Posted on by Fraser Cain

Charon has no Atmosphere

An artist’s conception of Pluto and its moon Charon. Image credit: NASA Click to enlarge
If you want to learn something about a place that’s billions of miles away, it helps to be in the right place at the right time.

Astronomers from MIT and Williams College were lucky enough to watch as Pluto’s largest moon, Charon, passed in front of a star last summer. Based on their observations of the occultation, which lasted for less than a minute, the team reports new details about the moon in the Jan. 5 issue of Nature.

A second paper from another group, led by French astronomer Bruno Sicardy, also appears in this issue of Nature.

The MIT-Williams team was able to measure Charon’s size to an unprecedented accuracy and determine that it has no significant atmosphere. The atmosphere on Pluto, on the other hand, has been very well established.

“The results provide insight into the formation and evolution of bodies in the outer solar system,” said lead author Amanda Gulbis, a postdoctoral associate in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

Specifically, the team found that Charon has a radius of 606 kilometers, “plus or minus 8 kilometers to account for local topography or possible non-sphericity in Charon’s shape,” Gulbis said. That size, combined with mass measurements from Hubble Space Telescope data, show that the moon has a density roughly one-third that of the Earth. This reflects Charon’s rocky-icy composition.

The team also found that the density of any atmosphere on the moon must be less than a millionth of that of the Earth. This argues against the theory that Pluto and Charon were formed by the cooling and condensing of the gas and dust known as the solar nebula. Instead, Charon was likely created in a celestial collision between an object and a proto-Pluto.

“Our observations show that there is no substantial atmosphere on Charon, which is consistent with an impact formation scenario,” Gulbis said. Similar theories exist about the formation of the Earth-moon system.

The success of the MIT-Williams team in observing the Charon occultation bodes well for future adaptations of the technique the researchers used.

“We are eager to use (it) to probe for atmospheres around recently discovered Kuiper Belt objects that are Pluto-sized or even larger,” said James Elliot, co-author of the Nature paper and a professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences and in the Department of Physics. Elliot has been observing stellar occultations by bodies in the solar system for more than three decades.

Jay Pasachoff, Williams College team leader and a professor in its Department of Astronomy, said, “It’s remarkable that our group could be in the right place at the right time to line up a tiny body 3 billion miles away. The successful observations are quite a reward for all of the people who helped predict the event, constructed and integrated the equipment and traveled to the telescopes.”

In addition to Elliot and Gulbis, members of the MIT team were Michael Person, Elisabeth Adams and Susan Kern, with support from undergraduate Emily Kramer. The Williams College team included Pasachoff, Bryce Babcock, Steven Souza and undergraduate Joseph Gangestad.

The work was supported by NASA.

Original Source: MIT News Release

Update: Pluto isn’t a planet. Why isn’t Pluto a planet?

Posted on by Fraser Cain

When a Meteor Slashed Mars

Color view of ‘butterfly’-shaped crater at Hesperia Planum. Image credit: ESA Click to enlarge
These images, taken by the High Resolution Stereo Camera (HRSC) on board ESA?s Mars Express spacecraft, show a large elliptical impact crater in the Hesperia Planum region of Mars.

The HRSC obtained these images during orbit 368 with a ground resolution of approximately 16.7 metres per pixel. The scenes show the region of Hesperia Planum, at approximately 35.3? South and 118.7? East. A large elliptical impact crater is visible within the scene, measuring approximately 24.4 km long, 11.2 km wide and reaching a maximum depth of approximately 650 metres below the surrounding plains.

Ejecta from this impact can be seen extending away from the crater, including two prominent lobes of material north-west and south-east of the crater.

The large circular feature, partly cut off by the border of the image, has a diameter of roughly 45 km.

This appears to be an impact crater that was subsequently resurfaced by lava flows, preserving the outline of the underlying crater. The curving features visible in the north of the image, known as ?wrinkle ridges?, are caused by compressional tectonics.

