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COMING EVENTS Special Presentation

Sept. 04, 2014 7:30 PM

Dr. Paul Sorenson Eldridge Extended

From the Smallest to the Biggest

Club Meeting

09/19/14, 7:30 PM 2001 Malott

President

Rick Heschmeyer rcjbm@sbcglobal.net

Webmaster Howard Edin

howard@howardedin.com Observing Clubs

Doug Fay dfay@ku.edu

ALCOR William Winkler

billwink10@yahoo.com

Report from the Officers The last attempt of the summer for observing at the band con-certs failed, once again, due to the weather. But don’t get too frustrated, the Fall will be here before you know it and the weath-er should improve with the cooler temperatures. In the mean time, while there is little we can do about the weather, except wait, an equally challenging obstacle to observing that we can impact is

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Volume 40 Number 08 AUGUST 2014

INSIDE THIS ISSUE

Officer Report (continued) 2

Early Earth Life Destroyed 2

Meteorite Mystery Solved 3

NASA Space Place 4

September EVENT Poster 5

Enceladus Geysers 6

Enceladus (continued) 7

Meteorite (continued) 7

Alien Worlds (continued) 7

Precise Alien Worlds 8

Dry Exoplanets 9

Dry Exoplanets (continued) 10

Life Destroyed (continued) 10

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About the Astronomy Associates of Lawrence

The club is open to all people interested in sharing their love for astronomy. Monthly meetings are typically on the second Friday of each month and often feature guest speakers, presentations by club members, and a chance to exchange amateur astronomy tips. Approximately the last Sunday of each month we have an open house at the Prairie Park Nature Center. Periodic star parties

are scheduled as well. For more information, please contact the club officers: president, Rick Heschmeyer at

rcjbm@sbcglobal.net; webmaster, Howard Edin, at howard@howardedin.com; AlCor William Winkler, at

billwink10@yahoo.com; or faculty advisor, Prof. Bruce Twarog at btwarog@ku.edu. Because of the flexibility of the schedule due to holidays and alternate events, it is always best to check the Web site for the exact Fridays and Sundays when events are

scheduled. The information about AAL can be found at http://www.physics.ku.edu/aal/

Copies of the Celestial Mechanic can also be found on the web at http://www.physics.ku.edu/aal/celestialmechanic

light pollution. Please note the international event scheduled for the week of Aug. 9, a celebration of starlight, which has the goal of reclaiming the Starry Night Sky: One Star at a Time. If you have an interest in taking part and taking the pledge, go to www.starry-night.org for more details. With summer coming to a close and the Fall semester soon upon us, we have the first public event of the season, a lecture sponsored by the Department of Physics and Astronomy. Dr. Paul Sorensen, an internationally known particle physicist, will give a talk on how modern research explores the link between the smallest and largest scales of the Universe. Note the location of the talk at the new Eldridge Extension at 8th and Vermont in downtown Lawrence. It should be a very informative and enter-taining evening. Our first club meeting of the semester is tentatively scheduled for 7:30 PM Friday, Sept. 19 in 2001 Malott. Our planned speak-er is Prof. Emeritus Ed Wiley, an exceptional amateur astronomer, who will regale us with tales of his observing adventures at Kitt Peak National Observatory this summer. We will post more details as they become available. Finally, if you attempted to access the club web site recently, you may have come to a dead end. The KU Club and Organiza-tions administration three weeks ago dropped all club web sites from their server. Our temporary home for now is http://www.physics.ku.edu/aal/ . We will let you know if this changes in the future.

Any suggestions for improving the club or the newsletter are always welcome.

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Early life probably fell victim to massive space rocks

Space rocks larger in diameter than Spain bombarded the early Earth, probably repeatedly eradicating emerging life. The last of these death rocks struck around 4.3 bil-lion years ago, scientists estimate in the July 31 Nature, providing an upper limit to when life first took hold on Earth.

From Earth’s origin around 4.6 bil-lion years ago until 3.8 billion years ago, the planet was such a hellish place that geologists call this eon the Hadean after Hades, the Greek

god of the underworld. Debris left over from the solar system’s creation regularly slammed into Earth, boiling away the early ocean and coating the planet with molten rock. But it was during this chaotic period that scientists think life arose on Earth. “If life on Earth emerged before [a] final sterilizing impact, it may have been completely erased,” says planetary scientist and lead author Simone Marchi of the Southwest Research Institute in Boulder, Colo. “Life would have had to start all over again.” Enough material struck Earth during the Hadean to extend the planet’s surface by the height of Mount Everest. These impacts

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Young sun's violent history solves meteorite mystery

In spite of their tranquil appearance in the night sky, stars are scorching furnaces that spring to life through tumultu-ous processes – and our 4.5 billion-year-old Sun is no exception. To glimpse its harsh early days, astronomers gather clues not only in the Solar System but also by studying young stars elsewhere in our Galaxy.

