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Overview of MESSENGER's science objectives
Posted: July 28, 2004

For a world such a relatively small distance from Earth, Mercury remains a big mystery. The planet is hard to study: Its average distance from the Sun is just 58 million kilometers (36 million miles), or about two-thirds closer than Earth's orbit. Mercury is visible from Earth only for several weeks a year, just after sunset or before sunrise, and astronomers have trouble observing it with ground telescopes through the sunlit turbulence of our atmosphere. Even the Hubble Space Telescope cannot view it because stray sunlight could damage its sensitive electronics. As such, many aspects of what we think we know are enigmas, perhaps unique in the solar system.

Artist's impression of the MESSENGER spacecraft in orbit at Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Thermal and dynamical obstacles challenge any spacecraft bound for Mercury, since it resides deep in the Sun's gravitational well. So far only NASA's Mariner 10 has visited the planet, flying past it three times in 1974-75 but seeing the same sunlit side on each pass. And Mariner 10 was unable to conduct the sort of global reconnaissance scientists now know is needed to put any planet into context.

We know less about Mercury than any of the other planets except Pluto - but what information we do have shows this extreme, odd member of the inner planet family has an incredible, fascinating story to tell. As the first rock from the Sun it has the shortest year and endures more solar radiation than any planet. It is the smallest and densest of the four rocky (or terrestrial) planets - which also include Venus, Earth and Mars - and its battered surface is perhaps one of the oldest in the solar system. It experiences the largest daily range in temperatures; at its hottest (about 450 degrees Celsius, or 840 degrees Fahrenheit) the surface temperature would melt lead, and during its long nights the cold (dipping toward -212 Celsius, or -350 Fahrenheit) could turn oxygen from a gas to liquid.

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission is designed to examine the planet in detail, conducting an in-depth global investigation organized around six key science questions. The answers not only will provide information specifically about Mercury, but offer a clearer, general picture of the origins and comparative evolution of all the terrestrial planets - and perhaps hint at what to look for in planetary systems beyond our own.

Learning how Mercury ended up the densest planet (after correcting for internal pressure) will tell us much about how planets form near their parent star. Discovering how Mercury has sustained a magnetic field while larger bodies either lost theirs (as Mars did) or show no sign of ever having one (like Venus) will help us understand how our own planet generates its protective magnetic field. Documenting the nature of Mercury's thin, tenuous atmosphere and the composition of mysterious radar-reflecting deposits near its poles - thought by many scientists to be water ice - will provide new insight into the volatile materials that exist on and around the inner planets.

Key Science Questions

Question 1: Why is Mercury so dense?

Mercury's enormous iron core distinguishes it from every other planet in our solar system. Each terrestrial planet has a dense, iron-rich core covered by a rocky mantle, but Mercury's core takes up more than 60 percent of its total mass - twice as much as Earth's. Why is this so? Is it related to Mercury's proximity to the Sun?

The planet's iron heart makes it incredibly dense, which results in a surface gravity about the same as Mars - a considerably larger planet. Scientists have several theories that could explain the reason for the large core. One is that as the planets formed from the disk-shaped cloud of gas and dust known as the solar nebula, dense particles (such as metallic iron) condensed and were preferentially retained in the innermost regions near the Sun, forming Mercury. Another possible explanation is that tremendous heat from the Sun vaporized part of the outer rock layer on a young Mercury, leaving it a metal-rich cinder. Yet a third idea is that a giant object - perhaps an asteroid - slammed into Mercury soon after it formed, blasting away much of its early crust and upper mantle.

Finding the answer: The evidence for solving the mystery of Mercury's density lies in its crustal silicate chemistry, and the amounts of certain elements (particularly iron, sodium, calcium and titanium) on the surface will tell much about the planet's evolution. Without geochemical remote-sensing tools, Mariner 10 could not provide any information on the chemical makeup of Mercury's surface. MESSENGER's spectrometers will examine the composition of the rocks on the surface and determine which minerals and elements are present - and which are conspicuously absent. This approach has been profoundly effective for the Moon and Mars.

