Mercury’s First Surface?

The planet Mercury has always been a challenge to observe. As the closest planet to the Sun, it’s usually hidden within our parent star’s bright glow. Occasionally, it moves far enough out of the glare to be seen in a twilit sky, just after sunset or just before dawn. Due to its small size and great distance, it looks tiny, even through telescopes, and not much surface detail can be seen.

In 1974 and 1975, NASA’s Mariner 10 spacecraft gave us our first close-up view of the planet, flying past it three times and sending back views of sinuous cliffs called lobate scarps, an enormous impact feature now known as Caloris Basin, and what, for lack of a better term, scientists labeled “weird terrain”—jumbled blocks of fragmented crust thought to be related to the Caloris impact.

It was nearly four decades before we would see Mercury that closely again. In 2011, another NASA spacecraft—the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission—went into orbit around Mercury, providing global coverage for four years, relaying images of the surface at high resolution until it deliberately crashed into the surface in April 2015. During its four year mission, it observed several intriguing craters surrounded by dark halos. Most likely, the halos are subsurface material exposed by the impacts that formed the craters, but exactly what they were composed of was unknown. Using data from an onboard instrument called a Gamma-Ray and Neutron Spectrometer, scientists analyzed the dark material and concluded that it’s a form of carbon, specifically graphite.

Yes, the same stuff that’s inside pencils.

It’s theorized that rather than being brought to the planet by comets or asteroids, the graphite was part of the ancient crust. Models suggest that the young planet was covered by an ocean of magma, or molten rock. Most minerals sank to lower levels, but a layer of buoyant graphite remained at the top and cooled to form Mercury’s first solid crust. This was covered by later activity, then re-exposed by impact processes. The dark halos seen by Mariner 10 may therefore be remnants of that early planetary surface and a glimpse of a time when our solar system was in its infancy.

The next mission to Mercury will be BepiColombo, a joint project of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), scheduled for launch in January 2017. –Bing Quock

A Swiftly Tilting (Red) Planet

The best and worst thing about big discoveries is that they can throw a wrench into the whole system. Einstein’s General Relativity, recently supported by the detection of gravitational waves, meant we had to re-evaluate the force of gravity and the concepts of space and time. The theorized Planet Nine means we need to take a hard look at how solar systems form (a concept you can explore more deeply in the new show Incoming!,now playing in Morrison Planetarium). And a recently-published study of Marsmay drastically alter how we think about the timeline of the Red Planet and whether life could have existed there.

Tharsis is a vast volcanic plateau that covers about 25 percent of the martian surface. It’s home to four of the largest volcanoes in the Solar System, including Olympus Mons, an extinct volcano that is the Solar System’s largest mountain. Tharsis is located near the equator of the planet, but it may not have started that way. According to recent computer models developed by the French National Centre for Scientific Research (CNRS) and their partners, Tharsis formed about 3.7 billion years ago at a latitude of about 20°N. For the next 200-700 million years, it remained there and grew, adding more volcanic material until it formed a massive bulge, an enormous weight on the side of the planet. How massive? One sextillion tons. That’s 1018 tons. Or 1/70th the mass of Earth’s Moon.

The Tharsis bulge became so huge that it actually shifted down to the equator where it would be in equilibrium with Mars’s rotation. But it’s not like Tharsis just relocated across or under the surface: it brought the surface with it, and the entire crust and mantle of Mars rotated around the core.

While this is mind-boggling to imagine, it actually makes a lot of sense, because the martian surface has always been a bit off. For example, Mars has huge underground reservoirs of water ice that seem consistent with polar ice caps, except they’re not at the poles: it turns out they’re where the poles used to be. The hypothesis also accounts for the placement and formation of ancient river networks.

This discovery was a huge undertaking. It involved modeling Mars’s rotation before Tharsis formed, how the rotation around the core (called True Polar Wander, or TPW) would have happened, how it would have affected geological stresses on the martian surface, how it would have altered the climate—which would in turn have affected those aforementioned ancient rivers—as well as the general influence on the martian environment. Some of this work had been undertaken previously by Isamu Matsuyama of the University of Arizona, another author on the paper, who independently modeled a Mars without the Tharsis bulge in 2010. Matsuyama’s models and those of CNRS matched nearly identically.

The ramifications are staggering. This period of martian history, from 3.0–3.5 billion years ago, is a major focus of study for scientists. At the time, Mars’s core was likely more active and the planet would have had a strong magnetic field that could protect a thick atmosphere. This atmosphere would have provided a better environment for liquid water, such as those ancient rivers and a vast ocean. Many scientists believe that Mars during this era was more conducive to life than Earth was at the time. If the planet was undergoing a massive shift during the same period, that could explain or negate many of our ideas of what Mars was like and whether life was possible.

As more models are created and tested, the theory of True Polar Wander and Tharsis’s shift will be scrutinized heavily. Likely, it will end up being tweaked and modified to more perfectly fit the observations we see on the martian surface. But as we dive more deeply into the history of one of the most promising habitats for ancient extraterrestrial life, we will have to take Tharsis into account. This once not-so-little super-volcano could very well mean we need to rethink everything we know about the history of our neighboring world. –Colin Elliott

Heading to Mars

Speaking of the Red Planet, this week brought news of two upcoming Mars missions that could help tweak these theories: ESA’s and Roscosmos’s Trace Gas Orbiter (TGO), potentially launching Monday, and NASA’s InSight, originally scheduled to launch this month but now postponed to May 2018.

If all goes well Monday, TGO, part of the ExoMars mission, will begin orbiting Mars in October to make a detailed inventory of atmospheric gases, with particular interest in rare gases like methane, which might imply the presence of microbial life on the Red Planet. If methane is discovered, TGO will also try to determine whether it stems from a geological or biological source.

TGO also carries a lander, Schiaparelli (named for the19th-century Italian astronomer who claimed to have observed “canali” on Mars), that will test a range of technologies as it descends and lands on Mars in preparation for future missions. TGO will also serve as a data relay for the second ExoMars mission, comprising a rover and a surface science platform, planned for launch in 2018.

The primary goal of NASA’s Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission is to help us understand how rocky planets—including Earth—formed and evolved by studying the deep interior of Mars. The spacecraft had been on track to launch this month until a vacuum leak in its prime science instrument prompted NASA to suspend preparations late last year. NASA’s Jet Propulsion Laboratory (JPL) will redesign, build, and conduct qualifications of the new vacuum enclosure for the Seismic Experiment for Interior Structure (SEIS), the component that failed in December.

The seismometer will help answer questions about the interior structure and processes within the deep martian interior. The SEIS instrument has three high-sensitivity seismometers enclosed in a sealed sphere. The seismometers need to operate in a vacuum in order to provide exquisite sensitivity to ground motions as small as the width of an atom.

This week, NASA announced that the new launch window begins May 5, 2018, with a Mars landing scheduled for November 26, 2018. –Molly Michelson

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