Can a planet's gravity rip out its moon's atmosphere?

Can a planet's gravity rip out its moon's atmosphere?

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We all know that solar flares can rip a planet's atmosphere if the magnetic field is weak.Solar flares travel at tremendous speed so when they hit the gas molecules of a atmosphere the molecules are ejected from the gravitational pull and thrown away.

Can a giant planet's gravity pull out its moons atmosphere?Is gravity enough to pull its atmosphere or do we need an instantaneous force/impact to blow away an atmosphere?

It's probably rare that moons have atmospheres to begin with. Planets don't generate solar flares. Planets emit thermal radiation as a result of their heat and during formation and collision planets can get significantly hot. Earth is believed to have glowed red hot following the theoretical giant impact and that heat actually may have helped give the Moon its lopsided crust. See the Earthshine part of this article. The Moon, which also likely glowed red hot after it formed, was unlikely to have ever had an atmosphere, but in theory, a hot planet could thermally strip a nearby moon if that moon had an atmosphere.

A planet with a surface temperature of a couple thousand degrees could heat the near side of its moon more than the star that they both orbit, perhaps giving off enough heat to cause the loss of the moon's atmosphere through thermal or Jeans escape which is basically a product of upper atmospheric temperature and escape velocity. It might not even be that uncommon for moons to receive more heat from the planets they orbits than the star they both orbit when the planet is undergoing sufficient bomboardment.

A second way that a planet might strip a moon's atmosphere is if the moon was close to the planet's Roche limit. The atmosphere is less gravitationally bound and even if the moon comfortably holds itself together outside the Roche limit, its atmosphere would extend miles above its surface and be less gravitationally bound where gradual atmospheric stripping would be possible.

Moons rarely have atmospheres to begin with, but ice moons at the right temperature can generate atmospheres through outgassing. Pluto, for example, has a tenuous atmosphere from the thawing of frozen nitrogen and other ices on its surface and Pluto's low gravity means this atmosphere is very dispersed and it can extend quite far from the planet, far enough that Pluto and its moon Charon may exchange atmosphere (grain of salt warning with that article, to say they share an atmosphere is an interpretation that I'm not 100% comfortable with, but the article does make the point that an atmosphere can be tenuously held by a comparatively low gravity object like Pluto, and as a result, affected and even shared with a nearby object in a close orbit).

A third theoretical way a planet could strip a moon of its atmosphere would be through magnetism. Jupiter's enormous magnetic field generates a radiation belt that extends beyond its inner moons, certainly beyond the inner 3 of its 4 Galilean moons. The high temperature of the charged particles in a planet's radiation belt could strip a moon of its atmosphere in a similar way that the solar wind can strip planets of theirs.

You just asked about gravity and I gave you three possible scenarios; radiation belts and thermal radiation aren't gravity but they could theoretically strip a moon of its atmosphere. The Roche limit scenario is essentially driven by gravity, so the answer is yes, it's possible. It's not actually the gravity but the tidal forces, which is a product of the gravity that can strip the atmosphere of a moon close to its planet's roche limit.

Moons with atmospheres are probably pretty rare and moons that orbit close to their planet's Roche limits are also fairly rare. So it might be a rare scenario where the two situations coincide.

Possibly an extreme case of the Roche limit (described aptly by userLTK) a moon with atmosphere in a low orbit could lead to tidal forces deforming the atmosphere to the point that at least some of the gas is pulled free from the moon's grav. field. I wouldn't count on it, since it's a bit hard to imagine the planet itself being atmosphere-free.

Does the Moon have a tidal effect on the atmosphere as well as the oceans?

The short answer is yes, and at various times this question of lunar tides in the atmosphere occupied such famous scientists as Isaac Newton and Pierre-Simon Laplace, among others. Newton's theory of gravity provided the first correct explanation of ocean tides and their long known correlation with the phases of the moon. Roughly a century later it was also used to predict the existence of atmospheric tides when Laplace developed a quantitative theory based on a tidal equation now bearing his name. Laplace's equation describes the motions of an ocean of uniform depth covering a spherical Earth [ see illustration ].

