Analysis of Charon’s spectrum at 2.21-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si59.svg" display="inline" id="d1e2257"><mml:mrow><mml:mi mathvariant="normal">μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math> from New Horizons/LEISA and Earth-based observations