While the majority of impact craters are relatively circular, the elliptical shape of this impact crater suggests a very low impact angle (less than 10 degrees).

The long axis of the impact crater is viewed as the impacting direction of the projectile. Similar elliptical craters are observed elsewhere on Mars, as well as on our Moon.

The colour scenes have been derived from the three HRSC-colour channels and the nadir channel. The perspective views have been calculated from the digital terrain model derived from the stereo channels.

Color view of ‘butterfly’-shaped crater at Hesperia Planum. Image credit: ESA Click to enlarge

The 3D anaglyph image was calculated from the nadir and one stereo channel. Image resolution has been decreased for use on the internet.

Original Source: ESA Mars Express

Posted on by Fraser Cain

Superbubble Complex N44

Superbubble complex N44 as imaged with GMOS. Image credit: University of Alaska Anchorage. Click to enlarge
Known as the N44 superbubble complex, this cloudy tempest is dominated by a vast bubble about 325 by 250 light-years across. A cluster of massive stars inside the cavern has cleared away gas to form a distinctive mouth-shaped hollow shell. While astronomers do not agree on exactly how this bubble has evolved for up to the past 10 million years, they do know that the central cluster of massive stars is responsible for the cloud’s unusual appearance. It is likely that the explosive death of one or more of the cluster’s most massive and short-lived stars played a key role in the formation of the large bubble.

“This region is like a giant laboratory providing us with a glimpse into many unique phenomena,” said Sally Oey of the University of Michigan, who has studied this object extensively. “Observations from space have even revealed x-ray-emitting gas escaping from this superbubble, and while this is expected, this is the only object of its kind where we have actually seen it happening.”

One of the mysteries surrounding this object points to the role that supernova explosions (marking the destruction of the most massive of the central cluster’s stars) could have played in sculpting the cloud. Philip Massey of Lowell Observatory, who studied this region along with Oey, adds “When we look at the speed of the gases in this cloud we find inconsistencies in the size of the bubble and the expected velocities of the winds from the central cluster of massive stars. Supernovae, the ages of the central stars, or the orientation and shape of the cloud might explain this, but the bottom line is that there’s still lots of exciting science to be done here and these new images will undoubtedly help.”

The Gemini data used to produce this image are being released to the astronomical community for further research and follow-up analysis. Note to astronomers: Data can be found at the Gemini Science Archive by querying “NGC 1929”. The image provides one of the most detailed views ever obtained of this relatively large region in the Large Magellanic Cloud, a satellite galaxy to the Milky Way, located some 150,000 light-years away and visible from the Southern Hemisphere. The images captured light of specific colors that reveal the compression of material and the presence of gases (primarily excited hydrogen gas and lesser amounts of oxygen and “shocked” sulfur) in the cloud.

Multiple smaller bubbles appear in the image as bulbous growths clinging to the central superbubble. Most of these regions were probably formed as part of the same process that shaped the central cluster. Their formation could also have been “sparked” by compression as the central stars pushed the surrounding gas outward. Our view into this cavern could really be like looking through an elongated tube, which lends the object its monstrous mouth-like appearance.

The images used to produce the color composite were obtained with the Gemini Multi-object Spectrograph (GMOS) at the Gemini South Telescope on Cerro Pachon in Chile. The color image was produced by Travis Rector of the University of Alaska Anchorage and combines three single-color images to produce the image.

Original Source: Gemini Observatory

Posted on by Fraser Cain

A Supernova Every 50 Years

An artist’s illustartion of the sequence of radioactive decay that gives out gamma rays. Image credit: MPE Click to enlarge
Using ESA’s Integral observatory, an international team of researchers has been able to confirm the production of radioactive aluminium (Al 26) in massive stars and supernovae throughout our galaxy and determine the rate of supernovae – one of its key parameters.

