The Hitchhiker's Guide to Planets - EXPLORE: MERCURY VENUS SATURN AND MORE!
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The Hitchhiker’s Guide
to Planets
EXPLORE:
MERCURY
VENUS
SATURN
AND MORE!
NASA/JPL
Voyage to a world
A world of
both fire and
ice, Mercury
excites and
confounds
scientists. The
BepiColombo
probe aims
to make
sense of this
mysterious
world.
of extremes
by Ben Evans
Color explodes from Mercury’s surface in this enhanced-color mosaic taken through several filters.
The yellow and orange hues signify relatively young plains likely formed when fluid lavas erupted from
volcanoes. Medium- and dark-blue regions are older terrain, while the light-blue and white streaks
represent fresh material excavated from relatively recent impacts. ALL IMAGES, UNLESS OTHERWISE NOTED: NASA/JHUAPL/CIW
2
M
Mercury is a land of contrasts. The solar system’s Upper left: Explosive eruptions
smallest planet boasts the largest core relative to its size. driven by superheated volcanic
gases left behind these bright
Temperatures at noon can soar as high as 800 degrees yellow deposits in Mercury’s
Fahrenheit (425 degrees Celsius) — hot enough to melt southern hemisphere. This
lead — but dip as low as –290 F (–180 C) before dawn. cluster of volcanic vents ranks
among the largest on Mercury.
Mercury resides nearest the Sun, and it has the most
eccentric orbit. At its closest, the planet lies only 29 mil-
Lower left: When planetary
lion miles (46 million kilometers) from the Sun — less scientists first saw this
than one-third Earth’s distance — but swings out as far oddly bumpy and grooved
as 43 million miles (70 million km). Its rapid movement landscape, they informally
dubbed it “weird terrain.” The
across our sky earned it a reputation among ancient region formed when seismic
skywatchers as the fleet-footed messenger of the gods: Italian scientist Giuseppe “Bepi” Colombo waves from the mammoth
helped develop a technique for sending a space impact that created the Caloris
Hermes to the Greeks and Mercury to the Romans. probe to Mercury and having it execute multiple Basin converged on the planet’s
Even though Mercury lies tantalizingly close to flybys. The European Space Agency honored opposite side.
Earth, it is frustratingly hard to get to. Only two space- his contributions by naming the BepiColombo
spacecraft after him. ESA
craft have ever visited this barren world. But that is Right: Bright blue depressions
set to change October 19, when the international litter the floor and mountain
BepiColombo spacecraft begins a decade-long odyssey Before the Space Age, telescopic observa- peaks in the Raditladi
impact basin. These shallow
to unlock the secrets of a planet that seems to defy tions indicated that Mercury was tidally “hollows” typically have
common sense. locked to the Sun, rotating once for each smooth floors unmarked by
The mission’s namesake — Italian scientist, math- 88-day orbit and thus perpetually show- impact craters, suggesting
that they are among Mercury’s
ematician, and engineer Giuseppe “Bepi” Colombo ing the same hemisphere to its parent youngest features. Scientists
(1920–1984) — was instrumental in devising a means to star. But in the 1960s, radar measure- created this five-image mosaic
deliver a spacecraft from Earth, via Venus, to Mercury. ments pegged its actual rotation at by merging high-resolution
black-and-white photos with
Scientists already knew that a planet’s gravitational field 58.6 days, two-thirds of its orbital period. a lower-resolution image in
could bend the trajectory of a passing spacecraft and In essence, the planet spins about its axis enhanced color.
enable it to rendezvous with another celestial body. In three times for every two solar orbits.
the early 1970s, Colombo showed that if a spacecraft As Colombo first described, this
encountered Mercury, it would end up with a period means a day on Mercury lasts twice as A STRANGE, OLD WORLD craters that occur in chains and
almost twice that of the planet’s orbital long as its year. Day and night each last Mariner 10 revealed an ancient terrain of clusters, covering the highlands.
period. He suggested that a pre- a mercurian year apiece, with new rugged highlands and smooth lowlands, In contrast, the sparsely cratered
cisely targeted flyby would sunrises arriving every 176 days strikingly reminiscent of our Moon. Yet lowlands formed near the end of the
present a possibility for — the same as the six-month the similarities aren’t even skin deep. Late Heavy Bombardment, about
an economical second interval between Mariner Mercury’s craters differ markedly from 3.8 billion years ago. Mariner 10 data
encounter. 10 flybys. So, the Sun lunar ones, because their impact ejecta suggested that the lowlands formed
NASA confirmed illuminated the same blankets a smaller area, partly due to either from volcanic activity or from the
the idea and used it to hemisphere of the planet’s much stronger gravity. The molten material splashed onto the sur-
send the Mariner 10 Mercury during all highland regions are less saturated with face after large impacts. Although the
spacecraft past the three encounters, craters; instead they are mixed with roll- spacecraft found no obvious smoking
innermost planet and the spacecraft ing “intercrater plains” that constitute gun for volcanism — such as lava
three times. The was able to map only one of the oldest-known surfaces on the flows, volcanic domes, or volcanic
probe encountered about 45 percent of terrestrial planets. cones — it did uncover strong cir-
Mercury in March the planet’s surface. The plains were laid down some cumstantial evidence.
