def M2afrho1(M1):
    """Convert JPL's absolute magnitude, M1, to Afrho at 1 AU.

    Based on an empirical correlation between M1 and Afrho as measured
    by A'Hearn et al. (1995).  There is easily a factor of 4 scatter
    about the trend line.

    Parameters
    ----------
    M1 : float
      Comet's absolute magnitude from JPL.

    Returns
    -------
    Afrho1 : float
      Afrho at 1 AU.  [cm]

    """
    return 10**(-0.208 * M1 + 4.687)