Using Herschel to survey the chemical composition of regions where stars are being born today, a team of astrono-mers has noticed that one object in particular is different.

The unusual source is a prolific stellar nursery called OMC2 FIR4, a clump of new stars embed-ded in a gaseous and dusty cloud near to the famous Orion Nebula.

"To our great surprise, we found that the proportion of two chemical species, one based on carbon and oxy-gen and the other on nitro-gen, is much smaller in this object than in any other protostar we know," says Dr Cecilia Ceccarelli, of the Institute de Planétologie et

d'Astrophysique de Grenoble, France, who lead the study with Dr Carsten Dominik of the University of Amsterdam in the Netherlands. In an extremely cold environment, the measured proportion could arise by one of the two compounds freezing onto dust grains and becoming undetectable. However, at the relatively 'high' temperature of about –200°C found in star-forming re-gions like OMC2 FIR4, this should not occur.

"The most likely cause in this environment is a violent wind of very energetic particles, released by at least one of the embryonic stars taking shape in this proto-stellar cocoon," Dr Ceccarelli adds.

The most abundant molecule in star-forming clouds, hydrogen, can be broken apart by cosmic rays, energetic particles that permeate the entire Galaxy. The hydrogen ions then combine with other elements that are present – albeit only in trace amounts – in these clouds: carbon and oxygen, or nitrogen. Nor-mally, the nitrogen compound is also quickly destroyed, yielding more hydro-gen for the carbon and oxygen compound. As a result, the latter is far more abundant in all known stellar nurseries.

Strangely enough, though, this was not the case for OMC2 FIR4, suggesting that an additional wind of energetic particles is destroying both chemical spe-cies, keeping their abundances more similar.

Astronomers think that a similarly violent wind of particles also gusted through the early Solar System, and this discovery might finally point to an explanation for the origin of a particular chemical element seen in meteorites.

Meteorites are the remains of interplanetary debris that survived the trip through our planet's atmosphere. These cosmic messengers are one of the few tools we have to directly probe the elements in our Solar System.

"Some elements detected in meteorites reveal that, long ago, these rocks contained a form of beryllium: this is quite puzzling, as we can't quite under-stand how it got there," explains Dr Dominik.

The formation of this isotope – beryllium-10 – in the Universe is an intricate puzzle of its own. Astronomers know that it is not produced in the interior of

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Violent wind gusting around proto-star in Orion. Credit: Herschel im-age: ESA/Herschel/Ph. André, D. Polychroni, A. Roy, V. Könyves, N. Schneider for the Gould Belt survey Key Programme; inset and layout: ESA/ATG medialab

Violent wind gusting around protostar. Credit:ESA/ATG medialab

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The Invisible Shield of our

Sun

By Dr. Ethan Siegel

Whether you look at the planets with-in our solar system, the stars within

our galaxy or the galaxies spread throughout the universe, it's striking how empty outer space truly is. Even though the largest concentrations of mass are separated by huge distances, interstellar space isn't empty: it's filled with dilute amounts of gas, dust, radiation and ionized plasma. Although we've long been able to detect these compo-nents remotely, it's only since 2012 that a manmade spacecraft -- Voyager 1 -- successfully entered and gave our first direct measurements of the interstellar medium (ISM).

What we found was an amazing confirmation of the idea that our Sun creates a humongous "shield" around our solar system, the heliosphere, where the outward flux of the solar wind crashes against the ISM. Over 100 AU in radius, the heliosphere prevents the ionized plasma from the ISM from nearing the planets, asteroids and Kuiper belt objects contained within it. How? In addition to various wavelengths of light, the Sun is also a tremendous source of fast-moving, charged particles (mostly protons) that move between 300 and 800 km/s, or nearly 0.3% the speed of light. To achieve these speeds, these particles originate from the Sun's superheated corona, with temper-atures in excess of 1,000,000 Kelvin!