At the point on the ocean's surface closest to the moon (point A in the illustration), the lunar gravitational attractive force is strongest and it pulls the ocean toward itself. On the opposite side of Earth (point B), its attractive force is weakest, which allows the ocean to bulge outward again, in this case away from the moon. As the planet rotates from west to east the two bulges tend to stay on the Earth-moon line. (The moon also revolves around Earth in the same direction as Earth's rotation but at a much slower rate.) For an observer stationed on the surface and revolving with it, the bulges would appear as a giant wave, which follows the apparent motion of the moon to the west and has two crests per lunar day.

Real ocean tides are of course complicated by the waters uneven depth and the presence of land. But Laplace's theory is perfectly applicable to the atmosphere if ocean depth in the tidal equation is replaced by a quantity called equivalent depth, characterizing the extent of the atmosphere above the surface. Just as our weight puts pressure on the ground beneath our feet, the weight of the atmosphere above us exerts pressure on the planet's surface and everything located on it (recall that pressure is defined as force per unit surface). This is the usual atmospheric surface pressure that we hear about in weather forecasts. It is clear then that Laplace's theory predicts two pressure maxima per lunar day corresponding to the two ocean bulges [ see illustration ]. One occurs approximately when the moon is directly overhead, the other half-a-day later. The dominant lunar tide in the atmosphere is therefore semidiurnal (half-daily).

Theory predicts stronger lunar pressure oscillations in the tropics but their amplitude rarely exceeds 100 microbars or 0.01 percent of the average surface pressure. Detection of such a tiny signal masked by much larger pressure variations associated with weather phenomena required the development of special statistical techniques and the accumulation of a long series of regular observations.

Surprisingly, such observations show that the sun also causes semidiurnal tides in the atmosphere, which are more than 20 times stronger, although the solar gravitational forcing is less than half that of the moon. After all, it is the moon that causes the dominant tides in the ocean, not the sun. (The average lunar day is about 51 minutes longer than the solar day because of the moons rotation around Earth and this allows scientists to reliably separate the two tides in long observational records.) Apparently, Laplace had suspected this, suggesting that the strong solar tide was primarily generated by solar heating and not by solar gravity. Scientists finally confirmed this hypothesis in the 1960s when it became possible to develop adequate models of solar atmospheric heating. As with the gravitational pull of a celestial body, the uneven solar heating on Earth's dayside distorts the spherical symmetry of the atmosphere, but in a more complex way. The thermal solar tide therefore consists of several dominant waves, the most prominent being the diurnal and semidiurnal ones.

Pressure variations cause tidal oscillations in other atmospheric characteristics as well. It is common for atmospheric waves to grow in amplitude with height as the air becomes thinner. The lunar tide, however, remains weak compared to the solar tide in the upper atmosphere. Still, at altitudes above roughly 80 kilometers (50 miles) lunar tides have been detected in winds, temperature, airglow emissions and a number of ionospheric parameters. Almost two centuries after atmospheric lunar tides were predicted and first observed, they are still studied. They represent a unique type of atmospheric motion whose forcing mechanism is known with great precision, allowing us to test our numerical models and theoretical predictions.

The Space & Beyond Blog

In 1898 it was suggested that Earth and the Moon were once a single body. This hypothesis was that a molten moon had been spun from Earth because of centrifuge-like forces and became the dominant explanation. There are three theories as to how our planet satellite could have been created: the giant impact hypothesis, the co-formation theory, and the capture theory.

The giant impact hypothesis, also known as the Theia Impact

The early solar system was a violent place and several newly formed bodies never made it to full planetary status. One of these could have crashed into Earth not long after the young planet was formed. Known as the giant impact hypothesis or the Theia Impact, the Moon formed from the ejecta of a collision between the proto-Earth and a Mars-sized planetesimal, approximately 4.5 billion years ago in the Hadean eon about 20 million to 100 million years after the solar system coalesced. Theia collided with Earth throwing vaporized chunks of the young planet’s crust into space. Gravity bound the ejected particles together creating a moon that is the largest in the solar system in relation to its host planet. This sort of formation would explain why the Moon is made up predominantly of lighter elements, making it less dense than Earth. The material that formed it came from the crust, while leaving the planet’s rocky core completely untouched.

