Sky & Telescope news
As we transition between seasons, Orion rides high in the evening sky — easily found by spotting the row of three bright stars in his Belt.
This is a month of transition. Clocks will change around the world, and we northerners switch to Daylight or Summer time — on March 8th in the U.S. and Canada but on the 29th across Europe. A second transition comes on March 20th at 6:45 p.m. Eastern Daylight Time, when Earth reaches one of the two equinox points in its year-long orbit. Equinox comes from the Latin word aequinoctium, meaning “equal nights.” On the equinox, the Sun rises due east and sets due west no matter where you are.
Once the Moon leaves the evening sky (it's full on the 5th), the view above will feature Venus well up in the west, Jupiter high up in the east, and the bright stars of winter in between. Sirius is the real "star" of the night sky this time of year, but as dusk deepens look due south to spot the mighty constellation Orion, the Hunter. Its two brightest stars are icy-white Rigel, which marks Orion’s lower-right foot, and orange-red Betelgeuse, his upper-left shoulder.
Midway between them, look for the three-star row of Orion’s Belt. Each member of this iconic trio has a name: Alnitak, on the left, means “the girdle” in Arabic; Alnilam, in the middle, translates as “string of pearls”; and Mintaka means “the belt.”
This is just a sample of the many March stargazing sights that await you even if you're just stepping outside for a casual look around. To get familiar with the bright stars and planets overhead, download our 7-minute-long stargazing podcast below.
There's no better guide to what's going on in nighttime sky than the March issue of Sky & Telescope magazine.
Western Colorado Astronomy ClubADDRESS
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The Western Colorado Astronomy Club is a non-profit organization founded
in 1989 and dedicated to science education. We serve the Grand Junction
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Deep Sky Colors
Premier astrophotographer Rogelio Bernal Andreo has published his first book, Hawai‘i Nights, documenting his nearly month-long photographic adventure visiting the islands. This landscape-format book measures 8.27 × 11.7 inches and is divided into informal chapters based on the nights he spent snapping breathtaking photos of the night sky on each of the main islands. The book features more than 70 photos of famous Hawai‘ian locations seen under the stars, many as two-page spreads. Hawai‘i Nights is available as a digital download for $24, in paperback for $49, and in hardcover for $65. 115 pages, ISBN 978-0-9906763-2-4.
SkyandTelescope.com's New Product Showcase is a reader service featuring innovative equipment and software of interest to amateur astronomers. The descriptions are based largely on information supplied by the manufacturers or distributors. Sky & Telescope assumes no responsibility for the accuracy of vendors statements. For further information contact the manufacturer or distributor. Announcements should be sent to nps@SkyandTelescope.com. Not all announcements will be listed.
Friday, February 27
Venus and Mars in the western twilight have widened to be 2.7° apart now. Look for faint Mars beneath Venus.
Saturday, February 28
Early this evening, the dark limb of the waxing gibbous Moon will occult (cover) the 3.6-magnitude star Lambda Geminorum for telescope users in North America east of the Mississippi and north of the deepest South. Some times: central Massachusetts, 8:00 p.m. EST; Washington DC, 7:56 p.m. EST; Chicago, 6:31 p.m. CST (in twilight); Kansas City, 6:21 p.m. CST (in twilight). See map and detailed timetables of both the disappearance and the (unobservable) reappearance; be careful not to mix these up when scrolling down the table.
Two mutual events among Jupiter's moons. Watch Europa occult (pass in front of) Io from 11:10 to 11:16 p.m. EST this evening. At the center of this time, their combined light is dimmed by 0.6 magnitude, not quite half.
Then less than an hour later, Europa casts its shadow onto Io from 12:02 to 12:09 a.m. EST, dimming Io by 0.9 magnitude at the mid-time of this eclipse.
Sunday, March 1
After dark, Jupiter is the bright "star" to the Moon's lower left, and Procyon is the real star to the Moon's right. Far lower right of there shines Sirius, the brightest star in the sky. Sirius is also the nearest star that's visible to the naked eye from northern latitudes, at a distance of 8.6 light-years.
