SKY REPORTER: Messages Well Received

The Last Image from Messenger taken on April 30, 2015.
(NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

On April 30 the Messenger spacecraft, its helium propellant needed for maintaining safe altitude exhausted, crashed onto the surface of Mercury concluding a highly productive 1,504 day orbital mission.

Messenger provided the first comprehensive survey of the speedy planet Mercury. It was only the second exploratory voyage from Earth to reach that world and the first to orbit—37 years after the Mariner 10 spacecraft made three quick fly-bys—returning data and images that revealed only limited areas of the planet’s surface.

A complex series of powered maneuvers and acrobatic gravitational assists was required during the spacecraft’s journey after launch in 2004. Successful orbiting of Mercury was among many firsts, both scientific and technological. To prevent Messenger from being roasted in the blaze of solar heat a six by eight foot sunscreen was required. 

The Messenger science team under the leadership of principal investigator Sean Solomon, Director of Columbia University's Lamont-Doherty Earth Observatory, gleaned remarkable details about Mercury’s nature. For example, there was substantial evidence that despite surface temperatures ranging as high as 530^oF, deep on the floors of Mercury’s polar craters, permanently hidden from direct rays of the Sun, there exists water ice. In those places, temperatures as low as minus 370^oF were recorded. The icy locations correspond to bright white spots previously revealed in radar images made with the huge Arecibo radio telescope.

Due to the intensive blast of sunlight, it was long assumed that small Mercury was incapable of holding an atmosphere. However Messenger confirmed existence of a thin exosphere constantly in flux, losing gas to outer space while surviving due to infusions of replacement molecules emerging from the planet’s interior. The craft’s Atmospheric and Surface Composition Spectrometer also observed seasonal variations in the intensity of a bright orange glow resulting from sunlight being scattered by sodium in the exosphere. A similar hue is seen in the nighttime aura above New York due to the city’s sodium vapor streetlights.

Consistent with rumpled surface features long attributed to global shrinkage, Messenger’s Laser Altimeter and Dual Imaging System revealed Mercury’s diameter has evidently contracted during cooling by more than eight miles, substantially more than previously believed.

As hard as it was for members of the Messenger team to say good-by, a final serendipitous gift from the dying craft has just been revealed—an insight made possible by the craft’s precipitous drop in altitude during recent months. Proximity to the ground allowed its magnetometer to detect weak previously unknown magnetic fields coming from surface rocks. This ancient vestigial magnetism is far less intense than the planet’s present active magnetosphere, even though the current field is only one percent as powerful as Earth’s magnetism. The present global magnetic field of Mercury is produced by dynamo effects in the planet’s liquid iron core. Comparisons with the ancient surface magnetism indicates Mercury’s field has evolved significantly during the past 3.7 to 3.9 billion years since surface rocks solidified thereby locking in imprints of magnetism that existed long ago. Discovery of these ancient residues ratchets up debate about why neither Venus nor Mars now have global magnetic fields although comparatively sized Earth and Mercury are cloaked in magnetism. 

The Moon

The Planets

On May 6, Mercury was at its greatest eastern elongation, 21 degrees of arc from the Sun.

All five planets visible to our unaided eyes, Mercury, Venus, Mars, Jupiter, and Saturn are above the horizon during evening twilight at mid-month, however low in the west, Mercury and Mars are challenging to see without binoculars.

Saturn now has visual magnitude about zero, rivaling the brightest stars in our night sky. Jupiter currently is at magnitude –2 six times brighter than Saturn, but Venus tops them both with a dazzling –4, and shines 40 times brighter than Saturn.  

The crescent Moon may be seen near Venus and Jupiter from May 20 through the 24. Nine p.m. Wednesday the 20, a two day old narrow lunar crescent is 16 degrees of arc (about one and a half fist lengths) above the west point of the horizon and 12 degrees beneath Venus.

Saturn is at opposition May 22, when the planet is visible all night rising around sunset and setting at sunrise the next morning. 

On the night of May 24, the Moon, one day before its first-quarter phase, is about 11 degrees from Jupiter.