How Many Planets are in the Solar System?

How Many Planets Are in the Solar System?
How Many Planets Are in the Solar System?

I’m just going to warn you, this is a controversial topic. Some people get pretty grumpy when you ask: how many planets are in the Solar System? Is it eight, ten, or more?

I promise you this, though, we’re never going back to nine planets… ever.

When many of us grew up, there were nine planets in the Solar System. It was like a fixed point in our brains.

As kids, memorizing this list was an early right of passage of nerd pride: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

But then in 2005, Mike Brown discovered Eris, an icy object thought to be about the same size as Pluto, out beyond its orbit.

That would bring the total number of planets to ten. Right? There’s no turning back, textbooks would need to be changed.
In order to settle the dispute, the International Astronomical Union met in 2006, and argued for, and against Pluto’s planethood. Some astronomers advocated widening the number of planets to twelve, including Pluto, its moon Charon, the Asteroid Ceres, and the newly discovered Eris.

In the end, they changed the definition of what makes a planet, and sadly, Pluto doesn’t make the cut:

Here are the new requirements of planethood status:

  1. A planet has to orbit the Sun. Okay fine, Pluto does that.
  2. A planet needs enough gravity to pull itself into a sphere. Okay, spherical. Pluto’s fine there too.
  3. A planet needs to have cleared out its orbit of other objects. Uh oh, Pluto hasn’t done that.

For example, planet Earth accounts for a million times the rest of the material in its orbit, while Pluto is just a fraction of the icy objects in its realm.

The final decision was to demote Pluto from planet to dwarf planet.

But don’t despair, Pluto is in good company.

Ceres. Image credit: NASA
Ceres. Image credit: NASA
There’s Ceres, the first asteroid ever discovered, and the smallest of the dwarf planets. The surface of Ceres is made of ice and rock, and it might even have a liquid ocean under its surface. NASA’s Dawn mission is flying there right now to give us close up pictures for the first time.

Haumea, named after the Hawaiian goddess of fertility, is about a third the mass of Pluto, and has just enough gravity to pull itself into an ellipsoid, or egg shape. Even though it’s smaller, it’s got moons of its own.

Makemake. Credit: NASA
Makemake. Credit: NASA
Makemake, a much larger Kuiper belt object, has a diameter about two-thirds the size of Pluto. It was discovered in 2005 by Mike Brown and his team. So far, Makemake doesn’t seem to have any moons.

Eris is the most massive known dwarf planet, and the one that helped turn our definition of a planet upside-down. It’s 27% more massive than Pluto and the ninth most massive body that orbits the Sun. It even has a moon: Dysnomia.

Pluto. Credit: ESO
Pluto. Credit: ESO
And of course, Pluto. The founding member of the dwarf family.

Want an easy way to remember the eight planets, in order? Just remember this mnemonic: my very excellent mother just served us noodles.

For all you currently writing angry tweets to Mike Brown, hold on a sec. Changing Pluto’s categorization is an important step that really needed to happen.

The more we discover about our Universe, the more we realize just how strange and wonderful it is. When Pluto was discovered 80 years ago, we never could have expected the variety of objects in the Solar System. Categorizing Pluto as a dwarf planet helps us better describe our celestial home.

So, our Solar System now has eight planets, and five dwarf planets.

Saturn’s Little Wavemaking Moon

Daphnis' gravity disturbs the edges of the Keeler Gap as it travels along

Captured on January 15, this narrow-angle Cassini image shows an outer portion of Saturn’s A ring on the left and the ropy F ring crossing on the right. The thin black line near the A ring’s bright edge is the Keeler Gap, a 22-mile-wide space cleared by the passage of Daphnis, a shepherd moon barely 5 miles (about 7.5 km) across. As it travels around Saturn within the gap its gravity perturbs the fine icy particles within the rings, sending up rippling waves both before and behind it — visible here near the upper center.

From Cassini’s distance of 870,000 miles (1.4 million km) Daphnis itself is just barely visible as a single pixel within the Gap — can you see it? If not, click below…

There it is:

Highlighting Daphnis inside the Keeler Gap
Highlighting Daphnis inside the Keeler Gap

While lacking the murky mystery of Titan’s atmosphere, Enceladus’ dramatic jets and the tortured and cratered surfaces found on Dione, Rhea, Mimas and many of Saturn’s larger icy moons, little Daphnis has always fascinated me because of the scalloped waves it kicks up within Saturn’s rings. Eventually these waves settle back down, but at their highest they can extend a mile or two above and below the ring plane!