1974, September 1974, 4.2 billion years ago during the Late Mariner 10’s successor, NASA’s
and March 1975. Its Heavy Bombardment, when remnants MESSENGER spacecraft, provided the
photographs gave Although Mercury appears from the solar system’s birth rained proof. During its initial flyby in January
humanity its first close-up mostly gray to the human eye, down on the infant planets. Mercury was 2008, the probe revealed a fractured
scientists often enhance the colors
views of the world, and the in images to heighten differences in only a few hundred million years old, region of ridges and furrows within the
last ones we would see for chemical, physical, and mineralogical and the plains obliterated older craters, huge Caloris Basin. MESSENGER would
a generation. properties among surface rocks. The circular buried several large basins, and carved go on to fly past Mercury twice more, in
tan feature at upper right is the giant Caloris
Sadly, Mariner 10 provided only a partial view Basin. The center of this hemispheric view lies many of the pits and bowls seen today. October 2008 and September 2009, and
because of a quirk in Mercury’s orbital parameters. at 0° latitude and 140° longitude. The plains boast groups of secondary then orbit the inner world for four years
4
suspect it could not retain an atmo-
sphere. But Mercury is full of surprises.
Mariner 10 discovered a thin layer of
loosely bound atoms, known as an exo-
sphere, albeit with a surface pressure tril-
lions of times less than that at sea level on
Earth. It contains hydrogen and helium
atoms captured from the solar wind —
the stream of charged particles emanat-
ing from the Sun — together with oxygen
atoms liberated from the surface by
micrometeoroid impacts. Spectroscopic
observations also revealed sodium, potas-
sium, calcium, magnesium, and silicon.
Caloris and the weird terrain appear to
be key sources of sodium and potassium,
The raven-colored rim of Poe Crater stands indicating that impacts can release gases
out from the smooth volcanic plains inside
Caloris Basin. Note the hundreds of tiny from below the surface.
blue-white hollows that dot the rim of this
48-mile-wide (77 km) crater. DIGGING DEEPER
Farther down, the planet’s interior
As Mercury’s interior cooled, the planet’s radius shrank by up to 4 miles (7 km). The contraction remains a puzzle. Before Mariner 10,
buckled the surface and left behind steep cliffs, including Carnegie Rupes, seen here cutting through starting in March 2011. While in orbit, scientists assumed Mercury had a solid
Duccio Crater. The colors in this perspective view highlight elevation changes, with red indicating
the highest terrain and blue the lowest. the spacecraft discovered at least nine interior that produced no intrinsic mag-
overlapping volcanic vents, each up to netic field. They did realize, however,
5 miles (8 km) across and a billion years that the planet has an inordinately high
Mercury Planetary Orbiter old, near Caloris’ southwestern rim.
Elsewhere on Mercury, MESSENGER
density. Overall, Mercury’s density aver-
ages 5.4 times that of water, close to those
MPO’s 11 instruments, some of which have uncovered residue from more than of the much larger Earth (5.5 times
multiple components, include cameras, an 50 ancient pyroclastic flows — violent water) and Venus (5.2 times water). But
altimeter, a magnetometer, a particle analyzer,
Instruments inside outbursts of hot rock and gas — tracing the gravity of these bigger worlds crushes
and several
thespectrometers.
spacecraft: It will concentrate
SIMBIO-SYS
(spectrometer) mainly onISA
Mercury’s surface chemistry and back to low-profile shield volcanoes, their interiors to far higher densities than
geology. (See text on p. 26 for instrument mainly within impact craters. they would have otherwise. Caloris Basin spans about 960 miles (1,550 kilometers) and ranks among the oldest and largest impact
MORE
descriptions.) features in the solar system. Lava eventually flooded the floor to a depth of about 1.6 to 2.2 miles
MGNS ASTRONOMY: ROEN KELLY, AFTER ESA
Caloris itself is an impressive relic The only reasonable way to explain (2.5 to 3.5 km). The lava appears orange in this enhanced-color mosaic; more recent impact craters
from Mercury’s tumultuous early days. Mercury’s high density is with the pres- exposed darker material (blue) from below.
