Understanding the plasma environment around Mercury.

As the closest planet to the Sun, has the most direct exposure to the solar winds of any planet in the Solar System. The planet has a weak magnetic field, with a magnetosphere strongly linked to the surface and exosphere (a zone around its solid surface within which individual atoms and molecules can be found moving freely, but seldom, if ever, interacting) by a range of processes. These are largely driven by the exchange of energy with the solar winds, and loss of material from the planetary surface. The magnetosphere contains a mixture of ions derived from the solar winds, primarily hydrogen⁺ and helium²⁺ ions, and others derived from the planet's surface via ionization of material in the exosphere. 

The Mariner 10 spacecraft made three flybys of Mercury in 1974 and 1975, detecting heavy ions thought to be derived from the planet's exosphere. Subsequent observations of Mercury by Earth-based telescopes were able to identify ions such as sodium⁺, potassium⁺, and calcium⁺. Between 2011 and 2015 the MESSENGER spacecraft orbited Mercury, during which time its Fast Imaging Particle Spectrometer was able to detect a variety of ions in the planet's magnetosphere, including hydrogen⁺ and helium²⁺ derived  from the solar winds, as well as heavier ions such as helium⁺, oxygen⁺, water group ions (hydroxide⁺, water⁺, and hydrogen peroxide⁺), sodium⁺, magnesium⁺, aluminium⁺, and silicon⁺, derived from the planet. The helium⁺ ions showed a relatively even distribution around the planet, but the other planetary-derived ions were concentrated around the planetary cusp (i.e. directly facing the Sun) and in the equatorial band of the nightside of the planet, although it was not possible to further define the distribution of individual types of ion.

The BepiColombo spacecraft is a joint project by the European Space Agency and the Japan Aerospace Exploration Agency which was launched in 2018, on aa trajectory which would lead it to make close flybys of Mercury in October 2021, June 2022, June 2023, September 2024, December 2024, and January 2025, before finally entering the planet's orbit in November 2026. 

In a paper published in the journal Communications Physics on 3 October 2024, a team of scientists led by Lina Hadid of the Observatoire de Paris, Sorbonne Université, Université Paris Saclay, École polytechnique, and Institut Polytechnique de Paris, present the result of a study of ion plasma observations made by BepiColombo's Mercury Plasma Particle Experiment instruments, during the flyby of Mercury made on 19 June 2023.

The 19 June 2023 flyby took BepiColombo to about 235 km above the surface of Mercury, enabling sampling of ions within the magnetosphere plasma at low altitudes along the spacecraft's trajectory. During the flyby BepiColombo approached Mercury from its dusk-nightside, passing through the post-midnight magnetosphere close to the equator of the planet, and moving away towards dawn-dayside. 

BepiColombo’s journey through Mercury’s magnetosphere. European Space Agency.

BepiColombo crossed the bow-shock of the planet inbound at 6.44 pm GMT, and outbound at 7.52 pm. Outside of this bowshock region, both before and after the crossing, ions with energies of about 10 and about 20 electronvolts were constantly observed, with mass-per-charge ratios of 1 and 16. This is consistent with hydrogen⁺ and oxygen⁺ ions, derived from water molecules outgassed from the planet.

Projections of BepiColombo’s third Mercury flyby trajectory in the aberrated Mercury–Sun magnetospheric (aMSM) coordinate system. (a) X'–Z' and (b) X'–Y' planes, all expressed in Mercury radii (The radius of Mercury is 2440 km). Note the displacement in (a) of the magnetopause relative to the planetary centre because of the northward offset of the magnetic dipole by approximately 0.2 of the radius of Mercury. In traditional MSM coordinates, the X-axis and Z-axis point to the sun and north pole, respectively, and the Y-axis completes a right-hand system. In the aberrated coordinates, Mercury’s orbital velocity is considered. The X-axis is anti-parallel to the solar wind direction in the rest of the reference frame of Mercury. The aberration angle varies between about 5.5° and about 8.4° assuming a solar wind speed of 400 km per second. The black arrows indicate the viewing direction of instrument during this flyby. The magenta and cyan crosses represent the observed inbound (and outbound) bow shock and magnetopause crossings, respectively. The red dot highlights the closest approach of BepiColombo to Mercury. The black solid and dashed lines represent the modelled dayside bow shock and magnetopause that are obtained from the statistical distribution of observed crossing points, respectively. Hadid et al. (2024).