Daphnis' wake casts peaked shadows on the rings
Daphnis’ wake casts peaked shadows on the rings

This effect was most pronounced during Saturn’s spring equinox in August 2009 when sunlight was striking the rings edge-on, creating strong shadows from any areas of relief.

Imagine the impressive view you’d have if you were nearby, positioned just above the rings as Daphnis approached and hurtled past, the rings rising up in mile-high peaks from the moon’s gravity before smoothing out again. Incredible!

Daphnis seen by Cassini in June 2010 (NASA/JPL/SSI)
Daphnis seen by Cassini in June 2010 (NASA/JPL/SSI)

And I’m not the only one to imagine such a scene either — apparently artist Erik Svensson is also intrigued by Daphnis, enough to have been inspired to create the image below. How very cool!

Future explorers watch Daphnis speed past at the edge of Saturn's A ring (© Erik Svennson, all rights reserved. Used with permission.)
Future explorers watch Daphnis speed past at the edge of Saturn’s A ring (© Erik Svennson, all rights reserved. Used with permission.)

Like its larger shepherd moon sister Prometheus, Daphnis may be little but still has a big effect on the icy stuff that makes up Saturn’s iconic rings.

And for lots more views of Daphnis click here (but watch out, it may just become your favorite moon too!)

Image credits: NASA/JPL-Caltech/Space Science Institute.

Cosmic C.S.I.: Searching for the Origins of the Solar System in Two Grains of Sand

Composite Spitzer, Hubble, and Chandra image of supernova remnant Cassiopeia A. A new study shows that a supernova as far away as 50 light years could have devastating effects on life on Earth. (NASA/JPL-Caltech/STScI/CXC/SAO)
Composite Spitzer, Hubble, and Chandra image of supernova remnant Cassiopeia A. A new study shows that a supernova as far away as 50 light years could have devastating effects on life on Earth. (NASA/JPL-Caltech/STScI/CXC/SAO)

“The total number of stars in the Universe is larger than all the grains of sand on all the beaches of the planet Earth,” Carl Sagan famously said in his iconic TV series Cosmos. But when two of those grains are made of a silicon-and-oxygen compound called silica, and they were found hiding deep inside ancient meteorites recovered from Antarctica, they very well may be from a star… possibly even the one whose explosive collapse sparked the formation of the Solar System itself.

Researchers from Washington University in St. Louis with support from the McDonnell Center for the Space Sciences have announced the discovery of two microscopic grains of silica in primitive meteorites originating from two different sources. This discovery is surprising because silica — one of the main components of sand on Earth today — is not one of the minerals thought to have formed within the Sun’s early circumstellar disk of material.

Instead, it’s thought that the two silica grains were created by a single supernova that seeded the early solar system with its cast-off material and helped set into motion the eventual formation of the planets.

According to a news release by Washington University, “it’s a bit like learning the secrets of the family that lived in your house in the 1800s by examining dust particles they left behind in cracks in the floorboards.”

A 3.5-cm chondrite meteorite found in Antarctica in Nov. 1998. Dark meteorites show up well against the icy terrain of Antarctica. (Carnegie Mellon University)
A 3.5-cm chondrite meteorite found in Antarctica in Nov. 1998. Dark meteorites show up well against the icy terrain of Antarctica. (Carnegie Mellon University)

Until the 1960s most scientists believed the early Solar System got so hot that presolar material could not have survived. But in 1987 scientists at the University of Chicago discovered miniscule diamonds in a primitive meteorite (ones that had not been heated and reworked). Since then they’ve found grains of more than ten other minerals in primitive meteorites.

The scientists can tell these grains came from ancient stars because they have highly unusual isotopic signatures, and different stars produce different proportions of isotopes.

But the material from which our Solar System was fashioned was mixed and homogenized before the planets formed. So all of the planets and the Sun have the pretty much the same “solar” isotopic composition.

Meteorites, most of which are pieces of asteroids, have the solar composition as well, but trapped deep within the primitive ones are pure samples of stars, and the isotopic compositions of these presolar grains can provide clues to their complex nuclear and convective processes.

