By Todd Ratcliff

The Apollo 15 (left) and Apollo 11 (right) lunar laser ranging retroreflector arrays. Image credit: NASA/D. Scott

Everyone knows that Earth’s Moon is a cold, dead chunk of rock, right? Hold on there, Sparky, not so fast! While it’s true that it was once thought that the moon was an inert, lifeless ball, scientists have known since the days of the Apollo astronauts that there’s much more going on inside our moon than meets the eye.

On Earth, seismology is one of the best ways to probe what’s happening inside our planet. We can “see” the layers and boundaries deep inside Earth, similar to how a sonogram lets us see the shape of a baby in a pregnant woman’s belly. Seismology is what lets us know that Earth’s core is divided into a solid inner core and liquid outer core.

So it only made sense for Apollo astronauts to place seismometers on the moon in order to measure the rumblings and grumblings of the lunar interior. Unfortunately, the locations of the moonquakes and positions of the seismometers didn’t allow for a nice “sonogram” of any possible lunar core. Despite almost a decade of measuring moonquakes, we never got a good peek at the core.

Diagram of the moon’s interior structure. Image credit: NASA

Another Apollo experiment left panels of retroreflectors — essentially giant mirrors (about a square meter, or about 11 square feet, in size) — on the moon’s surface. Observatories on Earth fire laser beams at the reflectors and measure how long it takes light to make the round-trip. Lunar Laser Ranging (LLR) gives us a remarkably accurate measure of the Earth-moon distance, good to within a couple of centimeters (a little less than an inch). LLR has also let us know that our moon probably isn’t completely solid. The moon most likely has a liquid iron core and possibly, like Earth, a solid iron inner core.

Now lunar scientists have revisited and applied modern seismic analysis techniques to the more than 30-year-old lunar seismic data set. Their analysis supports the idea that the moon does indeed have an inner core of solid iron surrounded by an outer core of liquid iron. Similar to (but very much smaller than) Earth’s core!

If all goes well, the soon-to-be launched GRAIL mission, whose twin orbiters will measure gravity in exacting detail, will shed even more light on the nature of our moon’s interior. Things are getting pretty interesting for this “cold, dead chunk of rock!”

By Julie Cooper

Each month in “Slice of History” we’ll be featuring a historical photo from the JPL Archives. See more historical photos and explore the JPL Archives at https://beacon.jpl.nasa.gov/.

Cesium-Lithium Test System — Photograph Number 383-5651Ac

As early as 1961, JPL’s Propulsion Division was working on a new type of power system for future spacecraft that would have to travel great distances and operate for long periods of time. The goal was to convert nuclear power to electric power without the use of moving mechanical parts. During the 1960s various magnetohydrodynamic (MHD) generator configurations and fluids such as liquid metal were tested in an effort to develop the most efficient power conversion system. This October 1970 photo shows a test system which used cesium and lithium and was referred to as an erosion loop. At left is the vacuum chamber that was moved into place over the erosion loop and sealed before testing. The project was cancelled in 1973 and this test equipment was put into storage.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.

By Erik Conway, writing for My Big Fat Planet

Mars has been a grand scientific mystery ever since the first modern images were beamed back from the Mariner 4 spacecraft in 1965. Those snapshots showed a moon-like, cratered surface — not what we expected. Scientists had assumed that Mars would have an Earth-like atmosphere, composed mainly of nitrogen and with traces of carbon dioxide and water vapor. What they found instead was a cold desert world, one that possessed a thin wisp of an atmosphere containing only carbon dioxide.

Subsequent missions to the Red Planet detected tiny amounts of water vapor in Mars’ atmosphere, and better images began to unveil what looked like river channels and deltas on the surface. Indeed, spacecraft launched in the late 1990s and 2000s found water on Mars in the form of ice, bound into the planet’s soil and in great underground deposits. Water used to flow on the surface of Mars. But how? And where did it all go?