The team, led by Roland Diehl of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, determined that gamma rays from the decay of Al 26 originate from the central regions of our galaxy, implying that production of new atomic nuclei is an ongoing process and occurs in star-forming regions galaxy-wide.

Our environment is composed of chemical elements formed long ago by nuclear fusion reactions in stellar interiors and supernovae. This process of ‘nucleosynthesis’ leads to the emission of gamma rays, which easily reach us from all regions of our galaxy. ESA’s Integral observatory has been measuring such gamma rays since October 2002.

Roland Diehl and his colleagues were able to measure the Al 26 gamma-ray emissions along the plane of the inner galaxy.

However, because the disc of the galaxy rotates about its central axis, with the inner regions orbiting faster, gamma rays from decaying Al 26 observed from these regions should be moderated by the Doppler effect in a characteristic way. It is this characteristic pattern that has been found by Integral.

From this measurement, the team found that Al 26 decay gamma rays do indeed reach us from the inner regions of the galaxy, rather than from foreground regions along the same line of sight possibly caused by local and peculiar Al 26 production. These regions would not have the observed high relative velocity.

From these new observations, it is possible to estimate the total amount of radioactive Al 26 in our galaxy as is equivalent to three solar masses. This is a lot, given that Al 26 is an extremely rare isotope; the fraction estimated for the early Solar System is 5/100 000 of Al 26, in proportion to its stable aluminium isotope (Al 27).

Because astrophysicists had inferred that the likely sources are mainly massive stars, which end their lives as supernovae, they could estimate the rate of such supernova events. They obtained a rate of one supernova every 50 years – consistent with what had been indirectly found from observations of other galaxies and their comparison to the Milky Way.

Integral’s study of gamma rays will continue to operate for several more years. Astrophysicists hope to increase the precision of such measurements. Project leader Roland Diehl said, “These gamma-ray observations provide insights about our home galaxy, which are difficult to obtain at other wavelengths due to interstellar absorption.”

Original Source: ESA Portal

Posted on by Nancy Atkinson

Leading the Way Back to the Moon

Computer illustration of the CEV in orbit around the Moon. Image credit: NASA. Click to enlarge.
Jeff Hanley was only 8 years old on July 20, 1969 when Apollo 11 landed on the moon, but he can recall every detail of that day and all the specifics of that historic mission. Each of the Apollo missions to the moon made such a big impact on Hanley that space exploration became his life’s passion, ultimately becoming his profession. Now, Hanley has been appointed to lead NASA’s new program to return astronauts to the moon and prepare to send human expeditions to Mars.

Hanley started working at NASA while he was still in college and eventually became a flight controller in Houston’s Mission Control for 13 years, and then became a flight director in 1996. He oversaw two of the complex missions to refurbish the Hubble Space Telescope and was the lead flight director for the first expedition crew to the International Space Station in 2000. He led the Space Station Flight Director Office for two years before being promoted to chief of flight directors for all space missions in January of 2005.

Hanley has served in his current position as manager for NASA’s new Constellation Program since October 2005. His tenure thus far has been a series of constant meetings, briefings and trips around the country to the various NASA centers. His job is to lead the development of a new spacecraft and launch system, the focal point of NASA’s Vision for Space Exploration.

“We have not developed a new crew launch system from scratch since the space shuttle in the late 1970’s,” Hanley said. “That’s a generational gap we have to overcome, so we’re building a bridge from what we have today to what we want in the future.” The space vehicles that Hanley and his team are designing are combinations of the best elements from both the space shuttle and the Apollo spacecraft with significant improvements that come from advances in technology.

The new Crew Exploration Vehicle (CEV), while reminiscent of the Apollo blunt-body capsule, is three times larger with the capacity to carry four astronauts to the moon. It also has the ability to dock with the International Space Station, and the same crew vehicle will eventually carry astronauts to Mars. The separate lunar module will be able to land anywhere on the moon, including the poles, unlike the Apollo spacecraft that could only land near the equator. Initially, crews will stay up to 7 days on the moon’s surface.

