A Google translation of suggests that the spacecraft experienced a sudden, unexpected rotation 2.5 minutes after the orbit-insertion manoeuvre began, apparently due to errant thruster firings. In response, an onboard computer turned off the braking rocket and immediately put the spacecraft in an emergency standby mode.

Unfortunately, the premature shutdown came well short of the minimum burn time (9.3 minutes) needed to be captured by the planet, so the spacecraft continued past Venus and into interplanetary space.

There is not enough fuel to try to correct the miscue by reversing course. Instead, Akatsuki will coast along a solar orbit that will return it to the vicinity of Venus in about seven years. Right now that reunion will bring the craft no closer than 3.7 million kilometres. But project manager Masato Nakamura thinks enough fuel remains on board to close that gap by changing course and to attempt another orbit-insertion manoeuvre.

Dangerous journey

The question then is whether the spacecraft and its instruments can survive a long interplanetary cruise. Within the next few years solar activity will peak and subject the craft to intense bursts of high-energy charged particles. The team hopes to test the spacecraft’s cameras periodically, perhaps by using them to map the distribution of interplanetary dust.

Akatsuki means “Dawn” or “Daybreak” in Japanese (not “Somehow,” as Google translates it), but it has also been called Planet C and Venus Climate Orbiter. Its boxy, desk-size structure, weighing about a half ton, carries five cameras to study the planet’s dense, cloud-choked atmosphere as never before.

Three of these cameras record infrared light at 1, 2, and 10 microns, wavelengths that probe specific altitudes and can reveal the surface. A wide-field ultraviolet imager is designed to track motions of sulphur dioxide in the stratosphere, and a visible-light camera to monitor the planet’s night side for airglow emissions and lightning. ( is a really good summary of the mission and its instruments.)

What went wrong

Launched in May, Akatsuki was supposed to operate at least two years from a looping near-equatorial orbit that ranged from 300 to 80,000 km above the planet. Its scientific mission would have nicely complemented that of the Europeans Space Agency’s , which has been orbiting the planet since April 2006.

Japan’s space agency has established an investigation committee to determine what went wrong and why. Nakamura says he is sorry that his team “could not meet the expectations of everyone in the nation”.

The whole situation is eerily reminiscent of the fate that befell another Japanese interplanetary probe almost exactly seven years ago. Ground controllers lost control of on 9 December 2003 – just five days before the spacecraft was to enter orbit around Mars.

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The bragging rights in the outer solar system may have shifted over the weekend. If the early results hold up, this time it’s the dwarf planet Eris’s turn to be demoted, and Pluto might have just regained its status as the largest object in the Kuiper Belt, the ring of icy bodies beyond Neptune.

The scene for this drama is high in the Chilean Andes, where on Saturday three teams of astronomers caught Eris in the act of passing directly in front of a star. Astronomers had known about this occultation well in advance, but the predicted path had shifted up and down along South America, leaving astronomers unsure who – if anyone – might see the event.

Well, the news from Chile is that three observing teams, all using relatively modest telescopes, saw the star blink out. Emmanuël Jehin of the University of Liège, Belgium, tracked the event with the 60-cm TRAPPIST telescope at La Silla Observatory. “I got seven frames where the light of the star is gone,” he says. Since the images were taken 4.5 seconds apart, Jehin says the occultation lasted about 27 seconds.

Then word came that the occultation had been witnessed by two more telescopes some 740 km to the north. Sebastian Saravia, Alain Maury, and Caisey Harlingten saw the star disappear for 76 seconds through Harlingten’s 50-cm PlaneWave telescope at the San Pedro de Atacama Celestial Explorations Observatory. The outage was also recorded by a remotely operated 40-cm telescope in another dome at the SPACE site, under the control of Jose-Luis Ortiz of the Institute of Astrophysics of Andalusia, Spain.

Tricky measurements

Any positive result would be cause enough to celebrate – never before has an occultation involved an object so far away. But successful observations from widely separated sites create two chords across Eris’s shadow that yields a unique solution for its diameter (assuming that the object is spherical).