The layered structure of a star about to go supernova; different layers contain different elements (Wikimedia)
The layered structure of a star about to go supernova; different layers contain different elements (Wikimedia)

Some models of stellar evolution predict that silica could condense in the cooler outer atmospheres of stars, but others say silicon would be completely consumed by the formation of magnesium- or iron-rich silicates, leaving none to form silica.

“We didn’t know which model was right and which was not, because the models had so many parameters,” said Pierre Haenecour, a graduate student in Earth and Planetary Sciences at Washington University and the first author on a paper to be published in the May 1 issue of Astrophysical Journal Letters.

Under the guidance of physics professor Dr. Christine Floss, who found some of the first silica grains in a meteorite in 2009, Haenecour investigated slices of a primitive meteorite brought back from Antarctica and located a single grain of silica out of 138 presolar grains. The grain he found was rich in oxygen-18, signifying its source as from a core-collapse supernova.

Finding that along with another oxygen-18-enriched silica grain identified within another meteorite by graduate student Xuchao Zhao, Haenecour and his team set about figuring out how such silica grains could form within the collapsing layers of a dying star. They found they could reproduce the oxygen-18 enrichment of the two grains through the mixing of small amounts of material from a star’s oxygen-rich inner zones and the oxygen-18-rich helium/carbon zone with large amounts of material from the outer hydrogen envelope of the supernova.

In fact, Haenecour said, the mixing that produced the composition of the two grains was so similar, the grains might well have come from the same supernova — possibly the very same one that sparked the collapse of the molecular cloud that formed our Solar System.

“It’s a bit like learning the secrets of the family that lived in your house in the 1800s by examining dust particles they left behind in cracks in the floorboards.”

Ancient meteorites, a few microscopic grains of stellar sand, and a lot of lab work… it’s an example of cosmic forensics at its best!

Source: Washington University in St. Louis

Cassini Says “Senkyo Very Much”

Narrow-angle camera image of Titan from Cassini (NASA/JPL-Caltech/Space Science Institute)

In this image acquired on January 5, Cassini’s near-infrared vision pierced Titan’s opaque clouds to get a glimpse of the dark dune fields across a region called Senkyo.

The vast sea of dunes is composed of solid hydrocarbon particles that have precipitated out of Titan’s atmosphere. Also visible over Titan’s southern pole are the rising clouds of the recently-formed polar vortex.

For a closer look at Titan’s dunes (and to find out what the name Senkyo means) keep reading…

In the image above north on Titan is up and rotated 18 degrees to the right. It was taken using a spectral filter sensitive to wavelengths of near-infrared light centered at 938 nanometers.

The view was obtained at a distance of approximately 750,000 miles (1.2 million kilometers) from Titan.

Titan’s hydrocarbon dunes are found across the moon in a wide swath within 30 degrees of the equator and are each about a kilometer wide and tens to hundreds of kilometers long… and in some cases stand over 100 meters tall. (Source: Astronomy Now.)

Titan dunes Jan 2007
Radar image of Titan’s dunes acquired on Jan. 13, 2007. This view is 160 kilometers (100 miles) high by 150 kilometers (90 miles) wide. (NASA/JPL)

Observations of the dunes with Cassini and ESA’s Huygens probe during its descent onto Titan’s surface have shown that the moon experiences seasonally-shifting equatorial winds during equinoxes, similar to what occurs over the Indian Ocean between monsoon seasons.

The name Senkyo refers to the Japanese realm of serenity and freedom from wordly cares and death… in line with the IAU convention of naming albedo features on Titan after mythological enchanted places.

Click here for an earlier view of Senkyo, and follow the Cassini mission here.

Color-composite of Titan made from raw Cassini images acquired on April 13, 2013 (added 4/17) NASA/JPL/SSI. Composite by J. Major.
Color-composite of Titan made from raw Cassini images acquired on April 13, 2013 (added 4/17) NASA/JPL/SSI. Composite by J. Major.

Hydrogen Peroxide Could Feed Life on Europa

Reprocessed Galileo image of Europa's frozen surface by Ted Stryk (NASA/JPL/Ted Stryk)
Reprocessed Galileo image of Europa's frozen surface by Ted Stryk (NASA/JPL/Ted Stryk)

According to research by NASA astronomers using the next-generation optics of the 10-meter Keck II telescope, Jupiter’s ice-encrusted moon Europa has hydrogen peroxide across much of the surface of its leading hemisphere, a compound that could potentially provide energy for life if it has found its way into the moon’s subsurface ocean.