“Apollo’s purpose was to send a man to the moon and return him safely to the earth,” Hanley said. “We go a substantial step beyond that with this architecture in terms of the capacity to deliver large amounts of mass to the moon and that’s really sending the signal that we’re serious about exploration and serious about coming to stay.” Developing a sustained presence on the moon will be the ultimate goal of the lunar missions, to demonstrate that humans can survive for long periods of time on another world.

Computer illustration of the CEV in orbit around the Moon. Image credit: NASA. Click to enlarge.
Instead of launching the entire system at once, the CEV and the lunar module launch separately. “In NASA shorthand we call it the 1.5 launch solution,” Hanley said. “The big heavy booster brings the lunar module and the upper stage to orbit and we’ll follow it with the crew launch vehicle, which launches on a smaller rocket, and the two vehicles will rendezvous and dock. Then we’ll light the Earth departure stage and send it on the way to the moon.”

Hanley continued, “We also want a quantum leap in safety and reliability in our launch systems over anything we have today.” Based on an engineering study, the new launch system will be 10 times safer than the space shuttle. The crew compartment sits on top of the rocket, unlike the space shuttle which is strapped to the rocket’s side. This allows for an escape system that can be used at anytime during launch.

The rockets will combine the reliability and power of solid rocket motors and the space shuttle main engines. The crew launch vehicle will be a single four-segment solid rocket motor with one shuttle main engine, which can lift 25 metric tons. The heavy cargo launch system will consist of two five-segment solid rockets and five shuttle main engines, which can boost 106 metric tons to orbit. A cargo-only mission could bring 21 metric tons of supplies to the moon.

Hanley anticipates the new spacecraft will be ready for its first launch in 2012, but he is challenging his team to have the spacecraft ready as soon as possible. “Our ideal is to make as small a gap as possible between the last shuttle flight [scheduled for 2010] and the first human flight of this system,” he said. “If we have things break our way and utilize good management practices in putting this together, I think we can do it.”

Hanley disagrees with the critics of NASA’s new program who say that returning to the moon is a waste of time and resources when the ultimate human destination is Mars, or perhaps other moons or asteroids. “That would be like the first explorers trying to circumnavigate the earth the first time they set out on the ocean,” he said. “That seems a little na?ve to me. The moon is three or four days away with the current rockets we have. Mars is months away. Once you light off the engines on the Mars transfer vehicle, there’s no turning back. You must have incredibly reliable systems to commit to those kinds of journeys.”

Hanley feels the only way to build up robustness and reliability of a spacecraft is through repeated use over time. “You’ve designed them, built them, and flown them over a period of time such that you’ve weeded out the ‘unknown unknowns,’ as we call them,” he said. “The moon gives us a natural platform to learn from when we get to the point when there’s no turning back from going to Mars.”

In addition, Hanley says, the exploration of other planets will only be successful if we learn to live off the land. “If you look in general at the history of exploration,” he said, “it wouldn’t have been possible without being able to live off the land. We have to learn how to use the available assets, like lunar soil and ice and convert that into rocket fuel and air, cultivating a way station, if you will, from which to test out systems for future exploration.”

Hanley believes that the successful international cooperation that has been forged through the International Space Station program should continue and expand through returning to the moon. “One of the unsung successes of the ISS program is the strong international team that has been cultivated,” he said. “The partnership has endured strains and come through them in great shape. The kinds of relationships and understandings we have today are a great basis on which to build more relationships for exploration.”

“Really,” he continued, “we have no choice but to partner with others to create a really robust program. NASA’s budget in the timeframe we are talking about just won’t be big enough to do all the things that possibly could be done, such as building habitats, rovers, and scientific stations. So there’s a huge opportunity for partners to come in and add value, robustness and capabilities.” Hanley said there have already been discussions at high levels with other space agencies on these matters.