SERENA The Sun illuminated only half the basin ence of heavy elements — some 70 per-
during Mariner 10’s visits, so it was left cent iron and nickel overall — with most
to MESSENGER to fully reveal its struc- of them concentrated in the planet’s giant rotational axis by 20 percent of Mercury’s temperatures can drop as low as
ture. Caloris spans 960 miles (1,550 km), core. This makes Mercury by far the radius and suggested that the planet pos- –370 F (–225 C). In November 2012,
MIXS
placing it among the largest impact fea- most iron-rich planet in the solar system. sesses a partially molten outer core that MESSENGER identified up to 1 trillion
MERTIS tures in the solar system, and it is ringed Scientists think the winding cliffs that surrounds a solid inner core. tons of water ice near the poles —
by a forbidding chain of mountains that reach up to a mile high and run for Astronomers still aren’t sure what enough to encase Washington, D.C., in
SERENA
SIMBIO-SYS rises 1.2 miles (2 km) above the sur- hundreds of miles formed when the keeps the core in an electrically conduc- a frozen block 2.5 miles (4 km) deep.
(high-resolution BELA
camera) SIMBIO-SYS
roundings. Beyond its walls, ejecta radi- surface buckled as the interior cooled tive, semi-liquid state. Perhaps it is the
Instruments inside
(stereo camera)
SIMBIO-SYS the spacecraft: ate in meandering ridges and grooves for and contracted. Despite this shrinkage, slow decay of the radioactive elements BEPICOLOMBO
(spectrometer) ISA more than 600 miles (1,000 km). The MESSENGER revealed that Mercury’s Mercury was born with. The Sun’s grav- COMES ON THE SCENE
MORE
MGNS impact that created Caloris was so glob- core stretches to within 250 miles ity, which raises tides as the planet fol- Despite Mariner 10’s and MESSENGER’s
ally cataclysmic that strong seismic waves (400 km) of its surface. lows its eccentric orbit, could flex the incredible discoveries, scientists still have
SERENA pulsed through Mercury’s interior and Scientists also were surprised when interior and play a contributing role. many questions about this enigmatic
MPO-MAG fragmented the landscape on the planet’s Mariner 10 discovered a magnetic field Despite this internal heat and the world. That’s where BepiColombo comes
SIXS opposite side, leaving a region of jumbled together with a small magnetosphere that blazing Sun above, Mercury also appears in. The European Space Agency (ESA)
rocks, hills, and furrows that some scien- weakly deflects the solar wind around to be a land of ice. In the 1990s, ground- initially envisioned three spacecraft for
tists have dubbed “weird terrain.” the planet. A solid, slowly rotating planet based radar observations revealed a this ambitious venture: The Mercury
MIXS Despite Caloris’ huge dimensions, shouldn’t be able to generate the strong number of bright spots within 6.5° of Planetary Orbiter (MPO) and Mercury
MERTIS
Mercury itself is quite small — just internal dynamo needed to create an the planet’s north and south poles. Many Magnetospheric Orbiter (MMO) would
SERENA 3,032 miles (4,879 km) in diameter. The intrinsic field, even one that’s just 1 per- scientists interpreted these findings as work in tandem to unlock Mercury’s
SIMBIO-SYS
(high-resolution BELA PHEBUS planet’s small size and high temperature cent as strong as Earth’s. MESSENGER evidence for ice deposits on the floors mysteries from above, and the Mercury
camera) SIMBIO-SYS led mid-20th-century astronomers to showed the field is offset along the of permanently shadowed craters, where Surface Element (MSE) would explore the
(stereo camera)
6 W W W.A S TR O N O MY.CO M 7
Beneath two planets’ skin
MESSENGER took this mosaic in October 2008,
Mercury moments after it flew past Mercury for the second
time. The probe captured the first image (at left)
nine minutes after closest approach; subsequent
Mercury’s interior differs significantly from Crust images came with the probe farther away (and
Earth’s. The inner world’s gigantic core starts Mantle thus show more area) and the Sun higher in the
just 250 miles (400 kilometers) below the planet’s sky. This equatorial swath spans about
Solid iron-sulfide layer
surface and is surrounded by a relatively thin 1,200 miles (1,950 km).
mantle and crust. Most of Earth’s volume Liquid middle core
resides in its mantle. The liquid parts of both Solid inner core
planets’ cores help generate their magnetic
fields. ASTRONOMY: ROEN KELLY
Earth
Crust
study the role of the solar wind in weath-
Upper mantle
Lower mantle
ering the planet’s surface.
Liquid outer core MPO carries two instruments to help JAPAN LENDS A HAND
Solid inner core understand why Mercury has so much The mission drew more international mercurian surface; the Mercury Plasma hold regular joint meetings. Some of our
iron and what this reveals about its evo- collaboration when the Japan Aerospace Particle Experiment (MPPE), to scrutinize science goals can only be reached if we
lutionary history. The Italian Spring Exploration Agency (JAXA) joined the the planet’s magnetic field and its interac- work closely together.”