That number, according to Bruno Sicardy of the Paris Observatory, is hard to pin down exactly because timings derived from the three telescopes’ light curves have some uncertainty. Even so, Sicardy notes in an email, “Almost certainly Eris has a radius smaller than 1170 km.” That would make it ever-so-slightly smaller than Pluto, whose radius is thought to be 1172 (+ or – 10) km. Don’t be surprised if the final value gets pushed another 50 or 60 km lower.

“This is huge,” notes occultation guru David Dunham. And you’ll get no disagreement from Mike Brown of Caltech, who headed the team that discovered Eris several years ago.

“Most of the ways we have of measuring the sizes of objects in the outer solar system are fraught with difficulties,” Brown notes on his . “But precisely timed occultations like these have the potential to provide incredibly precise answers.”

Growing and shrinking

We’ve gotten comfortable thinking that Eris is larger than Pluto, and why not? Images taken in December 2005 by Brown and others with the Hubble Space Telescope indicated a diameter of 2400 km, just 5 per cent larger than Pluto’s. But the true size remained uncertain because even Hubble’s super-sharp gaze is only barely able to resolve Eris’s disc. (Remember: it’s some 14 billion kilometres from the sun, twice as far away as Pluto.)

Observations made with the Spitzer Space Telescope yielded a diameter nearer 2600 km, and another group, using the IRAM radio telescope in Spain, upped the value to something closer to 3000 km. However, astronomers now realise that Eris’s spin axis is pointing toward the sun, an aspect that would keep the sunlit hemisphere warmer than average and skew any infrared measurements toward higher values.

The relatively brief occultation (it had been predicted to last nearly 2 minutes), combined with negative observations from a pair of telescopes in Argentina, suggested right away that Eris wasn’t as large as thought. So the outcome from Chile is actually more in line with the Hubble result from 2005.

Dense rock

Eris’s mass, determined from the orbit of its moon Dysnomia, is about 125 per cent that of Pluto – and that’s unchanged. So if the occultation results hold up, then the density of Eris must be higher, 2.5 g/cm³ or more, and its albedo (reflectivity) at least 90 per cent.

“A year ago I would have declared that result to be thoroughly crazy,” Brown says, “as it just seems unreasonable that Eris would have a density that high.” But he adds, “The albedo is already so ridiculously high that just a little more ridiculousness is OK.” Earlier this year Brown and others found that the largish Kuiper Belt denizen 50000 Quaoar is essentially a dense rock, averaging at least 3 g/cm³. “So I would say that densities, too, are up in the air,” Brown says.

One more thing: occultations are very powerful probes of whether an object has an atmosphere, and for now all the team members I contacted are mum on whether Eris has one. The object’s high reflectivity, combined with its great distance, suggests that the surface must be absorbing too little sunlight to vaporise any ice present there. But during an occultation in 1988 astronomers were shocked to find that Pluto has a thin atmosphere – so stay tuned!

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A couple of weeks ago we celebrated the Hubble Space Telescope’s 20 years of discovery, and today astronomers gathered in Noordwijk, The Netherlands, to recognise another milestone by an up-and-coming space observatory.

It’s been almost a year since the European Space Agency launched its Herschel Space Observatory, which shared its ride to orbit with the Planck spacecraft. Both payloads are doing well: Planck has been busy compiling the most accurate map ever made of the cosmic microwave background (CMB), the relic radiation from the big bang.

But the focus at today’s gathering was Herschel and its trove of amazing observations. The spacecraft boasts a mirror 3.5 metres across, more than twice Hubble’s light-gathering power. But Herschel looks almost exclusively at infrared light, at wavelengths from 55 to 670 microns. Since a telescope’s resolution decreases at longer wavelengths, Herschel needs all that aperture to see fine-scale details in its targets.