At first sight, the facts defy logic. According to astronomers, the sun used to be dimmer (i.e. colder) than it is now, meaning that Mars (and Earth) should have been colder in the past, not warmer. But observations tell us that it was clearly warmer and wetter on Mars in the past — not colder and more frozen. How did Mars buck the trend and stay toasty in the past? The most likely answer is that it used to have some sort of “super greenhouse effect” going on, the like of which we see on Venus. On Venus, the thick carbon-dioxide-based atmosphere traps the sun’s heat, resulting in surface temperatures that are hot enough to melt lead. Scientists think that early Mars also had a thick, carbon-dioxide-rich atmosphere that provided warming.

That said, in a recent talk at the American Geophysical Union conference in San Francisco, Mars specialist Bruce Jakosky of the University of Colorado pointed out that heat-trapping carbon dioxide alone would not have been sufficient to make Mars warm enough and wet enough to match our observations. Carbon dioxide’s ability to trap heat would have at some point “saturated”, or maxed out. Other greenhouse gases, like methane or ammonia, might have helped trap more heat near the surface of Mars — but they would not have been sufficient either because the sun’s ultraviolet radiation would have destroyed them far too quickly. Ergo, some sort of ultraviolet-absorbing layer high in Mars’ atmosphere would have been needed to help trap the heat. (The Earth’s ozone layer, which dates back to somewhere between 2 and 2.7 billion years ago, performs this service for us now.)

There is, as yet, no evidence of the necessary chemicals on Mars to do this. Jakosky didn’t draw any firm conclusions about how the warmer Mars could have existed. But he did lay out possible future investigations that might help uncover parts of this mystery a little more clearly. One of those includes the MAVEN mission to Mars, scheduled for launch in 2013, which will study how Mars’ atmosphere and climate has changed over time.

As Jakosky has said, in some ways, Mars is a very Earth-like planet. By looking at conditions on other worlds, we can gain insights into how, and why, our own climate is changing here on planet Earth.

You can read more about the Mars Science Laboratory rover here. Scheduled for launch in the fall of 2011, the Curiosity rover will help determine whether Mars has in the past, or does today, harbor life.

This post was written for “My Big Fat Planet,” a blog hosted by Amber Jenkins on NASA’s Global Climate Change site.

By Amy Mainzer

With WISE, I roamed the skies — seeing everything from the closest asteroids to the most distant galaxies. When I was a kid, maybe 6 or 7, I remember reading the encyclopedia about Andromeda, Mars and Jupiter. After that, I spent a lot of my free time (and a fair amount of gym class) wishing that I could be “out there” exploring the stars, imagining what it must be like to get close to a black hole or the lonely, cold surface of a moon. Fast-forwarding several decades, I’ve just spent a tremendously satisfying and delightful year using some of our most sophisticated technology to see “out there” for real. It’s pretty cool when your childhood dreams come true!

Today, the operations team sent the command to kill the survey sequence and put WISE into a deep sleep. While I’m sad to see the survey stop, the real voyage of discovery is just getting started as we unpack the treasures that our spacecraft beamed back to us. Although I’m going to miss waking up to see a new slew of pictures fresh from outer space, what I’ve looked at so far is only a tiny fraction of the millions of images we’ve garnered. My colleagues and I are working nonstop now to begin the decades-long process of interpreting the data. But I can already say for certain that we’re learning that the universe is a weirder, more wonderful place than any science fiction I’ve ever read. If I could go back in time to when I was kid, I’d tell myself not to worry and to hang in there through the tough parts — it was all worth it.

A cast of hundreds, maybe thousands, of people have worked on WISE and deserve far more credit than they get. The scientists will swoop in and write papers, but all those results are squarely due to the brilliance, stubborn persistence and imagination of the technicians, managers, engineers of all stripes (experts in everything from the optical properties of strange materials to the orbital perturbations of the planets), and administrative staff who make sure we get home safely from our travels. Although we may not be able to fly people around the galaxy yet, one thing Star Trek got right is the spirit of camaraderie and teamwork that makes projects like WISE go. For the opportunity to explore the universe with such fine friends and teammates, I am truly grateful.