Accelerometer (ISA) and Mercury project. JAXA developed the 630-pound tion with particles in the solar wind and The final element of the spacecraft is
Orbiter Radioscience Experiment (285 kg) MMO spacecraft. Earlier this particles coming from Mercury; and the the Mercury Transport Module (MTM).
(MORE) will investigate the planet’s year, the space agency renamed the craft Plasma Wave It holds four British-built xenon-ion
PWI
global gravitational field to understand Mio, which comes from a Japanese word Investigation (PWI), engines, 24 chemical thrusters, and two
surface. ESA The Bepi the size and nature of the core as well as meaning “waterway” or “fairway.” to study the planet’s large solar arrays that will provide elec-
planned to Colombo Laser the structure of the mantle and crust. Mio carries five science instruments, electric and magnetic fields as well as look trical power to keep MPO and Mio alive
land the MSE Altimeter (BELA) MPO also houses one-half of the including the second half of MERMAG. for evidence of aurorae and radiation belts. during their seven-year journey to the
near the day-night and the Spectrometers and Mercury Magnetometer (MERMAG) Its other tools are the Mercury Sodium “The collaboration with our Japanese Sun’s closest planet. “Solar electric pro-
terminator and have it Imagers for MPO BepiColombo that will study the magnetic field for Atmosphere Spectral Imager (MSASI), to colleagues goes very well; we almost feel pulsion [SEP] allows very significant
survive for about a week in the Integrated Observatory System (SIMBIO- clues to the dynamo lurking inside. study the origin and extent of sodium in as one team,” says Reininghaus. “However, autonomous capabilities for readjusting
harsh environment. The lander would SYS) will create digital terrain models to MPO carries a 24.6-foot-long the exosphere; the Mercury Dust Monitor the two spacecraft were designed and built the interplanetary trajectory, avoiding
carry heat-flow sensors, a spectrometer, quantitatively map Mercury’s geology, (7.5 meters) solar array with integrated (MDM), to explore space dust in the plan- totally independently, although we had to altogether large midcourse maneuvers,”
a magnetometer, a seismometer, a soil- elemental composition, and surface age. optical reflectors designed to keep the et’s vicinity and how it weathers the agree on interfaces. In the science area, we says Reininghaus.
penetrating device, and a tiny rover. Together with the Mercury Radiometer spacecraft at a temperature below 390 F
Unfortunately, budget considerations and Thermal Infrared Spectrometer (200 C). When in orbit around Mercury,
forced ESA to abandon the lander in
November 2003. “The decision to cancel
(MERTIS), Mercury Gamma-Ray and
Neutron Spectrometer (MGNS), and
the array must continuously rotate to bal-
ance MPO’s power requirements with the
Mio (Mercury MMO-MAG
the lander was a loss for the mission,”
says BepiColombo project scientist
Mercury Imaging X-Ray Spectrometer
(MIXS), they will identify key rock-
need to keep the probe
under its redline tem- PWI
Magnetospheric MPPE
MPPE
Mio’s five instruments will focus on
Mercury’s magnetic field and the
Johannes Benkhoff. “What we miss is a
so-called ‘ground truth.’ We can do many
forming minerals, measure global surface
temperatures, and address competing
perature. Meanwhile,
a radiator angled toward the planet will
Orbiter) solar wind environment. Several
of the instruments have multiple
components. For example, the Mercury
MPPE Plasma Particle Experiment (MPPE)
things remotely with our instruments, theories of the planet’s origin and evolu- reflect the intense infrared radiation MPPE has six sensors, one of which has two
which are already on the other spacecraft, tion. These tools also will search for addi- coming from Mercury’s searing surface. components. (See text above for
but the measurements of a lander would tional ice deposits and other volatile “The solar arrays will be exposed to MPPE instrument descriptions.)
have been used to calibrate them, and substances at high latitudes as well as pro- high-frequency, high-intensity ultraviolet ASTRONOMY: ROEN KELLY, AFTER ESA
that can unfortunately not be recovered.” vide insights into the role of volcanism. radiation, combined with high tempera-
MSASI
The rest of the mission continued, To analyze the composition, structure, tures, which was discovered to induce an MDM
PWI
however. ESA led the development of the and formation of Mercury’s exosphere, unexpectedly fast degradation in solar- MMO-MAG
2,535-pound (1,150 kilograms) MPO MPO provides BepiColombo’s Probing cell performance,” explains BepiColombo
spacecraft. The probe’s 11 instruments of Hermean Exosphere by Ultraviolet project manager Ulrich Reininghaus.
were fabricated by 35 scientific and Spectroscopy (PHEBUS) and Search for “This was resolved by a complex method
industrial teams in Switzerland, Exosphere Refilling and Emitted Neutral of continuous solar array steering con-
Germany, Italy, the United Kingdom, Abundances (SERENA) instruments. trol, in order to maintain the temperature
Russia, Finland, Sweden, Austria, Meanwhile, the Solar Intensity X-Ray always below an allowed maximum, and MPPE
France, and the United States. and Particles Spectrometer (SIXS) will by a specific redesign of the solar cells.” MPPE MPPE
8 A ST R O N O M Y • N O V E MB ER 2018 9
PWI
Left: Despite its proximity to the Sun, Mercury
boasts some of the coldest spots in the solar
system. The colors in this view of Mercury’s south
pole show the fraction of time that specific
regions lie in sunlight. The black areas are
those in permanent shadow, the largest of
which is the crater Chao Meng-Fu. MESSENGER
found solid evidence that abundant water ice
exists in this crater.
