There Might Not Be Ice at the Moon’s Pole

Image credit: Cornell University

At the South Pole of the Moon, there is a region that is always in the shadow of craters which scientists have long believed could have deposits of water ice. Despite the fact that ice was detected by two spacecraft that orbited the moon, a new survey of the area by the giant Arecibo radio observatory has failed to find any surface deposits of ice. This doesn’t mean that the ice isn’t there, but it might be trapped in a large area under the surface, like lunar permafrost. Arecibo is a good instrument for detecting ice because it gives a very specific echo signature in the radio spectrum.

Despite evidence from two space probes in the 1990s, radar astronomers say they can find no signs of thick ice at the moon’s poles. If there is water at the lunar poles, the researchers say, it is widely scattered and permanently frozen inside the dust layers, something akin to terrestrial permafrost.

Using the 70-centimeter (cm)-wavelength radar system at the National Science Foundation’s (NSF) Arecibo Observatory, Puerto Rico, the research group sent signals deeper into the lunar polar surface — more than five meters (about 5.5 yards) — than ever before at this spatial resolution. “If there is ice at the poles, the only way left to test it is to go there directly and melt a small volume around the dust and look for water with a mass spectrometer,” says Bruce Campbell of the Center for Earth and Planetary Studies at the Smithsonian Institution.

Campbell is the lead author of an article, “Long-Wavelength Radar Probing of the Lunar Poles,” in the Nov. 13, 2003, issue of the journal Nature . His collaborators on the latest radar probe of the moon were Donald Campbell, professor of astronomy at Cornell University; J.F. Chandler of Smithsonian Astrophysical Observatory; and Alice Hine, Mike Nolan and Phil Perillat of the Arecibo Observatory, which is managed by the National Astronomy and Ionosphere Center at Cornell for the NSF.

Suggestions of lunar ice first came in 1996 when radio data from the Clementine spacecraft gave some indications of the presence of ice on the wall of a crater at the moon’s south pole. Then, neutron spectrometer data from the Lunar Prospector spacecraft, launched in 1998, indicated the presence of hydrogen, and by inference, water, at a depth of about a meter at the lunar poles. But radar probes by the 12-cm-wavelength radar at Arecibo showed no evidence of thick ice at depths of up to a meter. “Lunar Prospector had found significant concentrations of hydrogen at the lunar poles equivalent to water ice at concentrations of a few percent of the lunar soil,” says Donald Campbell. “There have been suggestions that it may be in the form of thick deposits of ice at some depth, but this new data from Arecibo makes that unlikely.”

Says Bruce Campbell, “There are no places that we have looked at with any of these wavelengths where you see that kind of signature.”

The Nature paper notes that if ice does exist at the lunar poles it would be considerably different from “the thick, coherent layers of ice observed in shadowed craters on Mercury,” found in Arecibo radar imaging. “On Mercury what you see are quite thick deposits on the order of a meter or more buried by, at most, a shallow layer of dust. That’s the scenario we were trying to nail down for the moon,” says Bruce Campbell. The difference between Mercury and the moon, the researchers say, could be due to the lower average rate of comets striking the lunar surface, to recent comet impacts on Mercury or to a more rapid loss of ice on the moon.

What makes the lunar poles good cold traps for water is a temperature of minus 173 degrees Celsius (minus 280 degrees Fahrenheit). The limb of the sun rises only about two degrees above the horizon at the lunar poles so that sunlight never penetrates into deep craters, and a person standing on the crater floor would never see the sun. The Arecibo radar probed the floors of two craters in permanent shadow at the lunar south pole, Shoemaker and Faustini, and, at the north pole, the floors of Hermite and several small craters within the large crater Peary. In contrast, Clementine focused on the sloping walls of Shackleton crater, whose floor can’t be “seen” from Earth. “There is a debate on how to interpret data from a rough, tilted surface,” says Bruce Campbell.

The Arecibo radar probe is a particularly good detector of thick ice because it takes advantage of a phenomenon known as “coherent backscatter.” Radar waves can travel long distances without being absorbed in ice at temperatures well below freezing. Reflections from irregularities inside the ice produce a very strong radar echo. In contrast, lunar soil is much more absorptive and does not give as strong a radar echo.