When Voyager 1 finally left the heliosphere, it found a 40-fold increase in the density of ionized plasma particles. In addition, traveling beyond the heliopause showed a tremendous rise in the flux of intermediate-to-high energy cos-mic ray protons, proving that our Sun shields our solar system quite effectively. Finally, it showed that the outer edges of the heliosheath consist of two zones, where the solar wind slows and then stagnates, and disappears alto-gether when you pass beyond the heliopause.

Unprotected passage through interstellar space would be life-threatening, as young stars, nebulae, and other in-tense energy sources pass perilously close to our solar system on ten-to-hundred-million-year timescales. Yet

those objects pose no major dan-ger to terrestrial life, as our Sun's invisible shield protects us from all but the rarer, highest energy cos-mic particles. Even if we pass through a region like the Orion Nebula, our heliosphere keeps the vast majority of those dangerous ionized particles from impacting us, shielding even the solar system's outer worlds quite effectively. NASA spacecraft like the Voyag-ers, IBEX and SOHO continue to teach us more about our great cosmic shield and the ISM's irregu-larities. We're not helpless as we hurtle through it; the heliosphere gives us all the protection we need!

Want to learn more about Voyager 1’s trip into interstellar space? Check this out: http://www.jpl.nasa.gov/news/news.php?release=2013-278.

Kids can test their knowledge about

the Sun at NASA’s Space place: http://

spaceplace.nasa.gov/solar-

tricktionary/.

Image credit: Hubble Heritage Team (AURA / STScI), C. R. O'Dell (Vanderbilt), and

NASA, of the star LL Orionis and its heliosphere interacting with interstellar gas and

plasma near the edge of the Orion Nebula (M42). Unlike our star, LL Orionis displays

a bow shock, something our Sun will regain when the ISM next collides with us at a

sufficiently large relative velocity.

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Cassini Spacecraft Reveals 101 Geysers and more on Icy Saturn Moon

Scientists using mission data from NASA’s Cassini spacecraft have identified 101 distinct geysers erupting on Saturn’s icy moon Enceladus. Their analysis suggests it is possible for liquid water to reach from the moon’s underground sea all the way to its surface.

Over a period of almost seven years, Cassini’s cameras surveyed the south polar terrain of the small moon, a unique geo-logical basin renowned for its four prominent "tiger stripe” fractures and the geysers of tiny icy particles and water vapor first sighted there nearly 10 years ago. The result of the survey is a map of 101 geysers, each erupting from one of the tiger stripe fractures, and the discovery that individual geysers are coincident with small hot spots. These relationships pointed the way to the geysers’ origin.

After the first sighting of the geysers in 2005, scientists suspected repeat-ed flexing of Enceladus by Saturn’s tides as the moon orbits the planet had something to do with their behav-ior. One suggestion included the back-and-forth rubbing of opposing walls of the fractures generating frictional heat that turned ice into geyser-forming vapor and liquid.

Alternate views held that the opening and closing of the fractures allowed water vapor from below to reach the surface. Before this new study, it was not clear which process was the dom-inating influence. Nor was it certain whether excess heat emitted by En-celadus was everywhere correlated with geyser activity.

To determine the surface locations of the geysers, researchers employed the same process of triangulation used historically to survey geological features on Earth, such as mountains. When the researchers compared the geysers’ locations with low-resolution maps of thermal emission, it became apparent the greatest geyser activity coincided with the greatest thermal radiation. Comparisons between the geysers and tidal stresses revealed similar connections. However, these correlations alone were insufficient to answer the question, “What produces what?”

The answer to this mystery came from comparison of the survey results with high-resolution data collected in 2010 by Cassini’s heat-sensing in-struments. Individual geysers were found to coincide with small-scale hot spots, only a few dozen feet (or tens of meters) across, which were too

small to be produced by frictional heating, but the right size to be the result of condensation of vapor on the near-surface walls of the fractures. This immediately implicated the hot spots as the signature of the geysering process.

“Once we had these results in hand we knew right away heat was not causing the geysers, but vice versa,” said Carolyn Porco, leader of the Cassini imaging team from the Space Science Institute in Boulder, Colorado, and lead author of the first paper. “It also told us the geysers are not a near-surface phenomenon, but have much deeper roots.”

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This artist's rendering shows a cross-section of the ice shell immediately beneath one of Enceladus' geyser-active fractures, illustrating the physical and thermal structure and the processes ongoing below and at the surface. Image Cred-it: NASA/JPL-Caltech/Space Science Institute

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Thanks to recent analysis of Cassini gravity data, the research-ers concluded the only plausible source of the material forming the geysers is the sea now known to exist beneath the ice shell. They also found that narrow pathways through the ice shell can remain open from the sea all the way to the surface, if filled with liquid water.