Below: This enhanced-color view shows the
half of Mercury centered at 0° latitude and
320° longitude. The bright bluish rays
that drape across this hemisphere
stretch more than 600 miles
(1,000 km) and emanate
from the relatively fresh
impact crater Hokusai
at upper right.
Although the solar electric thrusters 60 percent propulsive
provide low thrust, they operate over a greater than intervention
long time, delivering what rocket scien- Earth’s, from the
tists call high impulse. In fact, the demanding spacecraft.
thrusters will accumulate the greatest a substantial “These flybys
total impulse ever achieved by a space velocity depend on the
mission. This posed considerable chal- change and [arrangement]
lenges during preflight testing. “[We correspondingly of the planets,
resolved this through] multiple test cam- high fuel and that is the
paigns in different chambers and with consumption. reason for the long
different test articles, combined with a To overcome these duration,” says
sophisticated modeling approach that obstacles, BepiColombo ini- Benkhoff. “The flybys pro-
allowed us to accurately predict end- tially will enter an orbit similar to vide almost half of the needed energy
of-life performance of the thrusters,” Earth’s, using its high-impulse, low- to go to Mercury. The SEP engine will be
Molten lava once covered Mercury’s vast northern volcanic plains. Lava nearly filled the 181-mile-wide (291 km) Mendelssohn impact basin, which lies at
explains Benkhoff. thrust xenon-ion engines to slowly used for about 50 percent of the time.” the lower right of this enhanced-color image. As the lava cooled, it formed large ridges that appear particularly prominent at bottom left. Meanwhile, the
decelerate against solar gravity and Six flybys of Mercury between bright orange region near the scene’s top shows the location of a volcanic vent that unleashed one of the planet’s largest pyroclastic flows.
GETTING THERE adjust its orbital plane. “Solar electric October 2021 and January 2025 will slow
Like Mariner 10 and MESSENGER propulsion was the only option to reach BepiColombo’s inbound trajectory until
before it, BepiColombo will take a circu- Mercury,” says Benkhoff. “In principle, its orbit nearly matches that of the planet. from its protective sunshield and part least-known worlds in the solar system. gravitational field in detail and provide a
itous route to reach Mercury. The space- one can fly a mission to Mercury with Finally, in December 2025, Mercury will company with Europe’s MPO. Three MPO will occupy a looping, 2.3-hour orbit rigorous test of Albert Einstein’s general
craft will launch from Kourou, French chemical propulsion, but it all depends weakly capture the spacecraft into a months later, the pair will commence at a distance that ranges from 300 miles theory of relativity.
Guiana, atop a giant Ariane 5 rocket, on the thrust-to-mass ratio. SEP is about polar orbit that comes within 420 miles autonomous operations, the former con- (480 km) to 930 miles (1,500 km); Mio Although the spacecraft’s roundabout
perhaps as early as October 19 (the first eight times more efficient than chemical (675 km) of the planet’s surface and trolled from the Usuda Deep Space will follow a highly elliptical path that route to Mercury is hardly in keeping
chance during a six-week launch win- fuel. Thus, for BepiColombo, we would swings out to 110,600 miles (178,000 km). Centre in Nagano, Japan, and the latter will carry it as close to Mercury’s surface with the fleet-footed nature of the plan-
dow). It will depart Earth 7,770 mph have needed at least 2 tons more mass to This so-called weak-stability-boundary from the Cebreros ground station near as 365 miles (590 km) and as far away et’s mythological namesake, the mission
(12,510 km/h) faster than the escape accommodate this.” technique adds flexibility compared with Madrid, Spain. “However, from the as 7,230 miles (11,640 km) during a and the god do share some similarities.
velocity from our planet. Although The spacecraft will complete 1.5 cir- traditional approaches, where a single standpoint of science operations, coordi- 9.3-hour orbit. Both will deliver an abundance of learn-
impressive by many standards, this speed cuits of the Sun, returning to Earth in engine firing typically brings a spacecraft nation planning will be maintained Scientists expect the baseline mission ing, and both will accomplish their goals
is problematic for a spacecraft heading April 2020 to pick up a gravitational into orbit. BepiColombo’s chemical among the principal investigators of the to last until May 2027, but there’s a good through ingenuity, an element of trick-
directly into the Sun’s powerful gravita- boost. This will propel it to Venus for thrusters will stabilize the orbit gradually two spacecraft, and a certain amount of chance ESA will grant a one-year exten- ery, and a pinch or two of old-fashioned
tional field. In fact, the energy needed to rendezvous in October 2020 and August and, after traveling 5.5 billion miles joint observations will certainly take sion. As a bonus, BepiColombo will good fortune.