As BepiColombo entered the dusk magnetosphere of Mercury, it encountered ions with energies of around 20 000 electronvolts, but following this the energy of the ions fell to a few tens of electronvolts. Hadid et al. interpret this area as the low latitude boundary layer, the area along the magnetospheric side of a planet's low-latitude magnetopause where plasmas form the magnetosheath and magnetosphere are mixed. This kind of energy dispersion within the plasma mantle is typically seen at high latitudes; its presence close to the equator of Mercury suggests a relationship between the plasma mantle and the low latitude boundary layer, with convection carrying ions deep into the magnetosphere. 

Hydrogen⁺ ions were detected in the low latitude boundary layer region which Hadid et al. interpret as having derived from the duskside magnetosphere (i.e. the part of the magnetosphere where the Sun is setting). The low latitude boundary layer region is also likely to contain heavy ions derived from the dayside exosphere of the planet and transported over the polar caps, although Hadid et al. are careful to emphasise that determining the origin of the low latitude boundary layer is beyond the scope of the current study.

Model of the hydrogen⁺ ion trajectories. (a) Shows various particle trajectory projections in the equatorial plane traced backward in time. (b) Shows the particle kinetic energy versus time. The ions are launched from different locations (closed circles) along BepiColombo’s orbit, and their trajectories are traced backward in time. The colour code depicts the different magnetospheric regions, viz., the Low- Latitude Boundary Layer in green, the umbra in blue, the Plasma Sheet Horns in yellow, and ring current in red. The test hydrogen⁺ ion trajectories were computed using a modified Luhmann–Friesen model for the magnetic field combined with a two-cell convection pattern for the electric field. The full equation of motion was integrated backward in time using a fourth-order Runge–Kutta technique. Hadid et al. (2024).

As BepiColombia enetered the umbra (shadow) of Mercury and inner part of the low latitude boundary layer at 7.24 pm it encountered 'cold' ions with energies as low as 30-100 electronvolts. Hadid et al. suggest that this might be because negatively charged, causing low-energy ions from the exosphere to become attracted towards it. The ions encountered around the low latitude boundary layer include oxygen⁺ and calcium⁺ and/or potassium⁺ ions thought to have originated from the dayside of the planet and lighter hydrogen⁺ and helium²⁺ ions, probably of solar origin.

At 7.28 pm, shortly after leaving the low latitude boundary layer, BepiColombo began to encounter 'hot' ions with energies in the kiloelectronvolt range, in an area corresponding to the 'plasma sheet horns' detected by the MESSENGER spacecraft a decade previously. These ions are thought to originate from the tail of the magnetosphere, and to be accelerated towards the planet by convection currents.

After passing through this region at 7.32pm, BepiColombo encountered a region with intense ion fluxes, with ion energies in the 5-40 kiloelectronvolt range. Because this layer is present at low altitudes in the equatorial region, Hadid et al. interpret this as a tenuous ring current, in which charged particles could become trapped in orbits of the planet at altitudes of 1.3-1.5 times its radius. At this altitude a hydrogen⁺ could orbit Mercury in about four minutes, bouncing back and forth on either side of the equatorial plane throughout its motion around the planet. The presence of such a ring current had been suggested from the MESSENGER data, but the data was rather limited, with particles with energies of no more than 13 kiloelectronvolts being detected. The greater energy range detected in the BepiColombo data provides much better support for the presence of such a ring current, although again it is not sufficient to state definitively that this is what is being detected. 