Monday, March 2
The bright planet near the waxing gibbous Moon tonight is Jupiter. Looks are deceiving, however. Jupiter is actually 40 times larger than the Moon in diameter, but it's 1,660 times farther away (as of tonight).
Another mutual event among Jupiter's moons. Tonight Ganymede occults Io from 11:06 to 11:11 p.m. EST; their combined light dims by 0.6 magnitude at the center of this time. Later Ganymede casts its shadow onto Io, but just a few minutes beforehand, Io disappears behind Jupiter's edge from Earth's viewpoint! (at 12:17 a.m. EST).
Tuesday, March 3
Bright Jupiter shines above the Moon this evening. Spot fainter Regulus closer to the Moon's lower left (for North America).
Wednesday, March 4
A challenge for North Americans as twilight turns into night: Distant Uranus, magnitude +5.9, glimmers just below Venus, which is 8,000 times brighter at magnitude –3.9. Use good binoculars or a telescope. At the time of nightfall on the East Coast, Uranus is 0.3° below Venus. By nightfall on the West Coast it's 0.5° below. Nothing else of that brightness is that close under Venus.
Thursday, March 5
Full Moon (exact at 1:05 p.m. Eastern Standard Time). This evening the Moon shines below the dim hind feet of Leo.
And another! Io partially eclipses Ganymede with its shadow from 1:35 to 1:46 a.m. Friday morning EST. Ganymede will dim by a full 1.0 magnitude at mid-eclipse. They're both to Jupiter's west – with Callisto in the background between them! Callisto is normally 1.1 magnitude fainter than Ganymede (which is the one appearing closest to Jupiter). But at mid-eclipse Ganymede will look almost identical to Callisto, with Io clearly outshining them both.
Friday, March 6
As the stars come out at this time of year, look due south for Orion standing upright at his highest. As night deepens, dim Lepus, the Hare, emerges under his feet. Below Lepus is Columba, the Dove.
Saturday, March 7
Little Mars has sunk to 6° below Venus now in the west at dusk. The gap between them will continue to widen until Mars finally becomes lost in the sunset in late April.
Daylight-saving time begins at 2 a.m. Sunday morning for most of the U. S. and Canada. Clocks spring ahead one hour.
But wait, there's more! Now Europa occults Io, from 1:15 to 1:21 a.m. Sunday morning EST. The blend of the two dims by just 0.5 magnitude at mid-event. They're the pair closest to Jupiter. Then exactly one hour after the end of that event, Europa's shadow starts eclipsing Io for 6 minutes, dimming it by 0.8 magnitude at mid-eclipse.
Similar mutual events happen among Jupiter's moons through the rest of the month; see the March Sky & Telescope, page 53. If you're not in North America, here's where to get the whole list worldwide, sortable by visibility from your location. These "mutual event seasons" happen about every 6 years. The current one will trail off later this spring.
Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.
This is an outdoor nature hobby; for an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.
You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.
Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability, which means fairly heavy and expensive). As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."This Week's Planet Roundup
Mercury (magnitude 0.0) glimmers just above the east-southeast horizon during dawn, a little lower every day. Bring binoculars.
Venus (magnitude –3.9) and Mars (less than 1% as bright at magnitude +1.3) are in the west-southwest during evening twilight. Look for Mars below Venus.
Jupiter (magnitude –2.5, in Cancer) comes into view high in the east during twilight. By 9 or 10 p.m. Jupiter is nearly as high as it will get. In a telescope Jupiter is still a big 44 arcseconds wide at its equator.
Saturn (magnitude +0.4, at the head of Scorpius) rises around 1 or 2 a.m. It's best placed in the south as dawn begins. Lower left of Saturn by 8° is orange Antares.
Look just ½° below Saturn in early dawn for Nu Scorpii, a showpiece double star for telescopes. Less than 2° to their right is Beta Scorpii, an even finer telescopic double.
Uranus (magnitude 5.8, in Pisces) is in the background of Venus just after dark.
Neptune is hidden in the glare of the Sun.
All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.
Eastern Standard Time (EST) is Universal Time (UT, UTC, or GMT) minus 5 hours.
“This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours.”