get to Mercury is larger than it would be 2021, which will reduce BepiColombo’s (8.9 billion km), the mission will at last place,” explains Reininghaus. make precise measurements of Mercury’s
to reach Pluto and leave the solar system. perihelion to about the same distance as be underway. All told, the two spacecraft will bring orbital parameters. Because the planet British spaceflight writer Ben Evans authored
Moreover, Mercury’s orbital velocity Mercury. Critically, this ingenious use After the MTM separates from the about 275 pounds (125 kg) of scientific lies so close to the Sun, this should the multivolume History of Human Space
of 105,900 mph (170,500 km/h) is of gravitational fields requires little probes, Japan’s Mio will be spring-ejected instruments to bear upon one of the allow astronomers to chart our star’s Exploration, published by Springer-Praxis.
On the road to Launch: Earth flyby: First Second First Second Third Fourth Fifth Sixth Arrival at Mercury: End of nominal End of extended
Mercury
October 19, 2018 April 6, 2020 Venus flyby: Venus flyby: Mercury flyby: Mercury flyby: Mercury flyby: Mercury flyby: Mercury flyby: Mercury flyby: December 5, 2025 mission: mission?
October 12, 2020 August 11, 2021 October 2, 2021 June 23, 2022 June 20, 2023 September 5, 2024 December 2, 2024 January 5, 2025 May 1, 2027 May 1, 2028
Mission planners expect to launch BepiColombo as early as October 19, 2018. As long as
the mission commences by November 29, subsequent timeline dates will remain the same.
11
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
T
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-
mission 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
12
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 proposed DAVINCI spacecraft would parachute to Venus’ surface, missions come in. The Venus Express mission imaged clouds swirling above Venus’ south pole at dizzying
speeds. The high-altitude clouds, like those seen here, can travel 60 times faster than
taking data during an hourlong descent. NASA/GSFC
Many mission options the planet rotates, contributing to the polar vortex Venus Express studied in detail during
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 investigator on VERITAS. “We’re trying to blance to the Vega missions because it
The Akatsuki space probe, launched by Japan’s
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 (O2). 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
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
14 15
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- in 2011, five years into its planned two-year
with infrared imaging revealing hot spots along tion by creating a better picture of how the investigation. It finally plunged to its end in
the peak’s summit and cascading over the side in
flows. ESA/NASA/JPL upper atmosphere differs from the lower 2014, after five mission extensions. ESA/MPS/DLR/IDA
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, a way to resurface the planet periodi-
to 40 miles (60km) appear in ultraviolet imaging
(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
an extra neutron) to ordinary hydrogen to answering the question,” Smrekar says. Earth,” Glaze says. “The planets are so
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
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?”
16 17
Land O’Lakes
Secrets from
Titan’s
seas By probing “magic islands”
and seafloors, astronomers are
These images show Titan, from
left to right, in October and
December 2005 and January
2006. The view from December
learning more than ever about is roughly the opposite side
of the moon from the October
and January flybys, but careful
the lakes and seas on Saturn’s inspection of Titan’s polar regions
shows how dynamic and variable
largest moon. by Alexander G. Hayes the polar weather can be. NASA/JPL/
UNIVERSITY OF ARIZONA
IMAGINE YOURSELF standing at the shoreline and organic material like plastic shavings or Styrofoam beads. On
of a picturesque freshwater lake, surrounded by soft grass and leafy closer inspection, the lake holds not water, but a liquid not unlike
trees. Perhaps you are enjoying a peaceful lakefront vacation. In the natural gas. And you’d better be holding your breath because the
calm water, you see the mirror-like reflection of a cloudy sky just surrounding air has no oxygen.
before it begins to rain. Now, let the surrounding vegetation disap- If you can picture all of this, welcome to the surface of Saturn’s
pear, leaving behind a landscape you might more reasonably expect largest moon, Titan.
to see in the rocky deserts of the southwestern United States. The Titan is the only extraterrestrial body known to support standing
temperature is dropping too, all the way down to a bone-chilling bodies of liquid on its surface and the only moon with a dense atmo-
–295° F (92 kelvins). The air around you feels thicker, although you sphere. It is also an explorer’s utopia, supporting landscapes that are
yourself feel seven times lighter, courtesy of reduced gravity. As the uncannily similar to those found on Earth while also presenting a
clouds pass overhead, you notice that the lake surface now reflects seemingly endless supply of intriguing mysteries, with fresh ques-
a hazy orange sky with the brightness of early twilight. After the tions following each new discovery. Two recent findings in particu-
clouds have moved on, you finally begin to feel rain hitting your lar have revolutionized our understanding of Titan’s lakes and seas:
hands. However, the rain falls much slower than normal and the their unexpected transparency to microwave radiation and the
drops are bigger, with large splashes following each impact. The appearance of mysterious “magic islands,” which our research team
ground you stand on is a loose sandy mixture of broken-up water ice has been privileged to bring to light. But Titan’s environment
amazed well before these latest discoveries.
