VENUS AND MERCURY HOT, VOLATILE PLANETS

CONTENTS 2 Torrid Mercury’s icy poles The MESSENGER spacecraft reveals water ice lurking in deeply shadowed craters near the innermost planet’s poles. 8 Mercury: Land of mystery and enchantment With MESSENGER behind us and BepiColombo to come, planetary scientists seek to understand the puzzling innermost planet. 14 Venus Revisited New missions aim to untangle the mysteries of how the planet’s scorching surface and violent clouds came to be.

ESO/Y. BELETSKY A supplement to Astronomy magazine Fire and ice

The MESSENGER spacecraft reveals water ice lurking in Torrid Mercury’s icy poles deeply shadowed craters near the innermost planet’s poles. by James Oberg

Color explodes from Mercury’s surface in this enhanced-color mosaic. The yellow and orange hues signify relatively young volcanic plains, while blue represents older terrain. The planet’s equator runs horizon- he saga of water ice hiding in GEochemistry, and Ranging — started Planetary scientist David Paige of the works best when good observations are tally through the center of this image; the poles lie at top and bottom. NASA/JHUAPL/CIW the shadows on Mercury ranks orbiting Mercury in March 2011 and has University of California, Los Angeles, a guided by theory.” among the most fascinating been returning reams of data ever since. participating scientist on the MESSENGER During the past several decades, scien- chapters in the history of plan- The discovery of ice contains its own sur- project, stresses the wider significance of tists have come to realize that a conspiracy to the planet nearly 40 years ago, could not Untangling Mercury’s web etary exploration. And the prises and new mysteries. the ice story. “The Mercury discoveries of freak accidents involving Mercury’s make the observations necessary to prove It took centuries for astronomers to deci- story didn’t end a year ago Scientists had suspected for decades that demonstrate the power of theory and motion and orientation had created small the case. Still, radar observations from pher Mercury’s physical and dynamic when scientists using NASA’s water ice might survive in corners of the imagination in astronomy and planetary regions on the planet’s surface where it Earth did detect unusually reflective ­characteristics and to understand their MESSENGER spacecraft con- solar system’s innermost world. Theorists science,” he says. “However, just because ought to be cold enough for ice to form and regions in Mercury’s polar regions that implications. Although the planet lies rea- firmed the existence of ice in craters near realized that cold regions could exist in something might be there doesn’t necessar- survive for billions of years. Unfortunately, seemed to overlap the supercold regions sonably close to Earth, it is a difficult world Tthe planet’s poles. MESSENGER — short certain areas, and observers seemingly ily mean that it is. Careful observations instruments on NASA’s Mariner 10 space- theorists had calculated. This match pro- to observe in detail. As the closest planet to for MErcury Surface, Space ENvironment, backed up these theoretical computations. and analysis are required as proof. Science craft, which made the only previous visits vided a tentative hint of what might be. the Sun, Mercury never strays far from our

2 ASTRONOMY • VENUS AND MERCURY:© 2013 Kalmbach HOT, VOLATILE Publishing Co.PLANETS This material may not be reproduced in any form WWW.ASTRONOMY.COM 3 without permission from the publisher. www.Astronomy.com

© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com A chill at the poles But not all coincidences are bad news. With better observations in following years, sci- entists realized that Mercury’s axis tilts 89.99° to its orbital plane — almost pre- cisely perpendicular. Theory suggests that this near-perfect match of axis and orbit is a long-term effect of the planet’s gravita- tional coupling with the Sun, which would mean that Mercury’s rotation axis adjusts to changes in its orbital inclination over time. As a consequence, deep craters near the planet’s poles can remain in permanent shadow and serve as ice traps for a billion years or more. In 1991, researchers beamed radar sig- nals at Mercury. Using the Goldstone radio telescope in California and the Very Large Array in New Mexico, they detected unusu- This oblique view of the inner world’s north polar region shows the surface temperature averaged over ally strong radar returns from the planet’s two Mercury years. Blue and purple represent the coldest areas, where ice can persist. NASA/UCLA/JHUAPL/CIW polar regions. These areas come into view because Mercury’s orbit tilts 7° to Earth’s, NATIONAL ASTRONOMY AND IONOSPHERE CENTER/ARECIBO OBSERVATORY CENTER/ARECIBO IONOSPHERE AND ASTRONOMY NATIONAL NASA/JHUAPL/CIW which allows astronomers to peek over the Deep craters near Mercury’s north pole harbor water ice. The first evidence for A mosaic of MESSENGER images of Mercury’s north polar region appears M these deposits came from Earth-based radar observations in the early 1990s, beneath radar data of the same area seen at left. All of the large deposits sit inner world’s poles. The giant 1,000-foot 5 5 2 25 3 35 4 45 5 55 which revealed bright patches that reflected most of the incoming signal. on the floors or walls of impact craters, including Kandinsky and Prokofiev. (305 meters) radio dish at Arecibo Obser­ vatory in Puerto Rico later observed these same regions with similar results. star’s glare. The best observing conditions an elliptical orbit. (For the same reason, we First, each Mercury day, from one sun- Photographs taken during the three occur when it lies near one of its greatest see slightly more than half of the Moon’s rise to the next, lasts two Mercury years. Mariner 10 flybys in the mid-1970s showed elongations, which create brief periods when surface during a lunar month.) Astrono- Long phases of heating and cooling are the that, in some cases, the high-return regions the planet climbs well above our earthly mers accepted Mercury as another example rule across the planet’s surface. seemed to coincide with deep polar craters. horizon and appears approximately half-lit. of rotational lock well into the 20th century. Second, despite these long cycles, not all Although water ice seemed the most likely It wasn’t until the late 19th and early But further Earth-based observations longitudes experience the same share of cause, theorists developed several even 20th centuries that astronomers, most suggested that Mercury’s “dark side” was heating. Because the planet has a fairly more exotic explanations, including sulfur notably Giovanni Schiaparelli in Italy and far warmer than it should be. Theorists eccentric orbit, it travels much faster when snow, sodium ions, or an unknown feature Greek-born Eugène Antoniadi in France, proposed various ideas to explain the it lies closer to the Sun. of the supercold surface. (After all, this consistently observed the same markings apparent contradiction, including one Noontime heating in one longitudinal was the coldest surface radar scientists on the planet’s illuminated side. These fea- in which the planet possesses a massive zone can be twice that in another 90° had ever explored.) tures did not move over short periods and atmos­phere that away. The 3:2 resonance also means The highest temperature observed over two Mercury years gives a better idea of where ice can survive. The reappeared in the same locations at succes- spawns hurricanes that the same regions experience MESSENGER to the rescue coldest spots in permanently shadowed craters only reached –370° Fahrenheit (50 kelvins). NASA/UCLA/JHUAPL/CIW sive greatest elongations. to carry super- extra heating day after day after Into this uncertainty and mystery, NASA These observations implied that Mer- heated air into day. The two hottest launched the MESSENGER probe in 2004. cury likely is tidally locked with the Sun the perma- zones lie along It was a challenge to get a spacecraft to and always keeps the same face pointed nently shad- the equator on Mercury with a speed slow enough that the 2 3 4 5 toward our star. The Moon suffers a similar owed regions. opposite sides of biggest propulsion module possible could fate, in which Earth’s gravity locks the But none of the MESSENGER the planet. Tem- decelerate the craft into a stable orbit. After NASA/JPL-CALTECH Moon so that it rotates in the same period theories seemed peratures there soar as a seven-year voyage that included three it takes to revolve around our planet. The to fit the obser­vations. high as 845° Fahrenheit (725 kelvins). flybys of the inner world, the probe arrived observations of Mercury also suggested Then, in 1965, researchers bounced Third, Mercury’s true rotation period in its planned orbit March 17, 2011. that its rotation axis lines up nearly per­ powerful radar beams off Mercury. A sub- relative to the stars (58.8 days) coincidentally After a year of observations and anal­ pendicular to its orbital plane. tle shift in the wavelengths of the signals turns out to be approximately half of Mer- ysis, NASA announced the results in In Mercury’s case, being tidally locked returned from the planet’s edges didn’t cury’s 116-day synodic period, the time it November 2012. The ice is real, but it isn’t would create a scorchingly hot Sun-facing match an object rotating at the same rate takes the planet to return to the same orbital what scientists expected. If anything, it hemisphere, a perpetually dark and frigid as it revolved around the Sun — Mercury configuration as seen from our planet. proved even more interesting. region on the opposite side, and a fairly spins faster than anyone had suspected. This means that successive observation MESSENGER Principal Investigator broad twilight zone where the Sun would Even so, the planet is tidally locked, just periods from Earth occur when the same Sean Solomon, director of Columbia Uni- rise and set periodically in response to the not in a 1-to-1 ratio. Mercury rotates three side of Mercury faces the Sun. This dynam- versity’s Lamont-Doherty Earth Obser­ planet’s varying orbital speed as it follows times on its axis for every two revolutions ical accident is just bad luck for earthbound vatory, described how the science team it makes around the Sun. Although this astronomers, who noticed — and misinter- painstakingly developed its case for ice at This map of “permafrost” shows the same area as above and indicates the depths below Mercury’s surface James Oberg, a former NASA “rocket scientist,” so-called 3:2 resonance is stable over long preted — the repeated appearances of the the poles. First, the researchers tested the where water ice should be stable. Gray denotes regions that are too warm for any ice, color reveals areas now works as a space consultant for NBC News. periods, it has some bizarre implications. same surface features. hypothesis that the areas where scientists where subsurface ice can exist, and white shows spots cold enough for surface ice. NASA/UCLA/JHUAPL/CIW