Question 2: What is Mercury's geologic history?

Mercury has several mysterious landscape features that beg explanation, such as the relatively "young" plains seen as smooth deposits between surfaces that contain the planet's oldest craters. Many scientists believe flowing lava created the plains, but no one knows for sure.

Over time, bombardment from stray comets and asteroids changed Mercury's surface. Without encountering a significant atmosphere to burn up incoming debris, many objects slammed into the planet to form large and small impact craters. The largest impacts, like the one that formed the Texassized Caloris Basin, appear to have transformed entire regions of Mercury's surface, similar to ones on the Moon. The ramparts of Caloris span 1,300 kilometers (about 800 miles) and the tallest mountains climb past 3 kilometers (nearly 2 miles). Theory holds that shock waves from the Caloris impact created the area of chaotic terrain on the opposite side of the planet.

Other mysteries include hundreds of superimposed scarps - curving cliffs, typically hundreds of meters high and tens to hundreds of kilometers long. When did they form and in what sequence? It is possible these scarps formed as Mercury's interior cooled, causing the whole planet to shrink and its crust to contract. How much contraction, in turn, caused Mercury's crust to buckle and scarps to form? Similar features form here on Earth as lava flows cool and shrink.

Finding the answer: MESSENGER will shed unprecedented light on the forces that shaped Mercury's surface. Its X-ray, gamma-ray, and visible-infrared spectrometers will measure the major elements and minerals in Mercury's surface rocks. The camera will photograph all of the planet, including the 55 percent that Mariner 10 missed - and at much higher resolution than Mariner 10's images. Nearly all of Mercury will be imaged in stereo to determine topographic variations and landforms across the globe. The laser altimeter will precisely measure the topography of surface features, and these data, when compared with gravity field measurements gathered by tracking MESSENGER's subtle movements in orbit, will help determine the thickness and structure of Mercury's crust.

Question 3: What is the structure of Mercury's core?

The biggest surprise from the Mariner 10 flybys was that Mercury has a global magnetic field, making it the only terrestrial (rocky) planet besides Earth to have one. Mercury's magnetic field is weak - about 100 times weaker than Earth's at the surface - but that it exists at all raises interesting questions about activity deep inside the planet.

Earth's magnetic field is generated by the swirling motions of molten liquid in our planet's outer core. But Mercury is so much smaller than Earth - 4,878 kilometers in diameter vs. Earth's 12,714 kilometers (3,031 miles vs. 7,900 miles) - that its core should have cooled and solidified long ago. Its many long scarps suggest that the planet has contracted and the core has cooled, so how could Mercury's now stagnant core generate a magnetic field? One potential answer is that the observed magnetic field is a fossil remaining from Mercury's earliest years; perhaps rocks were magnetized long ago when there was a magnetic field, and Mariner 10's magnetometer simply recorded leftover magnetization from the rocks. Another is that the core is indeed still liquid and actively generating the field.

Finding the answer: For insight into Mercury's insides, MESSENGER's laser altimeter will measure the planet's libration - the small amount it "wobbles" as it spins on its axis. By combining this measurement with what we learn about Mercury's gravity from radio science experiments, scientists will be able to deduce the size of the planet's core and how much of it is liquid or solid. The magnetometer should also be able to tell if the magnetic field stems from activity inside the planet, or from magnetic areas of the surface.

Question 4: What is the nature of Mercury's magnetic field?

The solar wind - the ever-expanding atmosphere of the Sun - forces constant change in Earth's magnetic field. We see the effects of these changes in the form of the aurora, electrical power blackouts, and TV and radio interference. Mariner 10 found that where the solar wind interacted with Mercury, the particles changed in a way that suggested the effects of an internal magnetic field. A better understanding of an internal magnetic field smaller, weaker and much closer to the Sun than Earth's will teach us more about our own magnetosphere - this is comparative planetology at its best.