Alexander G. Hayes is an assistant professor of astronomy at Cornell This Titan mosaic taken by the Cassini
University. He and his research group focus on comparative planetology Strange but familiar spacecraft shows the seas that speckle
its north polar region and sunlight that
and solar system exploration, specializing in the development and opera- In many ways, Titan’s landscapes are eerily similar to their ter- glints off of them. NASA/JPL-CALTECH/UNIVERSITY
tion of remote sensing instruments on unmanned planetary spacecraft. restrial counterparts. You can find sand dunes similar in both OF ARIZONA/UNIVERSITY OF IDAHO
18 19Hot, short southern summers
Ligeia Spring N
North Mare
Ligeia pole Titan
ASTRONOMY: ROEN KELLY, AFTER NASA/JPL-CALTECH/UNIV. OF ARIZONA/
Mare
Punga
Mare N
North Sun
pole
Evaporate Kraken Mare 9.06 AU
CASSINI IMAGING & RADAR SCIENCE TEAMS
N Winter
Kivu
Kraken-1 10.10 AU
Lacus
Punga
MacKay Mare Jingpo The Throat of Kraken
NASA/JPL/SPACE SCIENCE INSTITUTE
Lacus Lacus Summer
Kraken-2 N
Fall
Cassini’s RADAR instrument took this detailed Seasons on both Earth and Titan are caused by each world’s tilt, so that one side receives more
image of Titan’s north pole and the many lakes direct sunlight. But on Titan, Saturn’s eccentricity varies how far Titan orbits the Sun by more than
and seas that cover its surface. NASA/JPL-CALTECH/ASI/USGS an astronomical unit (AU; the average Earth-Sun distance), as well as speeding it up and down. Titan’s “purple haze” of an atmosphere is thanks
This means the hemispheres don’t share equal seasons, so the south has hotter, shorter summers to a thick shroud of methane, which separates
Many of Titan’s intriguing details are visible in than the north, driving liquid to the upper pole over eons. into distinct layers upon closer inspection.
this Cassini infrared image. The surface appears
largely in green, while dry lakebeds show up in
orange. The lakes and seas that dot Titan’s north-
ern hemisphere are the darkest regions. NASA/JPL- Cassini carries a radar mapper capable of obtaining images of the The Cassini RADAR discovered Titan’s lakes and seas in the
CALTECH/UNIVERSITY OF ARIZONA/UNIVERSITY OF IDAHO surface at a resolution of 1,000 feet (300m). The RADAR works by north polar region during a flyby in July 2006, during north-
sending out bursts of microwave energy and measuring how much ern winter. Since then, Cassini has discovered more than 300
reflects back. Cassini contains two additional infrared instruments liquid-filled depressions that range in size from moderately sized
size and shape to the largest in the dune fields of the Saharan and upper atmosphere). The methane splits into hydrogen, which escapes it uses to study Titan’s surface, but their resolution is usually less lakes at the limits of detection (about 90 acres, or 0.4 square km)
Namibian sand seas of Africa. Alluvial fans (cone-shaped sediment into space, and highly reactive compounds that quickly recombine than that of the RADAR. The Cassini orbiter also carried an ESA- to vast bodies larger than Earth’s Great Lakes. The three larg-
flows left behind by rivers, streams, and landslides) resemble those to form more complex hydrocarbons like ethane and propane. These provided probe, Huygens, which landed on Titan’s surface in early est, Kraken Mare, Ligeia Mare, and Punga Mare, hold the title
found in the Atacama Desert of central Chile, and mountain chains hydrocarbons rain out onto the surface and, over geologic time, 2005. Because at the time the surface of Titan was a mystery, engi- “mare,” which is Latin for sea. Collectively, the lakes and seas
are formed by tectonic forces similar to those responsible for the rework themselves into the solid particles that make up Titan’s dunes neers designed Huygens either to touch down on a solid surface cover 1 percent of Titan’s surface and lie mostly in the northern
Himalayas that span southern Asia. Perhaps most astonishingly, and coat the world’s surface. Carl Sagan referred to laboratory- or to land in an ethane sea. The probe touched down near Titan’s hemisphere, where they cover 35 times more area than in the
lakes and seas scatter the polar landscape with shoreline features generated versions of the kinds of compounds Titan’s atmosphere equator on what appears to be a flood plain strewn with rounded south. We believe Saturn’s eccentric orbit around the Sun causes
reminiscent of both marine and freshwater coastal environments generates as “tholins” and noted that they are similar to the organic cobblestones about 4 inches (10cm) in diameter. this contrast between north and south.