The high energy particles within this band appear to be hydrogen⁺ and helium²⁺ ions, but BepiColombo also encountered larger particles. The most common of these have energies of around 2 kiloelectronvolts, and are interpreted as oxygen⁺ ions, while more energetic particles, with energies of around 10 kiloelectronvolts, and are interpreted as being predominantly calcium⁺ and potassium⁺ ions, with some sodium⁺ ions also present. BepiColombo also encountered cold ions, with energies of about 15 electronvolts. These cold ions are presumed to have originated from the surface of Mercury, and peaked at the closest to the planet, an altitude of 332 km.

After passing through the post-midnight magnetosphere of Mercury, BepiColombo re-entered the planet's magnetosheath and then moved back into the solar wind. This solar wind comprises a compressed and heated stream of hydrogen⁺ and helium²⁺ ions, although outgasses water group ions could again be detected in this region.

Mercury’s magnetosphere during BepiColombo’s third flyby. European Space Agency.

The 19 June 2023 flyby of BepiColombo has provided us with our first reasonably detailed view of the structure of Mercury's magnetosphere, demonstrating that it is not greatly different from that of the Earth. Both low and high energy ions were observed in the planet's magnetosphere, including the deepest parts encountered, suggesting that ion sputtering (the dislodging of low energy ions from the surface of the planet by the impact of high energy ions from the Sun) plays a significant role in the system. The evidence collected by the spacecraft supports the presence of a ring current encircling Mercury, and for the first time demonstrate the presence of a low latitude boundary layer. 

The magnetosphere of Mercury will remain a subject of study for the remainder of the BepiColombo mission, including the planned further flybys and orbital stage, which should serve to greatly enhance our understanding of the planet's magnetic environment.

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Trying to understand the relationship between Sun-spot activity and Whale strandings.

Relatively little is known about the cues Whales use while migrating. Visual cues in the ocean are often limited, which may drive oceanic migrators to use other sensory modalities, such as the ubiquitous geomagnetic field. While it is impractical to perform behavioral assays on Whales, strandings have been recorded for decades, and may provide insight into whale migration. Many strandings document that the individual was neither ill nor injured and resumed normal activity following rescue. It is therefore possible that a portion of these animals stranded due to navigational errors.

In a paper published in the journal Current Biology on 24 February 2020, Jesse Granger of the Department of Biology at Duke University, Lucianne Walkowicz of the Adler Planetarium, and Robert Fitak and Sönke Johnsen, also of the Department of Biology at Duke University, examine records of Gray Whale, Eschrichtius robustus, strandings and records of Sun-spot activity to evaluate claims that there is a relationship between the two.

A Gray Whale stranded on Ocean Beach, San Francisco, in May 2019. Justin Sullivan/Getty Images.

Although many factors impact strandings (e.g. naval mid-frequency sonar, disease, etc.), Granger et al. focused on whether strandings can be used to study the potential for magnetoreception in migratory Whales. Previous studies have used spatial patterns in strandings to suggest the potential for magnetoreception in Cetaceans. They used 31 years of Gray Whale, Eschrichtius robustus, stranding data (186 individual events) to build on earlier work that found a positive relationship between strandings and sunspot counts. Sunspots are strongly correlated with solar storms, sudden releases of high-energy particles from the sun that modify the geomagnetic field and thus have the potential to disrupt magnetic orientation behavior. Granger et al. examined relationships between strandings and two aspects of Earth’s magnetosphere altered by solar storms, radio frequency noise and displacements in the Earth’s magnetic field. Their results suggest that the increase in strandings under high solar activity is best explained by an effect on the sensor, not on the magnetic field itself.

Granger et al. acquired U.S. Gray Whale stranding data spanning 1985–2018 from the National Oceanic and Atmospheric Administration. THey chose the species because it has one of the longest migrations of any Mammal, an extensive history in the stranding database, and is a near-shore migrator, suggesting that small navigational errors increase the risk of stranding. Each stranding was examined, and o nly those that likely stranded alive with no signs of injury, illness, emaciation, or Human interaction (e.g. entanglement, or boat strikes) were used. While the multi-factorial nature of strandings adds variation to this data set, Granger et al. hypothesize that isolating healthier Whales is a more efficient method to study navigational effects.