In the companion paper, the authors report the brightness of the plume formed by all the geysers, as seen with Cassini’s high resolution cameras, changes periodically as Enceladus orbits Saturn. Armed with the conclusion the opening and closing of the fractures modulates the venting, the authors compared the observations with the expected venting schedule due to tides.

They found the simplest model of tidal flexing provides a good match for the brightness variations Cassini observes, but it does not predict the time when the plume begins to brighten. Some other important effect is present and the authors considered sev-eral in the course of their work.

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by seismic waves moving within its interior. These readings encode precise information about the star's interior. The team leveraged them to narrowly gauge the star's radius, which is crucial for measuring the planetary radius.

Spitzer, meanwhile, confirmed that the exoplanet's transit looked the same in infrared light as in Kepler's visible-light observations. These corroborating data from Spitzer -- some of which were gathered in a new, precision ob-serving mode -- ruled out the possibility that Kepler's detection of the exoplanet was bogus, or a so-called false pos-itive.

Taken together, the data boast an error bar of just one percent of the radius of Kepler-93b. The measurements mean that the planet, estimated at about 11,700 miles (18,800 kilometers) in diameter, could be bigger or smaller by about 150 miles (240 kilometers), the approximate distance between Washington, D.C., and Philadelphia.

Spitzer racked up a total of seven transits of Kepler-93b between 2010 and 2011. Three of the transits were snapped using a "peak-up" observational technique. In 2011, Spitzer engineers repurposed the spacecraft's peak-up camera, originally used to point the telescope precisely, to control where light lands on individual pixels within Spitzer's infrared camera.

The upshot of this rejiggering: Ballard and her colleagues were able to cut in half the range of uncertainty of the Spitzer measurements of the exoplanet radius, improving the agreement between the Spitzer and Kepler measure-ments.

"Ballard and her team have made a major scientific advance while demonstrating the power of Spitzer's new ap-proach to exoplanet observations," said Michael Werner, project scientist for the Spitzer Space Telescope at NASA's Jet Propulsion Laboratory, Pasadena, California.

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This view looks across the geyser basin of Saturn's moon Enceladus, along fractures spewing water vapor and ice particles into space. Cassini scientists have pinpointed the source locations of about 100 geysers and gained new insights into what powers them. Image Credit: NASA/JPL-Caltech/SSI

stars, like some other elements, nor in the supernova explosion that happens at the end of a massive star's life.

The majority of beryllium-10 was formed in collisions of very energetic particles with heavier elements like oxygen. But since this isotope decays very quickly into other elements, it must have been produced just before it was incor-porated in the rocks that would later appear on Earth as meteorites.

In order to trigger these reactions and produce an amount of beryllium matching that recorded in meteorites, our own Sun must have blown a violent wind in its youth.

These new observations of OMC2 FIR4 give a very strong hint that it is possible for a young star to do this.

"Observing star-forming regions with Herschel not only provides us with a view on what happens beyond our cos-mic neighbourhood, but it's also a crucial way to piece together the past of our own Sun and Solar System," says Göran Pilbratt, ESA's Herschel project scientist.

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The Most Precise Measurement of an Alien World's Size

Thanks to NASA's Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the radius of a planet outside our solar system. The size of the exoplanet, dubbed Kepler-93b, is now known to an uncertainty of just 74 miles (119 kilometers) on either side of the planetary body.

The findings confirm Kepler-93b as a "super-Earth" that is about one-and-a-half times the size of our planet. Alt-hough super-Earths are common in the galaxy, none exist in our solar system. Exoplanets like Kepler-93b are therefore our only laboratories to study this major class of planet. With good limits on the sizes and masses of su-per-Earths, scientists can finally start to theorize about what makes up these weird worlds. Previous measurements,

by the Keck Observatory in Ha-waii, had put Kepler-93b's mass at about 3.8 times that of Earth. The density of Kepler-93b, de-rived from its mass and newly obtained radius, indicates the planet is in fact very likely made of iron and rock, like Earth.

"With Kepler and Spitzer, we've captured the most precise meas-urement to date of an alien planet's size, which is critical for understanding these far-off worlds," said Sarah Ballard, a NASA Carl Sagan Fellow at the University of Washington in Seat-tle and lead author of a paper on the findings published in the As-trophysical Journal.