4 ASTRONOMY • VENUS AND MERCURY: HOT, VOLATILE PLANETS WWW.ASTRONOMY.COM 5 How neutrons point to ice had observed bright radar returns were regolith [the layer of loose rock and soil How ice forms and survives positioned inside craters having the right found on the surfaces of most solar system shape and in locations where ice could sur- bodies] but is instead material with a reflec- North vive. “Imaging of both poles over multiple tance half that of Mercury’s average. The Sunlight Cosmic rays solar days on Mercury confirmed that all specific reflectance and limiting tempera- polar deposits are located in areas of per- ture are best matched by organic-rich mate- manent shadow,” says Solomon. And the rial found in comets and volatile-rich 550K Shadow craters are big enough so that light reflect- meteorites, and on the surfaces of outer 1 350K 80K ing from the Sun-facing rims does not solar system objects.” Solomon adds that flood the shadows with too much heat. these properties strongly suggest that the Second, the scientists used the craft’s water ice and the organic material found Neutrons neutron spectrometer, an instrument that their way to Mercury’s polar craters by a already had proved its worth during ice common process. searches on the Moon, to seek evidence of The researchers found the dark protec- hydrogen, two-thirds of the building blocks tive layer over ice in craters at some distance 2 for water molecules. “Neutron spectrometer from both poles. Its distribution clearly measurements [in the northern hemisphere] avoids the longitudes of the “hot spots” cre- showed that the polar deposits have a hydro- ated by Mercury’s peculiar 3:2 resonance. gen abundance consistent with a composi- The volatile deposits extend farther from Neutrons Ice layer tion dominated by water ice,” says Solomon, both poles along the coldest longitudes — “but only if most ice deposits are buried those 90° off the hot spot locations — pro- 3 beneath several tens of centimeters [at least viding further proof of their icy nature. several inches] of a low-hydrogen material.” But closer to both the north and south The spacecraft’s laser altimeter provided poles, the temperatures drop even lower. In the third line of evidence. This instrument, these regions, ice can remain stable without developed to measure the precise shape and any protection. The water ice is pure, and it Low-hydrogen layer 4 (4–8 inches [10–20cm] thick) elevation of the planet’s surface, also paid is naked to space. “We had expected to find This enhanced-color mosaic reveals Mercury’s south polar region. MESSENGER’s most detailed observa- dividends in the hunt for ice. “Reflectance evidence of ice, but bright ice on the surface tions show the planet’s northern half because the probe flies closer to that hemisphere. But images of measurements showed that most polar and right where the thermal models pre- the south pole show craters that lie in permanent shadow, just like those in the north. NASA/JHUAPL/CIW Pure ice

