ESA Rosetta Comet 67/P Mission: Rosetta Team Uncovers More Secrets

A colour image of Comet 67P/Churyumov-Gerasimenko composed of three images taken by Rosetta’s scientific imaging system OSIRIS in the red, green and blue filters; the images were taken on August 6, 2014 from a distance of 120 km from the comet. 

Image credit: ESA / Rosetta / MPS / OSIRIS Team / UPD /LAM / IAA / SSO / INTA / UPM / DASP / IDA.

The familiar shape of the comet has now had many of its vital statistics measured: the small lobe measures 2.6 × 2.3 × 1.8 km and the large lobe 4.1 × 3.3 × 1.8 km.

The total volume of the comet is 21.4 km3. Rosetta’s Radio Science Instrument has measured its mass to be 10 billion tons, yielding a density of 470 kg/m3.

By assuming an overall composition dominated by water ice and dust with a density of 1,500–2,000 kg/m3, Rosetta scientists show that the comet has a very high porosity of 70–80 percent, with the interior structure likely comprising weakly bonded ice-dust clumps with small void spaces between them.

The OSIRIS instrument has imaged some 70 percent of the surface to date: the remaining unseen area lies in the southern hemisphere that has not yet been fully illuminated since Rosetta’s arrival.

The scientists have so far identified 19 regions separated by distinct boundaries and, following the ancient Egyptian theme of the Rosetta mission, these regions are named for Egyptian deities, and are grouped according to the type of terrain dominant within.

The 19 regions identified on 67P/Churyumov–Gerasimenko are separated by distinct geomorphological boundaries; they are grouped according to the type of terrain dominant within each region. 

Five basic categories of terrain type have been determined: dust-covered (Ma’at, Ash and Babi); brittle materials with pits and circular structures (Seth); large-scale depressions (Hatmehit, Nut and Aten); smooth terrains (Hapi, Imhotep and Anubis), and exposed, more consolidated surfaces (Maftet, Bastet, Serqet, Hathor, Anuket, Khepry, Aker, Atum and Apis). 

Image credit: ESA / Rosetta / MPS / OSIRIS Team / UPD /LAM / IAA / SSO / INTA / UPM / DASP / IDA.

Five basic, but diverse, categories of terrain type have been determined: dust-covered; brittle materials with pits and circular structures; large-scale depressions; smooth terrains; and exposed more consolidated surfaces.

Much of the northern hemisphere is covered in dust. As the comet is heated, ice turns directly into gas that escapes to form the atmosphere or coma.

Dust is dragged along with the gas at slower speeds, and particles that are not traveling fast enough to overcome the weak gravity fall back to the surface instead.

Some sources of discrete jets of activity have also been identified. While a significant proportion of activity emanates from the smooth neck region, jets have also been spotted rising from pits.

The gases that escape from the surface have also been seen to play an important role in transporting dust across the surface, producing dune-like ripples, and boulders with ‘wind-tails,’ the boulders act as natural obstacles to the direction of the gas flow, creating streaks of material ‘downwind’ of them.

“Because comets have very little gravity, dust and gas flow freely into space. But we were surprised to find a cloud of particles orbiting the comet that are large and heavy enough to defy the Sun’s radiation pressure,” said Dr Dennis Bodewits of the University of Maryland.

The scientists were able to make this discovery thanks to OSIRIS’ very sensitive cameras.

“Each pixel is about 30 cm. You couldn’t see a coffee cup, but you could see a large lunchbox. The resolution is about 10 times higher than Google Earth.”

According to the team, 67P/Churyumov-Gerasimenko was releasing the earthly equivalent of 1.2 liters of water into space every second at the end of August 2014.

MIRO (Microwave Instrument for the Rosetta Orbiter)

Credit: ESA

“In observations, made by the Microwave Instrument for Rosetta Orbiter (MIRO), over a period of three months, the amount of water in vapor form that the comet was dumping into space grew about tenfold,” said Dr Sam Gulkis of NASA’s Jet Propulsion Laboratory in Pasadena.