When young Earth and this rogue body collided, the energy involved was 100 million times larger than the event believed to have wiped out the dinosaurs. This is the prevailing theory supported by the scientific community. Some evidence includes how Earth’s spin and the Moon’s orbit have similar orientations. However, at the end of the day, giant collisions are consistent with the leading theories of formation of the solar system.

Apollo moon rocks reveal the origin of the moon

This NASA moon rock was collected by astronauts on the Apollo 14 mission to the lunar surface in 1971. Photo: © NASA/Sean Smith

The Moon Once Had An Atmosphere

For centuries humans have dreamed of traveling to the Moon. We achieved that dream in 1969, but found our sister world to be a dry airless rock. Most of the early stories of a journey to the Moon painted a very different picture, giving the Moon a breathable atmosphere, and perhaps even exotic life. We now know the Moon is barren of life, but there was a time when the Moon had an atmosphere.

Our Moon doesn't have an atmosphere because it is too small and doesn't have a strong magnetic field. Any atmosphere it might have had would be stripped away by the solar wind that barrages the small world. In contrast, our planet has more mass to hold its atmosphere close, and a strong magnetic field to protect it. But that doesn't mean the Moon couldn't have had an atmosphere for a short time, and new evidence shows that it did.

About 3.5 billion years ago, the broad dark patches we see on the lunar surface first formed. Known as maria, they were created by large lava flows that later cooled to become basalt plains. During the Apollo missions of the 1960s and 1970s, astronauts brought back samples of these maria, and we found they contained traces of gas, such as carbon monoxide. This gas erupted from the Moon's interior at the same time as the maria formed.

Recently a team calculated just how much gas would have been released from this process, and found it was more than originally suspected. So much gas was released that it would have formed a thin atmosphere around the Moon. The atmosphere only lasted about 70 million years, which is brief for geologic scales, but it could have deposited ice and other molecules in the cold sunless regions of craters.

And that could be important for future astronauts. In order to build a permanent presence on the Moon, humans will need water and soil to sustain us. If water and other compounds can be found on the Moon, we won't have to bring it from Earth.

So our Moon is barren and airless today, but we might be able to live there thanks to the brief period when the Moon had a sky.

Saturn's largest moon is drifting off into space 100 times faster than researchers previously thought

Titan, Saturn 's largest moon, is distancing itself from its planet at a rapid speed, astronomers announced this week. The moon is drifting away into space much faster than previously predicted, the scientists said, possibly altering their understanding of our solar system.

Titan , which scientists believe could support life, is moving about 100 times faster than researchers previously thought, according to a study published this week in the journal Nature Astronomy. Using data from NASA's Cassini spacecraft, which observed Saturn for more than 13 years, astronomers found Titan is migrating at a rate of about four inches per year.

It's not unusual for moons to slowly drift from their host planets &mdash in fact, our own moon is constantly floating away from Earth at a rate of 1.5 inches per year. However, due to Titan 's distance from Saturn, scientists thought it was moving away from the planet more slowly.

According to NASA, as a moon orbits, its gravity creates a temporary bulge in the planet, causing tides as oceans move from side to side. Over time, the energy created by this interaction transfers from the planet to its moon, pushing it further away.

But don't worry about our moon. "Earth will not 'lose' the moon until both the earth and moon are engulfed by the sun in roughly six billion years," according to researchers at Caltech.

Larger than the planet Mercury, Titan is seen here as it orbits Saturn in 2012. NASA/JPL-Caltech/Space Science Institute

While scientists know that Saturn formed 4.6 billion years ago, the details on the formations of its rings and its system of more than 80 moons are less certain. Knowing that Titan is currently 759,000 miles from the planet, this new discovery suggests the whole system expanded relatively quickly.

Space & Astronomy

"This result brings an important new piece of the puzzle for the highly debated question of the age of the Saturn system and how its moons formed," lead author Valery Lainey said in a news release.

"Most prior work had predicted that moons like Titan or Jupiter's moon Callisto were formed at an orbital distance similar to where we see them now," said co-author Jim Fuller. "This implies that the Saturnian moon system, and potentially its rings, have formed and evolved more dynamically than previously believed."