“Europa has the liquid water and elements, and we think that compounds like peroxide might be an important part of the energy requirement,” said JPL scientist Kevin Hand, the paper’s lead author. “The availability of oxidants like peroxide on Earth was a critical part of the rise of complex, multicellular life.”

The paper, co-authored by Mike Brown of the California Institute of Technology in Pasadena, analyzed data in the near-infrared range of light from Europa using the Keck II Telescope on Mauna Kea, Hawaii, over four nights in September 2011. The highest concentration of peroxide found was on the side of Europa that always leads in its orbit around Jupiter, with a peroxide abundance of 0.12 percent relative to water. (For perspective, this is roughly 20 times more diluted than the hydrogen peroxide mixture available at drug stores.) The concentration of peroxide in Europa’s ice then drops off to nearly zero on the hemisphere of Europa that faces backward in its orbit.

Hydrogen peroxide was first detected on Europa by NASA’s Galileo mission, which explored the Jupiter system from 1995 to 2003, but Galileo observations were of a limited region. The new Keck data show that peroxide is widespread across much of the surface of Europa, and the highest concentrations are reached in regions where Europa’s ice is nearly pure water with very little sulfur contamination.

This color composite view combines violet, green, and infrared images of Europa acquired by Galileo in 1997 for a view of the moon in natural color (left) and in enhanced color (right). Credit: NASA/JPL/University of Arizona
This color composite view combines violet, green, and infrared images of Europa acquired by Galileo in 1997 for a view of the moon in natural color (left) and in enhanced color (right). Credit: NASA/JPL/University of Arizona

The peroxide is created by the intense radiation processing of Europa’s surface ice that comes from the moon’s location within Jupiter’s strong magnetic field.

“The Galileo measurements gave us tantalizing hints of what might be happening all over the surface of Europa, and we’ve now been able to quantify that with our Keck telescope observations,” Brown said. “What we still don’t know is how the surface and the ocean mix, which would provide a mechanism for any life to use the peroxide.”

Read more: Evidence for a Deep Ocean on Europa Might Be Found on its Surface

The scientists think hydrogen peroxide is an important factor for the habitability of the global liquid water ocean under Europa’s icy crust because hydrogen peroxide decays to oxygen when mixed into liquid water. “At Europa, abundant compounds like peroxide could help to satisfy the chemical energy requirement needed for life within the ocean, if the peroxide is mixed into the ocean,” said Hand.

(Source: NASA)

What’s notable to add, on March 26, 2013, the U.S. President signed a bill that would increase the budget for NASA’s planetary science program as well as provide $75 million for the exploration of Europa. Exactly how the funds will be used isn’t clear — perhaps for components on the proposed Europa Clipper mission? —  but it’s a step in the right direction for learning more about this increasingly intriguing world. Read more on SETI’s Destination: Europa blog.

MESSENGER Sees a Smoother Side of Mercury

A high-resolution view of a "silky" surface on Mercury

During its two years in orbit around Mercury — as well as several more years performing flybys — the MESSENGER spacecraft has taken over 150,000 images of the innermost planet, giving us a look at its incredibly rugged, Sun-scoured surface like never before. But not all areas on Mercury appear so harsh — it has its softer sides too, as seen above in an image released earlier today.

Here we see the smooth walls, floor and upper surfaces around an irregular depression on Mercury in high definition. The velvety texture is the result of widespread layering of fine particles, because unlike many features on Mercury’s  ancient surface this rimless depression wasn’t caused by an impact from above but rather explosively escaping lava from below — this is the rim of a volcanic vent, not a crater!

Previous images have been acquired of this irregularly-shaped depression but this is the highest resolution view MESSENGER has captured to date — about 26 meters per pixel.

A wide-angle view of the same depression, captured in July 2012
A wide-angle view of the same depression, captured by MESSENGER in July 2012

The full depression, located northeast of the Rachmaninoff basin, is about 36 km (22 miles) across at its widest. It’s surrounded by a smooth blanket of high-reflectance material — explosively ejected volcanic particles from a pyroclastic eruption that spread over the surface like snow.

Other similar vents have been found on Mercury, like this heart-shaped one in Caloris basin. The smooth, bright surface material is a telltale sign of a volcanic outburst, as are the rimless, irregular shapes of the vents.