— Neil deGrasse Tyson, 2014.
A supermassive black hole found only a billion years after the Big Bang adds to growing questions about how such black holes grew so quickly.
Astronomers have discovered one of the brightest quasars in the early universe. The source, SDSS J010013.02+280225.8 (hereafter J0100+2802), is powered by a supermassive black hole at a redshift of 6.3, meaning that its light left it 12.8 billion years ago.
(Note: due to the universe’s expansion, this does not mean that the quasar is 12.8 billion light-years away, as various press releases have claimed. Because determining the current physical distances to things in the early universe is confusing and inexact, we usually stick with look-back time and redshift, which we can more readily calculate.)
Xue-Bing Wu (Peking University, China) and colleagues found the quasar in an extensive survey of quasars in the early universe that combines visible-light data from the Sloan Digital Sky Survey (SDSS), near-infrared data from the 2-Micron All-Sky Survey (2MASS), and mid-infrared data from the Wide-field Infrared Survey Explorer (WISE). When luminous J0100+2802 stuck out, they attacked it with additional optical and infrared observations, measuring photometry and spectra from several different observatories around the world.
They determined that the quasar puts out energy at about 420 trillion times the rate of the Sun, making it the brightest known quasar beyond a redshift of 6. (Astronomers have detected about 40 quasars this far back in cosmic time.)
The astronomers estimated the black hole’s mass in two ways, first by extrapolating from the quasar’s total luminosity, and then by measuring the speed of clouds that orbit the behemoth. Both methods make assumptions and simplifications but give a mass of 12 or 13 billion solar masses.
As Bram Venemans (Max Planck Institute for Astronomy, Germany) writes in his perspective piece in the February 26th Nature, “It is not implausible to find a black hole of more than 10 billion solar masses within 1 billion years after the Big Bang. But it is still surprising.” To have grown so large so fast, the black hole must have been stuffing itself with gas at close to its maximum accretion rate for most of its existence — which is odd, because the outward push of radiation from the gas being swallowed should have cut off the black hole’s accretion after 10 million to 100 million years.
J0100+2802 is one of a handful of billion-solar-mass black holes seen in the first few hundred million years after the Big Bang. Its existence spurs a lot of questions. How massive is its host galaxy — will it prove to be supermassive, too? How did J0100+2802 grow to be so large so fast? When did it start growing, and from what?
The last question is part of a longstanding debate among black hole astrophysicists that might be gaining steam thanks to the recent Planck results. At a press conference at the American Association for the Advancement of Science earlier this month, theorist Priyamvada Natarajan (Yale) said that there’s now a real timing problem with growing the first supermassive black holes. Planck’s results suggest that galaxies didn’t start lighting up with stars in earnest until a redshift of about 8.8, or 13.2 billion years ago. But if the first supermassive black holes formed from smaller “seed” black holes created when stars went supernova (one of two competing ideas), that leaves only a few hundred million years for black holes the size of J0100+2802 to beef up — a short time span in astrophysics. Instead, the Planck results and discoveries such as J0100+2802 might push more astronomers in favor of the other, “direct-collapse” scenario, where pristine gas collapses on its own into black holes of 1,000 to 10,000 solar masses.
You can read more about the results in the various press releases put out by the authors’ institutions (there are at least four releases out there, by my count); the most comprehensive are from the Large Binocular Telescope and from the University of Arizona.
X.-B. Wu et al. “An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30.” Nature. February 26, 2015.
B. Venemans. “A giant in the young Universe.” Nature. February 26, 2015.
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The Astronomy Club is a new club in the American University in Cairo that will help develop and create a fun and challenging environment for those who love to create and those who like to think. This club basically aims its members to participate in outreach programs and learn a lot of things about astronomy not by the book but by hand on experience.
With the Moon riding high this week, what better time to look for its three best-known yet enigmatic "ring" craters?
We welcome back the waxing Moon this week. It's a chance for many of us to put dark-sky targets on the back burner and give some love to she who lights the night. During fall, the evening Moon "rides low" in the southern sky during its most attractive phases. Not until January does the ecliptic's steepening angle to the horizon finally loft the quarter Moon high in the southern sky, where it's viewed to best advantage.
