December 5, 2008
Using stereo topographic maps obtained by processing data from the high-resolution camera onboard NASA’s Mars Reconnaissance Orbiter, Caltech scientists Kevin Lewis, Oded Aharonson, and John Grotzinger, identified and measured layered rock outcrops within four craters in the Red Planet’s Arabia Terra region. The layering in different outcrops occurs at scales ranging from a few meters to tens of meters, but at each location the layers all have similar thicknesses and exhibit similar features.

Based on a pattern of layers within layers measured at Becquerel crater, the scientists propose that each layer formed during a period of about 100,000 years and that the same cyclical climate changes produced these layers.

In addition, every 10 layers were bundled together into larger units, which were laid down during an approximately one-million-year period. In the Becquerel crater, the 10-layer pattern is repeated at least 10 times. This one-million-year cycle corresponds to a known pattern of change in Mars’ obliquity caused by solar system dynamics .

“Due to the scale of the layers, small variations in Mars’ orbit are the best candidate for the implied climate changes. These are the very same changes that have been shown to set the pacing of ice ages on the Earth and can also lead to cyclic layering of sediments,” said Lewis.

Earth’s tilt on its axis varies between 22.1 and 24.5° over a 41,000-year period. The tilt is responsible for seasonal variation in climate, because the portion of Earth that is tipped toward the Sun receives more sunlight hours during a day and gradually changes throughout the year. During phases of lower obliquity, polar regions are less subject to seasonal variations, leading to periods of glaciation.

Mars’ tilt varies by tens of degrees throughout a 100,000-year cycle, producing even more dramatic variation. When the obliquity is low, the poles are the coldest places on the planet, while the Sun is located near the equator all the time. This could cause volatiles in the atmosphere, like water and carbon dioxide, to migrate poleward, where they’d be locked up as ice.

When the obliquity is higher, the poles get relatively more sunlight, and those materials would migrate away. “That affects the volatiles budget. If you move carbon dioxide away from the poles, the atmospheric pressure would increase, which may cause a difference in the ability of winds to transport and deposit sand,” Aharonson said. This is one effect that could change the layers’ deposition rate such as those seen by the researchers in the four craters.

The changing tilt would also change the stability of surface water, which alters the ability of sand grains to stick together and cement to form the rock layers.

“The whole climate system would be different,” Aharonson said.

However, such large climate changes would influence a variety of geologic processes on the surface. While the researchers cannot tie the formation of the rhythmic bedding on Mars to any particular geologic process, “strength of the paper is that we can draw conclusions without having to specify the precise depositional process,” Aharonson said.

“This study gives us a hint of how the ancient climate of Mars operated and shows a much more predictable and regular environment than you would guess from other geologic features that indicate catastrophic floods, volcanic eruptions, and impact events,” Lewis said. “More work will be required to understand the full extent of the information contained within these natural geologic archives,” he said.

“One of the fun things about this project is that we were able to use techniques on Mars that are the bread and butter of studies of stratigraphy on Earth,” said Aharonson. “We substituted a high-resolution camera in orbit around Mars and stereo processing for a geologist’s Brunton Compass and mapboard, and were able to derive the same quantitative information on the same scale. This enabled conclusions that have qualitative meaning similar to those we chase on Earth.”

December 5, 2008
Magnetic substorms on Earth disrupt orbiting satellites, including telecommunication satellites and global positioning systems. This mysterious phenomenon has been studied with the European Space Agency’s (ESA) Cluster satellites, comparing it with magnetic substorms on Jupiter for a better understanding.

Planets that have their own magnetic field, such as Mercury, Earth or Jupiter, are protected by the magnetic bubble that it generates.

During a magnetic substorm on Earth, particles located tens of thousands miles on the night-side of our planet are energized and hurled earthward within a few minutes. This creates colorful aurorae and excites the near-Earth environment. Despite decades of space-based research, several aspects of this phenomenon remain unknown.

One such unknown aspect is the mechanism that triggers these storms. It is not clear whether the storms are caused by processes internal to the magnetosphere or by other external processes.

Elena Kronberg and colleagues at the Max Planck Institute for Solar System Research, Germany, decided to look away from Earth, to other planets, to see if they could learn something new.

On Earth, a periodic substorm shows a gradual decrease followed by a rapid increase in the amount of particles that are hurled earthward. One such cycle takes 2-3 hours. At Jupiter, the same cycle takes 2-3 days. This duration is longer partly because of Jupiter’s stronger magnetic field and larger magnetosphere. The jovian magnetic field is so large that Jupiter’s magnetosphere envelopes the planet’s moons Io, Europa, Ganymede and Calisto.

After detailed analysis of data from several missions, Kronberg said, “We’ve found that at Earth and Jupiter, the magnetic field undergoes the same three steps during a substorm: growth, expansion, and recovery.”

Kronberg and colleagues studied data from NASA’s Galileo spacecraft. In 2007, they reported that periodic substorms at Jupiter were connected to the constant release of matter by the jovian satellite Io, which lies inside the jovian magnetosphere. This release of matter forms part of the mechanism that triggers the substorm. This means that the driver of this phenomenon is internal to the jovian magnetosphere.

“With Cluster, we’ve gained a better understanding of the processes taking place inside Earth’s magnetosphere; this has enhanced our understanding of how our solar system works. And now we’re delighted to learn more from gigantic Jupiter itself.” remarked Philippe Escoubet, ESA’s Cluster Project Scientist.

December 3, 2008
A pale yellow dot to the human eye, Venus comes to life in the ultraviolet and the infrared. New images taken by instruments on board the European Space Agency’s (ESA) Venus Express provide insight into our neighboring planet’s turbulent atmosphere.