“To be up close and personal with a comet for an extended period of time has provided us with an unprecedented opportunity to see how comets transform from cold, icy bodies to active objects spewing out gas and dust as they get closer to the Sun.”

NASA Messenger: Mercury contracted more than previous estimates

The planet Mercury, with only a small portion of its surface illuminated. 

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Carnegie Institution of Washington

New evidence gathered by NASA's MESSENGER spacecraft at Mercury indicates the planet closest to the sun has shrunk up to 7 kilometers in radius over the past 4 billion years, much more than earlier estimates.

The new finding, published in the journal Nature Geoscience Sunday, March 16, solves an apparent enigma about Mercury's evolution.

Older images of surface features indicated that, despite cooling over its lifetime, the rocky planet had barely shrunk at all but modeling of the planet's formation and aging could not explain that finding.

Paul K. Byrne

Now, Paul K. Byrne and Christian Klimczak at the Carnegie Institution of Washington have led a team that used MESSENGER's detailed images and topographic data to build a comprehensive map of tectonic features.

That map suggests Mercury shrunk substantially as it cooled, as rock and metal that comprise its interior are expected to.

Steven A. Hauck

"With MESSENGER, we have now obtained images of the entire planet at high resolution and, crucially, at different angles to the sun that show features Mariner 10 could not in the 1970s," said Steven A. Hauck, II, a professor of planetary sciences at Case Western Reserve University and the paper's co-author.

Mariner 10, the first spacecraft sent to explore Mercury, gathered images and data over just 45% of the surface during three flybys in 1974 and 1975.

MESSENGER, which launched in 2004 and was inserted into orbit in 2011, continues collecting scientific data, completing its 2,900th orbit of Mercury later this month.

Mercury contracted more than prior estimates, evidence shows

Mercury's surface is replete with cliff-like fault scarps. 

Here, one such scarp, Fram Rupes (image centre), lies near the terminator (the divide between day and night). 

The Fram Rupes scarp is almost 1.5 km high. 

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Carnegie Institution of Washington

Mercury's surface differs from Earth's in that its outer shell, called the lithosphere, is made up of one tectonic plate instead of multiple plates.

To help gauge how the planet may have shrunk, the researchers looked at tectonic features, called lobate scarps and wrinkle ridges, which result from interior cooling and surface compression.

The features resemble long ribbons from above, ranging from 5 to more than 550 miles long.

Lobate scarps are cliffs caused by thrust faults that have broken the surface and reach up to nearly 2 miles high.

Wrinkle ridges are caused by faults that don't extend as deep and tend to have lower relief. Surface materials from one side of the fault ramp up and fold over, forming a ridge.

The scientists mapped a total of 5,934 of the tectonic features.

The scarps and ridges have much the same effect as a tailor making a series of tucks to take in the waist of a pair of pants.

With the new data, the researchers were able to see a greater number of these faults and estimate the shortening across broad sections of the surface and thus estimate the decrease in the planet's radius.

Mercury's contraction much greater than thought

This image shows a long collection of ridges and scarps on the planet Mercury called a fold-and-thrust belt. 

The belt stretches over 336 miles (540 kilometers). 

The colours correspond to elevation -- yellow-green is high and blue is low. 

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Carnegie Institution of Washington

More information: Study paper: 'Mercury’s global contraction much greater than earlier estimates dx.doi.org/10.1038/ngeo2097

Martian clouds form in much more humid conditions

Clouds captured above a vast plain of sand by the Opportunity rover near Victoria Crater on Mars on October 2006. Credit: NASA

At first glance, Mars' clouds might easily be mistaken for those on Earth: Images of the Martian sky, taken by NASA's Opportunity rover, depict gauzy, high-altitude wisps, similar to our cirrus clouds.

Given what scientists know about the Red Planet's atmosphere, these clouds likely consist of either carbon dioxide or water-based ice crystals.

But it's difficult to know the precise conditions that give rise to such clouds without sampling directly from a Martian cloud.

Researchers at MIT have now done the next-best thing: They've recreated Mars-like conditions within a three-story-tall cloud chamber in Germany, adjusting the chamber's temperature and relative humidity to match conditions on Mars—essentially forming Martian clouds on Earth.