"The measurement is so precise that it's literally like being able to measure the height of a six-foot tall person to within three quar-ters of an inch -- if that person were standing on Jupiter," said Ballard.

Kepler-93b orbits a star located about 300 light-years away, with approximately 90 percent of the sun's mass and radius. The ex-oplanet's orbital distance -- only about one-sixth that of Mercury's from the sun -- implies a scorch-ing surface temperature around 1,400 degrees Fahrenheit (760 degrees Celsius). Despite its newfound similarities in composi-tion to Earth, Kepler-93b is far too hot for life.

To make the key measurement about this toasty exoplanet's radi-us, the Kepler and Spitzer tele-scopes each watched Kepler-93b cross, or transit, the face of its star, eclipsing a tiny portion of starlight. Kepler's unflinching gaze also simultaneously tracked the dimming of the star caused

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Using data from NASA's Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the size of a world outside our solar system, as illustrated in this artist's conception The diameter of the ex-oplanet, dubbed Kepler-93b, is now known with an uncertainty of just one per-cent.

According to this new study, the diameter of Kepler-93b is about 11,700 miles (18,800 kilometers), plus or minus 150 miles (240 kilometers) -- the approximate distance between Washington, D.C., and Philadelphia, Penn. Kepler-93b is 1.481 times the width of Earth, the diameter of which is 7,918 miles (12,742 kilometers).

The results confirm that the exoplanet is a "super-Earth." Although super-Earths are common in the galaxy, none exist in our solar system. Exoplanets like Kep-ler-93b are therefore our only laboratories to study this major class of planet.

With good limits on super-Earths' sizes as well as their masses, scientists can now start to theorize about what makes up these weird worlds. Previous meas-urements, by the Keck Observatory in Hawaii, had put Kepler-93b's mass at about 3.8 times that of Earth. The density of Kepler-93b, derived from its mass and newly obtained radius, indicates the planet is in fact very likely made of iron and rock, like Earth.

Despite its newfound similarities in composition to Earth, Kepler-93b is far too hot for life. The exoplanet's orbital distance -- only about one-sixth that of Mer-cury's from the sun -- implies a scorching surface temperature around 1,400 degrees Fahrenheit (760 degrees Celsius).

The methods employed in the new study could help nail down the sizes of other exoplanets, and improve our understanding of alien worlds.

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Hubble Finds Three Surprisingly Dry Exoplanets

Astronomers using NASA's Hubble Space Telescope have gone looking for water vapor in the atmospheres of three planets orbiting stars similar to the Sun — and have come up nearly dry.

The three planets, HD 189733b, HD 209458b, and WASP-12b, are between 60 and 900 light-years away. These giant gaseous worlds are so hot, with temperatures between 1,500 and 4,000 degrees Fahrenheit, that they are ideal candi-

dates for detecting water vapor in their atmospheres.

However, to the surprise of the re-searchers, the planets surveyed have only one-tenth to one one-thousandth the amount of water predicted by standard planet-formation theories.

"Our water measurement in one of the planets, HD 209458b, is the highest-precision measurement of any chemi-cal compound in a planet outside the solar system, and we can now say with much greater certainty than ever before that we've found water in an exoplanet," said Dr. Nikku Madhusudhan of the Institute of As-tronomy at the University of Cam-bridge, United Kingdom, who led the research. "However, the low water abundance we are finding is quite astonishing."

Madhusudhan said that this finding presents a major challenge to exoplan-et theory. "It basically opens a whole can of worms in planet formation. We expected all these planets to have lots of water in them. We have to revisit planet formation and migration models of giant planets, especially 'hot Jupi-ters', and investigate how they're formed."

He emphasizes that these results, though found in these large hot plan-ets close to their parent stars, may

have major implications for the search for water in potentially habitable Earth-sized exoplanets. Instruments on future space telescopes may need to be designed with a higher sensitivity if target planets are drier than predicted. "We should be prepared for much lower water abundances than predicted when looking at super-Earths (rocky planets that are sev-eral times the mass of Earth)," Madhusudhan said.

Using near-infrared spectra of the planets observed with Hubble, Madhusudhan and his collaborators from the Space Telescope Science Institute, Baltimore, Maryland; the University of Maryland, College Park, Maryland; the Johns Hop-kins University, Baltimore, Maryland; and the Dunlap Institute at the University of Toronto, Ontario, Canada, estimated the amount of water vapor in the planetary atmospheres based on sophisticated computer models and statistical tech-niques to explain the data.