deposits are dark at near-infrared wave- dicted it was a surprise,” says Paige. “Bright JOHNSON RICK : When high-energy cosmic rays strike Mer- lengths [1,064 nanometers in this case], but surface ice requires ongoing processes to cury’s surface, they liberate neutrons from some of the polar deposits at the highest deliver water to the polar regions faster But where it is cold, water ice is stable probe’s final year of operation, mission 5 ASTRONOMY subsurface atoms. Although these particles latitudes are much brighter than average than it can be buried by impact debris and for a long time. “There could definitely be planners intend to have it descend within normally escape into space after traveling Sunlight hits an impact crater near Mercury’s through the surface, hydrogen atoms in water for Mercury,” says Solomon. [ultraviolet] radiation.” billion-year-old ice on Mercury because the 15 miles (25 kilometers) of the surface at north pole at a shallow angle. Although ice block their route. An insulating layer low in Finally, the team matched the newly cold traps [have remained frigid for a major specific points, where it will be able to the sunlit rim grows hot, the opposite side remains in shadow (1). When comets strike the hydrogen protects the ice. ASTRONOMY: RICK JOHNSON make the most detailed observations yet. imaged candidate regions to actual cold Taking a bath fraction of the solar system’s history],” says planet (2), they spread water and organic com- areas. “Thermal models derived with topo- Considering NASA’s exobiology imperative Paige. Still, he remains cautious: “We don’t Until then, scientists will have to be con- pounds over a wide area, and some migrate to graphic maps constructed from altimeter to “follow the water” in search of extrater- know enough about the sources and tent with what the orbiter already has deliv- the poles and get trapped in craters (3). Water profiles showed that water ice is thermally restrial life, the question arises whether destruction rates of the ice to say how old ered. For Solomon, the water ice detection ice in the warmer areas vaporizes over time (4) but stays stable in the coldest spots, protected Neutrons tell the tale stable at the surface in those areas with water ice on Mercury opens a potential new any given piece of ice might be.” ranks high for two reasons. “First, the by a layer of organic material (5). bright reflectance, but is stable only if bur- habitat for biology. The answer, so far at Could the ice exist in successive layers, required measurements were difficult, Medium-speed neutrons ied by several tens of centimeters of another least, appears to be no. with the oldest at the bottom? If so, it could given that our spacecraft was far from the 1.02 still-volatile material for most [other] polar “The likelihood of persistent liquid provide future explorers with a time-lapse planet’s polar regions and the polar depos- deposit areas,” says Solomon. This uniden- water is remote,” says Solomon. Water in history of Mercury and, perhaps, solar its filled only a small fraction of the field of that polar ice on the Moon and Mercury 1.01 tified material appears to be less volatile Mercury’s polar craters either will be a sta- activity. In much the same way, terrestrial view of key remote sensing instruments,” reflect different phenomena. Mercury’s­ Pure ice No ice than water ice, which means it remains ble solid or in vapor form. “There may be geologists use ice cores from Antarctica and he says. “Second, the confirmation of water poles are warmer than the lunar poles, he 1.00 ­stable at higher temperatures. brief intervals where water ice may be Greenland to study Earth’s climate history. ice required that multiple lines of evidence points out, but the planet has a lot more 0.99 melted by sunlight or subsurface heat,” he “This is one possibility, but, unfortu- all pointed in the same direction. The water in much purer form. “It’s a continuing Relative ux Relative MESSENGER data A dark cover-up continues, “but there is no evidence that nately, we just don’t know the answer,” results came in one at a time, like plot mystery, the differences in the ice between 0.98 MESSENGER scientists didn’t anticipate water has modified the surface or near- Paige admits. He does speculate that the developments in a mystery novel, and the the Moon and Mercury,” he says. “We’ll discovering such a dark overcoat: “Finding surface in ways detectable from orbit.” apparently well-organized nature of Mer- solution came only on the final page.” need to go down and sample the surface.” 0.97 30° 40° 50° 60° 70° 80° 90° unusually dark material in association with “We don’t think there’s any possibility cury’s deposits suggests that they might Paige echoes those sentiments: “Scien- Actually reaching and analyzing Mer- North latitude the ice deposits was definitely unexpected,” for liquid water in association with these have migrated, mixed, and reformed as the tists have been studying polar ice on the cury’s polar ice is a challenge at the current MESSENGER’s neutron spectrometer surveys says Paige. “We hypothesized that these deposits,” adds Paige. “They are way planet’s orbit evolved over millions of years. Moon and Mercury since the 1960s. The limits of technological speculation. Perhaps the entire planet looking for neutrons. If no may be dark organic-rich deposits like we too cold. While surface and subsurface This could jumble any time history. MESSENGER observations are like a satis- a lander could do the job, or an impactor ice existed on Mercury, the number of neu- find in comets and primitive outer solar temperatures in the soil surrounding the fying end to a good story — or at least one paired with an orbiting collector made of trons would remain constant with latitude. If the radar-bright deposits were pure ice, system bodies. If this stuff really exists on ice deposits can be in the habitable zone To the next stage chapter in a longer story.” aerogel that captures the collision’s debris. the plot would follow the yellow curve. The Mercury, that would be pretty amazing.” [where conditions allow liquid water to The MESSENGER spacecraft continues to David Lawrence of the Johns Hopkins In other words, it’s an ideal goal for MESSENGER data closely match the pure-ice Solomon agrees: “The surprise was that exist], Mercury has no atmosphere, so any orbit Mercury, dipping closer to the surface University Applied Physics Laboratory, who young planetary scientists to consider. scenario. ASTRONOMY: RICK JOHNSON, AFTER NASA/JHUAPL/CIW the material covering most of those [ice] liquid water in these warmer regions would to gather more precise measurements of led the design team for the neutron spec- After all, the next chapter on Mercury’s deposits is not typical soil from Mercury’s quickly boil away into space.” particularly interesting regions. During the trometer that detected the hydrogen, stresses polar ice remains to be written.

6 ASTRONOMY • VENUS AND MERCURY: HOT, VOLATILE PLANETS WWW.ASTRONOMY.COM 7 First rock from the Sun

Land of mystery enchantmentAND With MESSENGER behind us and BepiColombo to come, planetary scientists seek to understand the puzzling innermost planet. by Jesse Emspak

The giant Caloris ended with a bang, not a whimper. On April 30, 2015, NASA’s Basin spans about MESSENGER spacecraft hurtled into Mercury’s surface at 960 miles (1,550 kilometers). In this around 8,750 mph (14,100 km/h). MESSENGER — short for enhanced-color MErcury Surface, Space ENvironment, GEochemistry, and mosaic, yellow and Ranging — had orbited the planet 4,104 times over 4 years, orange represent 1 month, and 12 days, giving humans their first complete reconnais- relatively young IT volcanic plains while sance of the innermost planet. (Mariner 10 flew past Mercury three blue signifies older times in 1974–5 and MESSENGER followed with three more flybys in terrain. ALL IMAGES: 2008–9.) The probe revealed a planet stranger than anyone expected. NASA/JHUAPL/CIW Luckily, scientists are planning another trip to Mercury. In 2017, the European Space Agency and the Japan Aerospace Exploration Agency will launch BepiColombo, which should arrive at Mercury in 2024. “BepiColombo will do more planetary physics and probe deeper,” says Wolfgang Baumjohann, principal investigator for the magnetom- eter instrument on BepiColombo’s Mercury Magnetospheric Orbiter (MMO). MESSENGER, in a way, was just a taste, says David Rothery, a planetary geoscientist at the Open University in England and lead co-investigator on BepiColombo’s Mercury Imaging X-ray Spectrom­ eter instrument. “We know what questions to ask now.” BepiColombo will be able to do a lot of things MESSENGER could not. First, the mission boasts two spacecraft. One is the Mercury Planetary Orbiter (MPO), which will focus on the planet’s surface chemistry and geology. The other is the MMO, which will concentrate on the inner world’s magnetic and solar wind environment. Both will

Jesse Emspak is a science writer who lives and works in New York City.