While the researchers were able to create clouds at the frigid temperatures typically found on Mars, they discovered that cloud formation in such conditions required adjusting the chamber's relative humidity to 190 percent—far greater than cloud formation requires on Earth.

The finding should help improve conventional models of the Martian atmosphere, many of which assume that Martian clouds require humidity levels similar to those found on Earth.

Dan Cziczo

"A lot of atmospheric models for Mars are very simple," says Dan Cziczo, the Victor P. Starr Associate Professor of Atmospheric Chemistry at MIT.

"They have to make gross assumptions about how clouds form: As soon as it hits 100 percent humidity, boom, you get a cloud to form. But we found you need more to kick-start the process."

Cziczo says the group's experimental results will help to improve Martian climate models, as well as scientists' understanding of how the planet transports water through the atmosphere.

He and his colleagues have reported their findings in Journal of Geophysical Research: Planets.

Seeding Martian clouds
The team conducted most of the study's experiments during the summer of 2012 in Karlsruhe, Germany, at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) facility—a former nuclear reactor that has since been converted into the world's largest cloud chamber.

The facility was originally designed to study atmospheric conditions on Earth. But Cziczo realized that with a little fine-tuning, the chamber could be adapted to simulate conditions on Mars.

To do this, the team first pumped all the oxygen out of the chamber, and instead filled it with inert nitrogen or carbon dioxide—the most common components of the Martian atmosphere.

They then created a dust storm, pumping in fine particles similar in size and composition to the mineral dust found on Mars.

Much like on Earth, these particles act as cloud seeds around which water vapor can adhere to form cloud particles.

Russian Ganymede Lander Mission more difficult than expected

Russia's proposed landing mission to Ganymede was discussed extensively last week at an international meeting hosted by the Space Research Institute of the Russian Academy of Sciences.

The mission to explore, and perhaps to drill, the Solar system's largest moon, presumably in close cooperation with the European Space Agency (ESA), would be a major challenge for Russia's space and science industries.

The project is generally approved, but success is far from assured.

The mission to Ganymede, now better known by the simple name of "Ganymede Lander", is the latest reincarnation of Russia's contribution to the Laplas project, promoted by the European Space Agency (ESA) in the early 2000s.

With Laplas becoming the single-spacecraft project JUICE (JUpiter ICy moon Explorer, until christened officially), Russian plans have also undergone major changes, although their main objective, sending a lander to Jupiter's biggest moon, remained intact.

The initial aim was to explore Europa, a smaller Jovian moon, where there is an ocean of liquid water beneath its frozen surface (around 10 km thick) and is therefore considered a good prospect for the exploration of habitable conditions.

Ganymede also holds liquid water, but much deeper, under an icy crust of around 130-150 km. On the other hand, this moon is farther from Jupiter with less radiation than Europa, putting spacecraft at a much lower risk.

However, the main argument for shifting to Ganymede was that the European mission now no longer plans to stay near Europa long enough to provide the high resolution images needed to select a landing site.

It is supposed now that the JUICE orbiting spacecraft will provide the Russian lander with preliminary reconnaissance data and perhaps act as a communication relay station for data sent between Ganymede and Earth.

Hence, the current scenario is that the European mission is developing independently while Russia's landing spacecraft has its own scientific payload and objectives.

As the success of the landing relies on many technical issues closely concerned with JUICE, the Russian equipment and goals must be taken into account from the start when designing the eventual lander.

A more detailed mission scenario was presented by Maksim Martynov, deputy general designer of the S.A. Lavochkin Association and head of the design bureau.

Following the "play safe" rule, it is supposed that Russia will send two spacecraft to Ganymede, a lander and a small additional orbiter to secure the landing site as a back-up option to information from JUICE.

Even though launched simultaneously from a Proton launcher, they will arrive separately. After reconnaissance and remote studies of the moon, the lander will be delivered to the surface to begin its studies.

The start is planned for 2022-23 with the completion in 2029-30 (JUICE is currently scheduled for 2022) and subsequent arrival on Ganymede within a few months.