The formation of cloud droplets on aerosol particles, technically known as the activation of cloud condensation nuclei (CCN), is the fundamental process driving the interactions of aerosols with clouds and precipitation. The Intergovernmental Panel on Climate Change (IPCC) and the Decadal Survey indicate that the uncertainty in how clouds adjust to aerosol perturbations dominates the uncertainty in the overall quantification of the radiative forcing attributable to human activities. Measurements by current satellites allow the determination of crude profiles of cloud particle size, but not of the activated CCN that seed them. The Clouds, Hazards, and Aerosols Survey for Earth Researchers (CHASER) mission concept responds to the IPCC and Decadal Survey concerns, utilizing a new technique and high-heritage instruments to measure all the quantities necessary to produce the first global survey maps of activated CCN and the properties of the clouds associated with them. CHASER also determines the activated CCN concentration and cloud thermodynamic forcing simultaneously, allowing the effects of each to be distinguished.
International audienceThe formation of cloud droplets on aerosol particles, technically known as the activation of cloud condensation nuclei (CCN), is the fundamental process driving the interactions of aerosols with clouds and precipitation. The Intergovernmental Panel on Climate Change (IPCC) and the Decadal Survey (NRC 2007) indicate that the uncertainty in how clouds adjust to aerosol perturbations dominates the uncertainty in the overall quantification of the radiative forcing attributable to human activities. Measurements by current satellites allow the determination of crude profiles of cloud particle size, but not of the activated CCN that seed them. The Clouds, Hazards, and Aerosols Survey for Earth Researchers (CHASER) mission concept responds to the IPCC and Decadal Survey concerns, utilizing a new technique and high-heritage instruments to measure all the quantities necessary to produce the first global survey maps of activated CCN and the properties of the clouds associated with them. CHASER also determines the activated CCN concentration and cloud thermodynamic forcing simultaneously, allowing the effects of each to be distinguished. Capsule CHASER proposes to revolutionize our understanding of the interactions of aerosols with clouds by making the first global survey of the fundamental physical entity linking them: activated cloud condensation nuclei
Based on experience gained from using the VIRTIS instrument on VenusExpress to observe the surface of Venus and the new high temperature laboratory experiments, we have developed the multi-spectral Venus Emissivity Mapper (VEM) to study the surface of Venus. VEM imposes minimal requirements on the spacecraft and mission design and can therefore be added to any future Venus mission. Ideally, the VEM instrument will be combined with a high-resolution radar mapper to provide accurate topographic information, as it is done on the VERITAS mission [1].The permanent cloud cover of Venus prohibits observation of the surface with traditional imaging techniques over most of the visible spectral range. Fortunately, Venus' CO 2 atmosphere is transparent in small spectral windows near 1 µm (Figure 1). Ground observers have successfully used these windows during the flyby of the Galileo mission at Jupiter and most recently for the VMC and VIRTIS instruments on the ESA Venus Express spacecraft. Observations have revealed compositional variations correlated with geological features, but existing data sets contain only a few channels. VEM offers an opportunity to gain significant information about surface iron-bearing mineralogy by virtue of having five different channels for surface observations. Surface mapping by VIRTIS on VEX: The VIRTIS instrument on the ESA mission Venus Express (VEX) was the first instrument to routinely map the surface of Venus using the near-infrared windows from orbit [3][4][5]. The instrument is the flight spare of the VIRTIS instrument on the ESA Rosetta comet encounter mission [6]. Originally designed to observe a very cold target far from the Sun, it was adapted to work in the Venus environment. The instrument's main purpose on VEX was to study the structure, dynamics and composition of the atmosphere in three dimensions. However, the idea of surface studies was introduced very late in the mission planning and VIRTIS was never specifically adapted for this purpose. For example, the wavelength coverage was not optimal and only the long wavelength flank of the main atmospheric window at 1.02µm could be imaged. Despite these issues, VIRTIS was an excellent proof-of-concept experiment and far exceeded our expectations. It provided significant new scientific results and showed, for example, that Venus had volcanic activity in the very recent geological past [7].The VEM concept: VEM is focused mainly on observing the surface, mapping in all near-IR atmospheric windows using filters with spectral characteristics optimized for the wavelengths and widths of those windows. It also observes bands necessary for correcting atmospheric effects [8]; these bands also provide valuable scientific data on cloud thickness, cloud opacity variations, and H 2 O abundance variations in the lowest 15 km of the atmosphere.VEM is a pushbroom multispectral imaging sys- Figure 2. Breadboard for VEM used for first testing.
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