It also needs help to keep its own “body heat” from swamping the faint signals from the depths of space. An onboard supply of liquid helium should keep the detectors hovering near absolute zero for another three years. Cooling the primary mirror is another story – it’s too big to refrigerate, so it stays hidden behind a sun shield to remain near -193 °C.

Herschel carries : a pair of cameras (PACS and SPIRE) and a ultra-high-precision spectrometer (HIFI). The mission’s one serious glitch came last August, when HIFI fell silent, but in January controllers switched to a set of backup electronics and it’s been working fine since then.

Bubble birth

At a news briefing on Thursday, team member Annie Zavagno of the Laboratory of Astrophysics of Marseilles in France focused on the little-understood process that spawns beefy stars with at least eight times the sun’s mass.

These heavyweights are rarely found, both because just-forming stars exercise lots of self-control when gobbling up interstellar gas and because massive stars are short-lived – they exhaust their hydrogen fuel in just a few million years.

Zavagno and her team have used Herschel to spot high-mass stars as they form, and it now seems that the trigger points occur along the margins of rapidly expanding ionised-hydrogen bubbles.

Meanwhile, the recently rejuvenated HIFI spectrometer has been tracking down concentrations of ionised water, which has a distinct and strong spectral signature.

“Water is an excellent diagnostic tool to probe the chemical and physical structure of the interstellar medium,” explained Alexander Tielens of Leiden University. In particular, he noted, water helps cool the gas and dust surrounding newborn stars by radiating infrared energy to space.

Dark horse

But what was really eye-catching was a side-by-side comparison of the famed Horsehead Nebula in Orion (see image). It turns out that the dark “voids” that give this showpiece its distinctive appearance in visible light are ablaze with star formation when spied in the infrared.

Steve Eales of the University of Cardiff showed the paired views to demonstrate the potential of the (short for Astrophysical Terahertz Large Area Survey), which will map 550 square degrees of sky in five infrared bands. Eales expects the 600-hour effort to reveal some 250,000 galaxies.

Herschel is showing us the “warm universe” as never before, and these results demonstrate that we’ve only scratched the surface of what this spacecraft will be revealing in the months and years ahead.

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It seems years ago – November 29, 2005, to be exact – since a Japanese spacecraft named Hayabusa touched down on a small asteroid in the hope of grabbing samples of its dusty surface and returning them to Earth. Had the mission gone according to plan, the precious bits from asteroid 25143 Itokawa would have reached waiting scientists in June 2007.

But the flight of Hayabusa, Japanese for “falcon,” has been anything but nominal. In fact, it’s been more of a train wreck.

The craft was nearly lost during its grab-and-go encounter due to a series of malfunctions that should have doomed the spacecraft. But it hung on, despite suffering a massive fuel leak, battery failure, and being incommunicado for two months. Then its attitude-control system failed. The loss of three of its four xenon-powered engines meant it would take three extra years to get the crippled craft home, nursed every step of the way by its dedicated team of engineers.

Well, folks, Hayabusa is almost home. from project manager Jun’ichiro Kawaguchi is that the sole remaining engine was commanded to shut down on March 27th, having gently accelerated the craft by 400 metres per second over the past year and nudged it onto a trajectory that will pass within several thousand kilometres of Earth. “What is left is a series of trajectory corrections,” Kawaguchi explains, “and the project team is finalizing the preparations for them.”

Unknown cargo

Barring an eleventh-hour setback, in mid-June a small 18-kilogram descent capsule will separate from the main spacecraft and slam into the atmosphere over south-central Australia. The larger craft will then manoeuvre to avoid Earth. Streaking through the darkness at 12.2 kilometres per second, the capsule should parachute to the ground somewhere along a target zone, measuring 100 by 15 km, in the remote .

After whisking it back to a clean room at the Japan Aerospace Exploration Agency (JAXA), scientists will carefully open the 40-cm capsule to learn, finally, whether it contains any asteroidal bits. It’s hardly a sure thing – despite sitting on Itokawa’s surface for 30 minutes, Hayabusa failed to fire two small tantalum pellets designed to kick surface material into a collection cone.

