StarDate

If you look at an elevation map of Mars, one thing really stands out: The northern hemisphere is a lot lower than the southern hemisphere. And planetary scientists are still trying to figure out why.

On average, the northern third of Mars is a mile or two lower than the rest of the planet. And there’s evidence that water once pooled there, forming a giant ocean.

There are several possible explanations for the difference in the two hemispheres.

One is that another planet-sized body slammed into Mars when it was young. The other world could have hit the northern hemisphere, excavating the deeper northern plains. Or it could have hit near the south pole, melting the upper layers of rock in the southern hemisphere. As the magma cooled, it left the southern hemisphere with a thicker crust.

Another idea is that the northern hemisphere was pounded by several big impacts.

And yet another says the cause was home-grown. Early Mars might have had plate tectonics like those on Earth. Motions in the crust and mantle could have piled up rocks in the southern hemisphere, making the crust there about 15 miles thicker than in the north.

Each of these ideas has problems, though. So there’s no final explanation for the “deep” difference in the Martian hemispheres.

Look for Mars near the Moon tonight. It looks like a bright orange star. It’s above the Moon at nightfall, and to the right of the Moon as they set in the wee hours of Thanksgiving.

Script by Damond Benningfield

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It’s quiet out there. We’ve been listening for radio signals from other civilizations off and on for decades. But we haven’t heard a peep.

Many scientists say there’s no reason to think Earth is the only planet with life in the Milky Way Galaxy. In fact, life could be common. And if there’s life, there’s a decent chance there’s intelligent life.

Many stars are much older than the Sun, so it’s possible that there are civilizations that are much older than our own. And if they were at all interested in exploring space, they could have colonized the entire galaxy in a few million years. So if anyone is out there, the reasoning goes, we shouldn’t have much trouble finding some evidence — a radio signal, a laser beam, an artifact, or something else.

There are many possible explanations for “the Great Silence.” It could be that civilizations are so far advanced that we don’t recognize them. Or perhaps Earth is off-limits — protected by the equivalent of Star Trek’s “prime directive.” Or perhaps we haven’t looked in the right places, with the right technology.

Some have suggested that we beam invitations to make contact out into the galaxy and see if anyone responds. Others say that such a move is risky — meetings between disparate civilizations never end well.

Perhaps, though, it’s all a moot point. One other possibility is that no one else is out there — that we’re the only technological civilization in the entire galaxy.

Script by Damond Benningfield

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If you enjoy a steam bath, the galaxy may offer a lot of planets for you to visit. These worlds may be completely covered by water. And they’re so close to their stars that a lot of the water evaporates, forming a deep, steamy atmosphere.

Astronomers have confirmed more than 4,000 planets in other star systems, with many more candidate worlds. Some of them are classified as “super-Earths.” These planets are a little wider and heavier than Earth. They’re also quite dense, so they’re probably balls of rock and metal, just like Earth.

Some other planets are “mini-Neptunes.” They’re about twice the size of the super-Earths, but they’re not nearly as dense. That could mean they have solid cores surrounded by layers of hydrogen and helium, the lightest elements.

But recent studies by research teams in France suggest another possibility: The planets are hot and wet, with atmospheres made of steam.

The mini-Neptunes in this category are quite close to their parent stars — millions to perhaps tens of millions of miles. At that close range, they’d be extremely hot. If they have deep oceans, all that heat would vaporize a lot of the water. The water vapor would form a puffed-up atmosphere. That would make the planets look a lot wider than they really are.

If that’s the case, then a lot of “mini-Neptunes” are really super-Earths in disguise — hidden below a blanket of steam.

Tomorrow: the Great Silence.

Script by Damond Benningfield

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Earth is four and a half billion years old. On the timescale of the universe, that makes it a youngster. The earliest planets probably were born more than eight billion years earlier — when the universe was only about one billion years old.

One of the oldest planets yet discovered orbits a pair of dead stars. The system is known as PSR B1620-26. It’s in an ancient star cluster that’s more than 12,000 light-years away.

The planet is bigger than Jupiter, the giant of our own solar system. And it has an interesting history.

