Weekend “Black Hole” full Moon brightest and closest of the year

Watch for a bigger and brighter full moon than usual tomorrow and Sunday nights. Photo illustration: Bob King

Wishing you a grand first day of summer today and clear nights ahead. This weekend we’ll see the full moon dangling like heavy fruit low in the southern sky. If it looks a tad bigger than last month’s Full Flower Moon, it really is. The moon reaches perigee – closest point to Earth in its orbit – at 6:11 a.m. CDT Sunday morning June 23 just a half hour before the moment of full moon. At the same time it will hover in front of the Milky Way’s central supermassive black hole.

With all those superlatives, you might think I’m setting you up for the end of the world. Trust me, I’m not. The full moon is near the same spot every June and perigees are almost as common as dandelions.

Like the planets do around the sun, the moon moves in an elliptical orbit around the Earth. Once a month it’s closest to us at perigee and farthest away at apogee. Next apogee will be at new moon on July 7. The moon’s average distance from Earth is 238,856 miles (384403 km).

The moon follows an elliptical path around the Earth with one side of its orbit some 31,000 miles (50,000 km) closer (perigee) to our planet than the other side (apogee). Since the moon orbits the Earth every 27 days it reaches perigee once or sometimes twice a month.

So what makes this one special? Well, not all perigees are alike. Some are closer than others and occur at times other than full moon. The closest perigees, like this Sunday’s, occur when the moon is either full or new – times when Earth, moon and sun are all lined up in a row. The sun’s gravity tugs more strongly on the moon at these phases, stretching its orbit and leading to extreme values for perigee and apogee.

No one notices an unusually large first quarter or crescent moon, but we all sit up and pay attention to a bigger-than-normal full moon.

The difference between the the moon at its most distant apogee and closest perigee dramatically illustrated in this pair of photos taken in 2006. The full moon can be as much as 14% bigger and 30% brighter when closest. The weekend’s moon will be 12% bigger than January 2014’s apogee full moon. Credit: Anthony Ayiomamitis

The moon’s most distant perigee (230,000 miles) happened at last quarter phase on March 5 this year. Sunday’s will be the closest of the year at just 222,000 miles (357,000 km). Some keen-eyed skywatchers might notice the moon looking a little larger than it will at apogee on Jan 14, 2014. I say might. Without a reference, it’s terribly hard to compare sizes. The moon illusion, an apparent bloating of our satellite when seen low in the sky, further complicates the view. Still, facts are facts. The moon will be bigger and brighter this weekend than on any other night this year.

Saturday night’s full “Black Hole Moon” will lie about one fist held at arm’s length left and above the center of Milky Way galaxy.  Astronomers call the spot Sagittarius A* (Sagittarius A star). Blocked by intervening dust and invisible to optical telescopes, it marks the site of a supermassive black hole. Created with Stellarium

Not only does the closest perigee of the year coincide with Full Moon, but the moon will be in the “Teapot” constellation Sagittarius in approximately the same direction as our galaxy’s 4-million-mass black hole. Look toward the moon and you can imagine it there munching its way on gas clouds and stars that pass that pass too close for comfort. Given that the beast is 26,000 light years (156 quadrillion miles) from us, neither moon nor monster have any gravitational inkling of the other.

Since full phase happens early Sunday morning, the moon will appear full to the eye both Saturday and Sunday nights. I hope you’ll get to enjoy the show.

Aurora watch for northern U.S. tonight June 20-21

Photo taken June 20 showing a large coronal hole (dark patch) in the sun’s northern hemisphere and the one likely responsible for tonight’s increased aurora chances. Notice that the sun’s magnetism and wind of particles is constrained in loops over sunspot groups but flows freely from the hole. These occasional high speed winds unbound by solar magnetism fly past Earth and can spark auroras. Credit: NASA

If you live along northern border of the U.S. and points north, there’s a 25% chance you’ll see a minor auroral storm tonight through tomorrow morning. Chances are 70% for a major storm at high latitudes across Canada and Alaska.

