Phobos: A Chip Off of Mars

NASA's Mars Reconnaissance Orbiter captured this view of the Martian moon Phobos on March 23, 2008, from a distance of about 4,200 miles. It's actually a false-color view, combining data from the camera's blue-green, red, and near-infrared channels.The smallest resolved features are about 65 feet across

Asaph Hall's discovery of Mars's two small moons in 1878 is one of the greatest success stories of observational astronomy. For nearly a century thereafter, however, Phobos and Deimos were little more than dynamical curiosities that traveled oh-so-close to their parent planet.

With the Space Age came the opportunity to see these objects at closer range, and a succession of spacecraft (beginning with Mariner 9 in 1971) has revealed Deimos and especially Phobos with ever-greater detail. Some particularly dramatic views have come from the HiRISE camera aboard Mars Reconnaissance Orbiter.

Yet, despite decades of careful scrutiny from space and from ground-based telescopes, astronomers still debate where these little worlds came from. They're very dark, implying a carbon-rich composition akin to bodies at the outer margins of the asteroid belt. Both also have a low density, suggesting they're riddled with internal cavities. All this evidence points to the notion that Mars somehow captured a couple of wandering objects that happened to stray too close by. (One big "gotcha": both moons have extremely circular orbits very close to Mars's equatorial plane — not exactly the outcome you'd get from two random encounters.)

Now a team of observers has stirred up the pot of possibilities. Yesterday, at the European Planetary Science Congress in Rome, hometown researcher Marco Giuranna (IFSI/INAF) argued that far-infrared spectra of Phobos acquired with the Mars Express orbiter don't match the composition of any known chondritic asteroid or meteorite type. (Chondrites are rocky bodies that have remained unaltered since the solar system's formation.)

As seen by a spectrometer aboard the Mars Express orbiter, the thermal-infrared spectrum of Phobos is a close match to certain phyllosilicates (clay minerals). Also, the position of the peak labeled CF suggests a composition rich in ultramafic (iron- and magnesium-rich) minerals.

What he and others have found instead is a distinct composition rich in dark ultramafic minerals (having lots of iron and magnesium) and clay minerals, called phyllosilicates. The clay signature appears strongest near the large crater Stickney, hinting that deposits were dredged up from deeper down.

Moreover, Martin Pätzold (Köln University) also announced at the ESPC that Phobos has a density of just 1.86 ±0.02 g/cm3. To be so low, the moon's interior must be incredibly porous, with voids taking up 25% to 35% of the total volume. The new density value is based on careful tracking of Mars Express during close flybys of Phobos, including a brush-by just 42 miles (67 km) away last March 3rd.

That mix of clay and ultramafic minerals might be rare among the asteroids, but it's likely a lot more common on the Martian surface down below. The upshot, Giuranna suggests, is that Phobos wasn't captured but more likely formed in place from debris blasted off the surface of Mars during a large, long-ago collision. Most of the debris would have escaped to interplanetary space, but enough of it (11 trillion tons, more or less) hung around to recollect into the two Martian moons.

It's a dramatic hypothesis, to be sure, but is it the correct one? First, as Giuranna's team points out, the new compositional clues don't rule out that Phobos (and Deimos) were captured. Observers have found plenty of asteroids with clay minerals on their surfaces, and ultramafic meteorites (achondrites) aren't exactly rare. Second, the moons' spongy interiors could have resulted if a single body broke apart during its capture by Mars and then then reassembled into a pair of not-quite-solid satellites. Finally, someone needs to run a computer simulation to see if this smash-and-dash scenario makes dynamical sense (according to impact modeler Robin Canup, no one's done it yet).

In any case, we might not have to wait too long to learn the Martian moons' pedigree. A Russian mission called Phobos-Grunt is being readied for launch late next year. The effort has encountered technical problems — it was supposed to head off toward Mars a year ago — but if it succeeds, scientists will have 100 to 200 grams of Phobos to analyze when the sample-return capsule lands in July 2014.

Jupiter Shines Extra Bright

If you look up on any clear September night, a big bright “star” will greet you. It’s low in the east after twilight, and higher in the southeast as the evening grows late. This is the planet Jupiter, and it's far brighter than any true star in the night sky.

Jupiter is always bright, but if you think it looks a little brighter than usual this month, you’re right. Jupiter is making its closest pass by Earth for the year. And this year’s pass is a little closer than any other between 1963 and 2022.