Hayabusa’s successful return would be a big deal in Japan, and plans for the welcome-home party are well under way. Kawaguchi has been careful not to divulge the exact date publicly, pending the engine shutdown and a sign-off from Australian authorities. “It is not at the beginning of June, and it is not at the end of June,” he teases. JAXA has produced an informative 21-minute video about the mission, in English, that you can view . There’s even a dramatic movie treatment: .

Airborne observatory

Because spacecraft rarely come down through the atmosphere so fast – Earth-orbiting satellites fall in about a third slower – there’s plenty of scientific interest in the re-entry itself. The capsule should create an artificial fireball beginning at an altitude of about 200 km and hit a peak brightness of magnitude -6.7 (several times brighter than Venus) before deploying its parachute.

For the past year, meteor specialist of the SETI Institute in California has been organising an international team to from an instrument-packed DC-8 jet flying near the recovery zone. Jenniskens mounted a similar effort for the return of the Stardust sample capsule in January 2006.

Will Hayabusa, despite all its problems, make it back to Earth? Will the capsule contain hard-won bits of asteroid Itokawa? Will Kawaguchi and his team get a ticker-tape parade through downtown Tokyo? Stay tuned for the final chapter of this remarkable mission!

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The mystery of a comet-like object circling among the main-belt asteroids has deepened, now that astronomers have slewed the Hubble Space Telescope (HST) over for a close-up look.

The release of a Hubble image taken on 29 January shows that this object, called P/2010 A2, is no normal comet. There’s a strange X-shape feature at its brightest end (I hesitate to call it a “nucleus”) that defies easy explanation. “This is quite different from the smooth dust envelopes of normal comets,” explains David Jewitt of the University of California at Los Angeles, who led the Hubble effort. The Hubble view also clearly shows a point-like object hovering nearby, about 1600 kilometres from the “X” and seemingly connected to it by a thread of material.

Comets within the asteroid belt aren’t unprecedented. comments that four others are known. Most notable among these is 133P/Elst-Pizarro, first spotted as an asteroid in 1979 and then found to have a cometary appearance in 1996.

Fast-spinning asteroid?

But apart from its long tail of debris, P/2010 A2 doesn’t look much like a comet, at least in the HST images taken on January 25th and 29th. Moreover, it lacks the emissions that are usually found in a comet’s coma and tail. “No gas yet,” Jewitt told me, “but we are still looking.” Unfortunately, a request to observe this curiosity with the Spitzer Space Telescope was turned down.

The most likely explanation for this object’s sudden appearance is that a pair of small, unseen asteroids collided, creating the trail of debris first spotted on January 6th by the LINEAR telescope in New Mexico.

So would a smashup involving two main-belt asteroids result in something like this? Perhaps. “Bill Bottke and I are thinking about what kind of modelling would be useful to do with this object,” notes of the Southwest Research Institute in Boulder, Colorado, “but we have nothing definitive so far”. Bottke notes that while a collision seems plausible, it’s also possible we are witnessing an asteroid that has been spun up (by gradual interactions with sunlight) to the point that it started shedding mass.

Jewitt says that there’s no handle yet on whether the “nucleus” is tumbling or spinning rapidly. But he says more Hubble sessions are planned in the months ahead.

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A press advisory notes that on Wednesday evening, “the President and First Lady will host an event at the White House for middle-school students to highlight the President’s commitment to science, engineering and math education as the foundation of this nation’s global technological and economic leadership and to express his support for astronomy in particular – for its capacity to promote a greater awareness of our place in the universe, expand human knowledge, and inspire the next generation by showing them the beauty and mysteries of the night sky.”

This stellar event is the brainchild of Audrey Fischer, a Chicago-area amateur who imagined uniting children around the world with one big stargaze. Early this year, her head full of stars after a visit to , she fired off an email to the White House to pitch the idea. “Every astronomer, teacher, and child I talked with [is] so enthusiastic,” she wrote. “They are just holding their breath for your response.”