For nearly 50 years, scientists estimated how fast a moon drifts from its planet under the assumption that outer moons migrate more slowly than closer moons because they are further away from the host planet's gravity. Four years ago, Fuller countered those theories, publishing research suggesting that a new orbit pattern would allow outer moons to migrate at a similar rate to inner moons.

To reach their new findings on Titan, researchers mapped background stars in images captured by Cassini in order to track the moon over a period of 10 years. They then compared their findings to an independent radio science dataset measuring Cassini's velocity as it was affected by the moon.

"By using two completely different datasets, we obtained results that are in full agreement, and also in agreement with Jim Fuller's theory, which predicted a much faster migration of Titan," said co-author Paolo Tortora.

With a diameter of 5,149 km, Titan is the second-largest moon in the entire solar system, larger even than the planet Mercury. It's the only moon with a dense atmosphere, and it's covered in rivers and seas made of liquid methane and ethane.

Under those bodies of liquid is a thick layer of ice. Data from Cassini revealed that a liquid water ocean lies even deeper, meaning Titan could potentially sustain life.

In 2026, NASA plans to further study the moon with its Dragonfly mission, which will arrive at Titan by 2034. The drone will monitor the moon for nearly three years to figure out if it could one day be habitable.

First published on June 10, 2020 / 3:21 PM

© 2020 CBS Interactive Inc. All Rights Reserved.

Sophie Lewis is a social media producer and trending writer for CBS News, focusing on space and climate change.

Revealed: How Mars Lost Its Atmosphere

Illustration of the Argyre impact basin in the southern highlands of Mars

One of the biggest challenges about flying to Mars is remembering why you went there in the first place. The Curiosity rover has been on the Red Planet for almost a year now, and the landing itself — an outrageous feat achieved by a stationary hovercraft that lowered the 1-ton Mars car to the surface by cables — was a global television event. But once the wheels touched the soil and all of the high fives had been exchanged, most people outside of the space community turned away.

Curiosity, however, went to Mars to work, and if its sister rovers Spirit and Opportunity — both of which arrived in 2004 and one of which is still chugging — are any indication, it should be at it for a long time. In the past year, Curiosity has already made some intriguing discoveries about the mineralogy of Mars and the planet’s watery past, and this week it delivered again. In a pair of papers published in the journal Science, investigators announced new findings from the spacecraft about one of Mars’ most long-standing mysteries: how it lost its atmosphere, and why.

Mars’ modern atmosphere is only 1% the density of Earth‘s, but the planet’s watery phase is believed to have lasted for the first billion of its 4.5 billion years, which means its air must have been around that long too. But things were never likely to stay that way. Mars has only half Earth’s diameter, 11% its mass and 38% its gravity, making it easy for upper layers of the original atmosphere to have boiled away into the vacuum of space and been blasted out by meteor hits. And that cycle would build on itself: the thinner the air became, the easier it would be for space rocks to hit the ground, unleashing still more explosive energy and, in effect, blowing still more holes in the sky.

But that’s only one mechanism. Planets can lose their air not just from the top up but also from the bottom down, as elements of the atmosphere bond with — and retreat into — the soil. Martian meteorites that landed on Earth have often been found to include gas bubbles from the Martian sky, evidence that this commingling was going on.

Curiosity scientists sought to settle the matter with the help of the rover’s Sample Analysis at Mars (SAM) instruments, a collection of sensors that sniff the air for its chemical makeup — particularly its mix of isotopes. Elements don’t come in just one form, but in different sizes and weights — such as carbon 12 and carbon 13 — determined by the number of neutrons in the nucleus. That weight issue is critical in atmospheric studies, because just as heavier metals sink downward and lighter ones rise as a molten planet is forming, so do gases stratify themselves in the atmosphere by weight.

Earlier measurements of Mars’ current atmosphere had always shown a high concentration of the heavy isotopes of carbon and oxygen — convenient elements to measure because Mars’ atmosphere is overwhelmingly made of carbon dioxide. Those findings differ from the isotopic makeup of the sun and the early solar system as a whole, in which lighter isotopes were more evenly represented. Mars, like Earth and all of the other planets, would have started out with that same relatively even mix. The fact that the heavy isotopes dominate the remaining Martian air means its lighter, high-altitude gases bled away first — supporting the top-down theory.