I suspect I’d see a glow along the northern horizon here in Duluth, Minn. right now (11 p.m.) if the sky were clear. Sadly, we’re socked in. A speedy blast of solar wind pouring from an opening in the sun’s magnetic field called a coronal hole is responsible for the enhanced activity. Take a look outside before going to bed tonight just in case.

Minnesota couple finds 33-pound space rock in their cornfield

Bruce and Nelva Lilienthal with their new-found iron meteorite. It was found on their farm in Arlington, Minnesota about an hour southwest of Minneapolis. Credit: Sheri Alexander.

Bruce Lilienthal has picked a lot of rocks from his cornfield in southern Minnesota but none like the 33-pound (15 kg) rusty slab he recently saw poking out of the soil. The odd, flat rock not only caught his eye but was unusually heavy for its size.

Bruce suspected it might be a meteorite so he called an expert with a description and then got a hold of Dr. Calvin Alexander, Earth Sciences professor and curator of meteorites at the University of Minnesota. Alexander drove out to the farm to visit Bruce and his wife Nelva.

The meteorite measures 16 x 11 inches (41 cm x 28 cm) and 1.6 inches (4 cm) at its thickest. Credit: Bruce and Nelva Lilienthal

“It has a very unusual shape,” said Alexander. The Lilienthals allowed him to remove a 0.6-gram sample in four small pieces. Back at the university, Alexander placed a fragment in an electron microprobe, a specialized instrument that determines the chemical makeup of a substance by bombarding it with beams of electrons. When excited by the little buggers, each element emits X-rays of a particular energy with a unique fingerprint.

The Widmanstatten pattern of interlocking nickel-iron crystals in a sliced and acid-etched sample of the Seymchan meteorite. The pattern is unique to iron meteorites. Credit: Wikipedia

The professor’s eyes must have lit up when he saw the results – the crumbs contained 8 percent nickel, an element rare in Earth rocks but common in meteorites and frequently used to tell the two apart.

Not only that, but the specimen flashed the telltale criss-cross pattern of interwoven iron-nickel crystals called the Widmanstatten patternunique to iron meteorites.

But what about that shape – why so flat? In the early solar system, nonstop meteorite impacts and heat from the decay of radioactive elements melted the larger asteroids, causing heavier materials like iron to sink to the core and lighter rocks to float to the surface and eventually harden into crust. Most iron meteorites originate in the cores of asteroids torn asunder by impacts from other asteroids. Not this one.

“It didn’t form in the interior,” said Alexander. “The object’s unusual shape indicates it probably formed as a thin layer or pool of melted surface rock created in an asteroid collision.”

A small slice of the 1894 Arlington meteorite showing iron-nickel crystals. Credit: Don Edwards

In the video below, Nelva describes two other meteorites found three miles either side of their cornfield. Indeed, in 1894 a farmer plowed up a similar flat stone weighing 19.7 pounds (8.9 kg) named Arlington and classified as a rare II-E anomalous iron meteorite. Translation: it probably formed as melted surface rock instead of inside the asteroid’s core. The bulk of this rock forms part of the University of Minnesota collection currently curated by the Smithsonian.

“I’m 99 percent sure it’s the same as Arlington,” said Alexander, who hopes to acquire the piece for the university’s collection and put it on display. If it does turn out to be one and the same, it will likely be named ‘Arlington II’.

KSTP-TV report on finding the Lilienthal space rock

For now, Bruce and Nelva haven’t  decided exactly what to do with their new-found rock from space. Wherever it ends up, they can say they’ve cradled a rock from the asteroid belt as old as the solar system itself thanks to their cornfield’s cosmic connections.

