The Week in Space and Physics: The Year of the Sun
On the solar maximum, Titan's strange icebergs, Japan's moon landing and a galaxy without any stars
In the dying days of 2023 two remarkable events took place close to our star. The first, and perhaps the more spectacular, was a vast solar flare; the most powerful to erupt from the Sun in six years. Fortunately its effects on Earth were minor. The flare merely grazed our planet, leaving little more than a vivid aurora in its wake.
The second, however, was an event of great technological accomplishment. On December 28 the Parker Solar Probe passed into the solar corona, touching a point just five million miles from the surface of the Sun. No other spacecraft has dared come so close to our star, and none, therefore, has ever given us such a detailed look as Parker just did.
That Parker was so close to the Sun when the flare erupted may seem like lucky timing. But in fact the event was not completely unexpected. Parker's latest voyage past the Sun came close to the solar maximum, a point in time when the Sun’s surface becomes blemished with spots and violent tempests rage across its surface.
The Sun’s activity has been rising for the last four years, as part of a regular ebb and flow in its behaviour. Roughly once every eleven years this activity reaches a peak, a moment known as the solar maximum. Afterwards its activity begins to weaken once more, descending towards a minimum some five to six years later.
For reasons still unknown, not all cycles reach the same peak. At times, indeed, the cycle has almost died down completely. Between 1645 and 1715, for example, sun spots became exceptionally rare and the astronomers of the day reported a weak and barely visible solar corona.
Predicting the strength of future cycles is a fool’s game. Yet the past cycle was considered a weak one, and the current one seems to be a strong one. Put together, that means the Sun is now more active than it has been in two decades - and that is offering modern telescopes and probes the perfect moment to study our star in depth.
For those of us who must remain on Earth, perhaps the most exciting opportunity of the year will be the solar eclipse of April 8. Over a period of five hours the Moon will pass directly in front of the Sun, carving a path of darkness over Mexico, across the United States and Eastern Canada and into the Western Atlantic.
As it does, the Sun’s outer atmosphere - the corona - will become briefly visible. For those watching, it will appear as a halo of white light surrounding the Moon, within which loops of pinkish red will be seen. Though the corona is visible in most total eclipses, that of April will be a rare chance to see the corona at full strength.
The Strange Icebergs of Titan
In many ways the surface of Titan resembles that of Earth. Lakes and seas are scattered across that faraway moon, fed by rivers flowing through a terrain of mountains and dunes. As on Earth, the presence of liquid has shaped its contours and fuelled an ever changing landscape.
Yet the rivers and seas of Titan are filled with methane, not water. Clouds of the stuff - which on Earth exists only as a gas - float serenely through its skies. From time to time they fall as rain, filling rivers and lakes with an oily liquid. This cycle of evaporation and rainfall is very Earthlike: but methane, it should be remembered, does not always act like water.
When the Cassini probe photographed Titan’s lakes in the 2010s, it found them to be strangely flat. Few waves were spotted rippling across them, which was odd, since we know winds should be blowing across them. At times features like islands seemed to appear within the lakes; only to have vanished by the time Cassini returned for a second look.
Since their discovery, some researchers have wondered if these islands might really be waves. If the seas of Titan are normally mirror flat, for whatever reason, then the occasional outbreak of waves could easily be mistaken for a temporary island.
A recent study favours a different explanation. As well as methane, Titan’s atmosphere contains more complex chemicals made of hydrogen, carbon and nitrogen. These should fall as snow across the moon, creating a thin layer of floating soot in the lakes. Most of that would quickly sink into the methane, but its presence would be enough to prevent large waves from forming.
On land, however, the sooty snow would pile up. Over time that could result in something like icebergs carving off and floating in the lakes. They wouldn’t be able to stay afloat for long - the study calculates they would gradually soak up methane and then sink. But they should last long enough to be photographed by a passing spacecraft, and so to be mistaken for an island.
Japan on the Moon
Japan last week became the fifth nation to land a probe on the Moon. Their SLIM spacecraft touched down on the lunar surface on Friday, targeting a precision landing south of the Sea of Tranquility.
Although Japan’s space agency confirmed that the probe had landed, they also reported that the spacecraft's solar panels were not working as expected. Without them, the probe was limited to the power contained in its batteries, which operators chose to turn off after three hours of use. It is possible, they say, that the panels could start to work in a few weeks once the Sun reaches the right point in the sky. That might allow SLIM to reawaken.
Still, SLIM seems to have been successful in its main goal of demonstrating precision landing techniques. The spacecraft was equipped with cameras and software capable of automatically bringing the probe to a desired landing site. In the end SLIM appears to have landed within one hundred meters of its target - well within the goals set for it.
In principle, the tools demonstrated by SLIM could be used to steer future probes towards areas of difficult terrain on the Moon or on Mars. Although such areas are often scientifically interesting, mission planners have so far preferred to land expensive spacecraft in easier places, and so reduce the risk of a failed landing.
Along with the lander, SLIM also deployed two small rovers to the surface. One, named SORA-Q, contained just enough power to turn itself on and send back a few photographs. The other, named LEV-1, is designed to hop about on the surface, collecting data about temperatures and radiation levels. Both seem to have worked as expected.
A Galaxy Without a Star
What is a galaxy without its stars? Of course, galaxies contain much more than their stars - they host gas clouds, black holes and, apparently, copious amounts of dark matter. But it is only through their stars that they become visible, and thus known to us.
In a chance discovery, however, astronomers recently spotted a galaxy that seems to have no stars at all. It was found by sheer accident: the astronomers had mistyped when pointing their telescope, and only realised their error when they later compared results with those obtained by a different telescope.
The galaxy seems to contain only hydrogen and dark matter, making it the darkest galaxy ever found. In form it could resemble the very earliest galaxies, albeit one in which star formation never got started. That may mean it is made of primordial hydrogen, say its discoverers, which would make it the only nearby object of its kind.
Good reading. Great job :-)