“As atmosphere was lost, the signature of the process was embedded in the isotopic ratio,” said NASA‘s Paul Mahaffy, principal investigator for the SAM team, in a statement. That was the theory anyway, but it took a suite of instruments like SAM to sample the air with enough sensitivity to prove the heavy-isotope imbalance. As the Science paper revealed, Curiosity indeed sealed that deal.

The findings are considered particularly reliable because Curiosity used two different instruments to do its work: the tunable laser spectrometer, which analyzes how Martian air pumped into a chamber reflects two different frequencies of infrared laser and the mass spectrometer, which, as its name suggests, measures the entire spectrum of elements present in an air sample according to their mass. “Getting the same results with two very different techniques increased our confidence that there’s no known systematic error,” said NASA’s Chris Weber, lead author of one of the new papers.

Mars’ lost air is never coming back, but the little bit it does have still makes the planet a chemically active place — and plays a major role in the combination of parachutes and braking-rockets spacecraft from Earth rely on to reach the surface safely. But change is a constant everywhere in the universe, and even today, the Red Planet’s atmospheric loss is thought to be continuing. How fast that’s happening will not be known until the arrival of NASA’s next Mars probe, the Mars Atmosphere and Volatile Evolution (MAVEN) mission, which is set for launch in November. The already harsh Mars, MAVEN may find, is fast becoming harsher still — one more reason to appreciate the improbably verdant Earth.

How’d that get here?

One potential explanation for this is that the planet formed at a cooler distance from the star and then migrated inward. But that would mean we've caught GJ 1132 b in a relatively narrow time window: between it getting close enough to the star to lose its atmosphere, but before all that atmosphere had been heated off into space. The odds are better that the planet had formed near where it is and has generated a second atmosphere after the first was lost.

Fortunately, the data the Hubble provided was able to provide some hint of what's in the atmosphere. The signature left on starlight by the molecules present in the atmosphere provides some indication of what they might be. These indications are complicated—there are many molecules that have signatures that partially overlap in some areas of the spectrum but not others. But it's possible to look at the signal from the planet's atmosphere and identify combinations of molecules that are compatible with that signal.

The researchers find there's likely to be some aerosols aloft in the atmosphere. And its composition really wouldn't be surprising on another planet: mostly methane, ethane, hydrogen, and hydrogen cyanide. But remember, the whole reason this atmosphere is interesting is because the planet should have lost its atmosphere early in its history—and all the hydrogen should have gone away with it.


Atmospheric pressure at a particular location is the force per unit area perpendicular to a surface determined by the weight of the vertical column of atmosphere above that location. On Earth, units of air pressure are based on the internationally recognized standard atmosphere (atm), which is defined as 101.325 kPa (760 Torr or 14.696 psi). It is measured with a barometer.

Atmospheric pressure decreases with increasing altitude due to the diminishing mass of gas above. The height at which the pressure from an atmosphere declines by a factor of e (an irrational number with a value of 2.71828. ) is called the scale height and is denoted by H. For an atmosphere with a uniform temperature, the scale height is proportional to the temperature and inversely proportional to the product of the mean molecular mass of dry air and the local acceleration of gravity at that location. For such a model atmosphere, the pressure declines exponentially with increasing altitude. However, atmospheres are not uniform in temperature, so estimation of the atmospheric pressure at any particular altitude is more complex.

Surface gravity differs significantly among the planets. For example, the large gravitational force of the giant planet Jupiter retains light gases such as hydrogen and helium that escape from objects with lower gravity. Secondly, the distance from the Sun determines the energy available to heat atmospheric gas to the point where some fraction of its molecules' thermal motion exceed the planet's escape velocity, allowing those to escape a planet's gravitational grasp. Thus, distant and cold Titan, Triton, and Pluto are able to retain their atmospheres despite their relatively low gravities.