Venus Express lets scientists compare what the planet looks like in different wavelengths, giving them a powerful tool to study the physical conditions and dynamics of the planet’s atmosphere.

Observed in the ultraviolet, Venus shows numerous high-contrast features. The cause is the inhomogeneous distribution of a mysterious chemical in the atmosphere that absorbs ultraviolet light, creating the bright and dark zones.

The ultraviolet reveals cloud structure and the atmosphere’s dynamical conditions in the atmosphere and the infrared provides information on the cloud tops’ temperature and altitude.

With data from Venus Express, scientists have learned that the equatorial areas on Venus that appear dark in ultraviolet light are regions of relatively high temperature, where intense convection brings up dark material from below. In contrast, the bright regions at mid-latitudes are areas where the temperature in the atmosphere decreases with depth. The temperature reaches a minimum at the cloud tops suppressing vertical mixing. This band of cold air, nicknamed the cold collar, appears as a bright ring in the ultraviolet images.

Infrared observations have been used to map the altitude of the cloud tops. Surprisingly, the clouds in both the dark tropics and the bright mid-latitudes are located at about the same altitude of about 45 miles (72 kilometers). At 60° south, the cloud tops start to sink, reaching a minimum of about 40 miles (64 kilometers), and form a huge hurricane at the pole.

Infrared images overlaid on ultraviolet images bring the giant hurricane’s eye at the planet’s south pole to life. Its center is displaced from the pole and the whole structure measures about 1,243 miles (2,000 kilometers) across, rotating around the pole in about 2.5 days.

This study reveals that variable temperature and dynamical conditions at the Venus cloud tops are the cause of the global ultraviolet pattern.

But the exact chemical species that creates the high-contrast zones still remains elusive, and the search is on.

December 2, 2008
Comet 96P/Machholz 1’s anomalous compositional characteristics help pinpoint its origin to one of three intriguing scenarios. David Schleicher, Lowell Observatory planetary astronomer, measured abundances of five molecular species in the comae of 150 comets and discovered that one comet, 96P/Machholz 1, has an unusual chemistry. The cause of this chemical anomaly remains unknown, but each of three possible explanations yields important but differing constraints on the evolution of comets.

One possible explanation is that Machholz 1 did not originate in our solar system, but escaped from another star. In this scenario, the other star’s proto-planetary disk might have had a lower abundance of carbon, resulting in all carbon-bearing compounds having lower abundances. “A large fraction of comets in our own solar system have escaped into interstellar space, so we expect that many comets formed around other stars would also have escaped,” said Schleicher. “Some of these will have crossed paths with the Sun, and Machholz 1 could be an interstellar interloper.”

The discovery of comet Machholz 1’s anomalous composition reveals the existence of a new class of comets. Astronomers identified two other classes in the 1990s. While Machholz 1 also has strongly depleted C2 and C3 carbon species, what makes it anomalous is that the molecule cyanogen (CN) is depleted. In Machholz 1, cyanogen is missing by about a factor of 72 from the average of other comets. “This depletion of CN is much more than ever seen for any previously studied comet, and only one other comet has even exhibited a CN depletion,” said Schleicher.

Another possible explanation for Machholz 1’s anomalous composition is that it formed even farther from the Sun in a colder or more extreme environment than other comets studied. The scarcity of such objects likely is associated with the difficulty of explaining how such comets moved into the inner solar system.

The third possibility is that Machholz 1 originated as a carbon-chain depleted comet but extreme heat altered its chemistry. While no other comet has exhibited changes in chemistry due to subsequent heating by the Sun, Machholz 1 has the distinction of having an orbit that now takes it to well inside Mercury’s orbit every 5 years. (Other comets get even closer to the Sun, but not as often). “Since its orbit is unusual, we must be suspicious that repeated high temperature cooking might be the cause for its unusual composition,” said Schleicher. “However, the only other comet to show depletion in the abundance of CN did not reach such high temperatures. This implies that CN depletion does not require the chemical reactions associated with extreme heat.”

Although comet 96P/Machholz 1 was first sighted in 1986, compositional measurements only took place during the comet’s 2007 apparition. Lowell Observatory’s program of compositional studies, currently headed by Schleicher, includes measurements of more than 150 comets obtained during the past 33 years. This research compares and contrasts Machholz 1 against this large database of 150 comets.

In the early 1990s, Lowell Observatory’s long-term program first identified the existence of two compositional classes of comets. One class, containing the majority of observed comets, has a composition called “typical.” Most members of this typical class have long resided in the Oort Cloud at the fringes of our solar system, but they are believed to have formed amidst the giant planets, particularly among Saturn, Uranus, and Neptune. Other members of this compositional class arrived from the Kuiper Belt, located just beyond Neptune.

The second compositional class of comets has varying depletions in two of the five chemical species measured. Because both depleted molecules, C2 and C3, are composed wholly of carbon atoms, this class was named “carbon-chain depleted.” Moreover, nearly all comets in this second class have orbits consistent with their having arrived from the Kuiper Belt. For this and other reasons, the cause of the depletion is believed to be associated with the conditions that existed when the comets formed, perhaps within an outer, colder region of the Kuiper Belt.

Comets are widely thought to be the most pristine objects available for detailed study remaining from the epoch of solar-system formation. As such, comets can be used as probes of the proto-planetary material that was incorporated into our solar system. Differences in the current chemical composition among comets can indicate either differences in primordial conditions or evolutionary effects.

Although the location of origin cannot be determined for any single comet, Machholz 1’s short orbital period means that astronomers can search for additional carbon-bearing molecular species during future apparitions. “If additional carbon-bearing species are also depleted, then the case for its origin outside of our solar system would be strengthened,” said Schleicher. The next opportunity for observations will be in 2012.