Granger et al. examined the frequency of live strandings in relation to sunspot count, and performed a permutation test that demonstrated strandings occurred more often on days with high solar activity. A plausible explanation for this result is that solar magnetic storms are disrupting features of Earth’s magnetosphere and, in turn, affect the whale’s navigational system. Solar storms could have two impacts on magnetic orientation. They could alter the geomagnetic fi eld, leading to false information, or disrupt the animal’s receptor itself, leading to an inability to orient. To better understand how this phenomenon may be affecting the Whales, they examined two effects of solar activity. The first, solar radio flux, is a globally averaged measure of radio frequency noise nominally measured at 10.7 cm wavelength (frequency: 2800 MHz). Solar storms cause an increase in broad-band radio flux noise. Granger et al used 10.7 cm radio flux noise because it has been reliably recorded for the longest time period.  Radio flux noise has been shown to affect magnetic orientation in several species, and thus acts as a proxy for disruption to the receptor itself. The second, Ap-index (daily average level geomagnetic activity), is a measure of displacement in Earth’s magnetic field, and thus acts as a proxy for the accuracy of magnetic information available.

 A solar storm. NASA.

Granger et al. examined the frequency of live strandings in relation to radio flux noise and performed a permutation test that demonstrated strandings occurred more often on days with high radio flux noise. Tthis test did not show that whales stranded more often on days with a higher Ap-index than random days.

A logistic regression was used to determine whether these effects were due to multicollinearity between solar activity and climate or seasonal effects. Granger et al. looked at the previously described solar variables (radio flux and Ap), along with season (measured as day-of-year), and a well-established parameter that encapsulates climate variability, the Pacific Decadal Oscillation. The Pacific Decadal Oscillation is a recurring pattern of ocean–atmospheric variability, similar to El Niño. It is known to impact tropical storm activity, rainfall patterns, marine ecosystem productivity, and global temperature patterns. Variance infl ation factors show no multicollinearity between these variables.

Finally, Akaike Information Criterion model selection (a statistical method for comparing the relative quality of models) was used to determine which variables best fit the data. The best model was Season plus radio flux and adding the Pacific Decadal Oscillation did not improve performance. Further analysis of the effects of Pacific Decadal Oscillation indicated it was not useful for predicting strandings. In addition, radio flux alone performed significantly better than season alone, Pacific Decadal Oscillation alone, or season plus Pacific Decadal Oscillation. Logistic regression (used to model the probability of a certain class or event existing), this gave a 2.3-fold increase in the likelihood of a stranding on high sunspot days, and a 4.3-fold increase for high radio flux-noise days.

Granger et al.'s work indicates that the relationships found by previous researchers between strandings and sunspots are robust to additional statistical methods that account for the underlying distribution of the data, collinearity with seasonal or climate variability, and autocorrelation between the time of strandings. There is a history of research on correlations between solar activity and migratory behavior; however Granger et al.'s study is the first to examine potential mechanisms mediating this correlation by examining geophysical parameters that are affected by solar storms. Specifically, they found that this relationship was best explained by increases in radio flux noise rather than alterations to the magnetic field. These results are consistent with the hypothesis of magnetoreception in Gray Whales, and tentatively suggest that the mechanism for the relationship betwee solar activity and live strandings is a disruption of the magnetoreception sense, rather than distortion of the geomagnetic field itself. While these results are consistent with the radical pair hypothesis of magnetoreception, which is predicted to be disrupted by radio flux noise, they do not preclude other possible receptors, such as one based on magnetite. This research is not conclusive evidence for magnetoreception in this species, and further research is still necessary to determine the mechanism for the increase in strandings under high radio flux noise. Finally, while Granger et al. examine only a few aspects that contribute to strandings, many other variables are known to influence live-strandings. 

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First results from the Messenger Mission to Mercury Published.

The Messenger Mission was launched on 3 August 2004 and moved into orbit around Mercury in March 2011, after a rather circuitous trip, involving a flyby of Earth, two of Venus and three of Mercury itself. It has been transmitting a steady stream of information back to Earth for six months now. This week NASA published the first findings derived from this information in a series of papers in the journal Science.