Since a collection of gas molecules may be moving at a wide range of velocities, there will always be some fast enough to produce a slow leakage of gas into space. Lighter molecules move faster than heavier ones with the same thermal kinetic energy, and so gases of low molecular weight are lost more rapidly than those of high molecular weight. It is thought that Venus and Mars may have lost much of their water when, after being photodissociated into hydrogen and oxygen by solar ultraviolet radiation, the hydrogen escaped. Earth's magnetic field helps to prevent this, as, normally, the solar wind would greatly enhance the escape of hydrogen. However, over the past 3 billion years Earth may have lost gases through the magnetic polar regions due to auroral activity, including a net 2% of its atmospheric oxygen. [4] The net effect, taking the most important escape processes into account, is that an intrinsic magnetic field does not protect a planet from atmospheric escape and that for some magnetizations the presence of a magnetic field works to increase the escape rate. [5]

Other mechanisms that can cause atmosphere depletion are solar wind-induced sputtering, impact erosion, weathering, and sequestration—sometimes referred to as "freezing out"—into the regolith and polar caps.

Atmospheres have dramatic effects on the surfaces of rocky bodies. Objects that have no atmosphere, or that have only an exosphere, have terrain that is covered in craters. Without an atmosphere, the planet has no protection from meteoroids, and all of them collide with the surface as meteorites and create craters.

Most meteoroids burn up as meteors before hitting a planet's surface. When meteoroids do impact, the effects are often erased by the action of wind. [6] As a result, craters are rare on objects with atmospheres. [ clarification needed ]

Wind erosion is a significant factor in shaping the terrain of rocky planets with atmospheres, and over time can erase the effects of both craters and volcanoes. In addition, since liquids can not exist without pressure, an atmosphere allows liquid to be present at the surface, resulting in lakes, rivers and oceans. Earth and Titan are known to have liquids at their surface and terrain on the planet suggests that Mars had liquid on its surface in the past.

A planet's initial atmospheric composition is related to the chemistry and temperature of the local solar nebula during planetary formation and the subsequent escape of interior gases. The original atmospheres started with a rotating disc of gases that collapsed to form a series of spaced rings that condensed to form the planets. The planet's atmospheres were then modified over time by various complex factors, resulting in quite different outcomes.

The atmospheres of the planets Venus and Mars are primarily composed of carbon dioxide, with small quantities of nitrogen, argon, oxygen, and traces of other gases. [7]

The composition of Earth's atmosphere is largely governed by the by-products of the life that it sustains. Dry air from Earth's atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (by volume), but generally a variable amount of water vapor is also present, on average about 1% at sea level. [8]

The low temperatures and higher gravity of the Solar System's giant planets—Jupiter, Saturn, Uranus and Neptune—allow them more readily to retain gases with low molecular masses. These planets have hydrogen–helium atmospheres, with trace amounts of more complex compounds.

Two satellites of the outer planets possess significant atmospheres. Titan, a moon of Saturn, and Triton, a moon of Neptune, have atmospheres mainly of nitrogen. When in the part of its orbit closest to the Sun, Pluto has an atmosphere of nitrogen and methane similar to Triton's, but these gases are frozen when it is farther from the Sun.

Other bodies within the Solar System have extremely thin atmospheres not in equilibrium. These include the Moon (sodium gas), Mercury (sodium gas), Europa (oxygen), Io (sulfur), and Enceladus (water vapor).

The first exoplanet whose atmospheric composition was determined is HD 209458b, a gas giant with a close orbit around a star in the constellation Pegasus. Its atmosphere is heated to temperatures over 1,000 K, and is steadily escaping into space. Hydrogen, oxygen, carbon and sulfur have been detected in the planet's inflated atmosphere. [9]

Earth Edit

Earth's atmosphere consists of a number of layers that differ in properties such as composition, temperature and pressure. The lowest layer is the troposphere, which extends from the surface to the bottom of the stratosphere. Three quarters of the atmosphere's mass resides within the troposphere, and is the layer within which the Earth's terrestrial weather develops. The depth of this layer varies between 17 km at the equator to 7 km at the poles. The stratosphere, extending from the top of the troposphere to the bottom of the mesosphere, contains the ozone layer. The ozone layer ranges in altitude between 15 and 35 km, and is where most of the ultraviolet radiation from the Sun is absorbed. The top of the mesosphere, ranges from 50 to 85 km, and is the layer wherein most meteors burn up. The thermosphere extends from 85 km to the base of the exosphere at 400 km and contains the ionosphere, a region where the atmosphere is ionized by incoming solar radiation. The ionosphere increases in thickness and moves closer to the Earth during daylight and rises at night allowing certain frequencies of radio communication over a greater range. The Kármán line, located within the thermosphere at an altitude of 100 km, is commonly used to define the boundary between Earth's atmosphere and outer space. The exosphere begins variously from about 690 to 1,000 km above the surface, where it interacts with the planet's magnetosphere. Each of the layers has a different lapse rate, defining the rate of change in temperature with height.