Explore this amazing, interactive 1.3 billion-pixel view of Mars

Click on the photo and you’ll be taken to the billion-pixel interactive photo. Take a spin around or burrow in for closeup views of rocks, the rover and other details. Don’t forget to click the “full screen” view when you’re there. Credit: NASA

Wanna go to Mars? NASA just released a wonderful way to get there. With a few clicks of the mouse, you can zoom into a 360-degree scene comprised of nearly 900 pictures taken by Curiosity Rover from the Rocknest site. This is where the rover gathered and examined its first scoops of Martian soil.

Extreme zooming into the mosaic reveals multiple rock layers in Mt. Sharp. Credit: NASA

The picture is a mosaic of 850 telephoto frames and 21 wide angle views taken with the high-resolution, mast-mounted cameras. An additional 25 black-and-white pictures, mostly of the rover itself, were shot with the Navigation Camera. Put it on your screen and enjoy almost being there.

Curious white object found while zooming in to the right of a large rock below the right side of Mt. Sharp, the peak dominating the distant horizon. Credit: NASA

All the photos were taken between Oct. 5 and Nov. 16, 2012. You’ll notice different levels of atmospheric clarity depending on the day the images were shot. Have fun. If you find any little green men, do let us know.

Celebrate summer’s start ’round about midnight Friday

A radiant sun shines through a cluster of Norway pine needles. This Thursday-Friday marks the summer solstice or first day of summer.  The season begins at 1:04 Eastern time June 21 and 10:04 p.m. Pacific  June 20. Credit: Bob King

“Summer afternoon—summer afternoon; to me those have always been the two most beautiful words in the English language.”  – Henry James

So it’s always seemed to me at the start of a summer vacation. Endless time laps ahead like a wave that never breaks. Those splendid hot stillnesses are returning. Come Friday June 21 at 12:04 a.m. CDT summer tiptoes through the dark to quietly unseat spring; the next three months belong to the high sun, iced drinks and late evening light.

Whichever end of Earth’s axis points toward the sun, it’s summer in that hemisphere. In June, the north polar axis tilts that direction and we experience summer (left). When it points away, it’s winter. At the fall and spring equinoxes, the planet is tilted neither toward nor away and day and night are equal. Credit: Tau Olunga

That’s in the northern hemisphere of course. Way down south it’s the first day of winter and the sun is never lower in the sky than on June 21. Here in the north, the sun beams from its highest point in the sky. Since it spends a great deal of time climbing to this lofty perch and an equally long time descending, summer days are exceptionally long. Daylight squeezes night into a narrow slot fewer than 9 hours long.  With darkness beginning after 10 o’clock, skywatchers are forced to choose between sleep and stars.

A mosquito from early Miocene times (~ 20 million years ago) frozen in time in Dominican amber. Credit: Didier Desouens

If the choice is stars, you’ll be sharing it with tiny, whining friends of the night. Mosquitos have been around for millions of years; our most distant human ancestors slapped and batted them away just like you and I do every time we look up in wonderment without protection on a pleasant June evening. But there are fireflies too and owls and frogs about, making a clear summer night as much a sonic experience as a visual feast.

All this summer stuff happens for one reason – the tilt of Earth’s axis. Simple as that. No need to bring in the experts, no special app required. Earth circles the sun tilted 23.5 degrees from vertical. Every June 20 or 21 the northern hemisphere points toward the sun, causing it to appear high in the sky. Not only do the days reach their maximum length, the sun’s high angle means the energy per unit area it pours over Earth’s surface is more than twice as intense as during the winter.

Six months later the north tilts away from the sun. A low sun and less intense surface heating means wintry consequences.

Male fireflies flash as they fly over the ground looking for a mate on a June night. Credit: Bob King

Spring and autumn fall between winter and summer extremes with Earth broadside to the sun and neither axis tilted toward or away. Day and night briefly agree to share the clock equally before charging off to the next season.

So yes, I’m ready for summer. Bring on the sweet smells of morning air, those endless afternoons and nights of fireflies tearing across the sky like biological meteors.