The ISS has also been criticized as wasting time and resources, but Hanley feels everything that has been learned through the ISS program is invaluable. “What we eventually want to do at Mars,” he said, “is build an outpost off the planet. The ISS already is an outpost off the planet. We’ve learned an incredible amount in creating it, sustaining it, and it will, by its very nature, inform us of what the best approaches will be to take the next step.”

“Station is helping us to expand our horizons,” Hanley continued. “We’re learning through the engineering of our systems and cultivating our capabilities at that outpost, so we’re learning about how to rely less and less on supplies from the planet. We’re building heritage. And as soon as we learn the lessons we need to learn on the moon, we will be setting our sights on Mars and I don’t think that will be very long into the future.”

Written by Nancy Atkinson

Posted on by Fraser Cain

Shadows on the Moon

The full moon. Image credit: Robert Gendler. Click to enlarge
The moon is utterly familiar. We see it all the time, in the blue sky during the day, among the stars and planets at night. Every child knows the outlines of the moon’s lava seas: they trace the Man in the Moon or, sometimes, a Rabbit.

This familiarity goes beyond appearances. The moon is actually made of Earth. According to modern theories, the moon was born some 4.5 billion years ago when an oversized asteroid struck our planet. Material from Earth itself spun out into space and coalesced into our giant satellite.

Yet when Apollo astronauts stepped out onto this familiar piece of home, they discovered that it only seems familiar. From the electrically-charged dust at their feet to the inky-black skies above, the moon they explored was utterly alien.

Thirty years ago their strange experiences were as well-known to the public as the Man in the Moon. Not anymore. Many of the best tales of Apollo have faded with the passage of time. Even NASA personnel have forgotten some of them.

Now, with NASA going back to the moon in search of new tales and treasures, we revisit some of the old ones, with a series of Science@NASA stories called “Apollo Chronicles.” This one, the first, explores the simple matter of shadows.

On the next sunny day, step outdoors and look inside your shadow. It’s not very dark, is it? Grass, sidewalk, toes–whatever’s in there, you can see quite well.

Your shadow’s inner light comes from the sky. Molecules in Earth’s atmosphere scatter sunlight (blue more than red) in all directions, and some of that light lands in your shadow. Look at your shadowed footprints on fresh sunlit snow: they are blue!

Without the blue sky, your shadow would be eerily dark, like a piece of night following you around. Weird. Yet that’s exactly how it is on the Moon.

To visualize the experience of Apollo astronauts, imagine the sky turning completely and utterly black while the sun continues to glare. Your silhouette darkens, telling you “you’re not on Earth anymore.”

Shadows were one of the first things Apollo 11 astronaut Neil Armstrong mentioned when he stepped onto the surface of the moon. “It’s quite dark here in the shadow [of the lunar module] and a little hard for me to see that I have good footing,” he radioed to Earth.

The Eagle had touched down on the Sea of Tranquility with its external equipment locker, a stowage compartment called “MESA,” in the shadow of the spacecraft. Although the sun was blazing down around them, Armstrong and Buzz Aldrin had to work in the dark to deploy their TV camera and various geology tools.

“It is very easy to see in the shadows after you adapt for a while,” noted Armstrong. But, added Aldrin, “continually moving back and forth from sunlight to shadow should be avoided because it’s going to cost you some time in perception ability.”

Truly, moon shadows aren’t absolutely black. Sunlight reflected from the moon’s gently rounded terrain provides some feeble illumination, as does the Earth itself, which is a secondary source of light in lunar skies. Given plenty of time to adapt, an astronaut could see almost anywhere.

Almost. Consider the experience of Apollo 14 astronauts Al Shepard and Ed Mitchell:

They had just landed at Fra Mauro and were busily unloading the lunar module. Out came the ALSEP, a group of experiments bolted to a pallet. Items on the pallet were held down by “Boyd bolts,” each bolt recessed in a sleeve used to guide the Universal Handling Tool, a sort of astronaut’s wrench. Shepard would insert the tool and give it a twist to release the bolt–simple, except that the sleeves quickly filled with moondust. The tool wouldn’t go all the way in.