You can imagine how many entreating emails West Wing staffers must slog through each day, but something about this one clicked. Desiree Rodgers, the White House’s social secretary (in charge of events like the annual ), called Fischer for more details. Hundreds of supportive messages poured in from around the country, and a volley of phone and email exchanges between Fischer and various Administration officials followed.

On Wednesday, President Obama will kick-off the event at about 2000 EDT (0000 GMT on Thursday) with a brief address that will be streamed live by the and by . Plans include viewing through more than 20 telescopes; presentations in portable planetariums; and various hands-on activities. (If it’s cloudy, there will likely be a secondary site to host the event indoors, perhaps the US Naval Observatory.)

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Update from at 1010 PDT (1710 GMT) on Tuesday: “Some good news. U.S. Forest Servie Fire Dispatch has informed us that as of 9:40 this morning ground crews were back at the Observatory. As of 8:00 am, air tankers were back in operation. The dispatcher expressed his opinion to Dave that as long as the fire continues to press the mountain from one direction ‘you are going to make it.’ Furthermore, there is some light rain developing in places in the Los Angeles basin, and there is a possibility for some thunderstorm activity that could lead to dry lightning. The humidity is up and the temperature is a bit lower, so, all in all, things are looking more promising than they have in the last few days.”

A rampaging fire in the Angeles National Forest in southern California is threatening a historic observatory at the summit of Mount Wilson, as well as a dense crowd of television transmitters there.

The “” fire began on Wednesday afternoon but has spread quickly, overnight, to an area covering more than 345 square kilometres. With tongues of flame stretching about 25 metres long, the fire is only 5 per cent contained and will likely take another week to contain completely.

By Sunday evening, it had moved to within 3.2 km of Mount Wilson Observatory. On Monday, fire officials fire had not reached the area, but they said the area still remained highly vulnerable.

Fire fighters were ordered to withdraw from Mount Wilson on Monday morning because it was too dangerous for crews to be in the area, according to the Los Angeles Times. Flame retardant will instead be dropped on the mountain from planes.

Throughout the day on Sunday, crews cleared tinder-dry brush from around the observatory complex and treated wooden structures to make them less vulnerable.

Historic discoveries

Located at an altitude of 1740 metres, Mount Wilson Observatory got its start in 1904 when George Ellery Hale signed a free, 99-year lease for 40 acres at the summit to build world-class telescopes.

Then Hale erected the Snow Solar Telescope (1905), a 60-inch reflector (the world’s largest when completed in 1908), the 150-foot Solar Tower, and finally the 100-inch Hooker Telescope (1918), which Edwin Hubble used to discover that the universe is expanding.

Despite being swamped with light pollution from the 13 million residents to its immediate south and southeast, the observatory has regained much of its scientific relevance.

In recent years, Mount Wilson has served as a test-bed for adaptive-optics and interferometric imaging. It’s the main facility of Georgia State University’s Center for High Angular Resolution Astronomy () and the site of the University of California’s Infrared Spatial Interferometer ().

Closed NASA site

Over the weekend, the Station Fire also kept NASA’s Jet Propulsion Laboratory closed to all non-critical personnel. Officially, JPL is in Pasadena, California, but technically it’s in the town of La Cañada Flintridge, which has been posting about the rapidly spreading inferno.

Building observatories on remote mountaintops places them at great risk from forest fires, especially where climates have turned dryer in the last decade or two.

This is not the first time Southern California’s notoriously frequent conflagrations have threatened a major astronomical facility. In November 2007, the Poomacha Fire came near, but did not damage, Palomar Observatory. Likewise the University of Arizona’s Steward Observatory had a close call during the Aspen Fire in June 2003.

Australia’s Mount Stromlo Observatory wasn’t so lucky – it was totally destroyed by a wildfire in 2003.

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For the past couple of years, our sun has been at the minimum of its 11-year activity cycle. Its face has been virtually spotless for months on end, and there’ve been no dire alerts of titanic solar storms about to slam into Earth.

