 116 miles (186 km) from the nucleus. This was likely created by the outburst of gas, forcing the solar wind to ‘stop’ further away from the comet and thus pushing the cavity boundary outwards beyond where Rosetta was flying at the time.
Soon afterward the outburst, the comet pressure sensor of ROSINA detected changes in the structure of the coma, while its mass spectrometer recorded changes in the composition of outpouring gases. Compared to measurements made two days earlier, carbon dioxide increased by a factor of two, methane by four, and hydrogen sulphide by seven, while the amount of water stayed almost constant. No question about it – with all that hydrogen sulfide (rotten egg smell), the comet stunk! Briefly anyway.
It was also more hazardous. In early July, Rosetta recorded and average of 1-3 dust hits a day, but 14 hours after the event, the number leapt to 30 with a peak of 70 hits in one 4-hour period on August 1. Average speeds picked up, too, increasing from 18 mph (8 m/s) to about 45 mph (20 m/s), with peaks at 67 mph (30 m/s). Ouch!
“It was quite a dust party!” said Alessandra Rotundi, principal investigator of GIADA (Grain Impact Analyzer and Dust Accumulator).
67P/C-G’s little party apparently wasn’t enough to jack up its brightness significantly as seen from Earth, but that doesn’t mean future outbursts won’t. We’ll be keeping an eye on any suspicious activity through perihelion and beyond and report back here.
Everything eventually dies, even galaxies. So how does that happen? Time to come to grips with our galactic mortality. Not as puny flesh beings, or as a speck of rock, or even the relatively unassuming ball of plasma we orbit.
Today we’re going to ponder the lifespan of the galaxy we inhabit, the Milky Way. If we look at a galaxy as a collection of stars, some are like our Sun, and others aren’t.
The Sun consumes fuel, converting hydrogen into helium through fusion. It’s been around for 5 billion years, and will probably last for another 5 before it bloats up as a red giant, sheds its outer layers and compresses down into a white dwarf, cooling down until it’s the background temperature of the Universe.
So if a galaxy like the Milky Way is just a collection of stars, isn’t that it? Doesn’t a galaxy die when its last star dies?
But you already know a galaxy is more than just stars. There’s also vast clouds of gas and dust. Some of it is primordial hydrogen left from the formation of the Universe 13.8 billion years ago.
All stars in the Milky Way formed from this primordial hydrogen. It and other similar sized galaxies produce 7 bouncing baby stars every year. Sadly, ours has used up 90% of its hydrogen, and star formation will slow down until it both figuratively, and literally, runs out of gas.
The Milky Way will die after it’s used all its star-forming gas, when all of the stars we have, and all those stars yet to be born have died. Stars like our Sun can only last for 10 billion years or so, but the smallest, coolest red dwarfs can last for a few trillion years.
That should be the end, all the gas burned up and every star burned out. And that’s how it would be if our Milky Way existed all alone in the cosmos.
Fortunately, we’re surrounded by dozens of dwarf galaxies, which get merged into our Milky Way. Each merger brings in a fresh crop of stars and more hydrogen to stoke the furnaces of star formation.
There are bigger galaxies out there too. Andromeda is bearing down on the Milky Way right now, and will collide with us in the next few billion years.
When that happens, the two will merge. Then there’ll be a whole new era of star formation as the unspent gas in both galaxies mix together and are used up.
Eventually, all galaxies gravitationally bound to each other in this vicinity will merge together into a giant elliptical galaxy.
We see examples of these fossil galaxies when we look out into the Universe. Here’s M49, a supermassive elliptical galaxy. Who knows how many grand spiral galaxies stoked the fires of that gigantic cosmic engine?
Elliptical galaxies are dead galaxies walking. They’ve used up all their reserves of star forming gas, and all that’s left are the longer lasting stars. Eventually, over vast lengths of time, those stars will wink out one after the other, until the whole thing is the background temperature of the Universe.