It probably was born around a Sun-like star. The system zipped through the densely packed center of the star cluster. There, it passed close to a binary — two stars locked in orbit around each other. One member of the system was a neutron star — the corpse of an exploded star. In a complex gravitational dance, the neutron star changed partners. It grabbed the Sun-like star and its planet, and kicked out its original companion.

Later, the planet’s original star reached the end of its life. It puffed up, so some of its gas spilled onto the neutron star. That caused the neutron star to spin faster. Today, it spins about a hundred times a second, beaming a “pulse” of energy in our direction with each turn. Slight changes in the timing of those pulses revealed the planet.

Today, the planet orbits both of these stars — almost 13 billion years after it was born.

More about exoplanets tomorrow.

Script by Damond Benningfield

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The youngest planet yet seen faces an uncertain future. It could live a long life near its parent star. Or it could fall into the star within a few hundred million years.

The planet orbits V830 Tauri, which is about 425 light-years away. The star is only about two million years old — so young that it’s not yet shining as a true star. It’s still collapsing from a cloud of gas and dust. In a few million years, its core will get hot enough to ignite nuclear fusion, and the star will enter the main phase of life.

The planet is V830 Tauri b. It’s about three-quarters the mass of Jupiter, the giant of our own solar system. It may be losing some of that heft, though. That’s because it’s less than six million miles from the star. It’s extremely hot, which makes it puffy. As a result, the young star’s winds and radiation may be blowing gas off the top of the planet’s atmosphere.

Right now, the star’s magnetic field and other factors could be causing the planet to move farther away. And it’s possible that the planet could assume a stable orbit and last as long as the star itself. On the other hand, it could reverse direction and spiral into the star — bringing its life to a premature end.

The system is in Taurus, which is quite low in the east shortly after nightfall. V830 is to the upper left of Aldebaran, the bull’s bright orange eye. But you need a telescope to see it.

We’ll talk about an old planet tomorrow.

Script by Damond Benningfield

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A planet that appeared to orbit the star Fomalhaut has blown away like dust on a stellar wind. Instead of a planet, the object may be debris from a collision.

Fomalhaut is low in the south as night falls, far to the lower left of the Moon. It’s the only bright star in its region of the sky, so it stands out.

The star is bigger, brighter, and heavier than the Sun. And it’s encircled by wide rings of dust. Gaps between rings suggest they’re being sculpted by the gravity of hidden planets.

In 2008, images of the system from Hubble Space Telescope appeared to show a planet — the first exoplanet ever photographed at visible wavelengths. A few years later, astronomers gave it a formal name: Dagon.

Some other telescopes, though, suggested Dagon wasn’t really a planet at all. And more-recent Hubble observations agree. They show that the “planet” is expanding and getting fainter. That suggests that the object is the aftermath of a collision between two comet-like bodies. Each of them was about 125 miles in diameter.

The collision probably took place not long before Hubble’s first pictures of the object. It could have been caused by the gravitational influence of one or more hidden planets.

After the impact, the debris spread out. And Fomalhaut’s wind of charged particles started blowing the cloud of dust out of the system. Today, it’s too spread out for Hubble to see it at all — a non-planet carried away on a stellar wind.

Script by Damond Benningfield

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One of Saturn’s moons is lucky to be alive. A collision with another body billions of years ago gouged a giant crater in its surface. And it may have created a long crack on the opposite hemisphere. The moon might have survived only because it was still fairly warm.

Tethys is Saturn’s fifth-largest moon. It’s about 660 miles in diameter. Its surface is quite bright. And it’s a fraction less dense than water — the least-dense major moon in the entire solar system. That means it’s made almost entirely of ice, with only a smattering of rock.

Tethys was hit when it was still quite young. It would have been much warmer than it is today, so its interior was at least partially molten. When it was hit, that “squishy” interior probably kept the moon from being blasted apart.

Even so, the impact carved a crater, named Odysseus, that’s 250 miles in diameter. It makes the moon look like a “Star Wars” death star. And there’s a giant canyon — Ithaca Chasma — on the opposite face of Tethys. It’s 1200 miles long, up to 60 miles wide, and three miles deep. Although its origin isn’t certain, it could be a “crack” caused by the impact — a scar on a moon that’s lucky to be around.