The problem is that this “quiet sun” has continued far too long – two years ago, a special task force predicted that the transition from the just-ended Cycle 23 to the upcoming Cycle 24 would come around March 2008. It didn’t. (To be fair, there was sharp disagreement within the group at that time.)

Much fanfare accompanied the appearance of a tiny high-latitude sunspot in early 2008, supposedly heralding Cycle 24’s arrival. Yet for months and months afterward the sun’s face remained spotless.

Knowing when the upturn in solar activity begins and, more importantly, how strong it will get at maximum has grown in importance over the years. When the sun gets agitated, it buffets our planet with huge “storms” of high-speed plasma (ionised gas), punctuated by threatening flares of relativistic protons (see Space storm alert: 90 seconds from catastrophe).

Rhythmic pulsations

But despite centuries-long records of sunspot counts and 50 years of mapping the sun’s magnetic fields, scientists still don’t understand what makes one cycle strong and another weak. The same task force that was bullish on Cycle 24 two years ago now believes the forthcoming activity could be the weakest in a century.

This week, two groups of researchers offered hope that we’re finally understanding the sun’s complex workings a little better.

The first comes from Rachel Howe and Frank Hill of the National Solar Observatory, who now believe that sunspots are linked to slow, eastward-moving “jet streams” about 7000 kilometres below the sun’s visible surface, or photosphere. They’ve analysed 15 years of observations made using helioseismology, which monitors rhythmic pulsations at the surface created by pressure waves bouncing around the solar interior.

“Think of the sun as a musical instrument,” Hill explains. “A piano has 88 keys, but the sun has five million ‘notes’ or modes of oscillation.”

Deep-seated currents

Howe and Hill find that a pair of deep-seated currents migrate from the solar poles toward the equator during each cycle, and that the migration has been unusually sluggish of late.

They took three years to shift 10° towards the equator, and only now have they reached a solar latitude of 22°, the point at which activity perks up and sunspots start to appear. They can’t yet tell whether the flow somehow causes sunspots, only that the two phenomena appear to be related.

“Had this analysis been available two years ago, we’d have seen the delayed onset of Cycle 24 coming,” notes solar physicist Dean Pernell of NASA’s Goddard Space Flight Center in Maryland.

The new findings were presented this week at a meeting of the American Astronomical Society’s Solar Physics Division, which had a special session on Cycle 24.

Sunspot model

The second announcement concerns sunspots themselves and the arrangement of the intense magnetic tangles within them. Writing in Science Express, Matthias Rempel of the National Center for Atmospheric Research and three colleagues used a supercomputer grinding out 76 trillion calculations per second to create the first comprehensive, 3D model of these mysterious dark patches’ inner workings.

The simulations reveal in detail how superheated gas streams along magnetic filaments from a spot’s dark, central umbra to the lighter penumbra surrounding it.

Solar physicists first recognised this outward flow about 100 years ago. But, as Rempel’s team notes, “The onset of these flows is closely related to the magnetic field inclination” and that outflows occur whenever the magnetic field is inclined more than 45° from vertical.

The hope is that a better understanding of sunspots will allow scientists to predict their behaviour more accurately and, in particular, to identify the ones most likely to trigger dangerous solar flares.

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Skywatchers have been trying to gauge the sun-Earth distance for thousands of years. In the third century BC, Aristarchus of Samos, notable as the first to argue for a heliocentric solar system, estimated the sun to be 20 times farther away than the moon. It wasn’t his best work, as the real factor is more like 400.

By the late 20^th century, astronomers had a much better grip on this fundamental cosmic metric – what came to be called the . In fact, thanks to radar beams pinging off various solar-system bodies and to tracking of interplanetary spacecraft, the sun-Earth distance has been pegged with remarkable accuracy. The current value stands at 149,597,870.696 kilometres.

Having such a precise yardstick allowed Russian dynamicists Gregoriy A. Krasinsky and Victor A. Brumberg to calculate, in 2004, that the sun and Earth are gradually moving apart. It’s not much – just 15 cm per year – but since that’s 100 times greater than the measurement error, something must really be pushing Earth outward. But what?