As long as galaxies have gas for star formation, they’ll keep thriving. Once it’s gonzo, or a dramatic merger uses all the gas in one big party, they’re on their way out.
What could we do to prolong the life of our galaxy? Let’s hear some wild speculation in the comments below.
Even robots can’t tear their eyes from a beautiful sunset. NASA’s Mars Curiosity rover pointed its high resolution mast camera at the setting Sun to capture this 4-image sequence on April 15 at the conclusion of the mission’s 956th Martian day. While it resembles an earthly sunset, closer inspection reveals alien oddities.
A day on Mars lasts 24 hours and 39 minutes, so sunrise and sunset follow nearly the same rhythm as they do on Earth. When we eventually establish a base there, astronauts should be able to adjust to the planet’s day-night rhythm with relative ease. Jet lag would be worse.
But sunsets and sunrises offer a different palette of colors than they would on Earth. For starters, the Sun only radiates the equivalent of a partly cloudy afternoon’s worth of light. That’s because Mars’ average distance from the Sun is 141.6 million miles or about half again Earth’s distance. Increased distance reduces the intensity of sunlight.
Not only that, but the solar disk shrinks from the familiar 0.5° across we see from Earth to 0.35° at Mars. Here on the home planet, your little finger extended at arm’s length would cover the equivalent of two Suns. On Mars it would be three!
What about color? Dust and other fine particles in the atmosphere scatter the blues and greens from the setting or rising Sun to color it yellow, orange and red. When these tints are reflected off clouds, sunset colors are amplified and spread about the sky, making us reach for that camera phone to capture the glory.
Things are a little different on Mars. The ever-present fine dust in the Martian atmosphere absorbs blue light and scatters the warmer colors, coloring the sky well away from the Sun a familiar ruddy hue. At the same time, dust particles in the Sun’s direction scatter blue light forward to create a cool, blue aureole near the setting Sun. If you were standing on Mars, you’d only notice the blue glow when the Sun was near the horizon, the time when its light passes through the greatest depth of atmosphere and dust.
On Earth, blue light from the Sun is scattered by air molecules and spreads around the sky to create a blue canopy. Mars has less the 1% of Earth’s atmosphere, so we only notice the blue when looking through the greatest thickness of the Martian air (and dust) around the time of sunset and sunrise.
Sunset on Mars photographed by the Opportunity Rover released earlier this year
The video above of the setting Sun was made using stills taken by Opportunity, NASA’s “other” rover that’s been trekking across the Martian landscape for more than 10 years now. You can see a bit of pink in the Sun just before it sets as in the Curiosity photos, but there’s something else going on, too. Or not going on.
When the Sun sets or rises on Earth, it’s squashed like a melon due to atmospheric refraction. Much thicker air adjacent to the horizon bends the Sun’s light upward, pushing the bottom of the solar disk into the top half which is less affected by refraction because it’s slightly higher. Once the Sun rises high enough, so we’re looking at it through less atmosphere, refraction diminishes and it becomes a circle again.
I’ve looked at both the Opportunity sunset and Curiosity sunset videos many times, and as far as I can tell, the Sun’s shape doesn’t change. At least it’s not noticeable to the casual eye. I bet you can guess why — the air is too thin to for refraction to make much of a difference.
Twilights linger longer on the Red Planet as well because dust lofted high into the stratosphere by storms continues to reflect the Sun’s light for two hours or more after sundown.
So you can see that sunset phenomena on Mars are different from ours because of the unique qualities of its atmosphere. I trust someone alive today will be the first human to see and photograph a Martian sunset. Hope I’m still around when that awesome pic pops up on Twitter.
“The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars,” Carl Sagan famously said in his 1980 series Cosmos. “We are made of starstuff.”
And even today, observations with NASA’s airborne SOFIA observatory are supporting this statement. Measurements taken of the dusty leftovers from an ancient supernova located near the center our galaxy – aka SNR Sagittarius A East – show enough “starstuff” to build our entire planet many thousands of times over.