Jupiter is nearest to Earth on the night of Monday, September 20th: 368 million miles away. But it remains nearly this close and bright (magnitude -2.9) throughout the second half of September.

At the closest point of its previous swing-by, in August 2009, Jupiter was about 2% farther from both Earth and Sun than this time. That made it 8% dimmer. At its next pass, in October 2011, it will be 0.4% more distant than now.



Jupiter on Sepetember 2, 2010
Jupiter on September 2nd, shown with south up, as it appears in a reflecting telescope. The Great Red Spot is prominent at upper left, and the South Equatorial Belt is almost invisible.

Also, according to legendary planetary observer Richard Schmude, Jupiter is an additional 4% or so brighter than usual because one of its brown cloud belts has gone missing. For nearly a year the giant planet's South Equatorial Belt, usually plain to see in a small telescope, has been hidden under a layer of bright white ammonia clouds.

Because Jupiter is so close to Earth, this is a great opportunity to view it through a telescope. Jupiter is most interesting when the Gred Red Spot is visible and/or when one of the moons is casting a shadow on Jupiter's disk.

Encounters with Comet Hartley 2

An icy visitor is positioning itself for easier viewing in the coming weeks. Periodic Comet 103P/Hartley 2 won't have the pizzazz of Comet Hale-Bopp or the unexpected spectacle of Comet Holmes. But it will be high in the evening sky when at its best, glowing at perhaps 5th magnitude. It should be dimly visible to the unaided eye from very dark locations, and visible in binoculars and telescopes from almost anywhere in the Northern Hemisphere. (Most of you in the Southern Hemisphere will be able to observe it from mid-October onward.)

Hartley 2's brightness, and its unusually fast slide across the constellations, both result from how closely it will approach Earth: by just 0.12 astronomical unit (11 million miles; 18 million km) on October 20th. This will be its closest approach since its 1986 discovery and one of the closest approaches of any comet in the last few centuries.

The comet has reached 9th magnitude and is brightening by 0.1 magnitude per day. So right now, before the Moon washes the sky with light, is an especially good time to look for this faint visitor.

By September 1st Hartley 2 had climbed north into a corner of Lacerta, where it spent a few days before crossing into northern Andromeda. On the night of September 8-9, at new Moon, the comet was less than 1° from 3.6-magnitude Omicron (ο) Andromedae.

The waxing Moon will brighten the evening sky from about September 15th to 26th. On the 22nd Hartley 2 should be 7th or 8th magnitude and within a few degrees of Lambda (λ) Andromedae.

October 1st finds the comet passing 1.5° south of 2.2-magnitude Alpha (α) Cassiopeiae, high in the northeast during moonless evenings. Perhaps 6th magnitude by then, it should remain at least this bright for the next nine weeks. But it's important to note that, with the comet now just 0.18 a.u. from Earth and closing, its light is no longer concentrated into a small dot but instead is more spread out. So even if you can sight a 6th-magnitude star with the unaided eye, Hartley 2 will be tougher. It's closest to Earth on October 20th at a distance of just 0.121 a.u.

On the night of October 7th in the Americas, when the comet should be 5th or 6th magnitude, it creeps less than 1° south of the Double Cluster in Perseus, magnitudes 4.3 and 4.4. This will make for a wonderful wide-field sight and a great astrophoto opportunity — particularly since it's again new Moon!

From here on Hartley 2 turns southeast, passing near the head of Perseus. On October 20th the fuzzy visitor passes just south of brilliant Capella. By the end of October the comet should still be around 5th magnitude — but now in Gemini. So it doesn't gain a high altitude until later in the night. Perihelion, 1.06 a.u. from the Sun, comes on the 28th — but that morning the nearly last-quarter Moon is just a few degrees away.

Moonless viewing times return around November 1st. But now, with the comet moving away from both the Sun and Earth, it fades by about a magnitude every two weeks. Besides — by then our attention should surely be turning to the exploits of (and pictures from) NASA's EPOXI spacecraft, which swoops by it on November 4th at a distance of just 600 miles (1,000 km).

A Comet's Tale

How could a short-period comet, visible to the unaided eye, go undiscovered until just 24 years ago? Read on.

Malcolm Hartley first spotted it on March 16, 1986, at magnitude 17 or 18 during a sky survey by the 1.2-meter U.K. Schmidt telescope at Siding Spring, Australia. A series of position measurements soon revealed it to be a short-period comet orbiting the Sun about every 6 years. It was the second short-period comet discovered solely by Hartley, hence the "2" in its name. The appellation 103P indicates that it was the 103rd comet with a known orbital period.

