Abstract
Acetylene (C2H2) and ethane (C2H6) are both produced in the stratosphere of Jupiter via photolysis of methane (CH4). Despite this common source, the latitudinal distribution of the two species is radically different, with acetylene decreasing in abundance towards the pole, and ethane increasing towards the pole. We present six years of NASA IRTF TEXES mid-infrared observations of the zonally-averaged emission of methane, acetylene and ethane. We confirm that the latitudinal distributions of ethane and acetylene are decoupled, and that this is a persistent feature over multiple years. The acetylene distribution falls off towards the pole, peaking at ∼ 30°N with a volume mixing ratio (VMR) of ∼ 0.8 parts per million (ppm) at 1 mbar and still falling off at ± 70° with a VMR of ∼ 0.3 ppm. The acetylene distributions are asymmetric on average, but as we move from 2013 to 2017, the zonally-averaged abundance becomes more symmetric about the equator. We suggest that both the short term changes in acetylene and its latitudinal asymmetry is driven by changes to the vertical stratospheric mixing, potentially related to propagating wave phenomena. Unlike acetylene, ethane has a symmetric distribution about the equator that increases toward the pole, with a peak mole fraction of ∼ 18 ppm at about ± 50° latitude, with a minimum at the equator of ∼ 10 ppm at 1 mbar. The ethane distribution does not appear to respond to mid-latitude stratospheric mixing in the same way as acetylene, potentially as a result of the vertical gradient of ethane being much shallower than that of acetylene. The equator-to-pole distributions of acetylene and ethane are consistent with acetylene having a shorter lifetime than ethane that is not sensitive to longer advective timescales, but is augmented by short-term dynamics, such as vertical mixing. Conversely, the long lifetime of ethane allows it to be transported to higher latitudes faster than it can be chemically depleted.
Highlights
The temperature of Jupiter’s stratosphere is determined by the balance between solar heating via absorption of sunlight in the visible and near-infrared, and radiative cooling by hydrocarbons, predominantly in the mid-infrared (Yelle et al, 2001; Moses et al, 2004).Photolysis of methane (CH4) produces acetylene (C2H2) and ethane (C2H6)
Melin et al / Icarus 305 (2018) 301–313 the pole. This was confirmed by Nixon et al (2007), who analysed mid-infrared observations taken with the CIRS instrument (Flasar et al, 2004a) during the Cassini flyby of Jupiter in 2000
There is a clear heating event in December-2014 and March-2015 data at 25°N. This heat has mostly dissipated by November 2015, remnants of this meteorological event are still visible during later observations – see Fig. 3. This heating at northern mid-latitudes may coincide with the expansion of the North Equatorial Belt (NEB, Fletcher et al, 2017), confirming the elevated 1 mbar temperatures identified by Fletcher et al (2016) in the December 2014 TEXES data
Summary
The temperature of Jupiter’s stratosphere is determined by the balance between solar heating via absorption of sunlight in the visible and near-infrared, and radiative cooling by hydrocarbons, predominantly in the mid-infrared (Yelle et al, 2001; Moses et al, 2004).Photolysis of methane (CH4) produces acetylene (C2H2) and ethane (C2H6). Melin et al / Icarus 305 (2018) 301–313 the pole This was confirmed by Nixon et al (2007), who analysed mid-infrared observations taken with the CIRS instrument (Flasar et al, 2004a) during the Cassini flyby of Jupiter in 2000. These two abundance distributions are surprising, given that both species are photochemical products of methane, and have an expected peak production rate about the equator, where the solar flux is the greatest (Moses et al, 2004)
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