Saturn-moon engagement plus a peculiar star worth watching

Look south this evening and you’ll see Saturn and Spica on either side of the moon. Farther down in the southeast, Antares sparkles with a reddish hue. Our featured star Delta is just in front. Created with Stellarium

Tonight look to the south and you’ll see the waxing gibbous moon paired up with two bright “stars”. One’s a real star – Spica in Virgo – and the other is the planet Saturn. If you let your gaze slide further south and east you’ll soon run into a third luminary – the red-orange heart of the Scorpius Antares (an-TAR-eez).

While Antares is one of the sky’s most interesting red supergiant stars, we’re going to turn our attention instead the three stars to its west that forms the head of Scorpius and in particular to the middle star Delta Scorpii. Delta is in the throes of a tantrum that’s lasted more than a dozen years.

The hot subgiant star Achernar, very similar to Delta Scopii, is flattened into an oval shape by its extreme rotation. Credit: Wikipedia

Back in June 2000, this unassuming 2nd magnitude star quite suddenly began to brighten until by 2001 and 2002 it had nudged up almost to 1st magnitude, the class which includes includes the brightest. It peaked in 2003-2004 at magnitude 1.6. Delta’s since faded back to about 2.0 magnitude (I checked two nights ago) which is still a third brighter than normal.

What could cause such an outburst? Astronomers think it may have to do with how fast Delta spins. Hot giant stars of its ilk rotate so rapidly – at least 155 miles per second vs. the sun’s 1.2 mile per second – they sometimes fling hot, luminous gaseous masses from their equators like a plump clown tossing candy at a parade.

The hydrogen gas forms a flattened bright disk around the star causing a temporary brightening. Complicating matters, Delta has a very close companion star that circles it every 10.8 years. Searching back through earlier data, astronomers found that a similar though weaker outburst occurred in 1990-91 a short time after the smaller star passed closest to Delta as it did again in 2000.

Every clear night I look to the southern sky to see what Delta’s up to. It’s so easy to do. If you’d like to try it yourself, compare Delta to nearby Beta Scorpii (magnitude 2.6) and Spica (mag. 1.0). If it’s exactly between the two, its magnitude is 1.8. For the moment, our featured star has yet to return to its original brightness, so you never know what’s next. That’s the fun of it of course.

Galactic cannibals devour hapless neighbors, don’t pick up after themselves

NGC 5907, also known as the Splinter Galaxy, is surrounded by loop de loops of stars and dust either ripped from a companion galaxy in an act of galactic cannibalism or spewed when two galaxies merged. Click to supersize. Credit: Jay Gabany,  Blackbird Observatory

Four billion years ago and 40 million light years away, an act of galactic cannibalism was committed. No human eyes saw the smaller galaxy shredded and ripped apart by the gravitational might of the larger. No tears were shed when its remains were finally devoured, but clues of the catastrophe remain to this day.

The Splinter Galaxy, an 11th magnitude edge-on spiral, is located high in the June sky in the constellation Draco. You can star-hop to it with your telescope using the Big and Little Dippers and the detailed map below. Edasich is also known as Iota Draconis. Maps made with Stellarium

High in the northern sky in the constellation Draco the Dragon ghostly ribbons of stars and dust swirling about the edge-on spiral galaxy NGC 5907 are all that remains of its one-time companion. Don’t expect to see those ethereal streams in a typical telescope; only long time-exposures reveal the remaining clots of dust and streams of stars torn from the companion galaxy during the close encounter and likely merger. The gigantic loops extend for more than 150,000 light years from NGC 5907, nicknamed the Knife-Edge or Splinter Galaxy.

In this closeup view, NGC 5907 is just a short star-hop from Edasich. Field of view is about 3 degrees.

Astronomers call these rivers of stars tidal streams. They’re created by disruptive gravitational tides induced when two galaxies pass near or through one another. As the companion orbited through the Splinter’s disk, repeated encounters stripped it of most of its goodies – stars, dust clouds and even dark matter – and flung the material into multiple tidal streams. When nothing but the compact stellar core was left it presumably merged (was eaten) by NGC 5907 and lost its individuality forever.