An animation showing the route Messenger took to Mercury.

The first two papers concerned the composition of rocks on the surface of Mercury, and the inferences that could be made about the origins of the planet from this. The first of these papers, by a team led by Larry Nittler of the Department of Terrestrial Magnetism at the Carnegie Institution in Washington DC focuses on the abundance of common elements in the crust. This reveals that the surface of Mercury has an extremely rich in magnesium and poor in aluminium and calcium compared to other terrestrial planets, and that it is also extremely rich in sulphur. The second paper, by a team led by Patrick Peplowski of the Applied Physics Laboratory at Johns Hopkins University concentrated on radioactive elements, finding the surface of Mercury to have an unusual abundance of radioactive potassium, relative to the radioactive elements thorium and uranium. These findings both suggest that the surface of Mercury has a highly evolved surface; that is to say that it is likely to have been formed from a medium with a more usual balance of elements, which was then separated out by some process, similar to the separation of elements that occurs in lavas cooling slowly within a volcano.

The third paper, by a team led by James Head of the Department of Geological Sciences at Brown University, studied the physical geology of Mercury's northern hemisphere. This revealed that a vast area of the planet, in excess of 6% of the total surface, was covered by vast areas of flood basalt. Flood basalts are areas where vast amounts of volcanic rock have erupted onto the surface of a planet at one time. On Earth these have been associated with mass extinction events; the Deccan Traps in India are associated with the end of the Cretaceous, and the larger Siberian Traps in Russia (less reliably) with the end of the Permian. However the extent of flood basalts far outstrips anything seen on Earth, and implies a catastrophic event on a scale far beyond anything Earth has ever seen (though rather less likely to have caused a mass extinction, given Mercury's lack of an atmosphere, and therefore life).

The Deccan Traps; layer after layer of volcanic rock covering a vast area of eastern India.

The fourth paper is by a team led by David Blewit, also of the Applied Physics Laboratory at Johns Hopkins University. This deals with a feature seen on the surface of Mercury that has never been seen on any other planet or moon. Many craters on Mercury are dotted with mysterious, highly reflective blue cavities, or hollows, ranging in size from a few tens of meters to several kilometers across. Blewit et al. theorize that these have formed where minerals have sublimated (passed from a solid to a gaseous phase, without ever being a liquid) in the intense heat at Mercury's surface.

Mysterious blue hollows on the surface of Mercury.

The fourth paper is by a team led by Brian Anderson, again of the Applied Physics Laboratory at Johns Hopkins University, and concerns Mercury's magnetic field. This is very week compared to that of Earth - about 1.1% the strength - but far stronger than those of Mars or Venus. The magnetic poles of Mercury are very close to the geographical poles (the points on the end of a hypothetical line about which the planet spins), only about 3% off. This suggests that the field is being generated by the spinning of a liquid iron core within the planet.

In the absence of an atmosphere this week magnetic field is the only protection that the planet has from the solar winds (a stream of charged particles constantly emitted by the sun) - and it isn't quite enough, as these winds actually touch the planet at the poles. This leads to the ionization of atoms at the surface, generating a planetary field of charged ions which is the subject of the fifth paper in the journal. In this paper a team led by Thomas Zurbuchen of the Department of Atmospheric, Oceanic and Space Sciences at the University of Michigan analyse this field, finding it to be rich in positively charged sodium and oxygen ions (i.e. sodium and oxygen atoms that have been stripped of one or more electrons), which on the side of the planet away from the sun rival the density of free protons (ionized hydrogen particles). There is also a considerable density of ionized helium atoms, though these are more evenly distributed.

The sixth paper also concerns Mercury's magnetic field. In this study a team led by George Ho of the Applied Physics Laboratory at Johns Hopkins University study the (intense) electron field about Mercury and come to the conclusion that the planet's magnetic field is to week to generate Van Allen radiation belts; torus shaped belts of charged particles that surround the Earth and other planets with strong magnetic fields, leading to Mercury having fields with less well defined shapes.

See also Saturn's moon Dione found to have an atmosphere, Running water on present day Mars and The surface of Vesta.