Original Source: Cornell News Release

Brightest Full Moon this Year


Image credit: NASA
The full moon on February 27 is going to be the brightest one of 2002. The moon’s orbit isn’t a perfect circle; over the course of its 28-day trip around the Earth, its distance varies from 406,700 km to 356,400. And today’s full moon happens to coincide with the closest point of that orbit, making it 20% brighter than an average full moon.

A pale ray of light shines through the bedroom window. In the distance, something howls. Eyes open. The clock ticks, it’s 2 a.m.. You’re wide awake — roused by a bright full Moon.

Don’t be surprised if this soon happens to you. The Moon will become full on Feb. 27th. It happens every 29.5 days, yet this full Moon is special: It’s the biggest and brightest of the year.

“Not all full Moons are alike,” says astronomy professor George Lebo. “Sometimes pollution or volcanic ash shades them with interesting colors. Sometimes haloes form around them — a result of ice crystals in the air.”

“This full Moon is unique in another way,” he says. “It will be closer to Earth than usual.”

Right: The apparent size of the Moon at perigee (top) and apogee (bottom).

“The moon’s orbit around our planet is not a perfect circle,” Lebo explains. “It’s an ellipse.” At one end of the ellipse (called apogee) the Moon lies 406,700 km from Earth. At the other end (called perigee) the Moon is only 356,400 km away — a difference of 50 thousand km!

When the Moon is full on Feb. 27th it will be near perigee — close to Earth. As a result the Moon will appear 9% wider than normal and shine 20% brighter.

The extra moonlight is caused, in part, by the Moon’s nearness to Earth. But that’s not all. The Sun is closer to Earth, too. Lebo explains: “Every year during northern winter, Earth is about 1.6% closer to the Sun than normal. (Like the Moon’s orbit around Earth, Earth’s orbit around the Sun is elliptical. Our closest approach to the Sun is called perihelion.) The Moon reflects sunlight, so the Moon is brighter during that time.”

This effect should not to be confused with the famous “Moon Illusion” — a trick of the eye that makes Moons rising near the horizon appear swollen. The nearby full Moon this week really will be bigger and brighter.

Below: The brightness of full Moons in 2002 relative to that of an average full Moon. In Feb., for example, the Moon will be 20% brighter than average; in Aug. it will be 12% dimmer. These values take into account the varying distances of the Moon from Earth and of the Earth from the Sun.

The first three full Moons of 2002 are all brighter-than-average. All three happen when the Moon is near perigee, and when Earth is relatively close to the Sun. Full Moons later this year will be smaller and dimmer by comparison. For example, August’s full Moon — an “apogee Moon” — will be about one-third dimmer than February’s.

But will anyone notice the difference?

“The human eye can easily discern a 20 or 30% difference in the brightness of two similar light sources,” says eye doctor Stuart Hiroyasu. By that reckoning, a sky watcher could tell the difference between a bright perigee Moon and a dimmer apogee Moon. But the two Moons would have to be side by side to effect the comparison — not likely except in a science fiction movie!

Below: Our Moon’s appearance changes nightly. This time-lapse sequence (Credit: Ant?nio Cidad?o) shows what our Moon looks like during a lunation, a complete lunar cycle. [more]

Even the dimmest full Moons are very bright, notes Lebo. They outshine Sirius, the brightest star in the sky, by twenty-five thousand times. They cast shadows, and provide enough light to read by. “There’s really no such thing as a faint full Moon. It’s all relative.”

Nevertheless, some sky watchers will sense that this Moon has something “extra” — particularly northerners. Many northern landscapes in February remain covered with snow. Snow reflects about two-thirds of the light that hits it, while bare ground reflects only about 15%. A snowy moonlit landscape always seems remarkably bright.

Perigee, perihelion, snowy terrain — they all add up to a big dose of Moonlight. Can you tell the difference? There’s only one way to find out: Go outside and look!

Original Source: NASA Science Story

Moon Could Still Have Molten Interior

Astronomers have calculated that the Moon, pulled by the gravity of the Earth and the Sun, may bulge as much as 10 centimetres over the course of its 27 day journey around our planet. The bulging could be caused by a molten slush surrounding the Moon’s core. The measurements were gathered by firing a laser pulse from the Earth to the Moon, and it measures the round-trip distance to an accuracy of 2 centimetres.