How Big is Mercury?
Mercury is the smallest planet in the solar system. (Pluto used to hold the title, but it was downgraded to a dwarf planet.) Although its surface resembles our moon, the tiny planet has a density that rivals Earth itself.
Radius, diameter and circumference
Mercury’s diameter is 3,030 miles (4,878 km), comparable to the size of the continental United States. This makes it about two-fifths the size of Earth. It is smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan.
But it’s not going to stay that size; the tiny planet is shrinking. When NASA’s Mariner 10 spacecraft visited the planet in the 1970s, it identified unusual features known as scarps that suggest the world is shriveling. As the hot interior of the planet cools, the surface draws together. Since the planet boasts only a single rocky layer, rather than the myriad tectonic plates found on Earth, it pushes on itself to create scarps.
A 2014 study of nearly 6,000 scarps taken by NASA’s MESSENGER spacecraft suggest that Mercury contracted radially as much as 4.4 miles (7 kilometers) since its birth 4.5 billion years ago. The discovery helped balance models of the planet’s interior evolution with observations at its surface.
“These new results resolved a decades-old paradox between thermal history models and estimates of Mercury’s contraction,” Paul Byrne, a planetary geologist and MESSENGER visiting investigator at Carnegie’s Department of Terrestrial Magnetism, said in a statement. “Now the history of heat production and loss and global contraction are consistent.”
The planet has a mean radius of 1,516 miles (2,440 km), and its circumference at the equator is 9,525 miles (15,329 km). Some planets, such as Earth, bulge slightly at the equator due to their rapid rotation. However, Mercury turns so slowly on its axis that astronomers once thought that the planet was tidally locked, with one side constantly facing the nearby sun. In fact, the planet spins on its axis once every 58.65 Earth days. Mercury orbits once every 87.97 Earth days, so it rotates only three times every two Mercury years. The slow spin keeps the planet’s radius at the poles and the equator equal.
Density, mass and volume
Mercury has a mass of 330 x 1023kilograms. This mass is contained in a volume of 14.6 trillion cubic miles (60.8 trillion cubic km). The mass and volume of Mercury is only about 0.055 times that of Earth.
But because Mercury’s small mass is enclosed inside of a tiny body, the planet is the second densest in the solar system, weighing in at 5.427 grams per cubic centimeter, or 98 percent of the density of our planet. Only Earth is denser. This high density in a planet that otherwise resembles the moon raises interesting questions about the composition of the planet’s interior.
Mercury’s small size makes it too weak to hold onto a significant atmosphere, especially with the constant bombardment it receives from the sun. The planet has a thin atmosphere, but it is constantly blasted into space by the solar wind. Without an atmosphere to help stabilize the incoming heat from the sun, the planet boasts some of the most varying temperature swings in the solar system.
Mercury’s surface greatly resembles that of Earth’s moon, with craters left over from the heavy bombardment early in the life of the solar system and during theplanet’s formation. Images taken by the Mariner 10 spacecraft show craters ranging from 328 feet (100 m) to 808 miles (1,300 km) across. Several of the planet’s scarps reach as high as 1.86 miles (3 km).
Mercury also boasts hollows—shallow, irregular depressions that seem unique to the planet. Hollows are some of the youngest and brightest features on the surface of Mercury, and range in size from 60 feet to over a mile across and 60 to 120 feet deep. With no atmosphere, the hollows weren’t carved by wind or rain. Instead, they may be caused by volatile minerals evaporating after suddenly being exposed by an impact.
“These hollows were a major surprise,” David Blewett, science team member from the Johns Hopkins University Applied Physics Laboratory, said in astatement. “We’ve been thinking of Mercury as a relic – a place that’s really not changing much anymore, except by impact cratering. But the hollows appear to be younger than the craters in which they are found, and that means Mercury’s surface is still evolving in a surprising way.”
Ironically, the planet closest to the sun contains ice on its surface. (It’s not the hottest planet; that honor is reserved for Venus.) The northern and southern poles lie in constant shadows, allowing ice to build up on their floor. Radar-bright deposits were spotted from Earth, and MESSENGER confirmed these regions lie in constant shadows. Eventually, the spacecraft was able peer directly into the craters and confirm that water ice is stable inside them.
“For more than 20 years the jury has been deliberating on whether the planet closest to the Sun hosts abundant water ice in its permanently shadowed polar regions,” MESSENGER’s primary investigator Sean Solomon, of Columbia University in New York, said in a statement.
