Search for active lava flows with VIRTIS on Venus Express

Kurzfassung

There are several lines of evidence suggesting that effusive volcanism still shapes the surface of Venus but the rate is not well constrained. The preservation of impact craters is consistent with a rate of 1 km³/year (comparable to Earth intraplate volcanism) as well as a rate several orders of magnitude less. It has been proposed to search images of the Venus nightside near 1 μm for the excess thermal emission of active lava flows. The scattering of photons in the optically thick cloud cover is similar in effect to a Gaussian blur with a Full Width Half Maximum (FWHM) of 90 km. This reduces the maximum intensity of the anomalies introduced by eruptions, making detection more difficult than in a clear atmosphere, but also provides larger targets for imaging. The VIRTIS instrument on Venus Express observed surface thermal emission at 1.02 μm wavelength between April 2006 and October 2008 at a spatial resolution of typically a few tens of km. We have searched the ~1000 images for the signatures of active lava flows in form of transient excess emission measured in several adjacent pixels. We estimate that eruptions with 1GW/μm/sr total excess specific thermal emission would be clearly visible compared to the instrumental and atmospheric noise but no such anomalies were detected. It has been shown that the total thermal emission of eruptions on Earth can be related to the lava discharge rate. We adjust this model to the Venus surface environment by adjusting observed lava surface temperature distributions and estimate that a discharge rate on the order of 1000 m³/s is required to produce an anomaly clearly identifiable in VIRTIS data. This is a relatively high value for effusive volcanism but a few historical eruptions on Earth surpassed it. Adopting a fit to the discharge rates of historical eruptions on Hawaii as a model, 4% to 10% of the eruptions in the field of view of VIRTIS images would have been detectable. In addition to the low detection probability, the low surface coverage of on average 1% of the surface per day over a period of 800 days indicates that the probability of an eruption imaged by VIRTIS was low to begin with. Therefore the VIRTIS data set fails to provide a useful constraint on the rate of volcanism on Venus. Venus Express continues to observe surface thermal emission with the Venus Monitoring camera. For future missions, near infrared imaging would profit from more frequent images of the same area than is possible from the excentric 24h orbit of Venus Express. The superrotation of the cloud cover allows to reduce atmospheric noise by averaging of images taken hours apart and the detection of eruptions with 100 m³/s lava or less might then be possible. Nevertheless, searching for lava flows emplaced between two observations of the same surface by high resolution radar imaging, altimetry or interferometry generally provides a better constraints than infrared imaging for the same observation duration and surface coverage. However, high resolution radar and low resolution infrared observations are highly complementary and could be accommodated on the same mission, preferably with a low circular polar orbit.