This illustration shows a stage in the predicted merger between our galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. The image represents Earth’s night sky in 3.75 billion years. Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull. After some 6 billion years the two will merge into a single elliptical galaxy. Click for more details. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

Or … it could have happened a different way when two equally mighty galaxies on a collision course yanked swirls of stellar debris from each other before their eventual merger into one serene spiral we see today. Here neither was the winner – two came together much as the Milky Way and Andromeda Galaxy will several billion years from now to meld into something altogether new.

Tidal streams of stars torn by gravitational tides exerted by the Milky Way on one of its small satellite galaxies the Sagittarius Dwarf. The dwarf’s core is shown to the right of  our galaxy’s disk. Credit: David R. Law, UCLA

As far as galactic cannibalism goes, we needn’t look any farther than our own Milky Way, which has been munching on the Sagittarius Dwarf Galaxy for at least the past billion years. Four separate whirls of stars peeling off above and below the plane of the Milky Way attest to our galaxy’s systematic preying on what was once a bright companion. Now half its contents are gone, dumped along the winding highway by an unrepentant galactic litter bug. Over time the dwarf and its contents will be fully absorbed by our galaxy with little left of its presence save a few stray globular clusters that once called it home.

Meet the James Webb Space Telescope, time machine extraordinaire

The James Webb Space Telescope, which will launch in October 2018, has a large, segmented mirror 255 inches across. A sunshield protects the telescope from heat so it can study the cosmos in infrared light. Credit: NASA

Remember when the 200-inch Hale Telescope at Mt. Palomar in California was the biggest in the world? It’s now surpassed by at least 18 other scopes, the largest of which is the Gran Telescopio Canarias with a mirror 410 inches in diameter. The bigger the mirror, the greater its light-gathering ability and farther we can see across the universe.

From the semi-stable L2 Langrangian point a million miles from Earth opposite the sun, the Webb can both block the sun, Earth and moon from view as well as study deep space 24/7. Credit: NASA

Soon we’ll recall when the Hubble with its 94-inch mirror was the biggest orbiting telescope.

In October 2018, NASA plans to launch the James Webb Space Telescope (JWST) with a behemoth 255-inch (6.5-m) mirror coated in gold.

The Webb will set up stakes one million miles from Earth at the L2 Lagrangian Point, a region of space where the sun’s and Earth’s gravities strike a balance, allowing a spacecraft there to “hover” indefinitely with only an occasional firing of its thrusters to maintain position.

James Webb Credit: NASA

Named after the NASA administrator James Webb, best known for his leadership in the Apollo moon program, it will be the most powerful space telescope ever built. It
will observe the most distant objects in the universe, including the very first galaxies and search for clues left behind by the earliest stars.

Closer to home, it will examine planets in our solar system as well as planets around nearby stars. The telescope will be able to determine the composition of an exoplanet’s atmosphere by studying the light of its host star filtering through the alien air. The Webb telescope is a joint project of NASA, the European Space Agency and the Canadian Space Agency.

NASA engineer Ernie Wright looks on as the first six flight ready James Webb Space Telescope’s gold-coated primary mirror segments are prepped to begin final cryogenic testing at NASA’s Marshall Space Flight Center. Credit: NASA’s MFSC/David Higginbotham

Its unique mirror, made of the lightweight metal beryllium and coated with a golf-ball’s worth of gold spread into an ultra-thin layer across 18-hexagonal mirror segments, is optimized for infrared light. That’s the invisible light just beyond the red of the rainbow that we sense as heat.

Light from celestial objects like galaxies receding rapidly from Earth is stretched and reddened. Light from objects approaching Earth appears bluer.