The sleeve made its own little shadow, so “Al was looking at it, trying to see inside. And he couldn’t get the tool in and couldn’t get it released–and he couldn’t see it,” recalls Mitchell.

“Remember,” adds Mitchell, “on the lunar surface there’s no air to refract light–so unless you’ve got direct sunlight, there’s no way in hell you can see anything. It was just pitch black. That’s an amazing phenomenon on an airless planet.”

(Eventually they solved the problem by turning the entire pallet upside down and shaking loose the moondust. Some of the Boyd bolts, loosened better than they thought, rained down as well.)

Tiny little shadows in unexpected places would vex astronauts throughout the Apollo program–a bolt here, a recessed oxygen gauge there. These were minor workaday nuisances, mostly, but astronauts were jealous of the minutes lost from their explorations.

Shadows could also be mischievous:

Apollo 12 astronauts Pete Conrad and Al Bean landed in the Ocean of Storms only about 600 yards from Surveyor 3, a robotic spacecraft sent by NASA to the moon three years earlier. A key goal of the Apollo 12 mission was to visit Surveyor 3, to retrieve its TV camera, and to see how well the craft had endured the harsh lunar environment. Surveyor 3 sat in a shallow crater where Conrad and Bean could easily get at it–or so mission planners thought.

The astronauts could see Surveyor 3 from their lunar module Intrepid. “I remember the first time I looked at it,” recalls Bean. “I thought it was on a slope of 40 degrees. How are we going to get down there? I remember us talking about it in the cabin, about having to use ropes.”

But “it turned out [the ground] was real flat,” rejoined Conrad.

What happened? When Conrad and Bean landed, the sun was low in the sky. The top of Surveyor 3 was sunlit, while the bottom was in deep darkness. “I was fooled,” says Bean, “because, on Earth, if something is sunny on one side and very dark on the other, it has to be on a tremendous slope.” In the end, they walked down a gentle 10 degree incline to Surveyor 3–no ropes required.

see captionA final twist: When astronauts looked at the shadows of their own heads, they saw a strange glow. Buzz Aldrin was the first to report “?[there’s] a halo around the shadow of my helmet.” Armstrong had one, too.

This is the “opposition effect.” Atmospheric optics expert Les Cowley explains: “Grains of moondust stick together to make fluffy tower-like structures, called ‘fairy castles,’ which cast deep shadows.” Some researchers believe that the lunar surface is studded with these microscopic towers. “Directly opposite the sun,” he continues,” each dust tower hides its own shadow and so that area looks brighter by contrast with the surroundings.”

Sounds simple? It’s not. Other factors add to the glare. The lunar surface is sprinkled with glassy spherules (think of them as lunar dew drops) and crystalline minerals, which can reflect sunlight backwards. And then there’s “coherent backscatter”–specks of moondust smaller than the wavelength of light diffract sunlight, scattering rays back toward the sun. “No one knows which factor is most important,” says Cowley.

We can experience the opposition effect here on Earth, for example, looking away from the sun into a field of tall dewy grass. The halo is there, but our bright blue sky tends to diminish the contrast. For full effect, you’ve got to go to the Moon.

Luminous halos; mind-bending shadows; fairy-castles made of moondust. Apollo astronauts discovered a strange world indeed.

Original Source: NASA News Release

Posted on by Fraser Cain

Tethys Floating Past Saturn

Tethys floating past the massive golden globe of Saturn. Image credit: NASA/JPL/SSI Click to enlarge
Tethys floats before the massive, golden-hued globe of Saturn in this natural color view. The thin, dark line of the rings curves around the horizon at top.