The numerous small craters that are seen inside the vent and on the smooth surrounding surfaces would be from meteorite impacts that occurred well after the eruption.

On March 17, 2011, MESSENGER became the first spacecraft ever to orbit the planet Mercury. It is capable of continuing orbital operations until early 2015. Find out more about the mission here.

Image credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

 

Enceladus’ Jets Reach All the Way to its Sea

Saturn's moon Enceladus sprays its salty sea out into space. Those plumes are rich in phosphates. (NASA/JPL/SSI/J. Major)

Thanks to the Cassini mission we’ve known about the jets of icy brine spraying from the south pole of Saturn’s moon Enceladus for about 8 years now, but this week it was revealed at the 44th Lunar and Planetary Science Conference outside Houston, Texas that Enceladus’ jets very likely reach all the way down to the sea — a salty subsurface sea of liquid water that’s thought to lie beneath nearly 10 kilometers of ice.

Enceladus’ jets were first observed by the Cassini spacecraft in 2005. The jets constantly spray fine particles of ice into space which enter orbit around Saturn, creating the hazy, diffuse E ring in which Enceladus resides.

Emanating from deep fissures nicknamed “tiger stripes” that gouge the 512-km (318-mile) -wide moon’s south pole the icy jets — and the stripes — have been repeatedly investigated by Cassini, which has discovered that not only do the ice particles contain salts and organic compounds but also that the stripes are surprisingly warm, measuring at 180 Kelvin (minus 135 degrees Fahrenheit) — over twice as warm as most other regions of the moon.

Read more: Enceladus’ Salty Surprise

Where the jets are getting their supply of liquid water has been a question scientists have puzzled over for years. Is friction caused by tidal stresses heating the insides of the stripes, which melts the ice and shoots it upwards? Or do the fissures actually extend all the way down through Enceladus’ crust to a subsurface ocean of liquid water, and through tidal pressure pull vapor and ice up to the surface?

"Baghdad Sulcus," one of many tiger stripe fissures on Enceladus (NASA/JPL/SSI)
“Baghdad Sulcus,” one of many tiger stripe fissures on Enceladus (NASA/JPL/SSI)

Researchers are now confident that the latter is the case.

In a presentation at the Lunar and Planetary Science Conference titled “How the Jets, Heat and Tidal Stresses across the South Polar Terrain of Enceladus Are Related” (see the PDF here) Cassini scientists note that the amount of heating due to tidal stress seen along Enceladus’ tiger stripes isn’t nearly enough to cause the full spectrum of heating observed, and the “hot spots” that have been seen don’t correlate with the type of heating caused by shear friction.

Instead, the researchers believe that heat energy is being carried upwards along with the pressurized water vapor from the subsurface sea, warming the areas around individual vents as well as serving to keep their channels open.

With 98 individual jets observed so far on Enceladus’ south polar terrain and surface heating corresponding to each one, this scenario, for lack of a better term… seems legit.

What this means is that not only does a moon of Saturn have a considerable subsurface ocean of liquid water with a heat source and Earthlike salinity (and also a bit of fizz) but also that it’s spraying that ocean, that potentially habitable environment, out into local space where it can be studied relatively easily — making Enceladus a very intriguing target for future exploration.

“To touch the jets of Enceladus is to touch the most accessible salty, organic-rich, extraterrestrial body of water and, hence, habitable zone, in our solar system.”

– Cassini imaging team leader Carolyn Porco et al.

Enceladus is actively spraying its habitable zone out into space (NASA/JPL/SSI)
Enceladus is actively spraying its habitable zone out into space (NASA/JPL/SSI)

Research notes via C. Porco, D. DiNino, F. Nimmo, CICLOPS, Space Science Institute at Boulder, CO, and Earth and Planetary Sciences at UC Santa Cruz, CA.

Top image: color-composite of Enceladus made from raw Cassini images acquired in 2010. The moon is lit by reflected light from Saturn while the jets are backlit by the Sun. 

Saturn to Shed its Spooky Spokes for Summer

Cassini image of Saturn's rings from Dec. 20, 2012 (NASA/JPL-Caltech/SSI)

As Saturn steadily moves along its 29.7-year-long orbit toward summertime in its northern hemisphere NASA’s Cassini spacecraft is along for the ride, giving astronomers a front-row seat to seasonal changes taking place on the ringed planet.