Others Edit

Other astronomical bodies such as sun, moon, Mercury, etc have known atmospheres.

Exploration of the Moon

Figure 2. Scientist on the Moon: Geologist (and later US senator) Harrison “Jack” Schmitt in front of a large boulder in the Littrow Valley at the edge of the lunar highlands. Note how black the sky is on the airless Moon. No stars are visible because the surface is brightly lit by the Sun, and the exposure therefore is not long enough to reveal stars.

Most of what we know about the Moon today derives from the US Apollo program, which sent nine piloted spacecraft to our satellite between 1968 and 1972, landing 12 astronauts on its surface. Before the era of spacecraft studies, astronomers had mapped the side of the Moon that faces Earth with telescopic resolution of about 1 kilometer, but lunar geology hardly existed as a scientific subject. All that changed beginning in the early 1960s.

Initially, Russia took the lead in lunar exploration with Luna 3, which returned the first photos of the lunar far side in 1959, and then with Luna 9, which landed on the surface in 1966 and transmitted pictures and other data to Earth. However, these efforts were overshadowed on July 20, 1969, when the first American astronaut set foot on the Moon.

Table 2 summarizes the nine Apollo flights: six that landed and three others that circled the Moon but did not land. The initial landings were on flat plains selected for safety reasons. But with increasing experience and confidence, NASA targeted the last three missions to more geologically interesting locales. The level of scientific exploration also increased with each mission, as the astronauts spent longer times on the Moon and carried more elaborate equipment. Finally, on the last Apollo landing, NASA included one scientist, geologist Jack Schmitt, among the astronauts (Figure 2).

Table 2. Apollo Flights to the Moon
Flight Date Landing Site Main Accomplishment
Apollo 8 Dec. 1968 First humans to fly around the Moon
Apollo 10 May 1969 First spacecraft rendezvous in lunar orbit
Apollo 11 July 1969 Mare Tranquillitatis First human landing on the Moon 22 kilograms of samples returned
Apollo 12 Nov. 1969 Oceanus Procellarum First Apollo Lunar Surface Experiment Package (ALSEP) visit to Surveyor 3 lander
Apollo 13 Apr. 1970 Landing aborted due to explosion in command module
Apollo 14 Jan. 1971 Mare Nubium First “rickshaw” on the Moon
Apollo 15 July 1971 Mare Imbrium/Hadley First “rover” visit to Hadley Rille astronauts traveled 24 kilometers
Apollo 16 Apr. 1972 Descartes First landing in highlands 95 kilograms of samples returned
Apollo 17 Dec. 1972 Taurus-Littrow highlands Geologist among the crew 111 kilograms of samples returned

Figure 3. Handling Moon Rocks: Lunar samples collected in the Apollo Project are analyzed and stored in NASA facilities at the Johnson Space Center in Houston, Texas. Here, a technician examines a rock sample using gloves in a sealed environment to avoid contaminating the sample. (credit: NASA JSC)

In addition to landing on the lunar surface and studying it at close range, the Apollo missions accomplished three objectives of major importance for lunar science. First, the astronauts collected nearly 400 kilograms of samples for detailed laboratory analysis on Earth (Figure 3).

These samples have revealed as much about the Moon and its history as all other lunar studies combined. Second, each Apollo landing after the first one deployed an Apollo Lunar Surface Experiment Package (ALSEP), which continued to operate for years after the astronauts departed. Third, the orbiting Apollo command modules carried a wide range of instruments to photograph and analyze the lunar surface from above.