Why infrared? Our expanding universe got its start in the Big Bang. Because light takes time to travel to our eyes from distant regions of the universe, we also peer back into time when we look into space. Since the universe is expanding, the farther back we look, the faster objects appear to be moving away from us. Like the sound of an ambulance siren dropping in pitch as it races to the hospital, light from a distant star or galaxy “drops in pitch” as it recedes from Earth, becoming redder in color. Astronomers say the star’s light is redshifted.

Since the Webb’s primary mission is to discover the farthest objects to light up the early universe, they’ll also be the ones receding most quickly. Light that left the earliest galaxies started out as visible and ultraviolet but has been redshifted by the expansion of the universe into the near and mid-infrared range, beyond the reach of the human eye and most telescopes.

Earth’s atmosphere happily lets visible light – colors of the rainbow – and radio waves pass to the ground but blocks most of the infrared, ultraviolet, X-rays and gamma rays.

Unfortunately we see little of that light from the ground. Our atmosphere acts as a barricade to much of the infrared beaming from space. The only way to sample this crucial slice of light is to loft a telescope above the atmosphere into space.

Gold is used as a mirror coating instead of the more typical aluminum because gold is an excellent reflector of yellow, red and infrared light. Think about why gold is golden-colored in the first place – it absorbs blue and green light and reflects that delicious buttery yellow back to our eyes.

If the Hubble Space Telescope’s 94-inch (2.4 m) mirror were scaled to be large enough for Webb, it would be too heavy to launch into orbit. The Webb team had to find new ways to build the mirror so that it would be light enough – only one-tenth of the mass of Hubble’s mirror per unit area – yet very strong. The sides of the mirror also fold back like leaves on a table for a compact fit in a rocket. Credit: NASA

Not only does infrared vision help astronomers see back to the universe’s teething years, it also penetrates dust to see otherwise hidden stars and planets cloaked in their dusty birth cocoons. The Webb will spy stars 10 to 100 times fainter than the Hubble Space Telescope. Click HERE for a nice summary of the mission’s primary science goals.

Because all things radiate some amount of heat or infrared light, including the telescope itself, everything must be kept very cold otherwise pictures would look like fogged film. That’s why a large, five-layered sunshield will be deployed toward the sun, blocking both visible and infrared light from the sun, Earth and moon that would otherwise heat up the telescope. Looking like a hi-tech Viennese layer cake, the vacuum of space between each layer serves as fabulous insulation.

Shielded this way, the Webb’s operating temperature will drop to a nippy -370 F (-223 C) or just 50 degrees above absolute zero. From the L2 vantage point described earlier, all three objects are almost in a straight line behind the space telescope and straightforward to block with the sunshield.

Group photo of the Webb Telescope team with a full-scale model of the James Webb Space Telescope at the Goddard Space Flight Center in Maryland. Credit: NASA

Getting a telescope with 255 inches (21.3 feet) of mirror into space means designing the craft and optics to fold up into a compact package that resembles a backpack. Once in orbit, the Webb will be carefully unfolded and tested before observations begin. Electricity generated by solar cells will provide the power needed to run this magnificent machine.

Delays and cost overruns have been part of the project to build the telescope, but work continues and a launch window has been set. The thought of looking back to the time when the universe’s lights first turned on not only gives me the chills but makes it worth the few extra bucks.

Big Dipper sure, but have you seen the Great Bear?

The Great Bear Ursa Major stands high in the northwestern sky at the end of evening twilight this month and into July. The moon is shown for tonight June 16. Created with Stellarium

The Big Dipper is the #1 most familiar star group in the northern hemisphere. Just about everyone has seen it. Orion comes in second and everything else a distant third. As many of you already know, the Dipper is only part of a constellation, what astronomers call an asterism. If you connect the rest of the dots you’ll make a bear up there by the name of Ursa Major. The Great Bear.

June and and the first half of July are good times to take a few minutes at the end of dusk and see if you can go beyond the familiar Dipper outline.

