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 is currently putting on a fine show in the east before dawn. Find a UK location with an unobstructed view due east an hour before sunrise to see the innermost planet some 6 degrees above the horizon from about 25 September—5 October. The very old crescent Moon lies just 2 degrees from Mercury at dawn on Thursday, 29 September.
Mercury attains a greatest easterly elongation of 18 degrees from the Sun on 15 March, the innermost planet’s best evening showing for Northern Hemisphere observers for the entire year. From 12–20 March, planets Mercury and Venus remain just 4 degrees apart low in the west 45 minutes after sunset as seen from the British Isles.
This multicoloured swirl of yellow and blue shows a prominent ring of gas near the North Celestial Pole. The pole appears to be fixed in place, while the rest of the night sky slowly circles around it because of Earth’s rotation. This image comes courtesy of ESA’s Planck satellite, which spent years mapping the entire sky in exquisite detail between 2009 and 2013.
