Tag Archives: Earth

Planetary perspectives inspire appreciation for the little things

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Venus is the bright “star” seen among Saturn’s rings in the photo taken by Cassini late last year. The bright arc is Saturn’s atmosphere glowing in backlit sunlight. Light reflected from the rings faintly illuminates the planet. Click to enlarge. Credit: NASA/JPL-CalTech

Nothing beats stepping back to gain a little perspective. NASA recently released a pretty picture of Venus peering through Saturn’s rings. The photo was taken on Nov. 10, 2012 when the Cassini spacecraft orbited the shadowed side of the ringed planet, so we see the rings and atmospheric edge of Saturn backlit by the sun. Venus is a pinprick of light nearly a billion miles away shining through the veil of icy bits that compose the rings.

Venus as seen this morning March 6, 2013 by SOHO’s camera. An opaque disk covers the sun (white circle) allowing astronomers to study the streaky solar atmosphere called the corona.  Venus is currently about 159 million miles from Earth. Credit: NASA/ESA

We also have a more recent photo of Venus taken by the Solar and Heliospheric  Observatory (SOHO) from a different point of view. This picture was taken earlier this morning and shows Venus nearby southwest of the sun. SOHO is parked near the L1 Lagrange point, a spot in space 1 million miles forward of Earth in the direction of the sun. Here the planet’s gravity balances that of the more distant sun allowing the craft to hover in equilibrium with its eye ever focused on the sun. Due to gravitational tugs from the moon and planets, SOHO fires its thrusters every few months to remain in position.

As Venus revolves around the sun, we see it pass through phases just like the moon. Today Venus is near the sun in the sky and appears like a nearly full moon. On the 28th it will be in conjunction and farthest from Earth on the opposite side of the sun. Illustration: Bob King

Venus might look like it’s in the foreground in the SOHO image, but it’s really in the background. On March 28 the planet will pass through superior conjunction when it will appear closest to the sun but located on the farside of its orbit behind it. After that date Venus begins its slow trek back into the evening sky as it comes round to the left or east of the sun. Watch for it to re-appear at dusk in late May.

Venus and Earth are nearly invisible in this wide angle view that includes the sun taken by the Voyager 1 spacecraft in 1990. Click to see the BIG version. Credit: NASA

A tight crop of the wide Voyager 1 photo clearly shows the pale blue dot of Earth. It’s caught in a streak of lens flare caused by the camera pointing directly at the sun. Everything we care about most deeply is contained in that minute fleck of light. Credit: NASA

Let’s pull back a bit more. What do Venus (and Earth) look like from 4 billion miles away, the way the Voyager 1 space probe saw them on Valentine’s Day 1990? Dots of course! Take a close look and you just might be able to see them in the photo. They’re much more obvious the full-resolution image, which can be had for a click.

Earth and the moon from 114 million miles away in the vicinity of Mercury photographed in 2010 by MESSENGER. Click to enlarge. Credit: NASA

Before we wrap up, let’s move in again a little closer and enjoy a picture of the dynamic duo of moon and Earth taken by the Mercury MESSENGER spacecraft from 114 million miles away. While not shot from the planet Mercury, the perspective is nearly identical. Doesn’t it make you feel a little exposed looking at these photos? I mean, there’s so much nothing out there compared to the bits of something. Time to hug my kids again.

Merry Christmas and may your nights sparkle with starlight

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A star in time for Christmas — frost on the window this morning. Photo: Bob King

Merry Christmas and a happy holiday to everyone! I’m grateful your interest in the blog as well as your many comments and observations. Thank you! I hope Santa brought you a gift certificate for a hundred clear nights this coming year.

Enjoy the spirit of the day, and don’t forget to go out and see the spectacular pairing of Jupiter and the moon in the eastern sky tonight. As you relish the sight, consider that the planet we call home spins ceaselessly day and night. Watch it below. 


The Earth spins at night. Pictures taken in infrared and visible light by the Suomi NPP satellite for nine days in April 2012 and 13 days in October 2012 were used to create a complete night map of Earth. It took 312 orbits to get a clear shot of every parcel of Earth’s land surface and islands.