The last human left the Moon in December 1972, just a little more than three years after Neil Armstrong took his “giant leap for mankind.” The program of lunar exploration was cut off midstride due to political and economic pressures. It had cost just about $100 per American, spread over 10 years—the equivalent of one large pizza per person per year. Yet for many people, the Moon landings were one of the central events in twentieth-century history.

Figure 4. Moon Rocket on Display. (credit: modification of work by David Morrison)

The giant Apollo rockets built to travel to the Moon were left to rust on the lawns of NASA centers in Florida, Texas, and Alabama, although recently, some have at least been moved indoors to museums (Figure 4). One of the unused Saturn 5 rockets built to go to the Moon is now a tourist attraction at NASA’s Johnson Space Center in Houston, although it has been moved indoors since the photo in Figure 4 was taken.

Today, neither NASA nor Russia have plans to send astronauts to the Moon, and China appears to be the nation most likely to attempt this feat. (In a bizarre piece of irony, a few people even question whether we went to the Moon at all, proposing instead that the Apollo program was a fake, filmed on a Hollywood sound stage. See the Link to Learning box below for some scientists’ replies to such claims.) However, scientific interest in the Moon is stronger than ever, and more than half a dozen scientific spacecraft—sent from NASA, ESA, Japan, India, and China—have orbited or landed on our nearest neighbor during the past decade.

Read The Great Moon Hoax about the claim that NASA never succeeded in putting people on the Moon.

Lunar exploration has become an international enterprise with many robotic spacecraft focusing on lunar science. The USSR sent a number in the 1960s, including robot sample returns. Table 3 lists some of the most recent lunar missions.

Table 3. Some International Missions to the Moon
Launch Year Spacecraft Type of Mission Agency
1994 Clementine Orbiter US (USAF/NASA)
1998 Lunar Prospector Orbiter US (NASA)
2003 SMART-1 Orbiter Europe (ESA)
2007 SELENE 1 Orbiter Japan (JAXA)
2007 Chang’e 1 Orbiter China (CNSA)
2008 Chandrayaan-1 Orbiter India (ISRO)
2009 LRO Orbiter US (NASA)
2009 LCROSS Impactor US (NASA)
2010 Chang’e 2 Orbiter China (CNSA)
2011 GRAIL Twin orbiters US (NASA)
2013 LADEE Orbiter US (NASA)
2013 Chang’e 3 Lander/Rover China (CNSA)

Mission to Mercury

Get the facts on Mercury, the closest planet to the sun.

Fast Facts

LOCATION: First rock from the sun

DISTANCE FROM THE SUN: 28,583,702 to 35,983,125 miles (46,001,200 to 69,816,900 kilometers)



GRAVITY: If you weigh 100 pounds on Earth, you’d weigh 38 pounds here.

Don’t panic when you peer from the porthole of your spaceship as it plummets to the surface of Mercury: You haven’t taken a wrong turn and touched down on the moon. Cratered, contoured with hills, and covered with dark dust, Mercury’s landscape certainly looks a lot like the moon’s. This itty-bitty planet—the smallest in the solar system—is only slightly larger than our moon, too. But one look up from Mercury’s surface at high noon will tell you you’re far from home. Seen from here, the sun appears three times larger in the sky than it does when viewed from Earth. You’re standing on the closest planet to the sun. Forget your sunglasses? Okay, now you can panic.

Mercury is a planet of extremes. By day (which actually lasts roughly 30 Earth days), it’s one of the solar system’s most scorching spots—more than four times hotter than boiling water. By night, temperatures plummet hundreds of degrees below freezing. (The planet’s weak gravity can’t keep a grip on a heat-trapping atmosphere.) But despite its harsh environment, this hot-and-cold rock offers perks for the spacefarer. A weak magnetic field protects visitors from the sun’s deadly solar radiation (Earth has a similar but much stronger field), and the crater-pocked surface may contain valuable minerals. The speediest of all the planets, Mercury orbits the sun every 88 days—which means any Earthlings born here could celebrate four birthdays for your every one.


• Don't bother scouring the sky here for any moons. Mercury doesn’t have any!

• NASA's MESSENGER spacecraft, which has been orbiting Mercury since 2011, carries a heavy-duty sunshade made from ceramic cloth.