February continues what January began. Each night, from now until early March, the "high-riding" Moon dares anyone with a telescope to brave the cold and behold an alien world up close. Each night, the terminator advances eastward, revealing thousands of new features, including three of the most remarkable craters you'll ever see — Hesiodus A, Crozier H, and Marth.
All three belong to a weird breed called concentric craters. Concentric craters are generally small, so you have to really look for them. None jumps out like, say, Copernicus or Tycho. There are about 58 all told, with diameters ranging from 3-20 km (average 8 km), each containing an inner ring usually about half as wide as the crater.
At first impression, you might think two unrelated impacts occurred precisely atop another, excavating not one but two crater rims. When I first saw Hesiodus A, the best and easiest of the trio, it struck me as artificial. How could something so symmetrical exist on the battered Moon?
Some of the rings look like bagels or donuts, others are clearly rings of hills. Some are elliptical like Crozier H, and others more circular like Hesiodus A. Most occur near the margins of the dark lunar seas (or maria) and the crater-scoured highlands where volcanic activity was intense in the distant past. And as we'll learn, their location provides a clue to their origin.
Hesiodus A, the easiest, can be seen in a 4.5-inch telescope. The others will require a 6-inch or larger scope. Remember, all are relatively small and their inner rings smaller, so don't shy from high magnification once you've found your target. Crank it up to 200x-250x. Unlike some lunar features which disappear in the shadowless sunlight of the full Moon, many concentric craters remain visible, looking like bulls-eyes.
Not too far from our first target, you'll find the 7 km-diameter crater Marth. This one's really small, but the bone-white inner ring pops out in an 8-inch scope. Crozier H is a bit easier to make out than Marth and occupies a busy cratered area along the edge of Mare Fecunditatis in the eastern half of the Moon.
A variety of hypotheses have been proposed to explain the origin of concentric craters' unique inner rings, everything from simultaneous impacts to mass wasting (rock and soil moving downslope under the force of gravity) to impacts in dual-layered lunar shield volcanoes better known as domes.
But based on the distribution, size, and other characteristics of concentrics, lava welling up through fissures or cracks in the craters' floors appears to be the best fit. Many lunar craters show evidence of lava flooding. Well-known examples include Plato, Archimedes, and the beautiful Bay of Rainbows aka Sinus Iridium, once a huge basin but nearly submerged by lavas that poured like hot oil across the lunar plains between 3.3 and 2.5 billion years ago.
Exactly why the lavas in concentric craters bubbled up to form donut-like rings remains unclear. It may have to do with cracks preferentially forming along the edges of crater floors, or the rate of lava flow as well as its texture. A plug of molten rock beneath the crater may have lifted up its floor, which later deflated in the center like a collapsed soufflé, leaving a ring. It's even possible that dense impact melt on the crater floor resisted uplift from lavas pushing up from below. Whatever the specific reason, most lunar experts agree that concentric craters owe their formation to volcanic processes.
If you've tackled our three examples and crave more concentricity, head over to this gold mine for additional challenges. Wishing you a "ringer" on your next night out!
Wondering what to call that crater? Use a Sky & Telescope Moon Globe to guide your explorations of the lunar surface!
Two total lunar eclipses occur this year, in April and September. Meanwhile, a total solar eclipse in March sweeps across remote Arctic waters on March 20th, and a partial event in September is likewise poorly placed for observing.
Any list of nature's grandest spectacles would certainly include eclipses of the Sun and Moon. Up to seven of them can take place in one year, though the last time that happened was 1982. The fewest possible is four, as will be the case in 2014. The solar eclipses — one total and one partial — are not observable from the Americas. Both lunar eclipses are total; April's favors the West Coast, while September's is best on the East Coast.Why Do Eclipses Happen? Few events in nature offer the drama and spectacle of a total solar eclipse, as demonstrated by this one seen over China on August 1, 2008. S&T: Dennis di Cicco
A solar eclipse, such as the one pictured at right, occurs only at new Moon, when the lunar disk passes directly between us and the Sun. Conversely, a lunar eclipse takes place during full Moon, when our satellite passes through Earth's shadow. These alignments don't happen at every new and full Moon because the lunar orbit is tipped about 5° to Earth's orbital plane — only occasionally do the Sun, Earth, and Moon line up exactly enough for an eclipse to occur. (The technical name for that, by the way, is syzygy.)