Saturn’s blue autumn

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Northern Minnesota reached peak fall color this past weekend. The view was sumptuous from an overlook on the Superior Hiking Trail near Little Marais, Minn. Photo: Bob King

Evidence of the changing seasons is all around. Leaves aflame with color, frost in the morning, snow pushing in from the west. Many of us feel wistful about seeing summer end but find renewed energy and a fresh point of view as fall sweeps it away.

Saturn’s largest moon Titan passes in front of the planet in a picture taken by the Cassini spacecraft last May. The rings (thin yellow line) casts broad shadows on Saturn’s cloudtops. The arrival of autumn in the southern hemisphere creates the blue cast. Click to enlarge. Credit: NASA/JPL-Caltech/Space Science Institute

I wonder what Saturnians think about their changing seasons? Saturn’s axis tilts 27 degrees, a few more than Earth’s, giving that planet a full round of seasons, too. While autumn on Earth blows by in 3 months, Saturn’s 29-year-long orbit means each season lasts for more than 7 years.

As on Earth, autumn manifests itself on the ringed planet as a change in color. Since there’s no solid surface or possibility for trees, we look to Saturn’s atmosphere for signs of fall and find an delightful autumnal blue.

Ultraviolet light from the sun works on the planet’s atmospheric chemistry to increase the amount of haze. During Saturnian summer, the intensity of UV is greater and the air gets hazier. In winter, UV light drops off and Saturn’s atmosphere clears. Air molecules can now better scatter the visible sunlight and tint the planet’s upper atmosphere blue. Our own sky is blue for the very same reason. Methane, which comprises almost half a percent of Saturn’s air and absorbs red light, further enhances the blue in a clearing atmosphere.

It’s all in the tip. As Earth – and other planets – orbit, first one hemisphere and then the other is oriented toward the sun. This changes the height of the sun in sky which changes the length of the day and drives the seasons.

Earth’s tipped axis is the cause of the seasons. On one side of our orbit, the northern hemisphere is oriented toward the sun, giving us summer; on the other side the hemisphere faces away from the sun, bringing us winter. Any planet with a fair amount of tilt experiences seasons.

Watch for a nice pairing of the moon and Jupiter in the east tonight and tomorrow night. The two will come up around 10 p.m. This map shows the sky around 11 o’clock. Created with Stellarium

Jupiter’s axis is tipped only 3.1 degrees or nearly straight up and down. Sunlight shines equally across the planet during an entire Jupiter year, which is equal to 12 Earth years. While we have to wait a while yet for Saturn to reappear in the dawn sky, Jupiter’s easy to spot. Tonight and tomorrow night the waning gibbous moon will pass very near it in the northeast. Clear skies!

What does Earth look like from Mars?

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Curiosity is inside Gale Crater far to the east of the planet’s most prominent telescopic feature Syrtis Major. Credit: NASA/ESA/ Hubble

Now that Curiosity’s safe and secure on the Red Planet and snapping photos of everything in sight, I hope it focuses its cameras on Earth sometime soon.

The rover sits inside the 96-mile-wide Gale Crater in Mars’ eastern hemisphere just 5.4 degrees south of the equator. I was curious what the sky looks from Curiosity’s location and fired up Stellarium to see.

It didn’t take long to find Earth, low in the northeastern sky in morning twilight not far from the planet Venus.

Seen from Gale Crater on Mars, Earth is a brilliant blue “star” in the constellation Pisces on August 10. Not far below shine the planet Venus. The view shows the sky facing east about 45 minutes before Martian sunrise. Maps created with Stellarium

Since Earth is an “inner planet” from Mars’ perspective, the same way Venus and Mercury are inner planets for us, it never strays too far from the sun and goes through phases just like the moon and other inner planets. Curiosity will see Earth best during morning and evening twilight. At Mars current distance from Earth of 154 million miles, our planet shines at magnitude -1.4 or nearly same as Sirius, the brightest star in the sky.

