Ashen light is a faint glow that some people say can be seen on the night side of Venus. Scientists have not yet confirmed this event, and many ideas exist about its cause. These include light from Venus itself or light effects inside the telescope used to observe it. A recent idea suggests the light might come from lightning, for which there is some evidence on Venus. However, this idea is no longer popular because the lightning does not create enough light to be seen. A newer theory is that the light comes from a type of short-lived aurora or airglow caused by very strong sunlight activity interacting with the top part of Venus's atmosphere.
History of observations
The discovery of the ashen light is often credited to Italian astronomer Giovanni Battista Riccioli. However, recent evidence suggests that German priest Athanasius Kircher may have observed the ashen light first during his only trip to Palermo, Sicily in the spring of 1638. Riccioli made the first clear and detailed record of the ashen light on 9 January 1643. He explained that the colors appeared because of the bending of light inside the telescope: "The colors arise from the various refraction of light in the glass, as it happens with trigonal glasses." This is likely a description of a phenomenon now known as chromatic aberration. Later, other scientists, including Sir William Herschel, Sir Patrick Moore, Dale P. Cruikshank, and William K. Hartmann, also reported observations of the ashen light.
By the late 19th century, people who saw the light described it as having the color of ash, similar to earthlight. Because of this, the phenomenon was called Venusian lumière cendrée in French or ashen light in English.
The ashen light is often seen when Venus appears in the evening sky, with the planet's evening terminator facing Earth. On 17 July 2001, Venus, which was 67% illuminated, reappeared from behind a 13% illuminated moon. Observers, including those using 61 cm (24 in) "Super RADOTS" telescopes, did not see the ashen light during this event. Video of the occurrence was recorded, but the camera was not sensitive enough to detect even the earthshine.
A good chance to observe the ashen light happened on 8 October 2015, when a 40% illuminated Venus reappeared from behind the unlit part of a 15% sunlit moon. The event was visible in dark skies across Central Australia and was recorded by David and Joan Dunham (of the International Occultation Timing Association) using a 10" f/4 Newton telescope and a Watec 120N+ video camera from a location near Alice Springs. They also observed the event with an 8" Schmidt–Cassegrain telescope. Neither the real-time visual observation nor a detailed review of the video showed any sign of the dark side of Venus.
Light source hypotheses
The Keck telescope on Hawaii observed a faint green glow and suggested it might be caused by ultraviolet light from the Sun splitting carbon dioxide (CO₂) molecules in Venus's atmosphere into carbon monoxide (CO) and oxygen (O₂). When oxygen recombines to form O₂, it emits a green light, but this light is too weak to explain the glow seen on Venus and is too faint to be seen with amateur telescopes.
In 1967, the Venera 4 mission discovered that Venus has a much weaker magnetic field than Earth. Venus's magnetic field is created by the interaction between its ionosphere and the solar wind, not by an internal dynamo like Earth's. This weak magnetic field offers little protection to Venus's atmosphere from cosmic radiation, which may cause lightning between clouds.
In 1957, Urey and Brewer proposed that CO, CO₂, and O₂ ions formed by sunlight could cause the glow. In 1969, scientists suggested the Ashen light might be an aurora caused by solar particles hitting the dark side of Venus.
During the 1980s, scientists believed the glow was caused by lightning on Venus. The Soviet Venera 9 and 10 missions found evidence of lightning through optical and electromagnetic data. The Pioneer Venus Orbiter recorded bright airglow in 1978. In 1990, Christopher T. Russell and J. L. Phillips supported the lightning idea, noting that repeated lightning strikes on Venus’s night side could create a visible glow. The European Space Agency’s Venus Express detected whistler waves in 2007, adding more evidence for lightning.
The Akatsuki spacecraft, launched by Japan’s JAXA, entered Venus’s orbit in 2015. It carries the Lightning and Airglow Camera (LAC) to search for lightning in visible light. The orbiter can view Venus’s dark side for about 30 minutes every 10 days. However, no lightning was detected during 16.8 hours of observation in July 2019.
Simulations suggest the lightning hypothesis is unlikely, as not enough light would reach Earth through Venus’s atmosphere. Some scientists believe the glow may be an illusion caused by how human eyes perceive bright, crescent-shaped objects. Spacecraft have not found clear evidence of the glow, leading some to think it is a myth.
A newer idea is that high solar activity might create auroral or airglow-like effects on Venus’s night side. After major solar storms, a green light (557.7 nm) has been seen in Venus’s upper atmosphere, similar to Earth’s auroras. This light usually appears only during solar events like coronal mass ejections (CMEs) or solar flares. However, weak emissions were observed in 2010 and 2013 during solar wind events called Stream Interaction Regions. In 2012, a CME hit Venus, causing a bright green glow. These events suggest charged particles, like those in Earth’s auroras, may be responsible.
In 2022, the Parker Solar Probe’s WISPR instrument captured images of Venus’s night side, showing the surface visible through clouds. Studies suggest nightglow, a faint light from the upper atmosphere, is more likely the source of the Ashen light. Human eyes are less sensitive to the red glow from Venus’s surface, which is mostly hidden by clouds. However, people with better sensitivity to red light might have seen this glow as the Ashen light.