Three types of lunar eclipse are possible (total, partial, and penumbral) depending on how deeply the full Moon plunges into or near the umbra, our planet's dark, central shadow. If it goes all the way in, we see a total lunar eclipse that's preceded and followed by partial phases. If the Moon skims part way into the umbra, only the partial phases occur. And if its disk passes just outside the umbra, it still encounters the weak penumbral shadow cast by Earth.
Interestingly, this year's two lunar events mark the conclusion of a series of four consecutive total lunar eclipses in 2014–15! Such eclipse tetrads are not common — the last one occurred a decade ago, but the next won't begin until 2032.
Fortunately, every lunar eclipse is observable anywhere on Earth where the Moon is above the horizon. (But there's still an element of luck involved: as described below, April's total lunar eclipse promises to be gorgeous from Los Angeles but completely unobservable from New York.)
However, solar eclipses more tightly restrict where you can see them because the Moon casts a smaller shadow than Earth does. If the Moon completely hides the Sun, the eclipse is considered total. With its brilliant disk completely covered, the Sun's ghostly white outer atmosphere is momentarily revealed for durations from seconds to several minutes. In November 2013, for example, planeloads of eclipse-chasers converged in a remote portion of northern Kenya to watch just 11 seconds of totality.
A completely eclipsed Sun can be viewed only from a narrow track or path on Earth's surface that's typically just 100 miles (160 km) wide. Outside of that path, about half of the daylit hemisphere of Earth is able to watch a partial eclipse as the Moon obscures a portion of the Sun.
Occasionally the Moon passes directly in front of the Sun but doesn't completely cover it. This circumstance is known as an annular eclipse, so-called because you can see a ring, or annulus, of sunlight surrounding the lunar disk. But an annular's path is likewise narrow, and outside of it observers see only a partial cover-up.The Four Eclipses in 2015
Below are brief descriptions of this year's eclipses of the Sun and Moon. You'll find more details for March's total solar eclipse and the two lunar eclipses both on this website and in Sky & Telescope magazine as the date of each draws near. Times are in Universal Time (UT) except as noted.
March 20: Total Solar Eclipse
Whenever the Moon covers the Sun, the narrow path of totality can be anywhere in the world — often traversing remote locations. The path of this year's total solar eclipse is doubly challenging — both to get to and to see once there. It's an especially wide track, up to 303 miles (487 km) wide, that offers a maximum of 2 minutes, 47 seconds of totality. But that's little consolation because the path is largely confined to the extreme North Atlantic and the open water between Greenland and Scandinavia. (Fun fact: the Moon's shadow leaves Earth as it crosses the North Pole!)
Totality will only be seen from the remote Faroe Islands (halfway between Iceland and Norway) and Svalbard (halfway between Norway and the North Pole). In these locations, diehard eclipse-chasers can expect 2 minutes of totality beginning at 9:41 UT and 2½ minutes beginning at 10:11 UT, respectively. These ice-swept outposts are so far north that in most of Longyearbyen, on the island of Spitsbergen, mountains will block views of the eclipsed Sun. Meanwhile, the prospects for clear skies from those bits of dry land — and for the tracts of sea around them — is relatively poor, with no site offering better than a 50:50 chance of clear sky at eclipse time.
Because the Moon is near perigee and thus look somewhat larger than usual in the sky, the area from which a partially covered Sun can be seen is also large: all of Europe, northwestern Asia, and northern Africa.
April 4: Total Lunar Eclipse
The first of this year's two lunar events only barely qualifies as a total eclipse. The Moon is completely immersed in Earth's dark umbral shadow for just 4½ minutes, centered on 12:00 UT. In fact, because the edge of totality is a fuzzy boundary, totality's duration depends on what definition for "umbra" is you adopt. For example, the U.S. Naval Observatory’s calculations (used by Sky & Telescope) yield a duration of 12.3 minutes, while Fred Espenak’s Fifty-Year Canon of Lunar Eclipses says 8.6 minutes.
