Bringing Space Science Down to Earth
Sifting through dirt for clues to planetary origins
Vincent Chevrier did not set out to study dirt. But as a geochemist at the Arkansas Center for Space and Planetary sciences, his research into the soil composition and chemistry of Mars has provided a fascinating challenge that made up for moving from rocks to mere bits of dust.
In fact, soil on Mars has become the focal point for much scrutiny, as it may contain clues as to the history of and current presence of water – and possibly life – on the planet.
“We know from our experience on Earth that life can evolve almost anywhere,” said Chevrier. Microbes have been found in most of Earth’s extreme environments – in radioactive sites, at freezing and boiling temperatures, in strong acids. “There are only two things you need. First, you need liquid water. Then, you need an energy source.”
How do scientists study the environment and possible inhabitants of a planet that lies between 36 million and 250 million miles away from Earth, depending upon the two planet’s orbits? At the University of Arkansas, scientists use the Andromeda Chamber, a planetary environmental chamber that can be set to the atmospheric conditions found on different planets throughout the solar system.
Studying the geological and chemical processes under Martian conditions in the environmental chamber helps researchers form ideas of the history of Mars, which is relevant to the search for current life there.
“We’re pretty sure there was a lot of water on Mars about four million years ago,” Chevrier said. “The question is: Where did the water go? We don’t know the answer yet.”
Scientists do know that huge ice deposits exist on the planet. They also have detected salts of various kinds, including the most recent discovery of perchlorates by the Phoenix Lander. Much as salt is used to lower the freezing point of water on icy roads, scientists suspect that salts on Mars may create brines, salty mixtures that could contain liquid water.
Chevrier and his team have designed experiments to understand the behavior of brines under Martian conditions. They have used different types of salts found on the planet and looked at what happens when they freeze and evaporate.
“From those experiments we’ve developed a comprehensive theory of brine behavior,” Chevrier said. This theory describes what happens when you have an exchange between the surface and the atmosphere.
The techniques and equations that Chevrier and his graduate students have developed for Mars have resulted in a $790,000 grant from NASA and the university to study Titan, the largest moon of Saturn.
Titan is the only satellite in the solar system that has a substantial atmosphere. The atmosphere is about 95 percent nitrogen, with small amounts of methane, hydrogen and hydrocarbons. The temperature of Titan’s surface is – 290 degrees F, so no liquid water exists on the moon’s surface. However, the space mission Cassini Huygens showed that liquid methane lakes, clouds and snow-topped mountains suggest a complex “hydrologic” cycle on Titan that involves organic molecules. Chevrier’s research will help determine the short and long term stability of light organic volatile compounds such as methane and ethane on the surface and subsurface of Titan. Knowing this could also help explain how the planet evolved and provide insights into the origins of the solar system.
Solving planetary puzzles one neutron at a time
Other scientists take a different approach to studying the solar system: They look at the little picture.
For instance, to get at the origins of the planet Earth, geochemist Fangzhen Teng studies the humble neutron.
Neutrons don’t get much respect in the atomic world; they don’t see much action, unlike protons and electrons. Chemically speaking, they don’t add or detract anything from the atoms they reside in.
Yet some elements contain slightly different numbers of neutrons. Called isotopes, these related but distinctly different entities originate in different ways from different sources, and studying them can lead scientists to new insights about the world we live in.
In elementary school, students learn that the planet we live on has a core, mantle and crust. But although scientists have a general idea of how the Earth formed, they know little about its evolution.
“There are three layers of the Earth, like an egg,” Teng said. He uses isotopes to study how the Earth’s crust has evolved over time, and how the crust and the mantle interact with one another.
“Isotopes are very sensitive to sources,” Teng said. “We can use isotopes as a tool to attack these problems.”
Teng uses the isotopes of three different elements to address different questions: magnesium, iron and lithium. Magnesium and iron isotopes behave in similar ways, but iron can separate into two oxidation states, which makes it useful in determining how planets formed.
In a paper published in Science, Teng and his colleagues reported that they found iron isotope fractionation, or separation, in basalt samples from the Kilauea Iki lava lake on the main island of Hawaii.
Teng likens the change in iron isotopic composition in basalts to the baking of a cake: With a cake, you start out with certain ingredients, but the baking process changes the ingredients and their proportions within the cake. In the same way, the process that makes basalt magma through partial melting of the mantle peridotites, or rocks, changes the iron isotope compositions.