A backtrack of Hartley 2's path revealed that it fell into its current orbit only recently. Three close encounters with Jupiter (0.33 a.u. in 1982, 0.09 a.u. in 1971, and 0.23 a.u. in 1947) had shifted the comet's track closer to the Sun. Prior to those encounters, Hartley 2 never came closer than 2 a.u. from the Sun, leaving it beyond visual detection.

Hartley 2's next return came in 1991, when it brightened that September to 8th magnitude. It did so again at its following return in December 1997. The 2004 apparition was a poor one, with the comet far from Earth.

Now it's arriving front and center for its best showing yet. So enjoy it while you can!

Hubble Revisits Supernova 1987A

Supernova 1987A's ring, about a light-year across,
was probably shed by the star about 20,000 years before it exploded.
The dozens of bright spots around the ring mark where a shock wave
unleashed by the stellar blast is slamming into the ring's material.

It's been more than two decades since we Earthlings got word that a star in the Large Magellanic Cloud had blown itself to smithereens.

Supernova 1987A peaked at 3rd magnitude, making it a snap to spot by eye. But, with its declination of -69°, the blast was invisible to virtually everyone north of the equator. I'm jealous of my southern astro-friends because I never got to see it. (In fact, I wonder how the popular perception of and appreciation for astronomy might be different had this event been in view from northern skies — a topic for another day!)

Fortunately, the Hubble Space Telescope started observing SN 1987A within months of its launch in 1990. Those first views revealed a ring of matter thrown out by the star about 20,000 years before its demise. The supernova's expanding shock wave eventually crashed into that ring, creating a necklace of bright knots first spotted in 1995 HST images. Observers have kept tabs on this ring ever since — primarily with an instrument called the Space Telescope Imaging Spectrograph, or STIS, which astronauts installed on Hubble in 1997.

Unfortunately, an electronic failure in 2004 rendered STIS inoperable, and astronomers had to make do without its services until last year, when spacewalking astronauts replaced a faulty circuit board and brought STIS back to life.

In the September 2nd edition of Science Express, a star-studded international team of astronomers describe observations of the supernova made with STIS earlier this year, the first in six years. The ejected ring is still there, now studded with about 30 hotspots. Over time, as the supernova's shock wave continues to barrel outward, these should merge into a single bright band.

More interesting is the insight being gleaned from a second shock wave, this one triggered by the ring itself and propagating back toward what's left of the progenitor star and through the supernova's expanding debris. The team reports that spectra of this inner shock reveal lots of hydrogen, as you'd expect, but they also see some other emissions that are probably from nitrogen and perhaps from carbon.

Essentially, the now-gone star has laid bare whatever was inside when it exploded, and over time careful observations by HST and other telescopes will, in Humpty Dumpty fashion, attempt to put the progenitor star back together again.

"I think a great thing here is the resurrection of STIS," notes coauthor Robert Kirshner (Harvard-Smithsonian Center for Astrophysics). "Astronauts zipping out 114 screws while wearing boxing gloves were not just doing it for the challenge! This paper shows that the instrument is back working, and that we're finding out new things about an object that is about the same age as HST."

One Star, Seven Planets

Many worlds around HD 10180
An artist's impression of the many worlds orbiting the Sunlike star HD 10180.

Nestled in the Chilean Andes at altitude of 7,900 feet (2,400 m), the European Southern Observatory's 3.6-m reflector at La Silla is neither the highest nor the largest telescope in the region. Far from it. However, coupled with the incredible HARPS spectrograph, this telescope is cranking out discoveries of extrasolar planets faster than any other observatory on Earth.

Today, at a gathering of planet-hunting astronomers in France, Christophe Lovis (Geneva Observatory) announced that his team has identified a solar system packed with planets — five for sure, and probably seven — using the La Silla facility. It's the most populous exoplanet system known.

The central star is HD 10180, a type G dwarf situated 137 light-years away in the southern constellation Hydrus. It's one of 400 nearby Sunlike stars that astronomers have been monitoring from La Silla for years. HARPS can't see these worlds directly; instead it detects minuscule Doppler shifts in the stars' light, caused by back-and-forth wobbles in their motion that, over time, reveal the presence of planets circling around them. In the case of HD 10180, the telescope amassed 190 nights of observations over six years.