8 ASTRONOMY • VENUS AND MERCURY:© 2016 HOT,Kalmbach VOLATILE Publishing PLANETS Co. This material may not be reproduced in any WWW.ASTRONOMY.COM 9 MERCURYform without permission from the publisher. www.Astronomy.com inevitable. But Mercury’s is a lot smaller with time. Ordinarily, you would expect it than scientists expected. to — Earth’s field changes quickly enough Johnson says there are several hypoth- that navigational charts must be updated eses to explain this, most centering on the every few years. Yet there was little evidence fact that Mercury is less massive so its core that Mercury’s field structure changed at exerts less pressure on the material inside all. Although MESSENGER got more data, it. That alters the way iron “freezes out” as it might not have been able to see changes the planet cools. because the onboard instruments weren’t And Mercury clearly had a stronger sensitive enough, says Johnson. The reason: magnetic field in the past, says Johnson. As Mercury’s internal field is only about four MESSENGER made close passes to the sur- times stronger than the external one gener- face before its final impact, it found mag- ated by the solar wind — 200 nanotesla netic anomalies in the crust — areas that compared with 50 nanotesla. So, on had been magnetized previously. This Mercury, changes in the external field can means any field Mercury had needed to be drown out signals from the internal one. strong enough to magnetize the rock and That’s not a problem on Earth, where the needed to last for 3 to 4 billion years. internal field registers 30,000 nanotesla and The colors in this view of Mercury’s south pole reveal how much sunlight various regions receive. The black areas represent those in permanent shadow, the largest of which is the crater Chao Meng-Fu. MESSENGER could not pin down the external one just 10 nanotesla. Unfortunately, MESSENGER did not view the planet’s southern hemisphere in as much detail as the whether the magnetic field is symmetric “Separating these field contributions northern, a situation BepiColombo should correct. around its axis. “The important thing about from these two sources is a very tricky the measurements BepiColombo will be problem,” says Karl-Heinz Glassmeier, carry instruments to measure Mercury’s Mercury’s equator. Instead, it is offset some getting is not just verifying MESSENGER, head of the Space Physics and Space The two-ringed crater Raditladi stands out for the many bright hollows that line the peaks along magnetic field, allowing for simultaneous 300 miles (480km) toward the planet’s but to see if there is smaller scale structure Sensorics group at the Technical University the crater’s inner ring. Scientists think hollows are features from Mercury’s recent past, and that readings from dif­ferent positions. north pole. That’s about 20 percent of the in the field — and can you see it over the of Braunschweig in Germany and principal they likely are still forming today. Orbital dynamics also plays a role in planet’s 1,516-mile (2,440km) radius. This southern hemisphere,” says Johnson. investigator of BepiColombo’s Mercury improving BepiColombo’s science. MES­ makes the field more than three times Magnetometer experiment. He adds that SENGER’s closest approach to the planet stronger in the northern hemisphere than A STATIC FIELD the relative strengths of the internal and (periapsis) was above the northern hemi- in the southern. Mercury’s magnetic field seems remarkably external fields leave open the possibility sphere at an initial altitude of about 125 The center of Earth’s magnetic field stable. Mariner 10 didn’t get enough data to that the external one affects the dynamo miles (200 kilometers). Its farthest point roughly coincides with our planet’s center, tell if the field’s overall structure changed process, or at least has an outsized effect (apoapsis) was some 9,440 miles (15,193km) though the magnetic axis tilts some 11° to on the fields themselves. above the southern hemisphere. This orbit the rotational axis. Mercury’s field tilts Another consequence of the relatively meant that observations of the southern only 4.5° to the rotational axis, but the off- small internal field is that events happen hemisphere came from much farther away, set is a major puzzle. “There are other plan- quickly, in mere seconds. This means that reducing the probe’s resolution. Although ets that have magnetic fields not centered whenever MESSENGER took readings of this arrangement allowed scientists to inves- on the center of the planet, and other plan- the field during a solar flare, it didn’t have tigate the solar wind environment, it pre- ets that have magnetic fields sort of aligned enough time to get to a different location to vented them from getting a close look at the [with the rotational axis],”says Steven measure what the effect might have been. “X” marks the spot in Mercury’s northern hemisphere where the MESSENGER spacecraft crashed magnetosphere near Mercury and reduced Hauck, a planetary scientist at Case In contrast, earthly magnetic events unfold April 30, 2015. The impact likely formed a crater 52 feet (16 meters) in diameter. the clarity needed for geological studies. Western Reserve University in Cleveland over many minutes or even hours. With BepiColombo’s two orbiters, on the and a member of the MESSENGER team. BepiColombo’s two spacecraft, one can other hand, will take up symmetrical “Mercury has both.” take measurements close to the planet’s there’s a mystery here, too. Sean Solomon compacts its material. Given the planet’s paths. Once they get to Mercury, they The magnetic field likely arises from a surface while the other does so farther out of Columbia University’s Lamont-Doherty proximity to the Sun and high temperature, will separate and enter polar orbits. The dynamo — the movement of electric cur- in the magnetosphere. Earth Observatory notes that Earth-based most scientists anticipated that Mercury’s MPO will be close in, with a periapsis of rents in the core’s liquid outer part — just MESSENGER also studied Mercury’s observations detected rapid changes in the surface would be rich in metals, with lots of 300 miles (480km) and an apoapsis of 930 like the one that surrounds Earth. “MES­ atmosphere. It contains atoms of sodium, amount of sodium in Mercury’s atmo- compounds containing iron. There would miles (1,500km). The MMO’s track will be SENGER confirmed that field predomi- calcium, and several other elements. On the sphere but MESSENGER didn’t see the be few volatile elements, those that vaporize more eccentric, with a minimum altitude nantly comes from the core,” says Hauck. side of the planet where the Sun is rising, same thing. The difference might arise at relatively low temperatures. of 365 miles (590km) and a maximum of The question is what kind of dynamo it calcium atoms appear in abundance, while because of the way earthly telescopes view MESSENGER found the reverse. There 7,230 miles (11,640km). Mission planners is. On Earth, convection drives the process. on the dusk side there seem to be far fewer. the innermost planet — from our home, are lots of volatiles on the surface and less designed both orbits so the probes will Heavy elements sink to the core and lighter Magnesium behaves similarly, though it’s you can’t see all of Mercury. BepiColombo’s iron than anyone expected. The abundance examine the northern and southern ones float to the top. The movement creates not as pronounced. Matthew Burger, a consistent, whole-world coverage should of iron in surface rocks hovers at around ­hemispheres in equal detail. Although a dynamo that generates the magnetic field. research associate at NASA’s Goddard Space help solve this problem. 2 percent. The amount of other metals also BepiColombo’s best visual resolution won’t Mercury is probably different, says Flight Center, says the degree of asymmetry differs from the Moon and the other inner be quite as good as MESSENGER’s was, the Catherine Johnson, a planetary scientist was unexpected based on observations A CHEMICAL MISFIT planets. For example, Mercury possesses coverage will be more comprehensive. at the University of British Columbia in from Earth. The amount of sodium atoms Which brings us to another weird thing more magnesium and less aluminum than Vancouver. One of the odd things about The large craters Poe, Sander, and Munch appears to stay relatively constant between about Mercury: Its chemistry is simply Earth, the Moon, or Mars. Meanwhile, the Mercury’s magnetic field is its strength, (top to bottom) lie in the northwestern part the dawn and dusk hemispheres. wrong. Before MESSENGER, planetary planet’s potassium and chlorine abun- MAGNETIC PERSONALITY of the Caloris Basin. The impacts that created Mercury’s magnetic field is about 1/100 or lack thereof. If a planet rotates above these features excavated darker material from The mechanism driving the abundance ­scientists knew the planet had a density dances are similar to volatile-rich Mars. the strength of Earth’s. MESSENGER a certain critical rate and has a liquid beneath the surface, which appears blue in this of sodium might be impacts from tiny dust second only to Earth, and it lagged behind Most surprising, however, is the pres- showed that the field doesn’t align with layer inside it, a dynamo is pretty much enhanced-color mosaic. grains in that part of the solar system. But our planet only because our greater mass ence of sulfur. Sulfur turns to gas at