To the eye, Earth would shine a pale ocean water blue. Venus would still be the brightest planet (magnitude -3.0) but distinctly dimmer than when viewed from Earth, because it’s farther from Mars than it is from our planet.

Earth and moon seen through binoculars from Curiosity’s landing site this morning.

According to my calculation, the moon would be slightly less than one arc minute from Earth and probably not visible as a separate point of light with the naked eye. However, you could easily see it directly below Earth through a pair of binoculars. The two would appear as a beautiful double planet!

The moon is much darker than Earth and would only shine at magnitude 2.5, about the same brightness as one of the Big Dipper stars. Through a small telescope magnifying around 60x Earth would appear as a tiny gibbous moon or a little more than 3/4 full.

More sky wonders await Curiosity’s cameras. Mars’ two moons cycle through the sky just like our moon.

Mars’ moon Phobos joins Earth and Venus shortly before sunrise on the morning of August 12.

Phobos, the larger, is 14 miles wide and orbits only 3,700 miles from Mars’ surface. It’s so close that it moves around Mars faster than the planet rotates. Instead of rising in the east and setting in the west, Phobos rises in the west and sets in the east. Nuts, right? It moves so fast it crosses the entire sky in just four hours and 15 minutes. If you could be there in person, you’d see it move in real time like a very slow satellite.

While Phobos is one of the darkest, least reflective bodies in the solar system, its proximity to the planet means it’s brighter than you’d expect, easily outshining Earth and Venus at magnitude -5 at midmonth. Wait a minute – that’s brighter than Venus is from Earth!

A gorgeous sight for Curiosity’s eyes – Venus, Earth, Phobos and Deimos in morning twilight on August 31. Foreground image shows the Opportunity rover’s solar panels. Sorry, I don’t have Curiosity in my software yet!

The smaller moon Deimos is about 7 miles wide and orbits far enough from the planet to rise in the east and set in the west like our moon does. Things really get fun later this month on the morning of 31st. That’s when Earth, Venus, Phobos and Deimos are all together in the eastern sky before sunrise. Wouldn’t it be cool if NASA pointed one of the high-resolution cameras for an awesome family portrait?

Mars’ north polar axis points toward Deneb and the Northern Cross which are part of the larger Summer Triangle. This view shows the sky from mid-northern latitudes on Mars. From roughly 10 degrees north of the Martian equator to the north pole, Deneb never sets.

One last tidbit. Mars’ axis is tipped 25.2 degrees, nearly the same as Earth’s 23.5 degrees. That’s why both planet’s have seasons. Despite similar inclinations, Mars’ axis points to a different direction in the Martian sky. Earth’s north polar axis points to the venerable North Star in the Little Dipper. Mars’ “north star” is close to Deneb, the bright star that marks the head of the Northern Cross or constellation Cygnus. Mars’ southern polestar is near the naked eye star Kappa Velorum.

It’s fun and fascinating to imagine how the planets and stars look on other worlds, especially the one we’re exploring with robotic eyes at this very moment. Seeing Earth from far away allows us to put our planet in perspective – we’re  a point of light dancing among the stars just like all the other planets.

Earth – that tiny point of light near the top – photographed by the Spirit Rover on Mars. Credit: NASA

Earth from afar – Tales of the Blue Marble

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A composite picture of the Earth's western hemisphere taken on January 4, 2012 by the Suomi NPP satellite. Click to enlarge. Credit: NASA/NOAA

Have you ever taken a portrait of yourself and a friend holding a camera at the end of your arm? Who hasn’t these days? It has to be the numero uno category of photos seen on Facebook. NASA got into the act too, only the “arm” is a satellite and the “friend” is planet Earth.