One idea is that the Sun is losing enough mass, via fusion and the solar wind, to gradually be losing its gravitational grip (see Astronomical unit may need to be redefined). Other possible explanations include a change in the gravitational constant G, the effects of cosmic expansion, and even the influence of dark matter. None have proved satisfactory.

Small tides

But Takaho Miura of Hirosaki University in Japan and three colleagues think they have the answer. In an submitted to the European journal Astronomy & Astrophysics, they argue that the sun and Earth are literally pushing each other away due to their tidal interaction.

It’s the same process that’s gradually driving the moon’s orbit outward: Tides raised by the moon in our oceans are gradually transferring Earth’s rotational energy to lunar motion. As a consequence, each year the moon’s orbit expands by about 4 cm and Earth’s rotation slows by 0.000017 second.

Likewise, Miura’s team assumes that our planet’s mass is raising a tiny but sustained tidal bulge in the sun. They calculate that, thanks to Earth, the sun’s rotation rate is slowing by 3 milliseconds per century (0.00003 second per year). According to their explanation, the distance between the Earth and sun is growing because the sun is losing its angular momentum.

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Impacts are the most ubiquitous geologic features in our solar system. Roughly (and countless lesser pits) scar the Moon’s ancient surfaces. On Earth, where wind and water continually wear down the land, the census of stands at just 176.

Mars, a mixed bag of ancient and modern terrains, lies somewhere in between. Over the years spacecraft have glimpsed ever-finer features in the Martian landscape. These days, the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter (MRO) can pick out objects only 0.3 metres in size; the High Resolution Stereo Camera on the European Space Agency’s Mars Express is no slouch either, with a ground resolution of 2 metres.

So HiRISE researchers were elated, but not particularly surprised, to discover some small, freshly gouged craters in images taken in 2008. Seen at five sites over a latitude range of 43° to 56° north, the excavations are typically 3 to 6 metres across and a third to two-thirds of a metre deep. One cluster must have appeared sometime between June and August, and a showed up between January and September.

What did astound the team were splashes of white seen in and around a handful of these craterlets. Could it be water ice? Colleagues operating the spacecraft’s CRISM instrument soon confirmed, for the one case large enough to yield a spectrum, that it was! Apparently fist-sized impactors had punched into a layer of ice hidden by a topping of dust about a third of a metre deep.

Disappearing act

In the months that followed, these snowy splashes gradually faded from view. Water ice isn’t stable at the craters’ latitudes, so most likely it gradually sublimated, or vaporised, into the atmosphere, leaving behind a veneer of any dust that had been mixed with it.

The disappearing act might also be due in part to a coating of dust blown in from the atmosphere. Either way, notes HiRISE investigator Shane Byrne of the University of Arizona, the icy deposits had to be at least a couple of inches (several centimetres) thick, and they couldn’t have been unearthed from more than a foot or two (0.3-0.6 m) down.

Byrne on Friday at the Lunar and Planetary Science Conference in The Woodlands, Texas. He points out that prior surveys, particularly one done by the neutron spectrometer aboard NASA’s Mars Odyssey orbiter, show that vast reservoirs of ice lay barely buried across most of the planet’s polar and mid-latitude regions.

So close

But scientists are only now realising just how near the surface the ice lies – and how easily it can be reached. When NASA’s Phoenix lander dropped onto a northern polar plain last May, its braking engine blew off a few inches of loose dirt and revealed slabs of nearly pure ice.

The irony in all this is that the Viking 2 lander, which arrived in September 1976, sits just 800 km southeast of the ice-splashed craterlet shown above, and scientists now realise that a layer of water ice almost certainly lies not far beneath its footpads.

“It’s probably just tens of centimetres down,” says HiRISE team leader Alfred McEwen. Had Viking’s sampling scoop been able to dig a little deeper, he adds, “we might have sampled ice on Mars 30 years ago.”

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