One of these fluctuations is the anticipated disappearance of the “spokes” found in the rings, a few of which can be seen above in an image captured on Dec. 20 of last year.

First identified by Voyager in 1980, spokes are ghostly streaks of varying size and brightness that stretch radially across Saturn’s ring system. They orbit around the planet with the ring particles and can last for hours before fading away.

Under the right lighting conditions spokes can appear dark, as seen in this image from Jan. 2010 (NASA/JPL/SSI)
Under the right lighting conditions spokes can appear dark, as seen in this image from Jan. 2010 (NASA/JPL/SSI)

One of the most elusive and transient of features found on Saturn, spokes are thought to be made up of larger microscopic particles of ice — each at least a micron or more — although exactly what makes them gather together isn’t yet known.

They are believed to be associated with interactions between ring particles and Saturn’s electromagnetic field.

“The spokes are most prominent at a point in the rings where the ring particles are moving at the same speed as Saturn’s electromagnetic field,” said Brad Wallis, Cassini rings discipline scientist. “That idea and variations of it are still the most prominent theories about the spokes.”

Other researchers have suggested that they may be caused by electron beams issuing outwards along magnetic field lines from lightning storms in Saturn’s atmosphere.

Regardless of how they are created, spokes are more often observed when sunlight is striking the rings edge-on — that is, during the spring and autumn equinoxes. Perhaps the increased solar radiation along Saturn’s equator increases the formation of lightning-generating storms, in turn creating more spokes? It’s only a guess, but Cassini — and astronomers — will be watching to see if these furtive features do in fact fail to appear during Saturn’s northern summer, the height of which arrives in 2016.

Read more about Saturn’s spokes here.

A New Look at Saturn’s Northern Hexagon

Raw Cassini image captured on 26 Feb. 2013 (NASA/JPL/SSI)

Freshly delivered from Cassini’s wide-angle camera, this raw image gives us another look at Saturn’s north pole and the curious hexagon-shaped jet stream that encircles it, as well as the spiraling vortex of clouds at its center.

Back in November we got our first good look at Saturn’s north pole in years, now that Cassini’s orbit is once again taking it high over the ringplane. With spring progressing on Saturn’s northern hemisphere the upper latitudes are getting more and more sunlight — which stirs up storm activity in its atmosphere.

The bright tops of upper-level storm clouds speckle Saturn’s skies, and a large circular cyclone can be seen near the north pole, within the darker region contained by the hexagonal jet stream. This could be a long-lived storm, as it also seems to be in the images captured on November 27.

About 25,000 km (15,500 miles) across, Saturn’s hexagon is wide enough to fit nearly four Earths inside!

The Saturn hexagon as seen by Voyager 1 in 1980 (NASA)
The Saturn hexagon as seen by Voyager 1 in 1980 (NASA)

The hexagon was originally discovered in images taken by the Voyager spacecraft in the early 1980s. It encircles Saturn at about 77 degrees north latitude and is estimated to whip around the planet at speeds of 354 km/h (220 mph.)

Watch a video of the hexagon in motion here.

The rings can be seen in the background fading into the shadow cast by the planet itself. A slight bit of ringshine brightens Saturn’s nighttime limb.

Cassini was approximately 579,653 kilometers (360,180 miles) from Saturn when the raw image above (W00079643) was taken.

Image credit: NASA/JPL/Space Science Institute

 

Take a Spin Around Mercury

Color map of Mercury's varied surface. The 1,550-km-wide Caloris Basin can be seen at upper right.

Created by the MESSENGER mission team at the Johns Hopkins University Applied Physics Laboratory and the Carnegie Institution of Washington, this animation gives us a look at the spinning globe of Mercury, its surface color-coded to reflect variations in surface material reflectance.

Thousands of Wide Angle Camera images of Mercury’s surface were stitched together to create the full-planet views.

While the vibrant colors don’t accurately portray Mercury as our eyes would see it, they are valuable to scientists as they highlight the many different types of materials that make up the planet’s surface. Young crater rays surrounding fresh impact craters appear light blue or white. Medium- and dark-blue “low-reflectance material” (LRM) areas are thought to be rich in a dark, opaque mineral. Tan areas are plains formed by eruption of highly fluid lavas. Small orange spots are materials deposited by explosive volcanic eruptions.

At this point, over 99% of the Solar System’s innermost planet has been mapped by MESSENGER. Read more about the ongoing mission here.

Image/video credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington