A Spin Through the Inner Solar System

More than 50 years of planetary exploration have yielded a rich harvest of data, including many volumes of pictures. These images have revealed the faces of nearly all the nearby worlds, which have turned out to be both forboding and inviting, alien and familiar. Everywhere there is beauty.

We have enough images, in fact, that for many planets, moons, and small bodies we can construct full, global maps. This has been true for places like Mars for decades. For others, such as Mercury and the asteroid Vesta, it has become possible only in the past few years, thanks to the ongoing work of robotic scouts throughout the Solar System and the dedicated people on Earth who fly them.

Following is a series of short videos showing the worlds of the inner Solar System spinning to show their various faces. With two exceptions, each video resulted from taking thousands of individual observations from spacecraft, and combining the data into a 3D computer model.

This look at the Sun comes from the Solar Dynamics Observatory in orbit around the Earth. It shows the Sun as it appeared over the course of the past few days, as seen by sensors tuned to three different wavelengths of extreme ultraviolet light. Later in the decade, new missions will actually fly close to the Sun for even more detailed shots.

This globe comes from thousands of obesrvations by the MESSENGER spacecraft in orbit around Mercury. MESSENGER carries cameras that can observe many wavelenths of light as it bounces off the planet, in order to spy different minerals on the surface. This map shows greatly exaggerated colors in order to highlight the diversity of geology.

Venus is entirely shrouded in dense clouds, of course, but in the 1990s the Magellan spacecraft mapped the surface anyway using radar. The video highlights two large “continents,” or highlands, Aphrodite Terra and Ishtar Terra, the Maxwell Montes mountain range, and Maat Mons, a large, currently dormant volcano. Notice the motion of the clouds at the beginning. Venus is the one inner planet that rotates “backwards” to the other planets.

This map of the moon comes from thousands of photos sent by the Lunar Reconnaissance Orbiter. Each was taken when the Sun was nearly directly overhead at the moment the image was taken. When all stitched together, they make a globe of striking crispness and clarity.

Here is Mars, showing both the actual relief of its surface features, and a version where the topographical data has been wildly stretched to draw out the planet’s complex landscapes.

Finally, we have Vesta, a tiny place compared to these other worlds, but a giant among the members of the asteroid belt. This rotation is not a computer model, but a series of images stitched together from photos sent by the Dawn spacecraft.

These spinning globes show how much we’ve explored. On the other hand, they serve as a reminder of how much remains unseen. There is no rotating map of the largest asteroid Ceres, or of Pluto. That situation will change next year. However, there are many other worlds in the outer Solar System where there are still blank spaces on the map. Sadly, there are no missions even on the drawing board to explore most of them.

There is still much to do.

This article originally appeared as a guest post on The Planetary Society site.


The Giant Spider of Mercury

Mercury will surprise you. At first glance, it’s gray and cratered, almost indistinguishable from the desolate far side of Earth’s Moon. But one of the Solar System’s most reliable rules holds sway at Mercury, too: take a closer look and you’ll find things you never expected.

For one thing, the First Planet is not really colorless. Sure, if you flew by and looked out a window that’s mostly what you’d see. But if you pass light reflected from Mercury’s surface through filters sensitive to different wavelengths, then combine the images, something more reveals itself.

Caloris in Color – An enhanced-color view of Mercury, assembled from images taken at various wavelengths by the cameras on board the MESSENGER spacecraft. The circular, orange area near the center-top of the disc is Caloris Basin. Apollodorus and Pantheon Fossae can be seen at the center-left of the basin. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

It turns out that different kinds of rock on the surface reflect subtly different colors. It takes special camera “eyes” to distinguish these hues, and there is just such a set of such instruments on board the robotic MESSENGER spacecraft now in orbit around Mercury. All deep space missions face dangers of one kind or another, but MESSENGER is an especially intrepid craft. The heat from the nearby Sun is so intense that the orbiter carries a large shield to protect its components. In fact, the heat reflected from Mercury’s surface alone, like a hot sidewalk on a summer day, could cause enough problems that MESSENGER follows a highly elliptical orbit. It regularly takes the spacecraft high above the planet so it can cool off and not spend too much time baking near the surface.

MESSENGER at Mercury – A rendering of the MESSENGER spacecraft in orbit around Mercury, created with NASA’s Eyes on the Solar System simulator. Credit: NASA / JPL
MESSENGER’s Orbit – MESSENGER’s elongated orbit around the Mercury keeps the planet’s intense radiated heat from frying the spacecraft. Rendered using NASA’s Eyes on the Solar System simulator. Credit: NASA / JPL

From its looping vantage point around Mercury, MESSENGER has mapped almost every square kilometer of the planet’s surface during the past couple of years of orbital flight. The spacecraft has seen some pretty striking landscapes along the way. Take Caloris Basin. Named, appropriately enough, for the Latin word for ‘heat’, this giant impact crater can be seen as the orange, circular area in the color image above. The Mariner 10 probe discovered Caloris in 1974, but at the time the feature was half hidden in the darkness of the Mercurial night side.

MESSENGER first flew by Mercury in 2008 before it settled into its final orbit, and one of the first things it saw was the Caloris Basin in its full glory. Including, right near the center, this strange land.

crater on Mercury
Pantheon Fossae – The striking troughs of Mercury’s Pantheon Fossae, the feature that MESSENGER scientists first called “The Spider” when they discovered it. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Mercury explorers saw an enormous set of grooved valleys, probably extensional faults, radiating from a 41-kilometer-wide crater. They called it “The Spider.”

Later on, the International Astronomical Union officially dubbed the region Pantheon Fossae. “Fossae” means “trenches”, and here those trenches resemble the inside of the dome in a Roman temple. For the same reason, the central crater was given the name Apollodorus, after the Pantheon’s architect.

Since then, scientists have continued to debate exactly what caused the faulting that pulled apart the rocks of Caloris Basin into such a distinctive pattern. They have also argued whether Apollodorus crater is, in fact, a coincidence unrelated to the grooves, since it appears to have formed later and not exactly at the center of the bullseye.

A variety of high-resolution scans of the area from MESSENGER could help get to the bottom of it. Here’s an arresting view, taken from an angle.

crater on Mercury
A New Angle on Pantheon Fossae – The MESSENGER orbiter provides us with an oblique view of the radial fault lines of Mercury’s Pantheon Fossae region. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington / Bill Dunford

And here’s a look right into Apollodorus. Like many craters on Mercury, it shows signs of “hollows”–bright patches where it’s thought that the unrelenting sun has actually blasted away once-solid rock, sublimating it into gas.

Apollodorus Close-Up – A look into Mercury’s Apollodorus Crater from the MESSENGER spacecraft. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington / Bill Dunford

“The Spider” is one of the most hauntingly beautiful spaces in all the Solar System’s strange wilderness. And to think it’s a place we never even knew existed until a few short years ago.

I can’t help but wonder what else the ongoing robotic reconnaissance has left to reveal.

This article originally appeared as a guest post on The Planetary Society site.


New Messages from Mercury

This post originally appeared as a guest blog entry on The Planetary Society web site.

You and I can do something very few people in history ever could: tour the planet Mercury. In fact, just last week NASA made available a new trove of images from the MESSENGER spacecraft. The orbiter circles the planet every eight hours, carrying a sun-blocking heat shield on one side and a suite of powerful cameras and instruments on another. I’ve been exploring the new pictures, and here are a few that caught my eye. These were all taken during this calendar year.

Mercury is sometimes just weird. I mean that in a good way, of course. The best example might be its famous water ice deposits, which defy the planet’s searing heat by hiding in deep craters near the poles. Lit only by starlight, never directly by the sun, the floor of the crater Petronius is one such place.

Petronius Crater
At the Edge of Darkness – The MESSENGER spacecraft returned this close view of Petronius Crater near the north pole of Mercury. The floor of the 36-km crater, which never sees the sun, hides deposits of water ice. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Sometimes the MESSENGER team commands the spacecraft to photograph the limb of Mercury at a highly oblique angle. These shots can reveal the shape the horizon, and are interesting just because they almost give the sensation of what it might be like to ride along with the orbiter as it buzzes over the surface of a harshly alien world.

Flying Over Mercury
Flying Over Mercury – This shot of Mercury’s limb from the MESSENGER orbiter gives a sense of what it might be like to fly over the landscape southwest of Debussy Crater. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury is a gray planet. If you did fly along with MESSENGER you’d see a surface that appears mostly colorless to your eye. But that’s not the whole truth. Varying kinds of minerals mark different parts of the landscape, depending on whether it formed in an eruption of lava, the impact of an asteroid, or some other process. Also, some terrain has been battered by space weather for longer than other places, depending on its age. All this leads to subtle color variations on the surface. MESSENGER’s cameras can draw these color changes out by shooting through filters sensitive to different wavelengths of light. Combine these images into the red, green, and blue channels of an ordinary color picture, and suddenly Mercury’s colors shine through.

It’s uncertain low long our MESSENGER at Mercury will fly. Its budget has already run out (it continues to operate on a pay-as-you-go basis, pending a possible mission extension) and its fuel won’t last forever. Meanwhile, it show us–in striking detail–places that previous generations of explorers could only imagine.

Mercury in 3D

Mercury in 3D
The crater Kertesz on Mercury, viewable in 3D using red-blue glasses. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The robotic spacecraft MESSENGER, now in orbit around the first planet, shot this image of the crater known as Kertesz. By combining two images into an anaglyph, this view can be seen in 3D with the use of red-blue glasses. The team that created this picture notes that better results may be achieved by tilting the head slightly to the left.

Notable in Kertesz are several examples of the mysterious hollows of Mercury.

Mercury Turns Out to be a Weird Little World

Unexplained "Hollows" on the planet Mercury
Enhanced-color view of the unexplained "Hollows" on the planet Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution

The robotic spacecraft MESSENGER, now orbiting the first planet, has found odd features on its surface, including unexplained, blueish ‘hollows’ that may be actively forming today. The MESSENGER team explains that this is a view of a larger section of the floor and peak-ring mountains of the Raditladi impact basin. The individual frames in the mosaic are about 20 km wide. The image was created by merging high-resolution monochrome images from MESSENGER’s Narrow Angle Camera with a lower-resolution enhanced-color image obtained by the Wide Angle Camera.

David Blewett, a staff scientist at the Johns Hopkins University Applied Physics Laboratory, and lead author of one of the Science reports, said in a teleconference today that the hollows remind explorers of the ice pits on Mars. Of course, Mercury’s pits are in rock, not ice. He speculated that the intense solar wind on the hot planet’s surface may play a role.

The new findings will be published this week in Science. Blewett said, “The conventional wisdom was that Mercury is just like the Moon. But from its vantage point in orbit, MESSENGER is showing us that Mercury is radically different from the Moon in just about every way we can measure.”