10 ASTRONOMY • VENUS AND MERCURY: HOT, VOLATILE PLANETS WWW.ASTRONOMY.COM 11 hemispheres, it should be able to tell if the distribution comes from thermal processes. Ea Inside two planets Volatiles heat up near the equator, especially Solid inner core at so-called hot poles where the Sun stays Liquid outer core directly overhead for longer periods. They Lower mantle then get deposited in cooler polar regions. “We’d expect a symmetrical distribution,” Upper mantle says Weider. “That would be a nice answer.” Crust Another notable feature MESSENGER ey revealed is that Mercury’s surface appears dark. On the Moon, dark areas often con- tain lots of iron-bearing compounds, which undergo space weathering — the gradual darkening of the surface as micrometeor- ites and solar wind particles bombard it. Other compounds, such as those con- Solid taining titanium, also darken under the inner core Liquid onslaught. But Mercury doesn’t have much middle core Crust iron or titanium in its crust and soil, so Solid iron-sul de layer Mantle there must be another answer. Carbon could be one solution, and The interiors of Mercury and Earth are quite different. Mercury’s gigantic core starts just 250 models indicate that the early Mercury miles (400 kilometers) below the surface, and it has a relatively thin crust and mantle. A major- ity of Earth’s volume resides in its mantle. The liquid parts of the cores of both planets help might have had a carbon-rich outer layer. generate their magnetic fields. ASTRONOMY: ROEN KELLY One scenario suggests that the molten rock covering the young Mercury was less dense The aptly named “spider” lurks near the center than the otherwise similar magma that of the mammoth Caloris impact basin. It com- existed on the early Moon and Earth Blewitt of Johns Hopkins University notes what makes Mercury dark. prises the sharp-rimmed crater Apollodorus sur- rounded by the radiating channels of Pantheon because it contained much less iron in the that the hollows are extremely young and Scientists designed the infrared spec- Fossae. Despite the proximity of the crater and This enhanced-color view shows Mercury’s south polar region. Ice likely lurks at the bottom of these material that eventually formed the mantle probably still actively forming. “The big trometer on the assumption that iron troughs, scientists don’t think their formation permanently shadowed craters, where temperatures can plunge to –370° F (–223° C). and crust. As a result, only the low-density surprise was that there was some sort of would be abundant, says Rothery, and it is is at all related. carbon compound graphite would be light modification of the surface,” he says. The tuned to a narrow range of wavelengths enough to float on it and make a crust. hollows likely form when volatile elements near iron’s signature. But the lack of iron Mercury’s temperature, so what existed Shoshana Weider of the Carnegie Insti­ sublimate. The surrounding soil can’t hold turned up by MESSENGER might be a if you’re looking at the target body. Asphaug there initially should have boiled away long tution for Science in Washington, D.C., says NOT-SO-SLEEPY HOLLOWS the structure and it caves in. blessing — a number of other elements imagines a situation where Mercury and ago. At the same time, there’s almost no that it’s possible the magnesium-rich region But perhaps the most surprising MESSEN­ Scientists want to know exactly what is radiate at nearby wavelengths. The absence Venus hit each other and then, on further oxygen bound up in the surface rocks. is an ancient impact crater, a spot hit so GER discovery are hollows — features akin sublimating. “There’s some component that of a strong iron signal will make it easier to passes, the two bodies compete for the now- These elemental abundances fly in the forcefully that the crust cracked and mantle to sinkholes that have sharp edges but were is unstable at the surface,” says Blewitt. detect minerals such as plagioclase, which stripped mantle material. In this scenario, face of the models planetary scientists material bubbled up. Just as important to not formed by impacts. No similar struc- “Between solar heating and space weather- forms much of the lunar highlands. That Venus would win out, leaving Mercury with worked with before MESSENGER. Pre­ the process could be the mantle itself. Some tures have yet been found elsewhere in the ing, you have these fresh-appearing fea- would reveal a lot about Mercury’s early a crust, a thin mantle, and a large core. viously, many researchers assumed that researchers think it could be heterogeneous, solar system. MESSENGER scientist David tures.” While the culprit is likely some crust because it will tell scientists what was He notes that the simulations he has Mercury was a planet whose mantle got unlike Earth’s mantle, because it is rela- volatile-bearing compound, MESSENGER floating on the top of the magma ocean run didn’t need to involve wildly improb- stripped off by a giant impact. It was the tively thin. “Even on Earth, you have places wasn’t able to determine which one. early in the planet’s history. able collisions or configurations. Most had most logical way, it seemed, to explain why that are different,” says Weider. Mercury’s BepiColombo will be able to explore about a one-in-ten chance of occurring. the planet has a core that starts a mere 250 thin mantle would be even more likely to these questions with a wider range of spec- A GIANT IMPACT? While that doesn’t sound like much, in the miles (400km) below the surface. In such have spots that differed from others. The troscopic instruments. The MPO will carry In the face of Mercury’s strange geology, context of an early solar system with lots of an impact, Mercury would have lost most composition of the terranes also would both an infrared and X-ray spectrometer. some planetary scientists have proposed large bodies flying around, it can work. of its volatile substances. depend on the amount of heating and cool- The first measures the amount of radiation ideas for the planet’s origin that don’t “Mercury surviving after a couple of hit MESSENGER also found regions with ing the mantle experienced. at specific infrared wavelengths to see which require a giant impact. Erik Asphaug and runs turns out to be exactly the luck unique geochemical compositions, called Because MESSENGER couldn’t achieve elements are present. The second uses fluo- of Arizona State University in Tempe you expect when you start with 20 Mars- “terranes.” The spacecraft’s spectrometers the same resolution in the southern hemi- rescence, the same process that causes a hypothesizes that Mercury itself is a “hit- like planets to accrete Venus and Earth, mapped the elemental abundances at Mer­ sphere, however, BepiColombo will play an black-light poster to glow when illuminated and-run” impactor. If that’s the case, the and have two unaccreted survivors [Mars cury’s surface. They found a magnesium- important role. The upcoming mission will by ultraviolet light, to measure X-rays given planet could have held on to its core and and Mercury]. The other kind of one-in- rich area in the northern part of the planet be able to see the chemical “map” of both off by various elements at the planet’s sur- lost a lot of mantle material while still ten luck is to be left alone, which Mars and an aluminum-rich region in the south- north and south with equal accuracy. face when they are exposed to solar X-rays. retaining volatile compounds. was,” he says. ern hemisphere as well as one in the area That two-hemisphere precision also can Because the orbiter will be able to examine “The main problem for Mercury wasn’t Solomon likes this idea. “I think he’s on around the giant Caloris Basin. Earth’s address the question of the surface distribu- A field of bright hollows lurks in the western both hemispheres close-up, these maps will the volatiles, but the problem of getting the right track — it’s not too big a stretch Moon has areas where the surface compo- tion of volatiles. MESSENGER picked up portion of the Zeami impact basin. Although an be more complete. MES­SENGER couldn’t rid of all that mantle rock after the giant of the imagination.” sition differs dramatically from the aver- high concentrations of volatiles near the individual hollow may be only a few hundred match the magnesium-rich area in the north impact,” he says. “Modelers had made val- feet across, fields like this can extend for several age, such as in the maria. But on Mercury, north pole — most famously in its discovery miles. Even bigger populations of these enig- to a geological feature, but BepiColombo iant attempts, but nothing worked.” the terranes don’t always seem to match up of water ice in permanently shadowed cra- matic features exist in the northern and north- might be able to find other elements that do Ordinarily, you might expect the vola- with visible surface features. ters. Since BepiColombo will look at both eastern parts of Zeami. correspond and determine, for example, tiles to disappear in that kind of impact —