Earth's eastern hemisphere compiled from photos taken by Suomi NPP. The hazy streaks are sunglint from reflection of the oceans. Click to enlarge. Credit: NASA/NOAA

The agency released two new pictures taken by the most recently-launched Earth observing satellite Suomi NPP that show our planet in all its green, brown and blue glory. Suomi NPP snaps images of Earth in 1,865-mile-wide swaths from a 512-mile-high orbit that takes it over the north and south poles. Each swatch is imaged through red, green and blue filters and later combined into a natural color photo. In the eastern hemisphere image, pictures from six orbits of the satellite on January 23 were compiled via special data processing to let us see Earth from nearly 8,000 miles away instead of the satellite’s strict 512 mile altitude.

Another composite map of Earth made in the early 2000s using pictures by the Terra satellite. Click and then select different resolutions to see larger maps for each month. Credit: NASA

Ten years ago thousands of photos snapped by the Terra satellite were stitched together with 3-D software to create monthly maps of the entire planet. To watch the changing seasonal colors and snowpack, click the photo above and you’ll be taken to a page with maps for each month. Click HERE to find out how it all was accomplished.

One of the few single images (not a composite) of planet Earth shot on color film by the Apollo 17 crew. It was taken shortly after the spacecraft left its parking orbit around Earth en route to the moon. Click to enlarge. Credit: NASA

Since satellites orbit relatively near Earth, to get a really good view of our world, you’ve got to step way back or have a very long arm. The very first picture showing the planet in its entirely was taken on December 7, 1972 by the Apollo 17 astronauts at a distance of 28,000 miles. Nicknamed the “Blue Marble”, it’s one of the most widely-circulated photos on, well … Earth.  Antarctica, Madagascar (right), the outline of Africa and Saudi Arabia all show beautifully.

Looking at Earth never gets old. It’s also refreshing. Missing in every photograph are all the borders and boundaries that can make life so confounding.

Earth and moon seen from far, far away

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A "faraway" photo of the Earth (left) and the moon taken on August 26 from a distance of 6 million miles by the JunoCam, Juno's onboard camera. Credit: NASA/JPL-Caltech

Ain’t much to look at, is it? Yet it contains everything precious in our lives. This photo of the Earth and moon was taken by the Jupiter-bound Juno space probe from a distance of about 6 million miles last Friday August 26. That’s 25 times the distance to the moon from Earth. As the craft headed outward on its long journey to Jupiter, the mission team tested out its instruments -  including the JunoCam – to make sure everthing was in working order.

The Earth and moon photographed by the European Space Agency's Mars Express probe on July 3, 2005 at a distance of about 5 million miles. Credit: ESA

At first glance, the picture seems to show little more than two dots – amazing in itself if you let it sink in – but there’s more to see.  The Earth shows a blue fringe, hinting at water, while the moon is gray-brown. It’s also apparent that the Earth is considerably larger than the moon — 3.7 times to be exact.

Still frames from the Japanese Kaguya spacecraft orbiting the moon show a colorful Earth rising over the forbidding lunar landscape. Credit: JAXA

The difference in brightness between the two worlds is striking. The Earth is so much brighter than the moon because of light reflected from snow, clouds and the oceans. As seen from the moon, the Full Earth would appear 100x brighter than than our full moon. In comparison, the moon’s rocky, dusty surface reflects the same amount of light as a fresh asphalt road. Imagine sitting at a window inside a lunar dwelling reading a book by the light of Earth. A delicious thought that makes me a tad jealous of the future.

Watch for the moon to return to the western evening sky tonight. This map shows the sky about 15 minutes after sunset. Created with Stellarium

If you’d like to connect with your local moon, you can do so tonight by facing the west around the time of sunset. The 3-day old crescent moon will appear about four fists held at arm’s length to the left of the sunset point and very low.

Because the moon’s evening path slants well to the south in late August and September, it’ll take a couple more days before the moon’s increasing elongation from the sun angles it high enough for more convenient viewing. I don’t mind a low moon though. It gives us the opportunity to see a distinctive celestial body mingle with things terrestrial, poking out from behind homes and trees like a kid playing hide-and-seek.