The planets were selected because they orbit relatively bright stars that provide enough radiation for an infrared-light spectrum to be taken. Absorption features from the water vapor in the planet's atmosphere are superimposed on the small amount of starlight that glances through the planet's atmosphere.

Detecting water is almost impossible for transiting planets from the ground because Earth's atmosphere has a lot of water in it that contaminates the observation. "We really need the Hubble Space Telescope to make such observations," said Nicolas Crouzet of the Dunlap Institute at the University of Toronto and co-author of the study.

The currently accepted theory on how giant planets in our solar system formed is known as core accretion, in which a planet is formed around the young star in a protoplanetary disk made primarily of hydrogen, helium, and particles of ices and dust composed of other chemical elements. The dust particles stick to each other, eventually forming larger and

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This is an artistic illustration of the gas giant planet HD 209458b (unofficially named Osiris) located 150 light-years away in the constellation Pegasus. This is a "hot Jupiter" class planet. Estimated to be 220 times the mass of Earth. The planet's atmosphere is a seething 2,150 degrees Fahrenheit. It orbits very closely to its bright sunlike star, and the orbit is tilted edge-on to Earth. This makes the planet an ideal candidate for the Hubble Space Telescope to be used to make precise measurements of the chemical composition of the giant's atmosphere as starlight filters though it. To the surprise of astronomers, they have found much less water vapor in the atmosphere than standard planet-formation models predict.

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larger grains. The gravitational forces of the disk draw in these grains and larger parti-cles until a solid core forms. This core then leads to runaway accretion of both solids and gas to eventually form a giant planet.

This theory predicts that the proportions of the different elements in the planet are en-hanced relative to those in their star, espe-cially oxygen that is supposed to be the most enhanced. Once the giant planet forms, its atmospheric oxygen is expected to be largely encompassed within water molecules. The very low levels of water vapor found by this research raises a num-ber of questions about the chemical ingredi-ents that lead to planet formation, say re-searchers.

"There are so many things we still don't know about exoplanets, so this opens up a new chapter in understanding how planets and solar systems form," said Drake Dem-ing of the University of Maryland, who led one of the precursor studies. "The problem is that we are assuming the water to be as abundant as in our own solar system. What our study has shown is that water features could be a lot weaker than our expecta-tions."

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This graph compares observations with modeled infrared spectra of three hot-Jupiter-class exoplanets that were spectroscopically observed with the Hubble Space Telescope. The red curve in each case is the best-fit model spectrum for the detection of water vapor absorption in the planetary atmosphere. The blue circles and error bars show the processed and analyzed data from Hubble's spectroscopic observations.

shaped the emergence of plate tectonics; however, few rocks older than around 3.8 billion years remain to provide a natural record of Earth’s early impact history. To reconstruct the barrage of rocks that assaulted early Earth, Marchi and colleagues looked to the relatively stagnant moon. Because the moon lacks the recycling action of plate tectonics, it still shows scars from early asteroid impacts. Scientists determine the ages of ancient lunar asteroid impacts using a method called crater counting. As a crater ages, falling meteors gradually blemish the impact site. Using the ages of moon rocks collected from lunar craters during the Apollo missions as calibration, scientists ap-proximate the age of large lunar craters by counting the number of smaller, fresher craters within it. Marchi’s team used such information about the moon to estimate the number, frequency and size of asteroids that impacted early Earth, assuming the two had a similar impact history. The team then ran a computer simulation of Earth’s early bombardment and observed that asteroid impacts be-came smaller and less frequent with time. The team also found that every bit of Earth’s surface was at some point covered in a magma-oozing crater created by an impact. The researchers found that between one and four aster-oids larger than 1,000 kilometers across probably struck Earth during this time, any of which could have vaporized all water and destroyed any life on Earth. The last of these life-sterilizing impacts took place 4.27 billion years ago, the researchers estimate. The oldest evidence of life on Earth is 3.8 billion years old, although that evidence is dis-puted .

Geochemist Jeffrey Bada of the Scripps Institution of Oceanography in La Jolla, Calif., believes that a better under-standing of early asteroid bombardment will help researchers studying the origins of life. “The window of when life appeared on Earth is sometime after these really traumatic impacts,” he says. “Life could not have started prior to that and survived.”

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