You could make a case that spotting alien planets has gotten a little ho-hum — after all, the total count (including those orbiting the Sun) now tops 500, and 15 systems involve at least three planets.

But HD 10180 definitely raises the bar. Its five sure-things, dubbed C, D, E, F, and G, have Neptune-class masses 13 to 25 times that of Earth. But all five are quite close to the star, in orbits that range from 0.06 to 1.4 astronomical units (5½ to 130 million miles out). So much mass packed so close together is bound to incite gravitational tussles among them, and future observations will follow the long-term evolution of the system.




Solar systems compared
A comparison of our solar system with that of HD 10180 and other stars encircled by at least three planets. Black lines indicate the range of distance due to eccentric orbits.

But wait — there's more! Lovis and his team are fairly certain there's a sixth planet, H, with at least 65 Earth masses (making it Saturn-ish) and orbiting 3.4 a.u. from the star. There's also strong evidence for a seventh sibling, B, zipping just 0.02 a.u (2 million miles) from the star. Although not yet confirmed, this innermost planet might be very close to Earth in mass. It causes a wobble in HD 10180 only about 2 miles (3 km) per hour — "slower than walking speed," notes team member Damien Ségransan — and is thus very hard to measure.

For all the details, read the team's article in the current issue of Astronomy & Astrophysics. At upper right is Figure 13, a concise comparison of solar systems with three or more planets (one is missing, however: the four-member set circling pulsar PSR 1257+12).

Now that we know of a system as crowded with planets as our own, isn't it about time we starting finding some that resemble Earth? That bar-raising exoplanet discovery may not be long in coming — NASA's Kepler spacecraft is looking for them right now.

The Incredible Shrinking Moon

At one point during the Apollo 17 mission, moonwalking astronauts Gene Cernan and Harrison "Jack" Schmitt tried to drive their lunar rover up the face of a 200-foot-high rise known as the Lincoln-Lee Scarp. It didn't seem that imposing a task, but the rover's wheels slipped so much that the astronauts were forced to climb it at an angle, much as a sailboat tacks into a stiff wind.


Thrust fault diagram
Thrust faults occur when the lunar crust is compressed laterally, breaking the rocky materials below and forming a long scarp on the surface.

Lincoln-Lee is the kind of feature created when one slab of rock overrides another due to horizontal compression (what geologists term a thrust fault). On Earth, good (if enormous) examples of thrust faults occur where crustal plates collide — think how the towering Andes rim the west coast of South America, and you get the idea.

What Cernan and Schmitt couldn't have known back in 1972 is that Lincoln-Lee is not an isolated feature but one of likely hundreds of small thrust faults all over the Moon. That revelation came to light only recently, thanks to the incredibly detailed images of the lunar surface being beamed to Earth by two Narrow Angle Cameras aboard NASA's Lunar Reconnaissance Orbiter. (These same cameras have taken snapshots of the historic Apollo 11 landing site and others.)


Map of lunar scarps
The distribution of lobate scarps known as of mid-2010. Black dots indicate previously known features, while white dots mark those found in images by the Lunar Reconnaissance Orbiter. Click here for a larger version.

It wasn't until LRO arrived on the scene that geologists realized the subtle fractures are pervasive. After poring over images from LRO, a team led by Thomas Watters (Smithsonian Institution) has identified 14 new scarps in addition to the three dozen already known. As the map here shows, half of the new finds are poleward of 60° in latitude. (Most of the previously recognized scarps had been spotted by cameras mounted into the orbiting command modules of Apollos 15, 16, and 17 — but these craft never strayed far from the lunar equator.)

In the August 20th issue of Science, the researchers reach a startling and unexpected conclusion: "We have now found that these lobate scarps occur everywhere on the Moon," Watters explains, "which means the Moon has been contracting or shrinking globally."


Scarp in Gregory crater
A thrust fault crossing the floor of Gregory crater on the lunar farside.

All told, the lunar diameter hasn't changed much, probably only about 700 feet (200 m). But the scarps look so fresh that they must have formed in the recent past, geologically speaking. "These scarps can't be any older than 800 million to 1 billion years," Watters noted during a press briefing yesterday, and they could be much younger. "We're finding the Moon is a truly dynamic planet," adds Michael Wargo, chief lunar scientist at NASA headquarters. "Who'd have thought that tectonic processes would still be occurring today?"