“Our observations reveal a particular cloud produced by a supernova explosion 10,000 years ago contains enough dust to make 7,000 Earths,” said research leader Ryan Lau of Cornell University in Ithaca, New York – the same school, by the way, where Carl Sagan taught astronomy and space science.
While it’s long been known that supernovae expel enormous amounts of stellar material into space, it wasn’t understood if clouds of large-scale dust could withstand the immense shockwave forces of the explosion.
These observations, made with the joint NASA/DLR-developed Faint Object InfraRed Camera for the SOFIA Telescope (FORCAST) instrument, provide key “missing-link” evidence that dust clouds do in fact survive intact, spreading outward into interstellar space to seed the formation of new systems.
Interstellar dust plays a vital role in the evolution of galaxies and the formation of new stars and protoplanetary discs – the orbiting “pancakes” of material around stars from which planets (and eventually everything on them) form.
The findings may also answer the question of why young galaxies observed in the distant universe possess so much dust; it’s likely the result of frequent supernova explosions from massive early-generation stars.
Source: NASA, Cornell, and Caltech
“We have begun to contemplate our origins: starstuff pondering the stars; organized assemblages of ten billion billion billion atoms considering the evolution of atoms; tracing the long journey by which, here at least, consciousness arose.”
– Carl Sagan, Cosmos (1980)
Just a day after skywatchers at mid- to upper-latitudes around the world were treated to a particularly energetic display of auroras on the night of March 17 as a result of an intense geomagnetic storm, researchers announced findings from NASA’s MAVEN mission of auroral action observed on Mars – although in energetic ultraviolet wavelengths rather than visible light.
Detected by MAVEN’s Imaging Ultraviolet Spectrograph (IUVS) instrument over five days before Dec. 25, 2014, the ultraviolet auroras have been nicknamed Mars’ “Christmas lights.” They were observed across the planet’s mid-northern latitudes and are the result of Mars’ atmosphere interacting directly with the solar wind.
While auroras on Earth typically occur at altitudes of 80 to 300 kilometers (50 to 200 miles) and occasionally even higher, Mars’ atmospheric displays were found to be much lower, indicating higher levels of energy.
“What’s especially surprising about the aurora we saw is how deep in the atmosphere it occurs – much deeper than at Earth or elsewhere on Mars,” said Arnaud Stiepen, IUVS team member at the University of Colorado. “The electrons producing it must be really energetic.”
To a human observer on Mars the light show probably wouldn’t be very dramatic, though. Without abundant amounts of oxygen and nitrogen in its thin atmosphere a Martian aurora would be a dim blue glow at best, if not out of the visible spectrum entirely.
This isn’t the first time auroras have been spotted on Mars; observations with ESA’s Mars Express in 2004 were actually the first detections of the phenomenon on the Red Planet. Made with the spacecraft’s SPICAM ultraviolet spectrometer, the observations showed that Mars’ auroras are unlike those found anywhere else in the Solar System in that they are generated by particle interactions with very localized magnetic field emissions, rather than a globally-generated one (like Earth’s).
(So no, it’s not a total surprise… but it’s still very cool!)
In addition to auroras MAVEN also detected diffuse but widespread dust clouds located surprisingly high in the Martian atmosphere. It’s not yet understood what process is delivering dust so high – 150-300 kilometers up (93-186 miles) – or if it is a permanent or temporary feature.
Source: NASA and Nature
Anyone who’s ever read a Charlie Brown comic strip knows “Pig-Pen”, the lovable boy who walks around in a constant cloud of his own dirt and dust. Every time he sighs, dust rises in a little cloud around him. Why bother to bathe? There’s dignity in debris, which “Pig-Pen refers to as the “dust of countless ages”. Comets shuffle around the Sun surrounded by a similar cloud of grime that’s as old as the Solar System itself.