12 ASTRONOMY • VENUS AND MERCURY: HOT, VOLATILE PLANETS WWW.ASTRONOMY.COM 13 Through the clouds

New missions aim to untangle the mysteries of how the planet’s scorching surface and violent clouds came to be. by Jesse Emspak

REVISITED

THE FIRST SPACECRAFT TO PULL might have much to tell us about our past, into Venus’ orbit in nearly a decade arrived our future, and even current exoplanets. in December 2015, hailing from Japan. Venus wasn’t always so unloved. From Akatsuki was five years late for its rendez- 1960 to 1984, more than 20 spacecraft vous, but Venus has gotten used to wait- investigated Venus — nearly as many as ing. The European Space Agency’s (ESA) Mars up to that point. The USSR’s Venera Venus Express visited the thickly shrouded and Vega programs resulted in no less than world in April 2006, and that was the first 18 orbiters and landers (though not all mis- Tmission to Venus since NASA’s Magellan sions were successful), and the U.S. added arrived in 1990. Named for the Roman five spacecraft. goddess of love, Venus wasn’t feeling much Two new NASA missions to Venus are of that from space agencies on Earth. Our in advanced planning stages, with their planet’s more favored neighbor, Mars, fates to be decided this year. Both ESA and had hosted roughly a dozen visitors in the the Russian Space Agency have designs on same period. the drawing board. And of course, there’s “Venus exploration is behind sched- the current science from Akatsuki finally ule,” says David Grinspoon, senior scien- streaming to Earth. All in all, things are tist at the Planetary Science Institute in looking up for Venus exploration, and Washington, D.C., and author of the book upcoming missions — mostly orbiters but Venus Revealed. “Our understanding of some with plans for landers or craft that Venus is about the same as it was with will dive into the atmosphere — could Mars in the 1970s.” answer fundamental questions that plan- Some planetary scientists are trying to etary scientists still have about Venus, and change that. For years, Venus lost out to provide hard evidence to nail down their Mars because of the tantalizing possibil- current theories. ity of finding life on the Red Planet. Yet in some respects, Venus is more similar to The twin paradox Earth than Mars is, and our inner neighbor Venus is often referred to as Earth’s twin Magellan arrived at Venus for a reason: The two planets’ most basic in August 1990 and orbited Jesse Emspak is a science writer who lives physical properties are nearly identical. for four years, imaging 98 percent in New York. “If we found Venus around a sunlike star, of the planet’s surface. NASA/JPL