Daylight saving time – love it or hate it

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Because of daylight saving time, you'll have to stay up an hour later to see Saturn in the east. Created with Stellarium

Daylight saving time. I don’t whether to love it or hate it. We lose an hour of early morning daylight but gain an hour in the evening. After nearly five months of winter, more light at the dinner hour is most welcome.

On the other hand, a later sunset means a later start to the night. Yesterday, twilight ended and true night began a little before 8 p.m. Tonight that becomes 9.  Saturn rose at 8 p.m. last night and will do so at 9 p.m. tonight. Since we’ll have to wait an additional hour for stars to come up in the east, daylight saving time has the effect of retarding every star’s rising by an hour.

Wait a minute. I was just getting used to seeing Saturn and the stars of early spring up in the east long before the 10 o’clock news. Does that mean we’re stuck with winter stars for a while longer? Yes. That’s the part of daylight time I don’t like. That and the hour of sleep I have to wait 7 months to get back.

As always, patience is necessary in astronomical pursuits. Soon enough, Earth’s journey around the sun will compensate for the later rising time. In only two weeks, Saturn and friends will be up where they used to be at 9 o’clock.

It works like this. Every night, the stars rise four minutes earlier than the night before. Over the days and weeks, the minutes accumulate into hours. When stars rise earlier, that means they also set earlier, causing them to drift westward over time.

View of Earth's orbit around the sun seen from above the northern hemisphere. As our planet moves to the left, the background constellations appear to drift to the right or westward. Credit: Bob King

The spring constellations are now low in the eastern sky, but in two months they’ll all be high in the south. As Earth travels in its orbit around the sun, we peer out into different sectors of the sky at night as the weeks and months pass. Think of sitting on one of those merry-go-round horses and looking out into the carnival crowd. As the merry-go-round turns, we look out at a different part of the fairground during our little spin. Now substitute the Earth for the horse and our orbit for the merry-go-round.

This time exposure shows stars trailing across the sky from east to west (left to right). This nightly motion due to Earth's rotation is different from the slow drift caused by Earth's revolution around the sun. Photo: Bob King

During Earth’s “little spin” around the sun, we see the stars and constellations drift from east to west across the sky as we pass them by.

This isn’t the same as the nightly rising and setting of stars – that’s due to Earth’s rotation. Every star you see makes a complete circle around the sky in 24 hours. The much more leisurely seasonal drift is superimposed on that pattern. It reveals itself to sky watchers who spend time regularly under the stars.

Frosty and blazing, fall steps forward

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Frost lines a turning leaf. Photo: Bob King

Yikes, fall begins tomorrow night (Sept. 22). Is the autumnal equinox already upon us? Must be the lulling effect a long summer has on the mind, because I didn’t pay attention to the calendar until this morning, the last full day of summer. Here in Duluth, the waning of the season puts many of us in a wistful mood, knowing that we’ll soon be spending five months with our hands cupped around a single candle, snapping icicles off our beards and pulling frozen bodies off the sidewalk. Oh, it’s not like that, really. Well, maybe sometimes.

At 10:09 p.m. Central Daylight time Wednesday, the sun will cross the celestial equator moving south. The celestial equator is simply the earthly version expanded into the sky against the background of stars. If you happen to live on the equator, you’ll see the sun overhead tomorrow around noon, and if you look down at your feet, you’ll see they completely cover your shadow. For everyone on the planet, the sun will rise due east and set due west. Daylight and night will achieve a perfect balance at 12 hours apiece no matter where you live. That’s the meaning behind the word equinox, derived from the Latin words for “equal” and “night”.

Two views of the sun's path at the autumnal equinox which begins tomorrow. The left side shows the view from the equator where the sun passes overhead. The right shows the view from the northern U.S. Credit: Tau'olunga

Depending on your latitude – how far north or south of the equator you are – the sun’s altitude at your location will vary. It shines overhead at the equator, about halfway between the horizon and zenith for mid-northern and mid-southern latitudes and directly on the horizon at the poles. Since the poles are at +90 and -90 degrees latitude, the celestial equator hugs the horizon in all directions.