Regardless, the event's timing favors the West Coast (mid-eclipse is at 5:00 a.m. Pacific Daylight Time). Anyone located east of the Mississippi gets to glimpse only the early partial phases before the Moon sets in brightening morning twilight. Because of the barely-there nature of this umbral crossing, the Moon's northern half (being closest to the umbral boundary) is certain to look brighter than its southern half. The partial phases begin at 10:16 UT and end at 13:45 UT.
September 13: Partial Solar Eclipse
Here's an eclipse that few can hope to see — unless you happen to be visiting Antarctica. Unlike March's solar eclipse, this time the Moon is relatively far from Earth, thus limiting the regional extent of visibility. The greatest coverage (79% of the Sun's diameter) occurs near Germany's Neumayer Station on the coast of Antarctica. Skywatchers in southern Africa can see a modest bite in the solar disk (up to 38% of its diameter) just after sunrise,
September 27–28: Total Lunar Eclipse
Observers throughout the Americas, as well as in western Europe and Africa, have the best seats for the year's final eclipse. Mid-eclipse is at 2:47 UT on the 28th, which corresponds to the evening of the 27th across the U.S. and Canada.
The total phase of this eclipse lasts from 10:11 to 11:23 p.m. EDT on the East Coast, where the Moon appears high up in the constellation Pisces. On the West Coast, totality runs from 7:11 to 8:23 p.m. PDT and begins with the darkened lunar disk low in the eastern sky in deep evening twilight. The partial eclipse begins at 9:07 p.m. EDT and ends at 12:27 a.m. EDT on the 28th. (Early phases of the eclipse occur before moonrise in western North America.)
Unlike the barely total event in April, this time the Moon plunges deeply into the southern half of the umbra. Consequently, the upper (northern) half of the lunar disk should be much darker than the lower half. The Moon is also at perigee, so the lunar disk appears about 13% wider than it did in April.
By the way, if you want to do more than simply gaze at the eclipsed Moon, several useful observing projects can enhance your experience.
Looking ahead, 2016 will again feature only four eclipses. But next year the emphasis switches: there'll be a total solar eclipse on March 9th (visible from Indonesia) and an annular eclipse on September 1st. The only lunar eclipses are penumbral, meaning the Moon grazes through Earth's outer shadow and never really gets dark, on March 23rd and September 16th.
An anomalous “cloud” imaged by amateurs in 2012 has puzzled astronomers, spurring some to suggest it was at inexplicably high altitudes above Mars’s surface.
A paper published February 16th in Nature chronicles an unusual cloud seen for several weeks at sunrise on Mars. Beginning on March 12, 2012, amateur planetary photographers reported the small “protrusion” along the morning terminator line in the Martian southern hemisphere, within the Terra Cimmeria region at roughly -45° latitude, 195°W longitude. It was visible for only a short period, lasting roughly 50 to 70 minutes, and disappeared once the Sun rose high enough to fully illuminate the landscape. The feature became more prominent over the following days, varying in size and shape each day, and remained through much of April 2012.
This “plume” was not detected by the MARCI instrument aboard the Mars Reconnaissance Orbiter, which records the planet’s surface and thin atmosphere in imaging strips much later in the Martian day, when the feature is not visible. In fact, observations were reported exclusively from amateurs using telescopes of 8- to 16-inch apertures and high-speed video cameras for 11 consecutive days from March 12th to the 23rd, and also from April 6th through the 16th. In all, 18 individual imagers detected the repeating cloud.
Agustín Sánchez-Lavega (Universidad del País Vasco and Unidad Asociada Grupo Ciencias Planetarias, Spain) and colleagues measured the brightness of these features using amateur images recorded through red, green, and blue color filters, and determined them to be brightest in shorter (blue) wavelengths. They used the sharpest amateur images to estimate the height of these plumes, producing a rough measurement of 200 to 250 kilometers above the planet’s surface.