14 ASTRONOMY • VENUS AND MERCURY:© 201 6HOT, Kalmbach VOLATILE Publishing PLANETS Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com to take the carbon dioxide out of the air and replace it with oxygen, as on Earth, the water and carbon dioxide — powerful heat-trapping gases — caused a runaway greenhouse effect. The situation wasn’t helped by the planet’s slow rotation. On Earth, our relatively rapid spin creates a dynamo effect in our planet’s iron core. This in turn generates a magnetic field that protects our home world from the solar wind, the stream of energetic particles the Sun flings in all directions. As the wind whipped by unprotected Venus, it stripped the hydrogen from the atmosphere, leav- ing fewer ingredients for the planet to have any hope of reforming its water, even if conditions were to miraculously become more temperate. The VERITAS spacecraft, if selected by NASA for approval, would fly in Yet the data from Venus Express and the early 2020s. Like the previous Magellan mission, VERITAS would orbit the Magellan probe don’t seem to tell the Venus, but it would study the planet in much greater detail. NASA/JPL-CALTECH whole story — and that’s where the new The Venus Express mission imaged clouds swirling above Venus’ south pole at dizzying The proposed DAVINCI spacecraft would parachute to Venus’ surface, missions come in. speeds. The high-altitude clouds, like those seen here, can travel 60 times faster than taking data during an hourlong descent. NASA/GSFC the planet rotates, contributing to the polar vortex Venus Express studied in detail during Many mission options its eight-year stay. ESA/VIRTIS-VENUS EXPRESS/INAF-IAPS/LESIA-OBS. PARIS/G. PICCIONI NASA is considering two missions this [astronomers would] be jumping up and the Sun is a diffuse year. The Deep Atmosphere Venus down saying we found another Earth,” says splotch of bright- Investigation of Noble gases, Chemistry, Colin Wilson, deputy project scientist on ness, appearing and Imaging (DAVINCI) will focus on Venus Express. as it does on an atmospheric chemistry. It involves an the atmosphere, but they couldn’t give sci- 2016 whether DAVINCI, VERITAS, or How similar are they? Venus has overcast day on atmospheric probe — “Huygens for Venus,” entists a good handle on the composition both will fly. a radius of 3,760 miles (6,050 kilome- Earth. That bright quips Lori Glaze, a scientist at NASA’s with respect to altitude, and that’s what’s From ESA, there’s EnVision, an orbiter ters), and Earth’s radius is 3,960 miles patch takes 117 Goddard Space Flight Center and the mis- needed to understand the kinds of reac- also equipped with advanced radar, and it (6,370km). Venus’ mass is 82 percent Earth-days to cross sion’s principal investigator — that will tions that occur in Venus’ cloudy skies. likely won’t launch until 2029, says Richard that of Earth, and its surface gravity is 91 the sky. Venus takes 243 Earth-days to features that look like dune fields exist. measure the atmosphere’s makeup at dif- NASA’s other option is the Venus Ghail, a lecturer in engineering geology at percent of the terrestrial norm. The two make a complete rotation — longer than Some areas have coronae — pancake-like ferent layers during an hourlong descent to Emissivity, Radio Science, InSAR, Imperial College London, who proposed planets’ densities are also almost identical. the planet’s year, which is 225 Earth-days. structures that can spread over 100 miles the venusian surface. Topography, and Spectroscopy (VERITAS) the mission. Aside from a more advanced That means their bulk composition should The daylight period is shortened slightly (160km). “It doesn’t need to survive hitting the mission. The VERITAS orbiter would set of radars than Magellan, EnVision be about the same, especially since both because the planet has a retrograde rota- Venus also is bone-dry. If the planet surface,” Glaze says. She notes that the operate similar to Magellan, but the big will be able to “spotlight” small areas to planets formed in the same region of the tion — the Sun rises in the west. The slow did form with similar amounts of water as Pioneer Venus and Vega missions looked at difference would be that its radars will image them in greater detail. “Instead of solar nebula. Their evolution also should rotation also means Venus lacks a magnetic Earth, as seems likely, it’s clear that water have much better resolution, able to spot 100-meter [300 feet] resolution, we can have been similar. field of any significance. isn’t there anymore. features as small as about 100 feet (30m) get down to 6 meters [20 feet],” Ghail says. But thanks to subtle differences, that When it rains on Venus, the droplets How did Venus become a toxic hells- across compared with Magellan’s more That is enough to see day-to-day changes didn’t happen. Present-day Earth has liquid evaporate before they reach the ground. cape while Earth stayed relatively cool? than 300 feet (100m). It also will measure on the surface. Spotlighting can get that water and an atmosphere dominated by Besides a forecast of “cloudy with a chance The prevailing model is that Venus’ water the planet’s gravity and how the surface resolution down to 10 feet (3m). nitrogen and oxygen. Argon accounts for of sulfuric acid rain,” there doesn’t seem to turned into vapor as the Sun, which was emits heat, which means it can see “inside” While there is some overlap with a nearly 1 percent, but carbon dioxide and be much in the way of weather at ground much dimmer billions of years ago, bright- some geological formations and discover, craft like VERITAS, Ghail says having the other gases exist only in trace amounts. level. Surface pressures are so high — ened and warmed the planet. While there’s for example, whether the coronae are filled VERITAS mission go actually would free The Venus of today is covered by a dense 90-plus atmospheres — that it’s like being some debate as to whether Venus ever with magma. VERITAS also will be able to up EnVision to do more spotlighting of atmosphere — 90 times more massive than underneath more than half a mile (900m) shared Earth’s vast oceans, it seems likely measure how the composition of surface specific areas and less global-scale map- Earth’s — consisting of 97 percent carbon of ocean, and the carbon dioxide there the planet was cool enough for substantial rocks differs. ping, since VERITAS will have accom- dioxide, with the rest as nitrogen and trace begins to behave as a supercritical fluid, a liquid water in its early days. But as the “We’ll be looking for surface mineral- plished that already. gases. Carbon dioxide is a powerful green- strange hybrid of liquid and gas. temperature climbed, any water evapo- ogy variations,” says Suzanne Smrekar of The Russian proposal is called Venera house gas that keeps Venus’ surface tem- Previous missions found that Venus’ rated, and once it reached the upper atmo- the Jet Propulsion Laboratory, the principal D. This spacecraft would bear some resem- perature at an average of 864° F (462° C). A terrain is as varied as Earth’s. Highland sphere, the Sun’s ultraviolet light broke The Akatsuki space probe, launched by Japan’s investigator on VERITAS. “We’re trying to blance to the Vega missions because it visitor to Venus could pour a glass of liquid regions called tesserae consist of ridges and apart water (H2O), and it quickly reformed space agency, entered venusian orbit in understand chemical variation, if there are involves a combined orbiter and lander. It zinc or lead. folds in the crust that extend for miles and into hydrogen (H2), hydroxide (OH), and December 2015. By March, the spacecraft was continents like on Earth, active volcanism might even include a balloon probe, also Unlike Earth, Venus’ surface is invis- form tile-like patterns. The lowlands seem oxygen (O ). Much of the oxygen stayed sending images back to Earth from its infrared . . . also to see if there are tectonic features, like the Vega missions. Like many missions 2 cameras, and in April it officially assumed full ible from above — at least in visible light. to be basalts, cut with what might be lava aloft because it is less dense than carbon science observations. This image shows Venus’ and to try to understand thermal evolu- before it, Venera D would focus on Venus’ It’s covered with highly reflective sulfuric channels. Some mountains appear peaked dioxide, but some descended and reacted night side from one of Akatsuki’s two near- tion — temperature variations in the litho- atmosphere and investigate the origins of acid clouds that never break. On Venus, with a kind of metallic “frost,” and even with surface rocks. Absent any biology infrared cameras. JAXA sphere.” NASA will decide in September the planet’s unusual atmospheric rotation

16 ASTRONOMY • VENUS AND MERCURY: HOT, VOLATILE PLANETS WWW.ASTRONOMY.COM 17 lightning in Venus’ clouds, so Akatsuki will try to clinch those observations. One of the problems Akatsuki will study is the “superrotation” of the atmo- sphere. Venus’ atmosphere zooms around the planet at hundreds of kilometers per hour in the upper regions. That’s not unusual — other planets show the same thing from time to time. But why the superrotation should be orders of mag- nitude faster than the planet’s rotation is unexplained. “We cannot yet accurately model superrotation numerically,” Limaye Venus Express captured this image of clouds Venus’ Idunn Mons is likely an active volcano, says. Akatsuki can help tackle this ques- with infrared imaging revealing hot spots along in 2011, five years into its planned two-year the peak’s summit and cascading over the side in tion by creating a better picture of how the investigation. It finally plunged to its end in 2014, after five mission extensions. ESA/MPS/DLR/IDA flows. ESA/NASA/JPL upper atmosphere differs from the lower and how the two interact.