Because the sun continues moving south of the celestial equator in the days following the fall equinox, it soon disappears below the horizon at the north pole and won’t reappear for another six months. Deep cold will follow quickly enough for high northern latitudes. The situation is reversed for the south pole, where the sun slowly climbs higher and remains above the horizon 24 hours a day for six months.

During Earth's revolution around the sun, we alternately face toward and away from the sun. We're sidelong at the spring and fall equinoxes. Credit: Tau'olunga

This crazy Earth. These variations are all caused by the tip of our axis. In summer, the northern hemisphere is tipped toward the sun, vaulting it high in the sky and making for long hours of daylight and a bounty of heat. During the winter, we’re tipped away, and a low sun means less daylight and subsequent loss of heating. Fall and spring are in-between times when neither hemisphere is tipped toward the sun. We all face it from the side, hence the equal day-night lengths and sun’s east-west path.

Two views of the sun taken around 8 a.m. this morning by the Solar Dynamics Satellite, one in ordinary light that shows the big sunspot #1108, and the other in far ultraviolet light that exposes a large coronal hole. Credit: NASA

While we’re on the topic of the sun, the folks at the NOAA Space Weather Prediction Center, are forecasting minor solar storm levels for the next two nights which could mean a chance of seeing northern lights at high latitudes. The cause of this bit of unsettled weather is a hole in the sun’s atmosphere called a coronal hole. Solar plasma – electrons and protons – are free to stream into the solar system from such holes where they can interact with the magnetic fields surrounding many of the planets and stimulate auroras. There’s also a substantial sunspot group in the sun’s southern hemisphere, but thus far it’s produced no major flares.

And the wheels on the bus go round and round

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The ancients imagined an immobile Earth at the center of the solar system with the sun and planets in orbit about it - the geocentric view. This was later disproved by Copernicus, who gave us the correct, heliocentric layout of the planets where rotating planets revolve around the central sun. Credit: Niko Lang

When you stay up under the stars late and long, you get into the slow rhythm of the turning sky. You may even start to feel the Earth rotating. This would have been absurd to the ancient Greeks. “The only thing moving,” they’d counter, “is the starry sphere above.” “The Earth is completely at rest.”

Our planet certainly feels motionless if you go strictly by your senses. After all, the ground stays put with no sensation of whirling or spinning. Accidentally drop a bag of groceries and it falls to the ground, not flying off to Kankakee. But imagine yourself in an airplane. Assume for a minute your flight is exceptionally smooth and you’re sitting in an aisle seat. How would you know you’re moving? Well, you wouldn’t, not physically at least. Though you’re traveling at over 500 mph relative to the ground, you could just as easily be on the tarmac with the engine running so long as you don’t look out the window. The in-flight magazine, that plastic cup of ginger ale and your seat companion are all moving at 500 mph, so your environment appears at rest and perfectly normal.

In a real airplane ride, turbulence and the occasional acceleration or deceleration give us clues that we’re in motion, but my point is this. The Earth’s rotation carries us all around at many hundreds of miles per hour – it varies from essentially 0 mph at the poles to over 1000 mph at the equator – but because everything around us is traveling at the same speed, we sense all is at rest. Thanks to Galileo’s studies of objects in motion and Jean Foucault’s pendulums, we know for a fact that Earth rotates west to east once every 24 hours. Because it’s so constant with no sudden changes in speed or direction, we’ll never quite feel the spinning in our bones, unlike the airplane, when turbulence makes us frightfully aware we’re in a flying machine.

Two pictures of the bright star Capella in the constellation Auriga the Charioteer taken four minutes apart last night demonstrate a rotating Earth. In the right frame, you can see that over that short time interval, it's risen a bit higher and closer to the tree branch. Photos: Bob King

We can still see the signature our planet’s rotation every clear night in the stars. As Earth spins from west to east, the stars in the eastern sky move upwards about 1 degree or two full moon diameters every four minutes, while the western ones sink horizon-wards by the same amount. It’s easy to see this for yourself by lining up a bright star with a power pole or tree and watching its slow creep. The movement is comparable to watching a distant cloud change shape. On nights when you stay up really late, there’s enough time for entire constellations to move from the eastern to the western sky and for the western stars to set out of sight. Of course you don’t have to go out at night to know the Earth’s turning. The sun’s arc across the daytime sky will serve as well.