This estimate is what has caught so many people’s attention: clouds should not exist this high in Mars’s atmosphere. Clouds and dust aerosols are all over the place up to about 50 km, with water vapor clouds being especially prominent when Mars is farthest from the Sun, as it was during these 2012 observations, says Todd Clancy (Space Science Institute). Smaller carbon dioxide clouds and hazes (both invisible from Earth because they’re so small and faint) also exist up to about 70 km (for the clouds) and 120 km (hazes). Above about 120 km from the surface, physical conditions in the atmosphere simply shouldn’t allow dust or ice aerosols to exist.
The authors acknowledge the surprising altitude and suggest that either water or CO2 ice clouds or possibly even aurorae activity could be responsible for these transient phenomena. The area of this event is known to have a strong magnetic anomaly, an ancient magnetic field frozen in the crust that has previously spawned aurorae at about 130 km up. Yet given the brightness of this plume, the aurora would have to be 1,000 times more intense than any seen on Earth to be visible in amateur instruments.
Clouds on Mars
While the 2012 observations are unusual and noteworthy, a survey of historical observations show they are not unique. Images taken by NASA’s Hubble Space Telescope record the occasional presence of clouds extending beyond the morning terminator on multiple instances in 1995, 1997, and 1999, though with the exception of the 1997 observation, these were typically less prominent than those captured by amateurs. Additionally, a similar protrusion was captured in roughly the same location on November 8, 2003, by Japanese imager Isao Miyazaki.
Going further back, a number of well-respected astronomers have reported visual observations of a similar nature in 1890, 1909, 1924, and 1929, including E. M. Antoniadi.
Taken together, these observations are certainly tantalizing. But the underlying topography of Mars might offer an explanation for the latest plumes. Given that Mars is relatively small, topography and meteorology would affect the shape of the night-day terminator line. If the land beneath the clouds is higher than the area at the sunrise terminator, the clouds might be illuminated well before the expected sunrise. The effect would make them seem higher than they are. The repeating nature of this 2012 event could then simply be a combination of the Sun’s position in the Martian sky as it enters the winter season in the southern hemisphere and illuminates thin morning clouds, which could arise for a few weeks as the season progresses. This doesn’t require the clouds to be at an unusual height at all.
Additionally, we need to consider the impact of amateur planetary photography techniques. Amateur planetary images often require significant sharpening, depending on the steadiness of the local atmosphere where the image is taken. One well-known artifact of sharpening is limb enhancement (explained here). Features along a high-contrast boundary (such as the edge between the planet and space beyond) are often bloated and exaggerated, depending on how aggressively the photographer applies sharpening. This effect is reduced on the sunrise or sunset terminator, where there is a gradual darkening towards the night side of the planet. Additionally, features along the limb of a planet will be blurred and expanded when stacking hundreds or thousands of individual frames taken in sub-par conditions, which can significantly reduce the accuracy of size measurements of features near a planet’s limb even before sharpening is applied. Among the images the authors used, those from imagers who reported excellent atmospheric conditions show the “plume” smaller and less pronounced than the others.
Note that images captured by the Hubble Space Telescope or orbiting spacecraft around Mars don’t require any sharpening at all to reveal high clouds along the edge of the planet, because these spacecraft are not inhibited by Earth’s shimmering atmosphere. This also explains why the Hubble images of such phenomena tend to be much smaller and fainter than those from amateur observations.
Sánchez-Lavega says that his team intensively discussed their image analysis, and that four coauthors with extensive planetary-image experience each independently measured the images. He also says that they looked at the local topography and concluded there were no depressions or other surface features large enough to affect the clouds’ apparent altitude.
Still, while there is no doubt that the feature captured in these amateur images is indeed real, there are still questions about the height measurements made for a feature along the edge of the planet, especially given the techniques required to reveal them. Clearly more research is needed.
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A red dwarf and its brown dwarf companion buzzed through the outer Oort Cloud some 70,000 years ago, around the time when modern humans began migrating from Africa into Eurasia.
Most of space is empty. So in a galaxy bustling with hundreds of billions of stars, there’s too high a separation between them for any physical run-ins. Even close encounters are few and far between.
