In this perspective view, we look west across Suisei Planitia (blue colours), the site of some of the crustal magnetic signals. The plains are comprised of volcanic lava flows that erupted and solidified several billion years ago, filling the low areas between the higher topography (red colours). The impact crater Kosho, 65 kilometres in diameter, is seen in the centre of the image (deep blue floor), and part of the crater Strindberg, 190 kilometres in diameter, is seen in the lower left at the edge of the image. The background image is Mercury Dual Imaging System global mosaic, coloured by surface elevation measured by the Mercury Laser Altimeter (MLA), both draped over a digital elevation model derived from MLA data. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.Mercury’s magnetic field, generated by a dynamo process in its outer core, has been in place far longer than previously known, a paper by Planetary Science Institute’s Catherine Johnson reports.
About 4 billion years ago, Mercury’s magnetic field could have been much stronger than today, as indicated by low-altitude observations made by NASA’s MESSENGER spacecraft that revealed evidence of magnetisation of ancient crustal rocks on Mercury.
The MESSENGER spacecraft crashed onto Mercury on 30th April after running out of fuel, but the mission provided a trove of new information on the planet closest to the Sun.
“From MESSENGER and Mariner 10 observations we already knew that Mercury has had a global magnetic field today and 40 years ago,” said Johnson, a senior scientist at the Planetary Science Institute and lead author of “Low-Altitude Magnetic Field Measurements by MESSENGER Reveal Mercury’s Ancient Crustal Field,” published May 7th in the journal Science.
In this cartoon of field lines and observed signal, the white lines show schematic magnetic field lines above the surface of Mercury from magnetised crustal rocks. The MESSENGER spacecraft passed over a region of crustal magnetisation and the Magnetometer instrument measured small variations in the magnetic field (illustrated by the blue wiggly line). Because the signals are small they were only observed when MESSENGER was very close to the planet. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.With MESSENGER orbiting Mercury closer than 100 kilometres (~60 miles) from the planet’s surface, the spacecraft’s magnetometer instrument that measures magnetic field strength and detection was able to resolve signals too small to be detected earlier at higher altitudes. The observed decrease in signal strength measured with changes in altitude from 15 to 80 kilometres (9 to 50 miles) confirms that the signals are due to the presence of magnetised crustal rocks, Johnson said.
Mercury is the only inner Solar System body other than Earth that currently possesses a global magnetic field generated by a dynamo in a fluid metallic outer core. In Mercury, as in Earth, the outer core is molten iron.
“Magnetised rocks record the history of the magnetic field of a planet, a key ingredient in understanding its evolution,” Johnson said. “We already know that around 3.7 to 3.9 billion years ago Mercury was volcanically and tectonically active. We now know that it had a magnetic field at around that time.”
“If we didn’t have the recent very low-altitude observations, we would never have been able to discover these signals,” said Johnson. “Mercury has just been waiting to tell us its story.”
Mercury attains a greatest easterly elongation of 19 degrees from the Sun on 1 April. This solar separation combined with a favourable inclination of the ecliptic to the western horizon an hour after sunset, means that the period 25 March to around 8 April offers the year’s best evening showing of the innermost planet for Northern Hemisphere observers.
On Monday, 9 May just after midday BST, suitably equipped observers in the British Isles can witness the start of a 7½-hour spectacle that hasn’t been seen for almost a decade — the silhouette of innermost planet Mercury crossing the face of the Sun. Here’s our online guide to observing this fascinating and comparatively rare event in complete safety.
If the current alignment of all the bright naked-eye planets — Mercury, Venus, Mars, Jupiter and Saturn — in the morning sky has prompted you to speculate where in the world might be the best place to view the spectacle, here’s a guide to using our upgraded interactive Almanac to help you find out.
