Just over a year ago, Japan’s Akatsuki spacecraft slid into orbit around Venus. What it saw that first month is making news today. A bow-shaped feature stretching nearly pole to pole over a mountainous region. 6,000 miles long.
On a planet where winds regularly approach speeds north of 200 miles per hour, this feature didn’t move. It didn’t budge for four days. That’s how long Akatsuki was able to watch the feature before continuing along its elliptical orbit.
The satellite soars above Venus’ surface at just 250 miles during its closest, but swings way out to 270,000 miles at its furthest point.
Did You Know: Akatsuki was initially supposed to be orbiting Venus in 2010. Instead of executing an orbital insertion burn for 12 minutes, the spacecraft’s engine burned for less than three minutes. JAXA tried it again five years later. By the time the second opportunity presented itself, the spacecraft was already passed its original 4.5-year mission. JAXA placed Akatsuki into safe mode to help make it last longer. The December 2015 attempt was a success. Four months later, another engine burn reduced the spacecraft’s orbital period from 13 days to nine.
One month later, Akatsuki dove back towards Venus and was ready to observe the feature again. This time, it was gone. Besides a few teasing glimpses in April and May last year, the bow-shaped feature hasn’t shown back up.
Image sequence showing the stationary bow-feature. The blue and yellow line represent the day/night line (morning and evening).
What were scientists looking at? They believe it was a “gravity wave.” Imagine throwing a stone into a pond. The ripples traveling horizontally are similar to gravity waves in Venus’ atmosphere. With one major difference. Atmospheric gravity waves move vertically instead of horizontally. We see them on Earth, and they are often accompanied by strong winds.
The one spotted above Venus appears above Aphrodite Terra, a highland region about the size of Africa. This region is home to mountain ranges, and it’s believed to be this geography that is responsible for the bow-shaped feature.
Scientists say numerical simulations do offer preliminary support for a gravity wave. “But the formation and propagation of a mountain gravity wave remain difficult to reconcile with assumed near-surface conditions on Venus,” the scientists write. They also don’t know why the prominent feature disappeared.
Maybe winds in the deep atmosphere are more variable than originally thought. Scientists just don’t have a firm answer yet.
Akatsuki was designed for in-depth study of Venus’ atmosphere. What makes the wind blow so hard? Are there lightning strikes? What about current volcanic activity? Those are the questions Akatsuki’s team hopes to answer during a two-year period of science operations that started in May 2016.
We’ll have to wait and see if the bow-shaped feature makes another appearance.