Undisturbed and with the hours flowing by under the serenity of a starry sky, I sometimes feel the Earth for what it really is – a planet like the all the others, rotating soundlessly in the vast void of space.

Our double planet captured from afar

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The Messenger spacecraft, on its mission to study the planet Mercury, took this photo on May 6 this year of Earth and moon against the starry blackness of space. The probe was looking for "vulcanoids" at the time. Credit: NASA/JHUAPL/CIW

This week NASA released a photo of Earth and moon in space taken by the Mercury Messenger spacecraft from a distance of 114 million miles.  This is how our “double planet” would appear to someone on Venus with a pair of binoculars. There’s nothing like the perspective afforded by a pair of distant, robotic eyes to stir our thoughts and fire our imaginations about where we fit in the scheme of things. This view reminds of looking at double stars through the telescope. The similarity is no coincidence — the moon is so large in relation to our planet, the Earth-moon system is often considered a “double planet”.

The character Spock from the planet Vulcan in the Star Trek TV series. Credit & copyright: Paramount Pictures

NASA’s reason to take the provocative photo was not connected to a higher purpose on this occasion. It was a delightful byproduct of a search for “vulcanoids”, small rocky asteroids that may be orbiting between Mercury and the sun. What, you say? NASA looking for Spock’s planet? Well, not really. No vulcanoids have been discovered yet, so they’re still hypothetical. The glare of the sun prevents easy hunting from Earth, so NASA directed the spacecraft, which is much closer and has a better view, to seek out this potential new class of solar system bodies. May it live long and provide prosperous imagery.

A scarp or fault from the shrinking moon has deformed or partially covered small ~125-foot craters (arrowed). The fault carried near-surface crustal materials up and over the craters, burying parts of their floors and rims. About half of the rim and floor of a 60-foot-diameter crater shown in the box has been lost. Since small craters only have a limited lifetime before they are destroyed by newer impacts, their deformation by the fault shows the fault to be relatively young. Credit: NASA/Goddard/Arizona State University/Smithsonian

There was more NASA news this week from the Lunar Reconnaissance Orbiter Mission (LRO) of the discovery of lobate scarps on moon. These cliffs on the lunar crust point to a moon that’s been shrinking, and not four billion years ago, but as recently as a couple hundred million years ago. The moon won’t be crunching itself into a tennis ball anytime soon since the total shrinkage is only on the order of 300 feet. Compared to its 2,160 mile diameter that’s a tiny fraction, but the consequences are evident on its surface. Even more intriguing, we’re waking up to a moon that’s not as dead as once thought. Since the Apollo missions, we’ve learned the moon has water ice embedded at many sites across its surface; now it also appears to have been geologically active in recent times.

These hills in the moon's Taurus-Littrow Valley, near the Apollo 17 landing site (arrowed), were thrust up when the moon contracted. Credit: NASA/Goddard/ASU/Smithsonian

After its formation some 4 billion years ago, the moon was mostly molten rock. As it cooled, the outer crustal rock solidified first. Later, magma beneath the crust also began to cool and solidify. As the molten materials went from a liquid to solid state, their density increased. If you take a certain volume of rock and increase its density, the amount of space or volume it occupies drops. As the moon cooled and grew denser, its internal rock slowly cooled and compressed itself into a smaller ball. The outer crust responded in the only way it could — it broke into faults and slid over itself in an attempt to form a tight fit over the ever-shrinking mantle and core. We see the results of this adjustment in the numerous lobate (rounded or lobed) scarps or cliffs hither and yon across the moon. And we know some are recent because they partially cover small, relatively fresh craters.

To learn more about the discovery, click HERE for the complete story or HERE to watch a short video.