But studies of a nearby, low-mass star hiding among the confusion of the galaxy’s disk shows that space might be a little less empty than previously thought.
A year ago, astronomer Eric Mamajek (University of Rochester) heard about a faint star, while chatting with his colleague. This star, nicknamed Scholz’s star, sparked his interest: it was close — only 20 light-years away — yet its proper motion was surprisingly slow, meaning that it inched sluggishly across the sky.
The latter doesn’t mean that the star isn’t moving, but that much of its movement is hidden in its radial velocity, the motion along our line of sight and into the plane of the sky. It became clear that the star had recently passed close to the Solar System and was now moving rapidly away.
Putting the star’s approximate distance and velocity into a “toy code,” Mamajek had a rough answer within 20 minutes: the star had almost certainly sped near the Sun tens of thousands of years ago.
To calculate the star’s trajectory more precisely, and to see just how close it had come, Mamajek needed data on the star’s current position and its motion, both along and into the plane of the sky. A team led by Adam Burgasser (University of California, San Diego) gathered the necessary data.
The star’s proper motion along the plane of the sky can only be measured by waiting long enough for the star to change its position appreciably. Luckily, images as far back as a photographic plate from 1955 had serendipitously captured the star. Between 1955 and 2014, the star had moved roughly 6 arcseconds. (For comparison, your little finger held up to the sky covers a full degree, or 60 arcseconds.)
Burgasser’s team measured the star’s parallax — that tiny back-and-forth motion we see as Earth moves from one end of its orbit to the other — to give the star’s current distance. And spectroscopy showed the slight Doppler shift in the star’s spectral lines as it moves away from us, providing the star’s radial velocity.
Most surprisingly, Burgasser’s team showed that Scholz’s star, a red M-class star, actually has a smaller brown dwarf companion.
With all the pieces of the puzzle, Mamajek and his colleagues were able to trace all the possible paths Scholz’s star may have taken. The team simulated 10,000 orbits for the star to take into account the uncertainties in the star’s position, distance and velocity, as well as the effect of Milky Way’s gravitational field.
Of all those simulations, 98 percent show that the star had passed through the outer Oort Cloud. Its closest approach was probably between 0.6 and 1.2 light-years away, when it scraped the Oort Cloud 70,000 years ago at 83 kilometers per second.
Until now, the top candidate for the closest flyby had been the so-called “rogue star” HIP 85605, discovered by Coryn Bailer-Jones (Max Planck Institute of Astronomy) in a study that analyzed the trajectories of 50,000 nearby stars. That star was predicted to pass 0.13 to 0.65 light-years from our Sun in 240,000 to 470,000 years.
Mamajek and his colleagues, however, demonstrated that the original distance to HIP 85605 was likely underestimated by a factor of ten. At its more likely distance, its newly calculated trajectory would not bring it within the Oort Cloud at all.
Bailer-Jones agrees with the team’s assessment of the rogue star. But he also warns that even though Scholz’s star currently holds the record, it doesn’t hold it by much. A second star, known as Gliese 710, has a more precisely calculated trajectory that shows it flying by almost as close as Scholz’s star. Both close approaches come within each other’s uncertainties.
Nonetheless, the discovery of another close-pass star proves an interesting point. “This is by no means a statistical survey,” says Mamajek. But, he continues, it’s an example of what are likely many more undiscovered nearby stars, whose trajectories might bring them close to the Sun.
The European Space Agency recently launched the Gaia satellite to map out the distances and velocities of billions of stars, bringing low mass stars into focus.
Close encounters could perturb comets in the Oort cloud, shaking them up and sending them our way. “But there is no need to worry,” says coauthor Henri Boffin (European Space Observatory). “Even if the Oort cloud was perturbed, it takes millions of years for a comet in the cloud to reach the Earth.”
Adam Burgasser et al. " WISE J072003.20-084651.2: An Old and Active M9.5 + T5 Spectral Binary 6 pc from the Sun." Astrophysical Journal. February 19, 2015.
Eric Mamajek et al. “The Closest Known Flyby of a Star to the Solar System.” Astrophysical Journal Letters. February 12, 2015.