You’ve probably noticed little flecks and streaks in photos returned by the Rosetta spacecraft in the blackness of space surrounding comet 67P/Churyumov-Gerasimenko. After a recent year-end break, the Rosetta team has returned with new updates on the comet including a series of four images recently released as a mosaic. The pictures were processed to highlight surface features; the space around the nucleus is black in comparison. But if we take a closer look at what first appears void, we soon discover it’s not empty at all.
In photos taken January 3rd, the writer of ESA’s Rosetta blog notes that “some of the streaks and specks seen around the nucleus will likely be dust grains ejected from the comet, captured in the 4.3 second exposure time.”
Using an image-editing tool like Photoshop, we can hold back the glare of the nucleus and “open up” the shadows around the comet. Jets of dust released by vaporizing ice are the most obvious features to emerge. The soft, low-contrast plumes plow into the vacuum around the nucleus wrapping it in a silky cocoon of gas and dust – a tenuous atmosphere that reflects sunlight far more weakly than the comet itself.
While staring at dust spots may not produce the same magical feelings as watching a sunrise, it’s fascinating nonetheless to contemplate what we’re seeing. If you’ve been struck by the beauty of a comet’s meteor-like head trailing a wispy tail, you’re looking at what countless individual grains of dust can do when sculpted by the master hand of the Sun. Perusing images of 67P, we see the process in its infancy as individual grains and small clots are released into space to be fashioned into something grander.
Rosetta’s Micro-Imaging Dust Analysis System or MIDAS measures the rate at which dust sweeps past the spacecraft and its size distribution. MIDAS catches dust grains by exposing a sticky target surface into space and waiting for a mote to drift by. It snatched its first one last November – a larger than expected mote measuring about 1/100 of a millimeter across with a complex shape and fluffy texture.
Analysis of the composition of another dust grain named “Boris” made by the COSIMA instrument has identified sodium and magnesium. Magnesium is no surprise as 95% of known minerals observed in comets resemble olivine and pyroxenes, common in meteorites and in the upper mantle of the Earth. Sodium has also been seen before in comas and tails, and originates in dust grains, but its mineral source remains uncertain.
As we might study the makeup of the dust Pig-Pen leaves in his wake to identify traces of earthly dirt, micro-organisms, pollen, pollution, and even recent volcanic eruptions, so we examine each mote that sprays Rosetta’s way, looking for clues to the origin of the planets and Solar System.
UPDATE: Watch a live webcast of the meteor shower, below, from NASA’s Marshall Space Flight Center during the night of Monday, May 5 to the early morning of May 6.
Halley’s Comet won’t be back in Earth’s vicinity until the summer of 2061, but that doesn’t mean you have to wait 47 years to see it. The comet’s offspring return this week as the annual Eta Aquarid meteor shower. Most meteor showers trace their parentage to a particular comet. The Perseids of August originate from dust strewn along the orbit of comet 109P/Swift-Tuttle, which drops by the inner solar system every 133 years after “wintering” for decades just beyond the orbit of Pluto, but the Eta Aquarids (AY-tuh ah-QWAR-ids) have the best known and arguably most famous parent of all – Halley’s Comet. Twice each year, Earth’s orbital path intersects dust and rock particles strewn by Halley during its cyclic 76-year journey from just beyond Uranus to within the orbit of Venus. When we do, the grit meets its demise in spectacular fashion as wow-inducing meteors.
Meteoroids enter the atmosphere and begin to glow some 70 miles high. The majority of them range from sand to pebble sized but most no more than a gram or two. Speeds range from 25,000-160,000 mph (11-72 km/sec) with the Eta Aquarids right down the middle at 42 miles per second (68 km/sec). Most burn white though ‘burn’ doesn’t quite hit the nail on the head. While friction with the air heats the entering meteoroid, the actual meteor or bright streak is created by the speedy rock exciting atoms along its path. As the atoms return to their neutral state, they emit light. That’s what we see as meteors. Picture them as tubes of glowing gas.