The scarps escaped notice until now because they're only a mile or two long and just tens of feet high — completely invisible to backyard telescopes and even to previous lunar-orbiting craft. Most likely, they result from the gradual contraction of the lunar interior as it cools, a process that apparently didn't end when the last maria filled with lava some 3 billion years ago.

Thrust faults appear on the surfaces of Mars and especially Mercury, but they're huge by comparison. Some of the Mercurian scars are hundreds of miles long and more than a mile high, implying that the planet shrank by at least a couple of miles as its molten interior cooled and contracted.

Yet at one time the Moon must have been really, really hot as well. After all, it likely accreted by picking up the white-hot pieces after something enormous collided early on with Earth. So shouldn't there likewise be giant thrust faults jutting skyward across lunar landscape? Some researchers think the Moon did undergo substantial contraction, but the surface scars from that have been erased over time. Watters thinks otherwise. "Our results are really more consistent with a cooler initial starting temperature," he explains, "one that didn't allow the entire Moon to melt."


Astronaut collecting lunar soil
Astronaut Harrison "Jack" Schmitt uses a special rake to collect lunar soil during the Apollo 17 mission in December 1972.

As I listened to yesterday's press briefing, I wondered what Cernan and Schmitt were thinking as they struggled up the slope of Lincoln-Lee Scarp all those years ago. So I asked one of them.

"We were well aware of the Lee-Lincoln scarp and that it is a potential thrust fault or wrinkle ridge," Schmitt recalls. "We drove up it to Station 2 at the base of the South Massif, and along and down it going to Station 4 [Shorty crater]. It was entirely covered by a light mantle or avalanche deposit. The avalanche probably flowed off the South Massif about 100 million years ago, as it appears to have been triggered by [nearby impacts from] Tycho ejecta. Lee-Lincoln would be at least older than the avalanche and I suspect much older than that."

Watters and other lunar geologists should eventually be able to say more about how and when all these lobate scarps formed. As of now, LRO has mapped only about 10% of the lunar surface at high resolution. But give it another three years (assuming the funding holds out), and there'll be enough coverage to inspect the entire lunar glove down to a resolution of just a few feet.

A Solar Tsunami

Solar flare on August 1, 2010
A false-color composite shows the
solar flare (bright area at lower left) and large magnetic disturbance
that rippled across the Sun's disk on August 1, 2010. Recorded by
NASA's Solar Dynamics Observatory at extreme-ultraviolet wavelengths,
the hues correspond to coronal gases at temperatures of 1 to 2 million
kelvins. Click here for a larger view.

After oversleeping for more than a year, our star is finally stirring from hibernation. Experts don't expect solar activity to peak until until mid-2013 (and a weak one at that), but the signs of awakening are clearly evident.

A few days ago the Sun let loose with a massive belch. On August 1st at 8:55 Universal Time, orbiting satellites witnessed a sizable flare erupting from the large sunspot region designated 1092. The strength of this outburst was pegged at C3, modest as flares go, but it still triggered an impressive coronal mass ejection, or CME, that shot out from the solar disk at more than 600 miles (1,000 km) per second. Watch the amazing video from NASA's STEREO spacecraft here.

When the flare erupted, NASA's recently-launched Solar Dynamics Observatory also looked on as the magnetic disturbance caused an enormous filament of superheated gas to pulse across the Sun's disk.

All this tumult occurred on the Earth-facing side of the Sun, and skywatchers at far northern and southern locations briefly enjoyed colorful auroral displays on the night of August 3-4.

The Sun has since quieted down, but the big spot in region 1092 has been joined by a second, smaller group (1093) that rotated into view yesterday. Go have a look — but, as always, never try to look at the Sun by eye or using optical aid without using a safe solar filter.

Meanwhile, when the Sun's activity finally does peak, solar physicists will have more spacecraft at their disposal than ever before — despite the news that NASA managers pulled the plug on the Transition Region and Coronal Explorer (TRACE) on June 21st after 12 years of operation.

The workhorse Solar and Heliospheric Observatory (SOHO), a collaboration between NASA and the Rueopean Space Agency, just keeps on chugging (it's nearly 15 years old). Even with the loss of TRACE, NASA still has SDO, STEREO, and ACE (Advanced Composition Explorer) in its arsenal of orbiting sentinels. (By the way, teh STEREO team has a great iPhone app showing the Sun in 3D.)

In addition, the Japan Aerospace Exploration Agency continues to receive great high-resolution images from its Hinode spacecraft.