Venus now A smoking gun for material — are dark. Venus Express’ cam- Current Venus research is already on a volcanism eras also caught some changes in surface delayed schedule. Japan’s Akatsuki probe Venus Express and Pioneer Venus both temperature that looked like signs of recent was supposed to arrive at Venus in 2010, found sulfur compounds — primarily lava flows. Finally, images from Magellan’s but it missed its orbital insertion. Instead, sulfur dioxide, which must continually radar maps show features that look pretty it took a cruise around the Sun for five enter the atmosphere somehow in order clearly like volcanoes, and even lava. years until engineers could steer it into an to be observed, because sunlight breaks But none of these is absolute proof, alternate orbit, more elliptical than origi- it up fairly quickly. “That provides pretty Glaze says. What’s needed is a picture from nally planned. Recovering the spacecraft at good evidence that Venus is volcanically one day to the next, or one week to the all was a major achievement. Akatsuki cur- active,” Grinspoon says. “There’s a lot of next, showing the changes in topography. Venus Express’ cameras sent back many views rently orbits Venus in a 13-day ellipse that sulfur dioxide in the atmosphere, and that That could show that volcanoes are active of Venus, revealing different layers of the planet’s clouds. Here, the spacecraft imaged takes it from closest approach at 260 miles sulfur would not stay without a source.” today, as opposed to in the distant past. the night side in infrared (upper, red layer), (400km) out to 273,000 miles (440,000km). Volcanism on the surface would do it. But Volcanic activity is a big piece of the Researchers constructed this elevation map of Venus using mosaicked data from Magellan. Blue showing clouds lower in the atmosphere, about Sanjay Limaye, senior scientist at the Space Venus Express hasn’t provided the smoking overall Venus puzzle because it offers ­represents lower elevations and red higher elevations. NASA/JPL/USGS 28 miles (45 kilometers) in altitude. Clouds closer Science and Engineering Center of the gun, as it were. “We see a lot of volcanoes, to 40 miles (60km) appear in ultraviolet imaging a way to resurface the planet periodi- (lower, blue) on the planet’s day side. ESA/VIRTIS- University of Wisconsin-Madison, says sci- but we don’t know if they are still active,” cally. Previous imaging missions showed VENUSX IASF-INAF, OBSERVATOIRE DE PARIS (R. HUESO, UNIV. BILBAO) ence goals actually will be enhanced by the Grinspoon adds. Venus doesn’t have many impact craters. than deuterium, and a high ratio of deute- rock. Basalt is a dark rock, granite is a light improvised orbit, which allows for longer There are three strong lines of evidence Assuming the craters are randomly dis- rium should mean the primordial hydrogen rock, and they have different temperatures periods of observation as Akatsuki swings of active volcanoes on Venus, Wilson tributed, that means something made older was stripped away somehow, probably by as a function of altitude.” as well as its chemistry. The lander would out to its farthest position. says. First is the way the detected sulfur ones disappear — the surface is getting the solar wind. The hydroxide is a product To make granite, you need water. “If allow for soil analysis, provided it survives Two of Akatsuki’s cameras work in the behaves. In the first year of Venus Express’ rebuilt every so often, perhaps as little as of the dissociation, or chemical breakup, of you don’t have water, you end up with long enough. The longest any lander has near-infrared and study the planet’s sur- observations, sulfur levels spiked and then every couple of hundred million years or as water by ultraviolet light. But Venus sur- things that approach granite but never lasted on Venus was barely two hours — a face, the motion of clouds, and the particles decreased tenfold over five to six years. much as 750 million to 800 million years. prised the scientists. “We’d expect it to lose get that far,” Ghail notes. Finding granite, record held by the Soviet Venera 13 mis- that comprise them. A long-wave infrared That points to a source that “burped” And given that the craters are on average water faster,” says Wilson. “But the escape therefore, would mean Venus once had sion, so history is on Russia’s side. The camera tracks the temperatures at the cloud sulfur, as volcanoes do. A second clue is hundreds of miles apart, whatever is resur- rate is less than on Earth. That came as a oceans — or at least enough water to allow Russian Space Agency hasn’t made any tops, about 40 miles (65km) above the plan- the infrared surface emission. Darker sur- facing the planet also must exist in the surprise.” Further atmospheric and geologi- for the reactions that make granite. firm commitments to the mission, but if it et’s surface. The other two instruments are faces emit more heat as infrared radiation same well-distributed pattern, Ghail says. cal studies might shed some light on this Ghail says the way the higher terrain did, it would be the first post-Soviet plan- an ultraviolet imager and a lightning and (think about asphalt on a hot day), and If there are active volcanoes — and by narrowing down the rate of outgassing looks — such as Aphrodite, Lakshmi, and etary mission of its kind. Launch wouldn’t airglow camera. Venus Express showed tan- fresh unweathered basalts — such as from Ghail thinks there are — then this resur- water from the surface, for example. Ishtar Terra — is tantalizing. “Aphrodite happen any earlier than 2024. talizing glimpses of what might have been recently spewed and hardened volcanic facing is a constant, steady process. But if Speaking of water, geological tests by looks like ancient, heavily formed conti- volcanoes aren’t currently active, then the VERITAS can help scientists understand nental-like material,” he says. Furthermore, resurfacing is likely to be something big better how much water Venus has now. these regions seem to cover about the same and sudden, covering huge chunks of the Such tests also could reveal if Venus once area that geologists think was covered by planet. VERITAS and EnVision could go a had something like plate tectonics or continental crusts on Earth soon after it long way toward providing a clear answer. formed a surface resembling that of early formed and the first oceans filled up. Earth. By looking closely at what kind of Knowing what Venus was like in the Where’s the water? rock makes up some of the higher-altitude distant past will offer a lot of insight into The other big question mark is water. Venus terrain, such as the tesserae, it will be pos- why Earth’s twin grew up so different from Express’ atmospheric analyses showed the sible to see if it is made of crust that looks its temperate sister. “Venus, in my mind, ratio of deuterium (hydrogen that carries like continents on Earth. “That is critical is an incredibly rich place to learn about Venera 13 survived on Venus for 2 hours, 7 minutes, and took this picture of the venusian surface (and parts of itself) on March 1, 1982. The Vega missions an extra neutron) to ordinary hydrogen to answering the question,” Smrekar says. Earth,” Glaze says. “The planets are so in June 1985 also deployed landers, but Vega 1’s instruments activated while it was still 12 miles (20km) above the surface, so it returned only limited data. is quite large, and that hydroxide is in the “What you’re measuring is surface tem- similar — how did biology form here and Vega 2 successfully transmitted data from the surface, but lasted only 56 minutes. NASA HISTORY OFFICE atmosphere. Ordinary hydrogen is lighter perature in relation to the composition of not there?”

18 ASTRONOMY • VENUS AND MERCURY: HOT, VOLATILE PLANETS WWW.ASTRONOMY.COM 19