Visible on Tethys (1,071 kilometers, or 665 miles across) are the craters Odysseus (top) and Melanthius (bottom). The view looks toward the anti-Saturn side of Tethys.

Images taken using red, green and blue spectral filters were combined to create this color view. Tethys is apparently darker than Saturn at these wavelengths. The edge of the planet appears fuzzy, which may indicate that we are seeing haze layers that are separated from the main cloud deck.

The images were acquired by the Cassini spacecraft narrow-angle camera on Dec. 3, 2005, at a distance of approximately 2.5 million kilometers (1.6 million miles) from Saturn. The image scale is 15 kilometers (9 miles) per pixel on Saturn and 13 kilometers (8 miles) per pixel on Tethys.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release.

Posted on by Fraser Cain

Pluto is Colder Than Charon

Pluto & Charon viewed from the surface of one of Pluto’s newly discovered candidate satellites. Image credit: David A. Aguilar (CfA). Click to enlarge
Mercury is boiling. Mars is freezing. The Earth is just right. When it comes to the temperatures of the planets, it makes sense that they should get colder the farther away they are from the Sun. But then there is Pluto. It has been suspected that this remote world might be even colder than it should be. Smithsonian scientists now have shown this to be true.

Scientists continue to discuss whether Pluto is a planet or should be considered a refugee from the Kuiper belt. Whatever its classification, Pluto and its moon Charon are certain to harbor secrets about the early history of planet formation. Charon is roughly half the diameter of the planet itself, and they form a unique pair in our solar system. How they came to be together remains a mystery.

Located thirty times farther away from the Sun than the Earth, sunlight reaching the surface of Pluto is feeble at best, with daytime resembling dark twilight here at home. Pluto’s temperature varies widely during the course of its orbit since Pluto can be as close to the sun as 30 astronomical units (AU) and as far away as 50 AU. (An AU is the average Earth-Sun distance of 93 million miles.) As Pluto moves away from the Sun, its thin atmosphere is expected to freeze and fall to the surface as ice.

Reflected sunlight gathered with instruments such as the Keck telescope in Hawaii and the Hubble Space Telescope suggested the surface of Pluto might be colder than it should be, unlike Charon’s. However, no telescope capable of directly measuring their thermal emission (their heat) was able to peer finely enough to distinguish the two bodies. Their close proximity presented a formidable challenge since they are never farther apart than 0.9 arcseconds, about the length of a pencil seen from 30 miles away.

Now, for the first time, Smithsonian astronomers using the Submillimeter Array (SMA) on Mauna Kea in Hawaii have taken direct measurements of thermal heat from both worlds and found that Pluto is indeed colder than expected, colder even than Charon.

“We all know about Venus and its runaway greenhouse effect,” said Mark Gurwell of the Harvard-Smithsonian Center for Astrophysics (CfA), co-author on this study along with Bryan Butler of the National Radio Astronomy Observatory. “Pluto is a dynamic example of what we might call an anti-greenhouse effect. Nature likes to leave us with mysteries – and this was a big one.”

During the observations, the SMA utilized its most extended configuration to obtain high-resolution interferometric data, allowing separate “thermometer” readings for Pluto and Charon. It found that the temperature of the ice-covered surface of Pluto was about 43 K (-382 degrees F) instead of the expected 53 K (-364 degrees F), as on nearby Charon. This fits the current model that the low temperature of Pluto is caused by equilibrium between the surface ice and its thin nitrogen atmosphere, not just with the incoming solar radiation. Sunlight (energy) reaching the surface of Pluto is used to convert some of the nitrogen ice to gas, rather than heat the surface. This is similar to the way evaporation of a liquid can cool a surface, such as sweat cooling your skin.

“These results are really exciting and fun as well,” said Gurwell. “Imagine taking something’s temperature from almost three billion miles away without making a house call!”

This research will be presented at the 207th meeting of the American Astronomical Society in Washington, DC.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: CfA News Release