Saturn appears near our Moon this evening. It looks like a fairly bright star to the right of the Moon at nightfall. The brighter planet Jupiter is a little farther from the Moon.

Tomorrow: A bright star loses a planet.

Script by Damond Benningfield

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The first people to drive on the Moon took their first jaunt 50 years ago this week. They weren’t actually on the Moon, though. Instead, they were driving from a control room in Russia.

Lunokhod 1 landed on November 16th, 1970, on a volcanic plain known as Mare Imbrium.

The rover looked like a bathtub with a lid on top. The lid opened during the day to expose its solar panels to the Sun. It closed at night to keep the rover warm.

Computer technology being what it was, the rover couldn’t drive itself. So using TV pictures, technicians drove it from Earth: a commander, driver, navigator, engineer, and antenna operator. It took more than four seconds from the time they gave a command until they could see Lunokhod respond. But during almost 11 months of operation, they drove it more than six miles.

The rover carried a device to reflect laser beams shot from Earth. Shortly after the rover expired, though, Earth lost it. It wasn’t found again until 2010, when it showed up in a picture snapped by a Moon-orbiting satellite. Scientists have used the reflector many times since then. Combined with other reflectors, it helps plot the distance to the Moon with an accuracy of less than a millimeter.

You don’t need that kind of accuracy to find the Moon this evening, though. It’s in the southwest at nightfall. Jupiter and Saturn line up to its upper left. Jupiter is closer to the Moon, and it’s by far the brighter of the two planets.

Script by Damond Benningfield

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When European sailors began exploring the southern hemisphere, they compiled maps of the stars. Astronomers used those maps to create some new constellations. And one of those constellations moves low across the south on November nights: Grus, the crane.

Grus was first featured on a celestial globe, in 1598. A few years later it was depicted in a famous star atlas, and it’s been around ever since.

The crane’s “head” and several other stars were taken from Piscis Austrinus, the southern fish. But the head retained its “fishy” name: Aldhanab, from a longer Arabic phrase that means “the tail of the fish.”

The star has passed the end of its “normal” lifetime, so it’s gotten much bigger and brighter. It’s more than four times wider than the Sun, and it emits almost four hundred times as much energy. It’s much hotter than the Sun, so most of its energy is in ultraviolet wavelengths, which are invisible to the eye.

Grus’s brightest star is Alnair — “bright one of the fish’s tail” — in the crane’s body. It, too, is at or near the end of its normal lifetime, so it’s also hundreds of times brighter than the Sun.

Grus hugs the southern horizon as night falls. From the northern half of the U.S., only its head and neck are visible. To see Alnair and the rest of the crane’s body, you need to be south of about Kansas City. And if you’re south of about Dallas, you can see its legs as well — the long legs of a celestial crane.

Script by Damond Benningfield

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Meteor showers often are tales of good news and bad news. And that’s the case for this month’s Leonid shower, which should be at its best tonight. On the good side, the Moon is just past new, so it won’t get in the way of the fireworks. On the bad side, there won’t be that many fireworks to interfere with.

The Leonids are spawned by a comet, 55P/Temple-Tuttle. When the comet gets close to the Sun, some of its icy crust vaporizes. That releases small bits of dust and rock, which surround the comet. Over time, some of this “comet dust” spreads out along Temple-Tuttle’s orbit. Earth flies through this trail of debris every November. Some of the grains plunge into our atmosphere at high speeds. They quickly vaporize, forming the glowing slashes of light known as meteors.

The clouds of dust are thicker near the comet. So the Leonids are better when the comet is passing through the inner solar system. At those times, the shower can produce a “storm” of thousands of meteors per hour. That last happened a couple of decades ago.

Today, though, Temple-Tuttle is racing away from the Sun, so the showers are much less intense. This year, you might see a peak rate of a dozen or so meteors per hour, in the wee hours of tomorrow morning.

The lack of moonlight, though, at least ensures that you can see whatever meteors the shower produces. But you need a skywatching site with dark skies, away from city lights, to see the Leonid meteor shower.

Script by Damond Benningfield

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