Earth-based and New Horizons measurements of Pluto’s atmospheric pressure conflict

Nature : According to measurements taken from Earth, Pluto’s atmosphere has been getting denser since 1988. But measurements taken by New Horizons as it flew by the dwarf planet in July show the opposite. A group of researchers led by Eliot Young of the Southwest Research Institute in Boulder, Colorado, say that on 29 June, just a few weeks before New Horizons’s arrival, Pluto’s atmospheric pressure was 0.022 Pa. New Horizons measured a pressure of just 0.005 Pa. The discrepancy could be due to the indirect measurement method used by the researchers, who based their finding on the fading of a star’s light as Pluto passed in front of it. Furthermore, the Earth-based calculation determines the pressure at an altitude of 50–75 km, whereas New Horizons calculated the pressure at Pluto’s surface. It does so by measuring the deflection of radio waves sent from Earth as they pass through Pluto’s atmosphere. To try to reconcile the two different measurements, the researchers plan to compare them with existing and new models of Pluto’s atmosphere.