The farther south you live, the higher the shower radiant will appear in the sky and the more meteors you’ll see. For southern hemisphere observers this is one of the better showers of the year with rates around 30-40 meteors per hour. With no moon to brighten the sky, viewing conditions are ideal. Except for maybe the early hour. The shower is best seen in the hour or two before the start of dawn.
From mid-northern latitudes the radiant or point in the sky from which the meteors will appear to originate is low in the southeast before dawn. At latitude 50 degrees north the viewing window lasts about 1 1/2 hours; at 40 degrees north, it’s a little more than 2 hours. If you live in the southern U.S. you’ll have nearly 3 hours of viewing time with the radiant 35 degrees high.
Northerners might spy 5-10 meteors per hour over the next few mornings. Face east for the best view and relax in a reclining chair. One good thing about this event – it won’t be anywhere near as cold as watching the December Geminids or January’s Quadrantids. We must be grateful whenever we can.
Meteor shower members can appear in any part of the sky, but if you trace their paths in reverse, they’ll all point back to the radiant. Other random meteors you might see are called sporadics and not related to the Eta Aquarids. Because Aquarius is home to at least two radiants, we distinguish the Etas, which radiate from near Eta Aquarii, from the Delta Aquarids, an unrelated shower active in July and August.
Wishing you clear skies and plenty of hot coffee at the ready.
Sunrise over the surface of the moon: a series of star tracker images taken by LADEE Saturday, April 12. The lunar horizon is ahead, a few minutes before orbital sunrise. Image Credit: NASA Ames.
NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) literally ‘saw the light’ just days before crashing into the lunar farside last Thursday April 17. Skimming just a few kilometers above the moon’s surface, mission controllers took advantage of this unique low angle to gaze out over the moon’s horizon in complete darkness much like the Apollo astronauts did from lunar orbit more than 40 years ago.
With the glow of Earth well-hidden, any dust in the moon’s scant atmosphere around the time of orbital sunrise should become visible. Scientists also expected to see the softly luminous glow of the zodiacal light, an extensive cloud of comet and asteroid dust concentrated in the flat plane of the solar system. The zodiacal light gets its name from the zodiac, that familiar band of constellations the planets pass through as they orbit the sun. Back on Earth, the zodiacal light looks like a big thumb of light standing up from the western horizon a couple hours after sunset in spring and before sunrise in fall.
So what did LADEE see? As you watch the animation above, comprised of images taken from darkness until sunrise, you’ll see a yellow haze on the horizon that expands into large diffuse glow tilted slightly to the right. This is the zodiacal light along with a smaller measure of light coming from sun’s outer atmosphere or corona. Together they’re referred to as CZL or ‘coronal and zodiacal light’. At the very end, the sun peaks over the lunar horizon.
What appears to be missing from the pictures are the mysterious rays seen by some of the Apollo astronauts. The rays, neatly sketched by astronaut Eugene Cernan of Apollo 17, look a lot like those beams of light and shadow streaming though holes in clouds called crepuscular rays.
Only thing is, Earth’s atmosphere is thick enough for cloud beams. The dust in the moon’s atmosphere appears much too thin to cause the same phenomenon. And yet the astronauts saw rays as if sunlight streamed between mountain peaks and scattered off the dust just like home.
It’s believed that dust gets lofted into the spare lunar atmosphere via electricity. Ultraviolet light from the sun knocks electrons from atoms in moon dust, giving them a positive charge. Since like charges repel, bits of dust push away from one another and move in the direction of least resistance: up. The smaller the dust particle, the higher it rises until dropping back down to the surface. Perhaps these “fountains” of lunar dust illuminated by the sun are what the astronauts recorded.
Unlike Cernan, LADEE saw only the expected coronal and zodiacal light but no rays. Scientists plan to look more closely at several sequences of images made of lunar sunrise in hopes of finding them.