Mythological depiction of Ursa Major on the 19th century Urania’s Mirror atlas.

While none of Ursa Major’s additional stars are as bright as the Dipper, they form distinct shapes, especially the long, furry legs and two-toed claws, although in the sky the whole works appears rather bony. The head is triangular and with a little imagination bears some resemblance to a bear’s. What you’ve always pictured as a dipper bowl is repurposed as back and belly, and the handle is an unnaturally long but still very believable tail.

The seven brightest stars of Ursa Major form the familiar Big Dipper. Many civilizations past and present recognized the form and often pictured it as a bear. Each star has its own name, all derived from Arabic.  The star in the bend of the handle is a true double star. Credit: Bob King

I’ve always been impressed with the large size of the Ursa Major constellation. In that regard it’s most like a real bear with a commanding physical presence. Checking a list of constellation size by area, I see that the Great Bear takes 3rd place behind Virgo and Hydra with 1,280 square degrees of heavenly territory. Good thing it isn’t trapped in its cage. Earth’s rotation day and night ensures our ursine friend gets plenty of exercise circling round the polestar Polaris.

Evening moon, popular planets and extreme sports on Mars

Face west-northwest tonight to see the moon near the star Regulus as well as a tight group of four bright sky objects – two stars an two planets. Created with Stellarium

The ambling moon is one day shy of first quarter phase tonight and lights up the sky near the star Regulus in Leo the Lion. Closer to the horizon, Venus and Mercury couple up with Gemini’s brightest stars Pollux and Castor, with bright Capella glimmering alone in the north.

Mercury and Venus join up for a conjunction (close pairing) on the 19th and 20th, while the moon passes near Saturn on June 18-19. Mars and Jupiter are both too close to the sun to see, but will soon return to morning twilight in the next several weeks.

A recent image from the Mars Odyssey spacecraft showing dark-bordered streaks caused by winds blowing around the dual craters’ walls. The dark areas are scoured of surface dust; the light zones are where the winds deposited their load of dust after being braked by the craters’ walls. Credit: NASA/JPL/ASU

Speaking of Mars, I came across some great images recently of wind streaks and dry ice “snowboard” trails on the Red Planet. Wind streaks can appear either dark or light-colored on Mars. When strong winds converge around craters and cliffs they can sweep away the lighter surface dust exposing the darker lava plains beneath. Craters can also slow down the winds, causing them to drop their loads of dust as light-colored streaks on the obstacle’s lee side. Sometimes both happen at the same time as in the photo above.

Mars Reconnaissance Orbiter photo of “linear gullies,” which may be explained by slabs of dry ice gliding down the slopes of sand dunes.  Different in form from other streaks and gullies on Mars, they can extend up to a mile (2 km) and end abruptly in pits.  Scale in meters at left. Click to enlarge. Credit: NASA/JPL-Caltech/Univ. of Arizona

While wind streaks make sense because of their earthly analogs, dry ice chunks gliding down the slopes of sand dunes on cushions of their own vaporizing gas sounds distinctly more alien. Yet that’s what NASA researchers believe is happening to create the zillions of narrow furrows seen along the slopes of some Martian sand dunes.

“I have always dreamed of going to Mars,” said Serina Diniega, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and lead author of a report published online by the journal Icarus. “Now I dream of snowboarding down a Martian sand dune on a block of dry ice.”

Dry ice gliding on sand dunes 

Carbon dioxide frost coats the dunes during the Martian winter which lasts about twice as long one on Earth. Over time, the ice accumulates and gets compressed into slabs which can break off and glide downhill during the spring season. As frozen carbon dioxide (dry ice) changes directly from a solid to a gas on contact with the warmer sand, the gas pushes against the surface to create a cushion of air. The block rides the cushion all the way to the bottom where it continues to vaporize, forming a little pit at the end of the gully. Be sure to watch the short video – I think you’ll be delighted at the experiment using dry ice on sand dunes here on Earth.