The Pinwheel Galaxy, swarming with dark matter.

The Pinwheel Galaxy, swarming with dark matter. Hubble Space Telescope, NASA

The north pole of Mercury. The dark area is ice.

The north pole of Mercury. The dark area is ice. NASA

Taurus the Bull.

Taurus the Bull. Library of Congress, “Urania’s Mirror”, published 1824

by Kim Malville


Venus has been dominating our evening skies these past few months. In March it drops suddenly from 32° in the western skies to 0° in four weeks. Venus passes the sun on March 25, when the planet sets 14 minutes after the sun at our latitude. Quite remarkably, it will also be visible in the morning skies for those folks who have a flat eastern horizon, rising 40 minutes before sunrise. It will be both an evening and morning “star”. At the end of the month, Venus will have made the complete switch from evening to morning and will be completely absent from our evening skies, rising in the east about an hour before sunrise.

March 1: As the skies darken, look for the slender crescent moon in the west. Nearby will be Mars and Venus

March 4: This evening the moon covers Aldebaran. A remarkable occultation of Aldebaran, the redshot eye of Taurus, (when the moon covers the star) will start around 8:30pm. It should be visible to the naked eye, but binoculars will help. The star will disappear behind the dark side of the lunar crescent. Get outside a little early and fix your eyes on Aldebaran. This is a special and rare event. Aldebaran is the brightest star that the moon ever occults.

March 10: The waxing gibbous moon (larger than half) is close to Regulus, the brightest star in Leo the Lion.

March 12: Daylight savings time starts at 2 am.

March 14: Jupiter, Spica (brightest star in Virgo) and the moon form a triangle.

March 20: Spring Equinox occurs at 4:29 am.

March 29: Just below and to the right of the slender crescent moon is Mercury

March 30: The moon passes Mars

March 31: The moon approaches Aldebaran again.

 Ancient ice on Mercury

Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 88 days is the shortest of all the planets in the Solar System. Mercury’s axis has the smallest tilt of any of the Solar System’s planets (about  1⁄30°), which means that sunlight never touches the insides of craters on its poles.  Having almost no atmosphere to retain heat, surface temperatures varies diurnally more than any other planet in the Solar System, ranging from a low at the poles of -370°F to a maximum of +800°F at high noon at the equator.

Two spacecraft have visited Mercury. Mariner 10 flew by the planet in 1974 and 1975. Messenger, launched in 2004, orbited Mercury over 4,000 times in four years before exhausting its fuel and crashing into the planet’s surface on April 30, 2015. As one of its most fascinating discoveries, Messenger detected layers of ice at the pole that may be more than 200 feet thick, a small glacier. The ice probably came from a series of comets that crashed on its surface. Only the ice of those comets that landed at its poles could remain. Perhaps the ice there now arrived more than a billion years ago when there were many more comets. Those comets may have provided not only the water in the oceans of Earth, the glacier of Mercury, but they also may have created oceans on Mars and Venus, which have now totally evaporated.

Could dark matter be made of black holes?

Last month I wrote about Vera Rubin’s discovery that galaxies are spinning too fast to hold themselves together. But they do keep from flying apart, and they have been doing that for billions of years. There appears to be an unknown source of gravity in these galaxies, which has been named dark matter. The nature of dark matter remains a perplexing puzzle, very perplexing because it must make up a whopping 84% of the mass in the universe.  What could it be?

A bizarre possibility (actually everything in astrophysics these days seems bizarre) is that dark matter in a galaxy consists of billions and billions of small black holes, which are too small to be individually detected. Black holes could have condensed out the unimaginably hot gas of the early universe when it was only one second old.  These primordial black holes may have diameters (size of their event horizons) of 100 miles and masses of 30 suns.

Black holes of this size were detected when gravity waves were announced in Feb. 2016 that had been produced by the violent merger of two black holes 29 and 36 times as massive as our sun. More detections of gravity waves in the next few years should reveal how many of these objects exist in our galaxy. A second burst of gravity waves was reported last June which was produced by black holes that were smaller, only 8 and 14 solar masses. Two additional events have been reported, and we await the details.

The Event Horizon Telescope

On April 5 the largest radio telescope on the earth, the Event Horizon Telescope, will be turned on for nine days. Consisting of 12 widely spaced telescopes spread across the earth from the South Pole, the Atacama Desert, and to northern Europe, it will have an effective size equal to that of the earth. Its goal will be to create an image of the black hole in the center of the galaxy, 26,000 light years away. During the lifetime of our Milky Way Galaxy, the black hole has gobbled up 4 million solar masses and has grown in size to 15 million miles. As large as that seems, as viewed from the earth its size is comparable to that of a grapefruit on the surface of the moon. It’s going to be a real challenge for this collection of telescopes to view the event horizon of the black hole, inside of which nothing can ever be seen.  Stay tuned, it should be exciting.