If basalts from the moon or Mars have similar iron isotope separation, it suggests that they formed through heat processes similar to those on Earth. However, if rocks from these planetary bodies do not have iron isotope separation, it suggests that they were formed in a different way.
More recently, Teng has examined magnesium isotopes in the Earth’s mantle and compared them to the isotopes from meteoritic material. Magnesium makes a particularly good marker for planetary origins because, first, isotopes of magnesium can be separated during evaporation and condensation in the solar system and, second and more uniquely, one isotope of magnesium, Mg26, is a decay product of Al26, which existed in the early solar system for less than 5 million years. Thus, materials with different origins and ages contain different amounts of Al26, which results in different amounts of magnesium isotope.
Teng’s group analyzed different types of rocks from different depths of the Earth’s mantle from a site in North China and compared the results to those of samples from chondritic meteorites. They looked at magnesium isotopes in samples from the whole rock, but they also separated out minerals from the rocks and examined the magnesium isotope composition of these minerals as well.
“The samples from Earth were slightly different from one another,” Teng said. Their compositions also matched closely with those of the meteorites, suggesting that Earth formed at the same time as much of the meteoritic material in the solar system.
Black holes, quasars, spiral arms and dark matter
While Chevrier and Teng concern themselves with planetary origins and evolution, astronomers Daniel and Julia Kennefick have more distant concerns – the closest being about 1,174,917,830,400,000,000 miles away. The Kenneficks study the origins and evolution of galaxies.
“We’re looking at how galaxies form, work and evolve,” Julia Kennefick said.
Together with astronomy professor Claud Lacy and astronomer Marc Seigar from the University of Arkansas, Little Rock, they are part of a research group known as AGES, the Arkansas Galaxy Evolution Survey. They are surveying galaxies with active, growing supermassive black holes, using techniques to “weigh” the black holes in their centers.
In 2008, researchers in the AGES group showed that a relationship exists between the mass of the black hole at the center of a galaxy and the angle of the spiral arms that spin out from the galaxy’s center. The smaller the pitch angle of the spiral, the larger the black hole mass at the galaxy’s core. The larger the pitch angle of the spiral, the smaller the black hole in the middle. The researchers will use this relationship to estimate the masses of galaxies from extensive archives of images provided by large telescopes, such as NASA’s Hubble Space Telescope. In addition to surveying galaxies and estimating the masses of black holes, the Kenneficks are trying to explain how the relationship between the spiral arm pitch and the black hole mass works.
“Now we’re really at the ‘why’ stage: How does a galaxy work, and why are these two things related?” Julia Kennefick said. “We’re trying to find a link between the two things.”
The AGES project will allow the researchers to study hundreds to thousands of galaxies and the black holes at their centers so they can begin to see patterns.
They also are trying to solve a mystery: The Case of the Missing Black Holes.
Looking back in time, scientists can detect massive black holes in distant quasars, which look like they weigh up to 18 billion solar masses. But younger galaxies appear to have smaller black holes, leading Julia Kennefick and others to ask: Where are all the large black holes now?
“They shouldn’t have disappeared. They should still be there,” she said. “They are there – we just can’t detect them anymore. They are quiescent.” But if the spiral structure of a galaxy gives scientists a way to estimate the size of the quiescent black hole hidden at the center, perhaps the largest black holes can be found after all.
The black holes may provide a way to trace the evolution of galaxies, but the black holes themselves remain a puzzle.
“We really don’t know how they work because we can’t see them directly,” Julia Kennefick said.
The mystery may be a matter – of dark matter, that is. “Dark matter” refers to the unseen, undetected material that seems to exert influence on the universe and cannot be explained by conventional matter as it is understood today.
“Dark matter doesn’t react to light or other electromagnetic radiation,” said Daniel Kennefick, “But it does have gravity.” A recent paper suggests that the density of dark matter controls how well a black hole can grow.
Dark matter also might help explain certain characteristics of galaxies that are poorly understood. For instance, researchers expect galaxies to behave as if their masses reside towards their centers, where most of the starlight comes from, but this turns out not to be the case. Also, the laws of gravity suggest that if two galaxies meet, they should pass through one another. Instead they form a deep “hole” from which they can’t escape.
“The dark matter may explain why galaxies merge and why you get clusters of galaxies,” Daniel Kennefick said. It also may explain why the spiral structure of the galaxy should be related to the size of its black hole. Astronomy professor Claud Lacy is studying binary black holes of merged galaxies to determine the characteristics of such combinations, again allowing researchers to better understand how dark matter, black holes and galaxy shapes interact with one another.