“MESSENGER has now supplied a unanimous affirmative verdict.”
How Far is Mercury From the Sun?
The closest planet to the sun, Mercury takes the least time to orbit the giant star. Its close proximity boosts the world’s temperature, and makes it difficult to hold on to its atmosphere. But just how close is close?
Mercury boasts an orbit that is the most elliptical of all of the planets, stretched out from a perfect circle. When it is closest to the sun, it is only 29 million miles (47 million km), but at its farthest, the distance to Mercury is 43 million miles (70 million km). (Pluto’s orbit is even more eccentric, but Mercury holds the “most elliptical planetary orbit” since the far-flung body was downgraded to a dwarf planet; Pluto’s downgrade also madeMercury the smallest of the eight remaining planets.)
This bizarre orbit confused early astronomers as they tried to piece together how the solar system worked. When Mercury is at its farthest from the sun, it is traveling the slowest. This gives Earth time to catch up to it about once every three and a half months. Astronomers charting the motion of Mercury across the sky would see it moving backward over the course of several nights in relation to other stars in the sky. It wasn’t until Nicolaus Copernicus established that planets traveled around the sun rather than around the Earth that the matter was cleared up.
Ironically, although Mercury is the closest planet to the sun, it is not the hottest;Venus boasts that honor. Mercury has wildly swinging temperatures, but only atthin atmosphere to trap heat on the planet, while the atmosphere of Venuscreates a runaway greenhouse effect that elevates its temperature higher than Mercury.
Because Mercury is so close to the sun, the Hubble Space Telescope, which has captured images of a number of bodies in the solar system, cannot photograph the planet’s rocky surface.
Mercury and the sun
Because Mercury lies between Earth and the sun, it undergoes phases much like the moon. However, the phases of Mercury are more difficult to detect than the phases of Venus, which also orbits between the Earth and the sun. Italian astronomer Galileo Galilei could detect the phases of Venus with his telescope, but not the phases of Mercury.
Like Venus, Mercury also undergoes transits, passing between the Earth and the sun about 13 times each century. But the inclination of Mercury’s orbit is such that a transit does not occur every time the two planets pass. Mercury only crosses the sun once every seven years. Astronomers used transits to help calculate the distance to Earth from the sun. (Venus’ transits are significantly more rare, occurring only once every 243 years.)
During the 2016 transit, three spacecraft observed the display.
“It used to be hard to observe transits. If you were in a place that had bad weather, for example, you missed your chance and had to wait for the next one,” Joseph Gurman said in a statement. Gurman is project scientist for NASA’s Solar and Heliospheric Observatory (SOHO) project scientist, one of the instruments. NASA’s Solar Dynamics Observatory and the Japanese Aerospace Exploration Agency-led Hinode solar mission also captured Mercury crossing the sun.
“These instruments help us make our observations, despite any earthly obstacles,” Gurman said.
How far is Mercury from Earth?
The constant motions of planets around the sun means that the distance between Earth and Mercury is in constant flux. When both planets are on opposite sides of the sun, they can achieve their maximum distance of 137 million miles (222 million kilometers). When they are at their closest, they are only 48 million miles (77.3 million km) apart.
How long does it take to reach Mercury?
Mercury isn’t that far away, as far as planets go. But only a few spacecraft have traveled to it. Traveling to a planet isn’t like driving across the country. Improvements in propulsion aren’t the most important determining factor in terms of how long the voyage will take. Space agencies often use gravitational boosts from other planets to gain speed without a high fuel cost. And unlike a road trip, the beginning and end point of solar system journeys are constantly changing position, sometimes ideally lined up but other times not.
Launched on November 3, 1973, NASA’s Mariner 10 slingshotted around Venus to reach Mercury. It performed its first of three flybys on March 29, 1974, taking just over four months to reach the planet. Mariner 10 was both the first spacecraft to travel to multiple planets and the first one to receive a gravitational boost.
The hot rocky body remained unvisited for over thirty years. On August 3, 2004, NASA launched the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission to the planet. MESSENGER performed a flyby of Earth and two of Venus before reaching Mercury on January 14, 2008, a voyage of nearly three and a half years.
The European Space Agency and the Japan Aerospace Agency plan to send a joint mission to Mercury known as BepiColombo. Scheduled to launch in 2018, the craft will flyby Earth once and Venus twice to arrive at Mercury in late 2024, having traveled in space for nearly six and a half years.