found across our planet. material that may have been important to the development of life Saturn is closest to the Sun during summer in Titan’s south-
However, the dunes are not silicate sand; they are instead organic on Earth. On Titan, these tholin-like materials form haze layers that Seasons and sunlight ern hemisphere, when it tilts areas below the equator toward our
materials more like plastic than quartz. Rather than rock fragments obscure the surface from visible-light cameras, such as those on The Saturn system tilts by 27° from the plane of its orbit, and star’s most direct light. Northern summer, on the other hand,
delivered by flowing water, alluvial fans on Titan are a mixture of board the Pioneer 11 and Voyager 1 spacecraft. thus Titan, like Earth, has seasons. Saturn and Titan, however, happens to occur when the Saturn system is farther from the
water ice and organic sediment delivered by flowing hydrocarbon take 30 years to circle the Sun, so their seasons are 7.5 years long. Sun. As a result, southern summers are both hotter and shorter,
liquids (methane and ethane). The mountains are broken-up sec- Close encounters with more intense sunlight than their northern counterparts.
tions of dirty water ice, and the lakes and seas are vast pools of liquid The presence of a thick atmosphere makes Titan unique among Over many seasons and years, the stronger, hotter sunlight in the
hydrocarbons. Despite these differences, the same mechanisms (such the moons in our solar system. It also made the saturnian moon south drives methane and ethane toward the northern hemi-
as wind and rain) sculpt and transport sediment across the land- one of the primary targets for exploration by Voyager 1. In fact, in sphere. But if this is the explanation for Titan’s lake distribution,
scape on Titan as they do on Earth. The similarities make Titan a order to reach Titan, Voyager 1 had to follow a specialized trajec- we should also note that it changes with time. The position of
natural laboratory for studying the processes that shape our own tory that eliminated the possibility of visiting Uranus or Neptune Titan’s seasons on Saturn’s eccentric orbit varies over periods of
planet, including extreme conditions impossible to recreate in earth- as Voyager 2 did on its “grand tour” of the solar system. While the 50,000 years. In fact, 35,000 years ago, the situation was the exact
bound laboratories. cameras on Voyager 1 were not able to see down to Titan’s surface, opposite of today’s scenario: Northern summers were hotter and
The forces that sculpt Titan’s landscapes resemble Earth’s water the spacecraft was able to use radio instruments to determine the shorter than southern summers. This suggests that the liquid in
cycle, except that the key liquid is methane. Near the surface, meth- surface pressure (1.5 times that of Earth) and temperature (92K). Titan’s polar regions shifts between the poles over timescales of
ane makes up 5 percent of Titan’s nitrogen-dominated atmosphere Following the Voyager encounter, scientists knew liquid methane 50,000 to 100,000 years. And, in fact, there are large-scale depres-
and, like water on Earth, condenses out of the atmosphere as rain and ethane were raining down and stable on Titan’s surface but sions in the south that include features reminiscent of old shore-
and can persist as a liquid on the surface. If all of the methane in had no idea how they were distributed. lines along their borders. These paleo-seas encompass an area
Titan’s atmosphere were to fall down to the surface, it would make a Prompted by the exciting results of the Voyager mission and similar to the northern maria and suggest that Titan’s south pole
global layer 23 feet (7 meters) deep. If you were to do the same thing the near two decades of ground-based imaging campaigns that once looked similar to the north. This orbitally driven mecha-
to the water in Earth’s atmosphere, the layer would be only 1 inch (3 followed, NASA and the European Space Agency (ESA) launched nism is analogous to the cycles on Earth that drive the frequency
centimeters) thick. On Titan, methane rain falls from the sky, flows the Cassini/Huygens mission to Saturn in 1997. As a multipur- and duration of the ice ages.
on the surface, cuts channels into the bedrock, and fills depressions pose mission, Cassini must divide its limited orbits around Saturn
to form polar lakes and seas. between many different moons (as well as the planet itself) and
Titan poses here in front of Saturn’s rings with its much smaller sibling
Wind and waves
High in Titan’s atmosphere, sunlight breaks apart methane in a carefully allot its close flybys, but the spacecraft came specially moon Dione. The fuzzy outline of Titan is due to its thick, hazy atmosphere. For most of Cassini’s mission, its instruments observed Titan’s
process called photolysis (this also happens to methane in Earth’s prepared for Titan. In order to penetrate Titan’s thick atmosphere, NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE lakes and seas to be calm and flat, with vertical deviations of
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