Earth has a dipolar magnetic field, shaped like a bar magnet's field, with positively and negatively charged poles. Mercury's field also appears to be dipolar. In contrast, the Moon and Mars lack a global dipolar magnetic field, but have local magnetic fields centered on different spots that are relicts. It's not clear how much of Mercury's field comes from smaller local fields (like on Mars or the Moon), and how much is indeed global, produced deep within the planet.

Finding the answer: MESSENGER's magnetometer will examine Mercury's magnetic field over four Mercury years (each 88 Earth days) to determine its strength and how it varies with position, altitude and time. The magnetometer and energetic particle and plasma spectrometer will also sense the magnetic field's responses to solar activity, and help separate the internal from externally induced components of the field.

Question 5: What are the unusual materials at Mercury's poles?

In the early 1990s, scientists using radar (i.e., microwaves) to observe Mercury noticed that something inside craters near its poles was strongly reflecting the radar pulses. To most experts the materials looked a lot like what would be expected from molecules such as water ice.

At first, it seems ludicrous to even think about water ice on a planet where "daytime" temperatures near the equator can soar to 450 degrees Celsius (840 degrees Fahrenheit). But since the planet does not tilt - its spin axis is nearly perpendicular to its equator - sunlight does not reach the floors and walls of polar craters, and temperatures inside these craters stay perpetually cold. Could water molecules from comets and meteorites have become trapped in the shadowy corners of these cold craters, frozen and accumulated over billions of years? Or, could water vapor have seeped out from inside the planet and frozen out near the poles? Such ice deposits could be insulated by thin layers of dust and other material ejected by impacts, but still visible to the penetrating waves of radar. Some scientists think the material isn't water ice but something else, such as sulfur, derived from minerals in the surface rocks. This enigma is an important topic in the comparative planetology of the Moon, Mercury and Mars.

Finding the answer: It will be a challenge to figure out what the deposits are, because they will be invisible to many of MESSENGER's instruments. The very shadows that preserve the ice deposits so close to the solar inferno keep them from being illuminated by the Sun. MESSENGER's gamma-ray and neutron spectrometers - designed to pinpoint key elements on Mercury's surface - will aim toward these polar craters and may be able to sense if they are lined with water ice or other materials. Looking in the same direction, the ultraviolet and energetic particle spectrometers could also detect hydroxyl (OH) or sulfur emissions from the deposits.

Question 6: What volatiles are important at Mercury?

Mercury is surrounded by an extremely thin layer of gas - so thin that, unlike in the atmospheres of Venus, Earth and Mars, the molecules surrounding Mercury don't collide with each other. Instead, they bounce from place to place on the surface, almost like rubber balls. (Such an atmosphere is also referred to as an "exosphere.")

We know of six elements in Mercury's atmosphere: hydrogen, helium, oxygen, sodium, potassium and calcium. These elements are relatively abundant and are particularly easy to detect with Earth-based telescopes. Each element in the atmosphere has a different origin. Hydrogen and helium come (at least in part) from the solar wind. Some of the hydrogen and oxygen may also come from ice that came aboard comets and meteorites that hit the planet. The sodium, potassium, calcium and some of the oxygen is thought to come from rocks on the surface.

Finding the answer: MESSENGER will measure the composition of Mercury's atmosphere with its ultraviolet and energetic particle spectrometers. By comparing these data with X-ray and gamma-ray measurements of the surface rocks, scientists will gain invaluable clues on the origin of each element in the planet's atmosphere, and learn more about where they came from.

Science Groups

The MESSENGER science team, which includes 23 investigators from 13 research institutions, is divided into four broad disciplinary groups. The Geology group, chaired by Dr. James Head III, Brown University, will interpret data on Mercury's geologic history. Geochemistry, led by Dr. William Boynton, University of Arizona, will interpret measurements of Mercury's surface composition. Geophysics, chaired by Dr. Maria Zuber, Massachusetts Institute of Technology, will cover the altimetry and gravity measurements. The Atmosphere and Magnetosphere group, led by Dr. Stamatios Krimigis, the Johns Hopkins University Applied Physics Laboratory, will analyze data on Mercury's magnetic field, atmosphere, and energetic particle and thermal plasma characteristics.

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